"If SOME is GOOD and MORE is BETTER then absolutely TOO MUCH should be just about RIGHT." Had some tee shirts made up years ago memorializing a meeting where a senior VP went around basically chanting the first two-thirds of this quote in his presentation. When I added the last third during a lull in the chanting, the VP just stared at me with a stunned look. The President took a liking to me right then and there and made sure I was included in more meetings, which were occasionally not fun.
When I was a physics grad student in the 80s I disagreed with a professor about an E&M problem - the prof was a real *sshole about it and I was sure I was right. I phoned up Feynman at his home (he was in the directory!) and asked him his opinion. He told me I was right (this story ended up doing the rounds at UCI) and he asked me the sprinkler problem. I gave a few different answers that I said were naïve answers (which are covered in your video!), and that I was unsure. He told me to call him back when I had my answer. Overall we had a 45 minute conversation - I felt very honored. I became disappointed in myself as I never got a fully convincing answer so never called him back, and he died in 1988. I felt like I had failed the great man - until I saw your video today!!!!
For the same reason it took them so long to.....just freaking build an apparatus and test it..... Because physicists aren't as smart as engineers ;) Id argue if this is the effect at play then they obviously could manipulate the tube runs to reverse the reversed reversal of the reversed flow....ya know, but backwards.
Am I the first person to notice that the description of Feynman's experiment is wrong? Actually, he tried to pump air into the top of the carboy to push the water backwards through the tubing; he didn't suck the water out of the tube. Eventually the pressure blew the carboy apart. See "Surely You're Joking, Mr. Feynman" at the end of Part 2: The Princeton Years.
Most mathematical problems that are waiting for a solution are named after smart mathematicians who could not find a solution. Q.E.D. Once the problem is solved, it does not take on the name of the successful first solver.
I wish they would have redesigned the test so the arms of the sprinkler don't have that central cavity for the vortexes to form. They could have brought the two tubes together in an upside down Y with the leg of the inverted Y pointing straight up in the center -- that should eliminate those vortexes that were contributing rotational forces.
Yeah, after watching the video, I still have the feeling that this massivly depends on thd design of the sprinkler. Of you'd add an infinite ammount of arms, you'd end up with something simmelar to a tesla turbine, which would possibly spin in the other direction. And a different hub layout might also form the center vortecies in a different way.
my thought exactly. they could even bend them parallel before joining along their sides to preserve laminar flow. to the point that even there the cumulative outer track of water would still move faster and might still cause slight asymmetries: then there is probably a way to angle the inlet jets entering the central chamber to compensate for the lopsided velocities. just angle them until all 4 vortices are equal. there's probably a million variations you could build, but I'm betting per design, there is a small adjustment which preserves the behavior with water flowing out, but which is balanced when water flows in. its not about principle, its about which of like 7 minute balancing acts your current design happens to be failing the most, and that is the latent rotation being seen.
@@clockworkvanhellsing372directional baffles could align all the vortices, I would think. Such a setup should eliminate now dampened speed, and making it more relative omnidirectionally.
Excellent thought ALSO the fact that the outer curve away from the center has more surface area against which the incoming fluid would press against would seem to be relevant too (I’m not sure why it would matter vs fluid already in the tube though to be clear…) - even altering the laminar vs turbulent friction with varying materials there would impact things etc!?
@@arstinoit’s more the surprise that it took so long for someone to build and test this. It’s… not hard to do. Especially if you simplify the problem. The real question is whether a fluid getting sucked into a curved pipe exerts a push or pull force on the pipe, if any force at all.
"almost entirely unburdened by modesty." That is the greatest description of Feynman! He wasn't so much arrogant as bereft of any desire to *not* be arrogant.
I think you're not quite right. He really really wished to avoid being given any credit for one skill on the basis of something that had nothing to do with it. He didn't put his name Feynman on the drawings and portraits he was good at.
There are two types of superiority. The psychological (complex) kind and the factual (real) kind Feynman was the real kind. He was better. That’s a fact. Was he an a\*\*hole about it? Nope, and I love it
This seems more a function of the specific design of the sprinkler internals. If the pipes were angled to have the vortices' sizes inverted it could be made to rotate in the other direction.
If there was a suitably shaped baffle inside the sprinkler head, it could be therefore made to rotate slowly in any direction, or not at all, it seems. Normal sprinkler rotation, including overcoming friction, is surely mostly rocket science - reaction to the mass of the water sprayed in the opposite direction of rotation of the sprinkler head.
They should have directed the internal jets upwards so all the incoming water streams are flowing in parallel before they are allowed to interact to avoid "spooky actions in a (hidden) vortex". IMO this experiment has not effectively addressed the original question.
Yea, I have no idea why they had two tubes in this experiment. I assume three would cause similar vortices but how about just one tube so there wouldn't even be a need for a chamber like that? Could even have just round corners so there shouldn't be any noticeable vortices at any point.
Yeah, that's what I was thinking. I want to see what happens if they design a system to nullify these internal forces, and focus only on the water in the arms of the sprinkler.
@@lisabenden If you nullify the internal forces then you aren't actually testing the problem :) The internal forces are a result of the bends in the pipe. If you nullify them, then you will have to by definition design your "reverse" sprinkler, to impart forces onto the sprinkler system to counteract the "natural" designs rotation. By having pipes that bend, you end up with water flow that is not "centrered" in the pipe. This "non uniform flow" is the effect that causes the rotation.
TLDR: The 100 year old answer of "depends on what engineering choices you pick to have the most effect" is the right one and nothing was actually discovered beyond why small house vacuums often have the intake opening on the side which was also known for quite a while.
Ye i thought the same. So if the sprinkler doesnt have cnc quality openings but instead a janky mold of some sort the answer would be completely different? How would the scenario play out if you used turbulent flow?
They basically engineered a sprinkler to get a result. That particular sprinkler design didn't exist until they made it. Its result is rather inconsequential. As the question was concise and the parameters were quite clear, the experiment used methods that eliminated mechanical friction, which exists in all functioning sprinklers. The mechanical friction was not a variable that needed elimination. The question was not "what would happen to a specially designed sprinkler submerged underwater if it sucked in water." It's a nice little experiment, but I don't think it actually answered the question. In fact, the design probably fails miserably at being an actual sprinkler.
No, the question is what is the result of the forces in that system. Mechanical friction and unaligned tubes would obscure the actual results, while this set up is what an "ideal" sprinkler would act like. This way, we discovered what actually had a predominant effect on the direction of rotation, which is the flow in the internal parts of the sprinkler, more than the liquid actually being sucked in or hitting that wall in the first bend in the tube
I would've loved it if they did other designs too to see how the angles and types of flow contribute to those vortexes, but this is still an interesting result
These are the types of videos that make me glad to study physics in college. I guessed right in the first part, surmised the opposite in the second part, and I was happy with the result in the third part. Always adapting to new information and ideas.
@@otm646 FLUID bearings are common, but I don't think this concentric-MENISCUS bearing is -- did you actually look closely at how it works? I don't think it would provide enough radial stiffness for any uses except sensitive instrumentation.
to the people saying they only got this answer because of the way they designed their sprinkler: they also did all of the calculations and math derivations so you can now predict the movement of many sprinkler designs, not just the one they actually built.
Yea that's the most important part of this study IMO. The results were obvious and it's embarrassing this wasn't "solved" earlier as vacuums knew and solved this internal vortex issue several decades ago, thus I already knew the results. I figured this video was going over something that was solved in the 1990s or something but it's pretty sad looking at that date..
Thank you. Opposing comments ended up voted higher because they believed that the scientists decided to stop after seeing any internal assymetry and concluded with a new hypothesis. People forget that the scientific method isn't just widely used, it is respected and followed through all the way to the final calculations.
I think it's because the geometry of the plenum wasn't specified, and the effect would disappear depending on the plenum's geometry. This seems more like experimental error unless the problem specifically states that the sprinkler has to have this specific plenum geometry. I assumed the sprinkler wouldn't move, but i also assumed the experiment would provide a suction via a 2:1 header with decent flow characteristics rather than dumping asymmetrical flow into an internal volume. Of course it would spin in revere in that case, but arbitrary changes to the internal volume can give any result. Care was taken to isolate the system from pump vibration, meniuscus bearing for lower friction, all this is pretty obvious and what i'd assume would be the setup, but i also would assume that the experiment would account for the internal geometry by using a low turbulence Y connection to the suction. As the problem was being described, i already thought of a siphon and meniscus bearing, as well as a fairly laminar internal structure. The experiment is specifically designed to give this result, and it can give the opposite result if the internal geometry contained baffles, guide fins, a rounded feature in which an axle/pivot bolt runs, or any number of possible configurations. When i design hovercraft hulls i use all kinds of tricks like this to negate lift motor torque by adjusting the plenum geometry.
I wonder, if they were able to perfectly match everything to minimize design biases influencing fluid dynamics, would we see outside forces acting to create a spin, such as Coriolis and gravitational effects from mass concentrations.
I'd like to see the arms be offset from the axis of rotation to create an internal vortex that is opposite the observed head rotation direction. Make the arms reversible as well so they can create the forward internal vortex and see if there is a difference in speed. To me the rotation is obviously attributed to different pressure at the suction face and reverse side of each arm. That is where the external system interacts most strongly with the internal system.
In this case, though, isn't the force ultimately from the different total curvature of the inside and outside paths of the pipes? I know that the differential vortices are apparent at the center, but it seemed to me that the force arises as an imbalance in the suction experienced across the inside walls of the pipes. (The different speeds and sizes of the central vortices are therefor a result and not a cause.) It's surely true that you could overcome that by introducing other geometric facts at other locations. However, it would seem to remain true, that in a truly symmetric system, that the result obtained here should remain. Do you not think it so?
@@fnamelname9077In this specific internal geometry a rotation is imparted. The question isn't asked in terms of the effects of the internal geometry, but in terms of what effect the suction has in terms of imparting rotation. In this configuration the signal to noise ratio doesn't allow a valid result in terms of suction. If the internal geometry isn't fixed as a constant and can be anything that causes suction through the arms, i can easily design a range of internal geometry configurations to impart any desired rotation. And as for the differential in the pipe, it becomes a larger effect only because there are colliding differential flows. This will not hold true if there is only one tube, or three, or seven, or if the tubes were flush to the inside, or if those tubes were tapered, or angled, or mitered at the ends or, or and on and on. Unless the internal geometry of all sprinklers are an ansi standard, then all sprinklers will act differently, and the answer only applies t this specific case. The answer is that they answered the wrong question and call it solved because noise caused the result. This might just bother me enough to run this experiment myself with a setup capable of giving a result, it's not like it's hard to 3d print some tubes and siphon some water. And in the case that the confounding factor of internal vortices is accounted for, there's a whole other can of worms with the nozzle shape. Is the sprinkler arm tip just a cut off section of pipe? Does it have an internal taper? Beveled outer edges inducing Coanda effect? Something else like decorative plastic flowers that the water shoots out from? Everyone in Feynman's class disagreed because they all have different brand sprinklers a home lol.
Are you telling me that not _one_ person decided to bend the tubes upward toward the pump-rather than just ending them at cavity where they point at each other-in order to basically remove the vortices entirely? It's like the question hasn't been answered at all, at this point.
Its simple, the video author explained about how the submerged sprinkler sucks in water and the individual legs of the sucking tubes are ending inside in a mutual opposite alignment (which is the reason for the submerged sprinler rotating backwards) But inorder to truly find out the sppinning effect by avoinding this new disturbance, both the sprinkler tubes can be bend 90 degrees and be taken entirely out from the water so that the problem of momentum interaction of water molecules inside the submerged sprinkler head will not arise. The true motive of the experiment can be served justice. Now did you get the idea ?@@marvin.marciano
It wouldn't necessarily remove the vortices, just change their orientation. Any asymmetry means they could still provide a net force. I would rather see a design with just one nozzle (and a counterweight for balance) with the pipe having, as far as practical, a constant diameter from pump to nozzle.
A while ago I saw a TH-cam video that immediately came to mind. My first thought was also that pressure is equally everywhere in every direction, by the way. But the video was about a simple vertical (PVC) pipe connected to a vacuum cleaner. It was mounted to the side of a table, but not actually fixated in place. When the vacuum cleaner turns on, the pipe moves up a bit. Conclusion was that the air that is right next to the pipe gets sucked in with a sling-shot motion and the centrifugal force that came with it, pulls the pipe up. It also heavily depends on the shape of the rim: a well rounded edge pulls less.
Well the optics might have been available to scientists 140 years ago but the lasers are a more recent invention. And the computer required to crunch the data for PIV even more so.
If that's what you think this is about, carry on. You can't learn if you already know everything. The problem wasn't posed for the proof, it was posed because it was hard to theorize. Now that the joke is dead, nice.
I remember hearing about a problem that early jet aircraft had, especially those with intakes in the nose. It was called "inlet lift". At high angles of attack the air entering the inlet had to turn a corner, and this created a nose up force. This made stall recovery tough because adding power, i.e. afterburner, increased the effect.
stall is a lack of lift. and you're saying now that increasing the total net lift is making stall worse.. are you daft? not to mention that same effect is happening at the bottom of the engine aswel in the opposite direction. and thus cancels itself out. you must be on drugs or something.
In the end, a form of asymmetric turbulence inside the common chamber, induces some reverse thrust into the underwater counter-rotating sprinkler. It is so easy to understand when you can physically see it. Compliments to the experimentalists who set up this device, and Dr. Ben Miles - that produced this great video with an outstanding explanation of the Feynman's sprinkler. Greetings, Anthony
I think it is possible to analyze the problem in a simpler way by breaking it down. 1/ pumping fluid quickly inside a simple tube with an entry and exit generates strong pushing thrust at the exit, and weak pulling thrust at the entry. this can probably be measured independently using load cells. the thrust can be converted into movement/rotation or a stationary force/torque, it doesnt matter. this is highlighted by jet ski having the jet exit direction controling the thrust, while the intake is directed forward and downward (not straight forward) and doesnt change direction for forward or reverse operation 2/ if you now have several exits, and several entries, the overall thrust will be approximately the sum of the exit thrusts 3/ if exit thrusts cancel each others approximately, then the intake thrusts can become prevalent 4/ if exits streams point at each or at fixed objects other weird turbulence and vortices will happen and create additional secondary effects way more complicated to study and probably cant be predicted without numeric simulation and understood through experimentation 5/ even it the main exit thrusts cancel each other, those secondary effect could still outweight intake thrusts. THIS IS probably the ONLY CONCLUSIION of this experiment? 6/ the rotating part of a sprinkler should be analyzed like a freely rotating system with entries and exits for fluid to be pumped through 7/ the traditional sprinkler has several exits which combined generate a clear torque, stronger than any effec onthe sucking side, the intakes don't matter 8/ the generic sucking sprinkler achieved using any sprinkler, with reversed pumping action, is designed wihout any attention to the blowing side , and because of this, has undetermined behavior 8/ the sprinkler shown in this experiment is seemingly designed to cancel the effects of the blowing side to show the effect of the sucking side (by using symetrical exits, pointing at the center, but failed to do so because asymetrical flows and resulting asymetrical vortices
Not so convinced.. since you talked about the opposite effects of sucking and inertial forces in the pipe corners, Reynolds should be an important factor. The pressure gradients involved in sucking are influenced by viscosity, while the force imparted on the tube due to the fluid changing direction are not. I expect it would spin in the normal way at sufficiently high Re.
I'm satisfied with the above explanation neither. Imagine sprinkler mouth sucking a thick jelly, so it will cut itself into a jelly. And water may behave like such a thin jelly in this regard.
Kind of a random addition to the experiment to allow the fluids to collide inside the sprinkler. This should not be part of the experiment, and mitigated with the pipes being fed / sucked by separate tubes. Or them being bend upwards before joining. The whole experiment lacks a certain clarity of its definition.
@@vast634 Yes, the turbulence effects inside of sprinkler should be eliminated by experimental arrangement in similar way, like the described experiment already does outside of it.
@@vast634 I agree, the internals are irrelevant to the problem. At the very least the internals should have been isolated to be nonconsequential. Otherwise too many variables.
I remember seeing a model of the inverse sprinkler years ago with air being sucked in. The result was that it was sensitive to disturbances and it was possible to get it going in both directions. It wasn't going that far to reduce the disturbances though.
Start of video hypothesis: the sprinkler should spin forwards. The water being sucked in makes contact with the sprinkler pushing it. It works similar to a sailboat. The wind(water) pushes the sail(nozzle).
It appears that I was incorrect. I feel like there are so many variables that can be altered to make the premise the same but the result different though.
This was already solved a decade ago at Harvard. They knew about the votices inside the hub and that the rotation depends on the geometry. Wang's contribution is more subtle and the new "solution" is pop science sensationalism
Seeing as the real solution (aka depends on what you engineer the sprinker for/emphasis on what forces) was already present during RPFs lecture, it was solved more than just a decade ago and not at Harvard.
It would be interesting to have the tubes inside the central cavity turn so that they are pointing vertically (up or down) compared with the axis of rotation.
Did Mach's theoretical sprinkler have the attitude of the internal ends of the tubes defined? Thanks to all who work on this problem, it has been spinning around my brain for decades now, since I read Feynman's book.
It would have been interesting for the researchers to simplify the validation of the force that rotates the "sucking" sprinkler backwards by building a second and third sprinkler that has the arms exiting the the reservoir body at both an obtuse and acute angle relative to the axis of rotation while the arm exit into the open water chamber is in the identical location as the main experiment. This would confirm that changing this specific variable alters the direction of the "sucking" sprinkler, without needing to visually interpret the laser-illuminated particle flows. Very cool and enlightening problem !
Well, it's a straightforward problem in the case of something like a bow thruster, which is a "ducted fan": a symmetrical arrangement of a propellor in the middle of a duct open at both ends. It's not hard to argue that most of the force transfer here is at the surface of the propellor itself, which in turn is transferred via the mounting frame to the vessel. In an open environment, very little force can be said to result from the thruster developing higher ambient pressure on one side of the vessel relative to the other. But that "very little" difference is still NONZERO and, significantly, it has the SAME SIGN as that of the blade thrust. The Feynman sprinkler, for obvious reasons, develops perhaps HALF of that pressure difference in the best case. We might say that the entire ambient environment is at a common pressure, but in the area close to the vent, the pressure is lower. If you put your finger over the vent, you can easily feel it being drawn toward the vent. That's a rough measure of the available motive force in this negative pressure scenario. The fluid in that region has mass and therefore resists being accelerated. The various resulting force vectors in the neighborhood cancel except for the component along the axis of the vent. In short, this force may be modest but it is NONZERO, and it has the SAME SIGN as the flow through the duct, which is inwards in the case of a Feynman sprinkler. A broadly conical vent will tend to contain this negative pressure and direct its force more in line with the vent axis. It will still be more diffuse, therefore less directed and effectively weaker, relative to what is possible with a positive pressure through the vent. But if you imagine making the vent into a diffuser, you can see how easily the positive pressure scenario can be weakened as well, until the two scenarios become quite closely comparable.
How does this apply in a situation where you suck on a straw? You are creating a pressure differential between your mouth and the water, at which point the water travels up the straw to enter your mouth thus balancing the differential. I would consider that a pull.
Ohh wait is it becasue the pressure of the world outside the straw is now greater than the pressure in your mouth and it pushes the water up the straw?
@@brianthibodeau2960 yeah, when you arent sucking the air pressure inside the straw and outside are the same. once you start sucking, theres less air pressing down on the liquid inside the straw vs outside, so the air outside is able to push the liquid up the straw to try and equalize the pressure. if you had a straw going all the way to space (so just a tall straw with a vacuum inside it) it would only be able to push the liquid up a certain distance before the weight of the water in the straw is too much for the atmosphere to keep lifting. so you could put a tube from the ocean to space and it wouldnt drain the ocean
When water is spit out it all goes one direction (due to its momentum inside pipe) but when sucked in, it comes in from almost every direction (except for the direction of the pipe) since its initial momentum is close to zero. This is why “put-put” boats work.
The simplest sprinklers are composed of hoses with holes punctured at regular intervals. Great presentation and delivery of the material. Only 4 minutes in but I appreciate the thought and craftsmanship that has been invested in communicating this problem.
It doesn't feel like this is really answering the question. I do not think the original hypothetical was supposed to consider the effect of the internal cavity of the sprinkler. That seems like the bearing resistance issue the experiment was trying to solve for. We establish initially that the normal sprinkler rotates because of the force of water going through the tubes, without considering what happens when the water first comes into the central cavity. So, to ask what happens when water is sucked out, it doesn't seem like we should be looking at what happens when the water enters that central cavity. What would happen if the water was sucked all the way out of the sprinkler system (for example, all the way to the side basin) rather than into the central cavity where the flows press into each other? Does that make a difference.
What if instead of the tubes ending pointing towards each other they were offset slightly to cancel this effect or were taken and pointed 90deg down, probably no cavity effect and no motion. When I try and think about it in simplistic terms, if you have a tank of fluid with no rotation and when it leaves the sprinkler, the exiting water has no rotation, I would have thought there would be no net change in rotational momentum and therefore no overall torque?? Seems like what they have here is an experimental quirk and haven’t answered the question
A vacuum cleaner can be made to suck or blow, however the suction is very local and directed into the head, but blowing is always at a distance, and the effects are much more random, which is why I object to council road and park maintenance operatives using fossil-fuel driven leaf blowers to scatter the leaves in a general direction, before being picked up by other means. If they had vacuum cleaners, the leaves would be sucked into receptacles on site or by hoses connected directly to their vehicle's leaf collector directly.
It's obvious that it wouldn't spin if the liquid is drawn uniformally from the system (which can be achieved through inner arrangement of the sprinkler) because there is no net change in the angular momentum of the water. Basically the way it spins depends on how the water is leaving the system, not how it enters. Same as with regular sprinkler. Edit: The answer given in the video is only correct if you want to know what forces do sprinkler arms contribute and ignore everything else. Which is not quite the same as the original question
Interesting and informative. I got it right at the pause, but I knew that I didn't know enough for sound reasoning. I was quite amazed at the answer, because so many of the components I'd thought of were present. Great video.
People saying that the impact of the particles on the bend will balance the suction have forgotten about 2 things: 1) that the bend is angled, only a component of the incident force would counter act the "suction" and more importantly 2) the particle would bunce off the bend and then hit the other side of the tube, cancelling out the tangential force on the apparatus.
I'm guessing a simpler explanation is - particles are moved by a pressure difference guiding them, and the impacts on the walls are too small to matter.
And the direction of rotation actually wouldnt be affected by the external angle of the pipes, assuming the vortices still formed in the same manner, right?
It seems like you could make it spin whichever direction you wanted by changing the direction of the inlets into the central chamber to something other than oppositional. If they had built the central chamber so the inlet pipes pointed up or down instead of oppositional or left or right you'd achieve a different result by modifying the way vortices form or don't form. They did all this work to basically prove nothing because the design of the system simply shifts the "blowing" effect from external to internal.
dear god I tired. I love the subject, your voice is decent, but I only got 2:11 in before I got sick of flashing to your face. I don't care about your face , it isn't a sprinkler or Feynman's face. I don't care about the room you're in. So i stopped watching and skipped to reading the paper, because that at least has the sense to make it about the physics and not their faces
Physicist: Let's try to theorize about how the sprinkler will spin underwater. Engineer: Let's put a sprinkler underwater, hook up a pump and see what happens.
My father has tried this when I was a kid. My older brother was a 7th grader. They were siphoning from the second floor to the ground floor through the bathroom window.Their result was it does not move. If moved backward it stops quickly, if they jolt it forward it slows down over a much longer time. This must have been in 1971 in a very rural area, so I bet a lot of other people must have tried it without ever becoming public with their result.
Theory, in a non-Newtonian fluid the sprinkler spins backward as the energy of the small holes sucking it in causes it to harden making it more like the sprinkler is spinning around to catch fluid while in a superfluid it would spin forwards as the fluid rushing in towards the sprinkler and combined with the sucking force it would spin forwards. In water it should stay stationary while it travels into the sprinkler
the best and most mind-blowing video I ever saw in my entire life and yet, it has a three percent dislike ratio?! I've seen terrible videos where cooked up explanations with no scientific basis had only a tenth of the dislikes this video got. I'm somewhat unsure what happened here. okay, the only critique I can find it that it never explains why the turbulent low pressure zone in front of the nozzle apparently cancel out with the bending momentum of the laminar flow inside the tube, which I thought would be more powerful and therefore make it spin backwards. okay, there's one other thing that I'm missing. to me, this all looked like as if minor construction differences formed those inconsistencies; meaning that a different setup could cause it to spin into the opposite direction. it's not clear why always those two corners would take the "upper hand." so, it could dive deeper into this topic, but the main takeaway is that hardly anyone, including some of the greatest minds, had internal vortices on their radar to ultimately consider.
0:49 Angular momentum conservation. The water has none. So sprinkler must spin so that water exits purely radially. In a pool it's the opposite. Water enters tips of the sprinkler from all directions, with pressure forces on inside and outside of the tube in balance. No force on the tube, no spinning.
Thank you for your charismatic presentation and the thorough content. I appreciate the illustrative visuals and all the effort you put into your videos. It's impressive how you manage to honor the hundreds of man-hours that scientists dedicate to their research throughout the years. Your work truly brings their contributions to life!
This is a fairly simple problem. First, we just need to simplify it a bit. Suppose instead of dealing with a submerged sprinkler, we are dealing with a submerged box that has a front side and a back side. The front side keeps getting the water there pumped out through a large hole so water keeps being taken away from that face as fast as surrounding water can replace the water that gets removed, so you effectively have zero pressure on the front side of the box. However, you still have the normal amount of water pressure on the back side. You then have pressure and thus force on the back side and none on the front side. That makes the box move forward. If we make this box be a nozzle of a sprinkler, then we are essentially just attaching the left or right side of the box to a stick that keeps the box going around in circles instead of going in a straight line, so the sprinkler head turns, moving backwards. Changing the shapes involved also is irrelevant so long as the nozzle direction stays the same relative to the sprinkler pivot point because the water pressure physics stay the same overall. This should not be a difficult physics problem.
I used to have an aquarium. When doing water changes you suck the air out of a hose you put in the water. If i remember correctly, this moved the hose "forward" in the direction of the opening first but then when the water hits the backside it basically bounces back, then little to no motion at all.. but it's not free-spinning like the sprinkler anyways. So it depends on which force is stronger then. The forward force of sucking in the water, the backwards force of it hitting the backside of the tube/sprinkler, or if they are the same strength it would not move at all. Dunno if this isnt an oversimplification, but i would assume the reverse to happen as if we run it normally.. it spinning the other way around as if we propelled water from it. Edit: Wow that was a cool explanation, and the green particle demonstration looked brilliant aswell!
My initial conclusion when hearing the problem was that it wouldn’t spin for the same intuitive reasons that explained why the force from sucking in fluid is much much weaker than expelling fluid. Now I also made an assumption that those tubes that went into the sprinkler housing, didn’t just terminate immediately into an empty cavity where vortices can form. I assumed the tubes would bend downwards. If the tubes did bend downwards once inside the housing, would the sprinkler rotate at all in this case?
i have a three-arm rotating sprinkler that has one behavior we haven't ever managed to explain: at low flux with the faucet barely on the sprinkler has a threshold to overcome after which it spins; as the flux is increased the sprinkler spins faster. So far this makes sense. What baffles us is that if the flux is increased from zero to maximum rapidly, the sprinkler doesn't turn at all, it just sits there spraying three streams of water -- while if the flux is increased to maximum slowly the sprinkler just keeps rotating, which shows that the flux is not the issue.
I'm guessing it's because of static friction and the slight delay between the water pressure reaching the sealed bearing vs the nozzles on the arms. There may also be some leaking water but only at lower pressures, and that could affect the static friction.
Best video of the year. It was something we discussed for a few weeks in the 1990s. I'm sure my colleagues will remember. Obviously we had no idea at the time. Well lots of ideas, no consensus and none of them correct in retrospect.
This to me makes perfect sense. And there's a clear issue with the question being asked here that makes it seemingly difficult to answer. When you talk about how a sprinkler rotates, you have to consider that a sprinkler in general takes a single, already directional thing (in this case fluid) and pushes it through something stationary with openings that are all facing in a direction that is optimized for spinning the sprinkler head. So the single directional "fluid" in this case is forced to change direction in 3 or more areas by "running it into" the curves of the outlets of the sprinkler head all at the same time and with the same directional change while forcing it out of the only exit(s). Then the moving fluid runs into a stationary surrounding, in this case the sprinkler head itself, and also a whole bunch of surrounding stationary fluid. (moving water with velocity hits water without velocity, and the water pushes back on the moving water, causing the tube it is coming from to move in the opposite direction of the water's velocity, meaning you are simply changing the direction) This question is so much easier to answer when you look at it from the other side and remove the bearing and the hose supplying it with water, lets make it EVEN easier and lets say the sprinkler is instead a simple single opening end that goes into 3 nozzles that come out at an angle, 90 degrees from the inlet, then rotated 30 degrees to angle them to spin it, a single plastic part. If you were to attach a big syringe full of water (maybe with a solenoid attached to push the plunger arm in or pull it out) directly to the single inlet and had pushed the plunger inward, squeezing the water and thus pushing it through the inlet, and then up through the nozzles it will try to spin the whole system. And, here's the fun part, if you take that syringe off the inlet, then attach 3 syringes to the 3 nozzles, then PUSH the water into the 3 nozzles [which forces it out the single inlet], it would LIFT the base of the inlet up like a rocket from the water exiting the single outlet. So with that in mind, now reverse it from push to pull. If you attached 3 syringes to the 3 nozzles and pulled water through it, it wont try to spin, it would just pull water through the single inlet because of the vacuum in the syringe tubes trying to pull inward. So if you pull fluid through the 3 outlets and it doesn't spin, then if you pull fluid by using the single syringe on the inlet, it still doesn't spin. The fact is, that the structure would crush itself before it could move because of the vacuum on the inside of the system. With a vacuum strong enough, it would collapse the walls of the sprinkler, and to the extreme, eventually crush it so hard, it would eventually become a black hole. So the question you are asking, we do not have an answer for. You are REALLY really asking: is there any velocity at the absolute center of a black hole. Back to reality though, the issue is you are really comparing Vacuum vs Thrust. Thrust is pushing, vacuum is pulling. The same thing applies here, if you are "pulling" you are creating negative "pressure" at the outlets of the sprinkler head, not *creating* "velocity" in the opposite direction. And so you are not *changing* the direction of a velocity that already exists. When you pull from the inlet, you create negative pressure within the sprinkler head, so the water is simply in the way of the inward force that is trying to pull on the inside of the sprinkler head, so it moves it out of the way, and the only place it can go is that new low pressure zone you just created by the inlet. And it just so happens that with that negative pressure at the 3 outlets, you pull the water in from all directions at the same time at all 3 outlets, creating a self cancelling system.
OK I'll play ball and engage because you showed me an interesting problem :) My hypothesis at the start of the video is that the sprinkle-sucker will rotate counter to its above-water counterpart. I visualized the forces of a space ship to arrive at this answer. The water jet of a sprinker has essentially the same properties as a rocket. It's just a jet of water instead of a jet of fire. So, the inverse seems to be the most likely outcome, since we have inverted the forces at play.
My first thought before watching the video is that it would not spin at all. There's some conflicting forces and resistance that I think would prevent it from moving. The water sucked in would push against the internal walls of the arm which would try to turn it forward, but the low pressure from the suction would be pulling it backwards, and then there's the resistance from being submerged in water. Edit: After watching, the answer for spinning backwards was much more complex and way cooler than I expected.
@@Jerkal yeah, that would be a nice touch, or at least add the name of the source in the corner while the footage is being shown. I think because it's a few seconds clip he might haven't bothered
What a great video and explanation in simple terms. Repetition of the experiment needs to occur, with changes to the sprinkler design. You need to use the same sprinkler under water as you would use out of water for consistency. Perhaps use magnetic frictionless bearings to eliminate any friction. Also the internal cavity where the arms extend from needs to be redesigned to eliminate internal vortexes. Perhaps extend the spinning arms directly down to where the water enters sprinkler , making sure that there are no internal spaces for water to accumulate above the bearing position.. However with a conventional water sprinkler that you would use to irrigate your grass or lawn operating under water with the pump working in reverse. The sprinkler head does in fact work in reverse as proved. Why it does so is a different problem. If the sprinkler is redesigned and used to irrigate grass and doesn’t work as the one shown wouldn’t then have you proved anything anyway?
I love science. I was thinking it won't move because suction isn't the reverse of blowing and intake is slow compared to repellent and it's just intaking inside the same material. But then of course, slight inconsistancies in fluent motion causing a slight spinning angle. It's so simple yet so complicated.
Ok for the start of the video challenge I've got a few potential ideas for arguments based around the sprinkler. First it will help to think about when the pump is on in the usual case as a rocket equation (ie assuming the hydrostatic pressure of the water the sprinkler is submerged in isn't too high, it should function as normal submerged, because it is still ejecting material) 1) Argument from equilibrium state. Consider the lack of presence of a pumping force at all with the pump submerged. We know that the pump does not spin. By pure stochastic happenstance we expect some water molecules to move from the tubing, through the sprinkler and out of the end. The effect of this is indistinguishable from jets coming out of the sprinkler only at smaller scale. We know since there is nothing doing net work the sprinkler should not accelerate into spinning so there must exist a counteracting torque. Since any material exiting contributes to the jet torque, we must conclude that stochastic motion into the sprinkler constitutes the counter-torque. Therefore the spin generated from backpumping should be expected to be in reverse direction. Were this not to be the case and both caused acceleration in the same direction we would see spontaneously induced rotational motion with no work having been done. 2) Argument from net momentum change. Really we just need to consider what's happening at the nozzle, since only motion perpendicular to the radial direction is relevant. In the driven case the sprinkler takes water initial moving (approximately) radially, then accelerates it to be perpendicular to the radial direction (or with a component that is). Equal and opposite reaction force means at the nozzle the sprinkler must experience a force in the other direction. Across all the arms this leads to rotational motion. Water heading the opposite direction would require that the net effect be the opposite, again leading to reverse direction of spin. So I'm tentatively in favour of reverse direction of spin, let's see how wrong I am.
I was taught that there are 5 possible methods to any kinematics problem. This one is readily solved with "conservation of angular momentum". Angular momentum is imparted to the water drawn into the sprinkler. The key question is where is this angular momentum shed. If it's also shed within the sprinkler then it won't spin (except initially when turned on). If it's shed outside the sprinkler then it will spin in reverse, as shown. Doesn't seem super hard. Of course, without "conservation of angular momentum", it becomes one of the hardest problems imaginable.
Since there are left and right openings causing four vortices, two big ones are form by collision of two inward jets. two inward jets are formed from left and right pipes (2 direction). The question to ask are 1. what happens if it is 3 direction or 4 direction sprinkles for these experiment? Doesn't it stops? 2. does the vortices being indirectly related to the mirrored direction of the bend pipes? If so can't we create a specific bend pipe that stops spinning 2 direction?
If the three pipes are still concentric, then you'll get three jets meeting at offset angles, which probably will still cause spin. That said, minor changes in the geometry of any part of the system could cause significant change in rotation speed, in its direction and even in whether it happens at all. You can almost certainly get a three-tube model to spin in reverse, in the same direction as when sprinkling or even to not spin, depending on how you build it.
I did a very simple experiment on this years ago to determine what happened. I cut the top off a 1.5L plastic sparkling water bottle to create a simple tube and drilled a 3mm hole in one side flank of each of the 6 flukes formed at the bottom of the bottle so that when filled with water it streamed out roughly tangentially. When this bottle was floated in water and then filled internally to a level higher than external the water exited the holes and the bottle rotated as expected. When the empty bottle was weighted and put back into water such that the outside level was higher than inside then water flowed into the bottle which then rotated in the opposite direction. The bottle needed a weight at the bottom for stability. No expense was spared with my method!
That leaves a couple of design to look at to verify the experiment. 1) Make the tubes perfectly straight. 2) Angle their direction entering the center portion.
Thank you. “Experimental design” questions answered that occurred to me as you were presenting the facts, possible solutions and attempted proofs. Very clearly demonstrated and well explained for a visual, life long learner.
I ain’t smart but I’m at 0:45 seconds and figure I’d give it my shot. I imagine it’s backwards from a combination of the lower pressure in the tube, the higher pressure on the outside and the adhesive water pulling other water around the surface of the outside of the nozzle. Now let’s find out how dumb and wrong I am!
While sucking air or fluid, air molecules are going in the pipe from everywhere, except the pipes cross section. That assymetry is driving the motion. I hope someone thought of this simple explanation and discarded it in favour of a fancy complex one, just to insult occam and his razor.
Just taking a guess here before getting to any answers - I’m betting it has to do with the center of the sprinkler which is a non-factor to this problem when pushing water out through the sprinkler since it is full of water which applies a mostly uniform distribution of forces in all directions aside from the outward flows, but when pulling water in this cavity would become a much more significant factor in the dynamics of how the system spins..
Put the sprinkler in a pressurized tank and pump water “backward” through the sprinkler that is vented to atmospheric pressure. This is to increase the operating differential pressure between the outlet and inlet of the sprinkler. A sprinkler operates on a differential between the hose and atmospheric pressures. Hose pressure (pushing water out) can be 3, 4, or 5 bar… there is no limitation on pressure. But there is a limitation on inlet pressure of a pump (suction, sucking water in): -1 bar gage (neglecting cavitation). The maximum differential pressure of a “reverse sprinkler” is less than 1 bar. But if the sprinkler is submerged in a pressurized tank (and vented to atmosphere), then the differential pressure can be set to any level: 3, 5, 10 bar or more. Viscous drag and density significantly decrease rotational speed of a submerged sprinkler. Increasing the differential pressure overcomes these losses and any observable effects will become more distinct at higher operating differential pressures.
Perhaps, but it isn't necessary. The lowered "suction" pressure is adequate to measure the effects. This video is correct though it is explained a bit poorly in places.
Wow! As a former fluid physics student, this was a lot of fun to see if my intuition would hold up. My gut predicted counterrotation, and giving it some thought I predicted a net rotation in the water to impart angular momentum. It was probably a lucky guess though!
One of the things that I noticed about the options given is that none of them considered the difference in mass that air has versus water. The folks predicting that it would stay still were the closest, but I didn't notice any mention that the water particles that are pushing their way back into the tube are pressing against a pipe that's been backstopped against pressurized water. I'm only at 8:45, so we'll see what they conclude, but I think that's likely why there was the difference when it's water being sucked rather than air being sucked in Feynman's experiment.
That's based on the sprinkler geometry in the experiment which creates specific vortex patterns. However, those vortexes could easily be eliminated by just bending the nozzles differently to get a more laminar flow from the nozzles to the pump (or the syphon tube). The core question of the Feynman Sprinkler Problem is therefore still open: do the forces in an "ideal" sprinkler cancel out, or is there an imbalance in the flow and in the momentums (due to viscosity) which causes the sprinkler to rotate backwards regardless of its geometry?
The water pressure on the submerged sprinkler arms is equal in all directions - until the suction starts. Once the suction starts, there is a low pressure zone at the sprinkler opening and thus there is a pressure imbalance on the sprinkler arms. So it’s not the ‘suction’ that causes the sprinkler tubes to get ‘pulled’ forward causing the reverse spin; rather, it’s the unbalanced pressure on the back side of the tube that ‘pushes’ the tube foward, causing the net reverse spin.
So, basically the reason it spins backwards is the same reason the outside wheel of an axle in a turn spins faster. The particles begin at the same point and end at the same point; however the ones on the outside of the tube are moving with more velocity. Because of the way the tubes are lined up in this experiment, the higher velocity particles are hitting the lower velocity particles from the opposite tube. This is what's creating the spin. I assumed the sprinkler heads would pull them selves in the opposite direction of flow, but my assumption was more based the velocity change between the particles being sucked in (outside the nozzle) vs the ones inside the nozzle. Similar to how compressors work. Interesting finding for sure.
I dont get why they debated this so long when Isaac Newton gave them the answer centuries ago with conservation of momentum. Since all of the fluid starts at rest, the motion of the sprinkler is a function of the angular momentum of the flow exiting it. If there is any swirl (or net angular momentum) remaining in the central pipe as flow is sucked through, the head would move. It could be either direction due to any asymmetries specific to how the test is designed. I think the purest form would be if they had stators (flow straighteners) in the central pipe in which case the head would not move at all. This phenomenon is super common place in hovering aircraft like the F-35 or the Harrier where the inlets usually point forward but they dont move forward since their exhaust is pointed directly downward.
14:40. This shows that there can be a torque depending on how the water enters the central drum. This means if you modify the design of the tubes entering the drum, you can make it spin *either* direction, depending on how much angular momentum is acquired by the water exiting the drain. This should probably be viewed as a flaw in the experiment. If you design the drum specifically to prevent the water from acquiring any angular momentum at the drain, it will not spin. As an example, turn the tubes in the drum straight downward so that water must exit without angular momentum.
The visualization of the vortices at the central hub of the sprinkler is edifying, but clearly the layout of those vortices will vary depending on the offset of the tubes? For tubes that enter the hub opposite each other, pointing straight at the axis, apparently you're getting a small torque, but for a different offset of the tubes the torque could be either amplified or reversed. So the bottom line, for me, is that the reversed sprinkler can turn in ANY direction, and it all depends on how the arms connect to the hub. Another point worth mentioning is that (if I remember correctly) Feynman&Co weren't only looking at water sprinklers, but also at superfluid helium: the most interesting experimental setup was a sprinkler made of glass (spider-like hub with bent arms and no central channel), simply resting on a pin instead of a bearing, in a bath of liquid helium in a superfluid state. A dark spot inside of the sprinkler was heated with a laser, causing the helium inside to go from superfluid to regular fluid. The regular helium would escape the hub through the arms, causing the sprinkler to rotate, and at the same time the hub would be replenished by superfluid helium being sucked in through the arms, i.e. moving through the same channels as the "blown" helium, at the same time but in the opposite direction.
I think if your sprinkler is underwater then your grass is probably wet enough.
Thats why you run it in reverse.
😂
This is why they want to pump the water back out... ;)
"If SOME is GOOD and MORE is BETTER then absolutely TOO MUCH should be just about RIGHT." Had some tee shirts made up years ago memorializing a meeting where a senior VP went around basically chanting the first two-thirds of this quote in his presentation. When I added the last third during a lull in the chanting, the VP just stared at me with a stunned look. The President took a liking to me right then and there and made sure I was included in more meetings, which were occasionally not fun.
Yeah. And besides needing a reverse sprinkler you'd have to invest in a seaweedwacker.
When I was a physics grad student in the 80s I disagreed with a professor about an E&M problem - the prof was a real *sshole about it and I was sure I was right. I phoned up Feynman at his home (he was in the directory!) and asked him his opinion. He told me I was right (this story ended up doing the rounds at UCI) and he asked me the sprinkler problem. I gave a few different answers that I said were naïve answers (which are covered in your video!), and that I was unsure. He told me to call him back when I had my answer. Overall we had a 45 minute conversation - I felt very honored. I became disappointed in myself as I never got a fully convincing answer so never called him back, and he died in 1988. I felt like I had failed the great man - until I saw your video today!!!!
that is awesome, having been able to ask feynman about your problem :D
I was a chem/physics student at UC Irvine in the 80s. Any chance you could hint at the prof's name? (Edited to clarify university).
Rest in Peace
Don’t be too hasty to give up on this problem. As I’ve posted elsewhere, I am not convinced we have solved this yet. 👍🖖
Why didn't they repeat the experiment with internal tubes pointing upwards to cancel the vortex?
For the same reason it took them so long to.....just freaking build an apparatus and test it..... Because physicists aren't as smart as engineers ;)
Id argue if this is the effect at play then they obviously could manipulate the tube runs to reverse the reversed reversal of the reversed flow....ya know, but backwards.
exactly my point - you said it clearer.
@@TankR Yeah they could've just used the local swimming pool at the deep end. Mythbusters would've.
I just wrote that above. You beat me to it lol. I have a couple of variations in my comment. One was to use pressure instead of suction.
yes, it shouldn't matter hey.@@ThePaulv12
Am I the first person to notice that the description of Feynman's experiment is wrong? Actually, he tried to pump air into the top of the carboy to push the water backwards through the tubing; he didn't suck the water out of the tube. Eventually the pressure blew the carboy apart. See "Surely You're Joking, Mr. Feynman" at the end of Part 2: The Princeton Years.
Makes a lot more sense, I'm sitting here wondering how on earth he broke the tank by just sucking in water
I am glad someone else spotted this. The subtitles of the inner hub interactions could lead to any number of outcomes.
@@DB-thats-meyou meant “subtleties” ?
@@TheYurubutugralb Damn lystexia. 😳😂👍
yeah, if it was just sucking the water in then it would be really obvious that it would spin towards the water it's pulling in
6:42 "Sucking is not the opposite of blowing" lol
Of course not, people that suck and people that blow are usually the same people.
If so, how come it isn't called 'suck job'?
In a way it's true 😂
"-i can take this conversation a lot of directions from here"
🤨🤨
I’m also glad that he had the exact same thought I did with Bart Simpson.
Imagine being so smart that a Problem gets Your name because you could NOT solve it.
Sounds more like an ego problem than anything else
I think Mr. Sprinkle got involved in case it gets to be known as the Sprinkle Sprinkler.
I don't see the ego problem when Feynman didn't name it himself.
Sure thing, Einstein
Most mathematical problems that are waiting for a solution are named after smart mathematicians who could not find a solution. Q.E.D. Once the problem is solved, it does not take on the name of the successful first solver.
"Feynman was keenly aware of his own abilities and almost entirely unburdened with modesty" - the sentence where I clicked Subscribe.
For me it was this one: "... and this year's entry for nominative determinism, Brennan Sprinkle."
Yep, that was firmly in second place for me too!
and a huge sexist at the same time
@@oxiosophy let me guess, you don't believe that Feynman was a great person academically and otherwise, right?
How does modesty burden one, except for the burden on the ego?
I wish they would have redesigned the test so the arms of the sprinkler don't have that central cavity for the vortexes to form. They could have brought the two tubes together in an upside down Y with the leg of the inverted Y pointing straight up in the center -- that should eliminate those vortexes that were contributing rotational forces.
Yeah, after watching the video, I still have the feeling that this massivly depends on thd design of the sprinkler. Of you'd add an infinite ammount of arms, you'd end up with something simmelar to a tesla turbine, which would possibly spin in the other direction. And a different hub layout might also form the center vortecies in a different way.
my thought exactly. they could even bend them parallel before joining along their sides to preserve laminar flow. to the point that even there the cumulative outer track of water would still move faster and might still cause slight asymmetries: then there is probably a way to angle the inlet jets entering the central chamber to compensate for the lopsided velocities. just angle them until all 4 vortices are equal. there's probably a million variations you could build, but I'm betting per design, there is a small adjustment which preserves the behavior with water flowing out, but which is balanced when water flows in. its not about principle, its about which of like 7 minute balancing acts your current design happens to be failing the most, and that is the latent rotation being seen.
@@clockworkvanhellsing372directional baffles could align all the vortices, I would think. Such a setup should eliminate now dampened speed, and making it more relative omnidirectionally.
The tip of the nozzle has a lower pressure than the surrounding water will pull the nozzles forward
Excellent thought ALSO the fact that the outer curve away from the center has more surface area against which the incoming fluid would press against would seem to be relevant too (I’m not sure why it would matter vs fluid already in the tube though to be clear…) - even altering the laminar vs turbulent friction with varying materials there would impact things etc!?
0:47 build it, test it, problem solved. And don't make the system so weak it explodes.
Yes, that's what they did. Did you watch the video? The results aren't as straightforwards as you think they are.
@@arstino yes I watched the video
@@deltacx1059 so, why did you even make the comment?
@@arstino it reflects my thoughts at that moment in the video. Not something to be concerned about.
@@arstinoit’s more the surprise that it took so long for someone to build and test this. It’s… not hard to do. Especially if you simplify the problem. The real question is whether a fluid getting sucked into a curved pipe exerts a push or pull force on the pipe, if any force at all.
"almost entirely unburdened by modesty." That is the greatest description of Feynman! He wasn't so much arrogant as bereft of any desire to *not* be arrogant.
I think you're not quite right. He really really wished to avoid being given any credit for one skill on the basis of something that had nothing to do with it. He didn't put his name Feynman on the drawings and portraits he was good at.
Ditto
An early Didier Raoult ...
Kind of like the difference between a nudist and a flasher: the flasher wants to be seen and have people react, the nudist just doesn't care.
There are two types of superiority. The psychological (complex) kind and the factual (real) kind
Feynman was the real kind. He was better. That’s a fact. Was he an a\*\*hole about it? Nope, and I love it
I'm giving you a thumbs up for excellent audio quality, no over powering music and clear responses. Great work here.
* overpowering
I don’t think he needs an explanation for every like
This seems more a function of the specific design of the sprinkler internals. If the pipes were angled to have the vortices' sizes inverted it could be made to rotate in the other direction.
If there was a suitably shaped baffle inside the sprinkler head, it could be therefore made to rotate slowly in any direction, or not at all, it seems. Normal sprinkler rotation, including overcoming friction, is surely mostly rocket science - reaction to the mass of the water sprayed in the opposite direction of rotation of the sprinkler head.
They should have directed the internal jets upwards so all the incoming water streams are flowing in parallel before they are allowed to interact to avoid "spooky actions in a (hidden) vortex". IMO this experiment has not effectively addressed the original question.
Yea, I have no idea why they had two tubes in this experiment. I assume three would cause similar vortices but how about just one tube so there wouldn't even be a need for a chamber like that? Could even have just round corners so there shouldn't be any noticeable vortices at any point.
Yeah, that's what I was thinking.
I want to see what happens if they design a system to nullify these internal forces, and focus only on the water in the arms of the sprinkler.
@@lisabenden If you nullify the internal forces then you aren't actually testing the problem :)
The internal forces are a result of the bends in the pipe. If you nullify them, then you will have to by definition design your "reverse" sprinkler, to impart forces onto the sprinkler system to counteract the "natural" designs rotation.
By having pipes that bend, you end up with water flow that is not "centrered" in the pipe. This "non uniform flow" is the effect that causes the rotation.
TLDR: The 100 year old answer of "depends on what engineering choices you pick to have the most effect" is the right one and nothing was actually discovered beyond why small house vacuums often have the intake opening on the side which was also known for quite a while.
Ye i thought the same. So if the sprinkler doesnt have cnc quality openings but instead a janky mold of some sort the answer would be completely different? How would the scenario play out if you used turbulent flow?
@@D3nn1sor if you had more than two intakes at various angles.
They basically engineered a sprinkler to get a result. That particular sprinkler design didn't exist until they made it. Its result is rather inconsequential. As the question was concise and the parameters were quite clear, the experiment used methods that eliminated mechanical friction, which exists in all functioning sprinklers. The mechanical friction was not a variable that needed elimination. The question was not "what would happen to a specially designed sprinkler submerged underwater if it sucked in water." It's a nice little experiment, but I don't think it actually answered the question. In fact, the design probably fails miserably at being an actual sprinkler.
No, the question is what is the result of the forces in that system. Mechanical friction and unaligned tubes would obscure the actual results, while this set up is what an "ideal" sprinkler would act like. This way, we discovered what actually had a predominant effect on the direction of rotation, which is the flow in the internal parts of the sprinkler, more than the liquid actually being sucked in or hitting that wall in the first bend in the tube
I would've loved it if they did other designs too to see how the angles and types of flow contribute to those vortexes, but this is still an interesting result
These are the types of videos that make me glad to study physics in college. I guessed right in the first part, surmised the opposite in the second part, and I was happy with the result in the third part. Always adapting to new information and ideas.
That meniscus bearing is cool. I wonder where this idea came from? Can this be used to create frictionless bearings for more practical applications?
Magnetic bearing: No.
Fluid bearings like this are common in industry, both water, oil and the classic air bearing.
@@otm646 FLUID bearings are common, but I don't think this concentric-MENISCUS bearing is -- did you actually look closely at how it works? I don't think it would provide enough radial stiffness for any uses except sensitive instrumentation.
Don't think they can support much load
to the people saying they only got this answer because of the way they designed their sprinkler: they also did all of the calculations and math derivations so you can now predict the movement of many sprinkler designs, not just the one they actually built.
Yea that's the most important part of this study IMO. The results were obvious and it's embarrassing this wasn't "solved" earlier as vacuums knew and solved this internal vortex issue several decades ago, thus I already knew the results. I figured this video was going over something that was solved in the 1990s or something but it's pretty sad looking at that date..
Ooh that's neat
So, what is the ubiquitously used solution in vacuums?
Thank you. Opposing comments ended up voted higher because they believed that the scientists decided to stop after seeing any internal assymetry and concluded with a new hypothesis. People forget that the scientific method isn't just widely used, it is respected and followed through all the way to the final calculations.
I think it's because the geometry of the plenum wasn't specified, and the effect would disappear depending on the plenum's geometry. This seems more like experimental error unless the problem specifically states that the sprinkler has to have this specific plenum geometry.
I assumed the sprinkler wouldn't move, but i also assumed the experiment would provide a suction via a 2:1 header with decent flow characteristics rather than dumping asymmetrical flow into an internal volume. Of course it would spin in revere in that case, but arbitrary changes to the internal volume can give any result.
Care was taken to isolate the system from pump vibration, meniuscus bearing for lower friction, all this is pretty obvious and what i'd assume would be the setup, but i also would assume that the experiment would account for the internal geometry by using a low turbulence Y connection to the suction. As the problem was being described, i already thought of a siphon and meniscus bearing, as well as a fairly laminar internal structure.
The experiment is specifically designed to give this result, and it can give the opposite result if the internal geometry contained baffles, guide fins, a rounded feature in which an axle/pivot bolt runs, or any number of possible configurations. When i design hovercraft hulls i use all kinds of tricks like this to negate lift motor torque by adjusting the plenum geometry.
Haha, I just read your comment after basically posting the same exact thing.
I wonder, if they were able to perfectly match everything to minimize design biases influencing fluid dynamics, would we see outside forces acting to create a spin, such as Coriolis and gravitational effects from mass concentrations.
I'd like to see the arms be offset from the axis of rotation to create an internal vortex that is opposite the observed head rotation direction. Make the arms reversible as well so they can create the forward internal vortex and see if there is a difference in speed.
To me the rotation is obviously attributed to different pressure at the suction face and reverse side of each arm. That is where the external system interacts most strongly with the internal system.
In this case, though, isn't the force ultimately from the different total curvature of the inside and outside paths of the pipes? I know that the differential vortices are apparent at the center, but it seemed to me that the force arises as an imbalance in the suction experienced across the inside walls of the pipes. (The different speeds and sizes of the central vortices are therefor a result and not a cause.)
It's surely true that you could overcome that by introducing other geometric facts at other locations. However, it would seem to remain true, that in a truly symmetric system, that the result obtained here should remain.
Do you not think it so?
@@fnamelname9077In this specific internal geometry a rotation is imparted. The question isn't asked in terms of the effects of the internal geometry, but in terms of what effect the suction has in terms of imparting rotation. In this configuration the signal to noise ratio doesn't allow a valid result in terms of suction.
If the internal geometry isn't fixed as a constant and can be anything that causes suction through the arms, i can easily design a range of internal geometry configurations to impart any desired rotation.
And as for the differential in the pipe, it becomes a larger effect only because there are colliding differential flows. This will not hold true if there is only one tube, or three, or seven, or if the tubes were flush to the inside, or if those tubes were tapered, or angled, or mitered at the ends or, or and on and on.
Unless the internal geometry of all sprinklers are an ansi standard, then all sprinklers will act differently, and the answer only applies t this specific case.
The answer is that they answered the wrong question and call it solved because noise caused the result.
This might just bother me enough to run this experiment myself with a setup capable of giving a result, it's not like it's hard to 3d print some tubes and siphon some water.
And in the case that the confounding factor of internal vortices is accounted for, there's a whole other can of worms with the nozzle shape. Is the sprinkler arm tip just a cut off section of pipe? Does it have an internal taper? Beveled outer edges inducing Coanda effect? Something else like decorative plastic flowers that the water shoots out from?
Everyone in Feynman's class disagreed because they all have different brand sprinklers a home lol.
Are you telling me that not _one_ person decided to bend the tubes upward toward the pump-rather than just ending them at cavity where they point at each other-in order to basically remove the vortices entirely?
It's like the question hasn't been answered at all, at this point.
Hey English isn't my first language and I didn't understand your suggestion. Could you draw it and send a link?
What are you yapping about
Its simple, the video author explained about how the submerged sprinkler sucks in water and the individual legs of the sucking tubes are ending inside in a mutual opposite alignment (which is the reason for the submerged sprinler rotating backwards) But inorder to truly find out the sppinning effect by avoinding this new disturbance, both the sprinkler tubes can be bend 90 degrees and be taken entirely out from the water so that the problem of momentum interaction of water molecules inside the submerged sprinkler head will not arise. The true motive of the experiment can be served justice. Now did you get the idea ?@@marvin.marciano
It wouldn't necessarily remove the vortices, just change their orientation. Any asymmetry means they could still provide a net force. I would rather see a design with just one nozzle (and a counterweight for balance) with the pipe having, as far as practical, a constant diameter from pump to nozzle.
@@chicklucas6682Are you genuinely unable to visualise what the OP describes?
A while ago I saw a TH-cam video that immediately came to mind. My first thought was also that pressure is equally everywhere in every direction, by the way. But the video was about a simple vertical (PVC) pipe connected to a vacuum cleaner. It was mounted to the side of a table, but not actually fixated in place. When the vacuum cleaner turns on, the pipe moves up a bit. Conclusion was that the air that is right next to the pipe gets sucked in with a sling-shot motion and the centrifugal force that came with it, pulls the pipe up. It also heavily depends on the shape of the rim: a well rounded edge pulls less.
i had a feeling the fan topic was gonna be brought up and lo and behold, 6:42 comes up
It took 140 years to put a sprinkler underwater.
lol
precisely.
Well the optics might have been available to scientists 140 years ago but the lasers are a more recent invention. And the computer required to crunch the data for PIV even more so.
@@salsamancer none of which is needed to put a sprinkler underwater
If that's what you think this is about, carry on. You can't learn if you already know everything. The problem wasn't posed for the proof, it was posed because it was hard to theorize.
Now that the joke is dead, nice.
I remember hearing about a problem that early jet aircraft had, especially those with intakes in the nose. It was called "inlet lift". At high angles of attack the air entering the inlet had to turn a corner, and this created a nose up force. This made stall recovery tough because adding power, i.e. afterburner, increased the effect.
stall is a lack of lift. and you're saying now that increasing the total net lift is making stall worse..
are you daft?
not to mention that same effect is happening at the bottom of the engine aswel in the opposite direction. and thus cancels itself out.
you must be on drugs or something.
8:35 of course, Brennan _had_ to work this problem.
fascinating
🖖
There's a long history in the UK of people who's namesake became their job. My metalworking teacher was called Mr. Bolt.
For some reason, YT wanted my “feedback” on this comment.
Knowing him, 90% of his motivation for this was the joke.
It was his calling
In the end, a form of asymmetric turbulence inside the common chamber, induces some reverse thrust into the underwater counter-rotating sprinkler.
It is so easy to understand when you can physically see it.
Compliments to the experimentalists who set up this device, and Dr. Ben Miles - that produced this great video with an outstanding explanation of the Feynman's sprinkler.
Greetings,
Anthony
I think it is possible to analyze the problem in a simpler way by breaking it down.
1/ pumping fluid quickly inside a simple tube with an entry and exit generates strong pushing thrust at the exit, and weak pulling thrust at the entry. this can probably be measured independently using load cells. the thrust can be converted into movement/rotation or a stationary force/torque, it doesnt matter. this is highlighted by jet ski having the jet exit direction controling the thrust, while the intake is directed forward and downward (not straight forward) and doesnt change direction for forward or reverse operation
2/ if you now have several exits, and several entries, the overall thrust will be approximately the sum of the exit thrusts
3/ if exit thrusts cancel each others approximately, then the intake thrusts can become prevalent
4/ if exits streams point at each or at fixed objects other weird turbulence and vortices will happen and create additional secondary effects way more complicated to study and probably cant be predicted without numeric simulation and understood through experimentation
5/ even it the main exit thrusts cancel each other, those secondary effect could still outweight intake thrusts. THIS IS probably the ONLY CONCLUSIION of this experiment?
6/ the rotating part of a sprinkler should be analyzed like a freely rotating system with entries and exits for fluid to be pumped through
7/ the traditional sprinkler has several exits which combined generate a clear torque, stronger than any effec onthe sucking side, the intakes don't matter
8/ the generic sucking sprinkler achieved using any sprinkler, with reversed pumping action, is designed wihout any attention to the blowing side , and because of this, has undetermined behavior
8/ the sprinkler shown in this experiment is seemingly designed to cancel the effects of the blowing side to show the effect of the sucking side (by using symetrical exits, pointing at the center, but failed to do so because asymetrical flows and resulting asymetrical vortices
"entry for nominative determinism" slayed me sir. bravo
Not so convinced.. since you talked about the opposite effects of sucking and inertial forces in the pipe corners, Reynolds should be an important factor. The pressure gradients involved in sucking are influenced by viscosity, while the force imparted on the tube due to the fluid changing direction are not.
I expect it would spin in the normal way at sufficiently high Re.
Yeah that and changing the dynamics of the center fluid removal would seem highly relevant etc
I'm satisfied with the above explanation neither. Imagine sprinkler mouth sucking a thick jelly, so it will cut itself into a jelly. And water may behave like such a thin jelly in this regard.
Kind of a random addition to the experiment to allow the fluids to collide inside the sprinkler. This should not be part of the experiment, and mitigated with the pipes being fed / sucked by separate tubes. Or them being bend upwards before joining. The whole experiment lacks a certain clarity of its definition.
@@vast634 Yes, the turbulence effects inside of sprinkler should be eliminated by experimental arrangement in similar way, like the described experiment already does outside of it.
@@vast634 I agree, the internals are irrelevant to the problem. At the very least the internals should have been isolated to be nonconsequential. Otherwise too many variables.
What a fascinating result. Fluid dynamics - elegantly simple rules that often defy expert intuition.
I remember seeing a model of the inverse sprinkler years ago with air being sucked in. The result was that it was sensitive to disturbances and it was possible to get it going in both directions. It wasn't going that far to reduce the disturbances though.
Start of video hypothesis: the sprinkler should spin forwards. The water being sucked in makes contact with the sprinkler pushing it. It works similar to a sailboat. The wind(water) pushes the sail(nozzle).
It appears that I was incorrect. I feel like there are so many variables that can be altered to make the premise the same but the result different though.
Plural of meniscus is unusually correct as menisci but singular of phenomena is phenomenon...
This was already solved a decade ago at Harvard. They knew about the votices inside the hub and that the rotation depends on the geometry. Wang's contribution is more subtle and the new "solution" is pop science sensationalism
The Harvard study did not include the internal geometry and how this can change the rotational direction
@@Ghredle You didn't read the paper
Thank you
@@shanent5793 no i did not …just had access to one drawing which shows the water intake… my assumption was based on incomplete knowledge,
Seeing as the real solution (aka depends on what you engineer the sprinker for/emphasis on what forces) was already present during RPFs lecture, it was solved more than just a decade ago and not at Harvard.
It would be interesting to have the tubes inside the central cavity turn so that they are pointing vertically (up or down) compared with the axis of rotation.
Did Mach's theoretical sprinkler have the attitude of the internal ends of the tubes defined?
Thanks to all who work on this problem, it has been spinning around my brain for decades now, since I read Feynman's book.
It would have been interesting for the researchers to simplify the validation of the force that rotates the "sucking" sprinkler backwards by building a second and third sprinkler that has the arms exiting the the reservoir body at both an obtuse and acute angle relative to the axis of rotation while the arm exit into the open water chamber is in the identical location as the main experiment. This would confirm that changing this specific variable alters the direction of the "sucking" sprinkler, without needing to visually interpret the laser-illuminated particle flows. Very cool and enlightening problem !
“Completely unburdened by modesty” nicest way I’ve ever heard someone called arrogant
Next experiment: stop those vortexes from forming within the center of the hub
That's what I thought.
also, there is no "sucking" only pressure differentials. meaning fluids always get pushed, never pulled.
Well, it's a straightforward problem in the case of something like a bow thruster, which is a "ducted fan": a symmetrical arrangement of a propellor in the middle of a duct open at both ends. It's not hard to argue that most of the force transfer here is at the surface of the propellor itself, which in turn is transferred via the mounting frame to the vessel.
In an open environment, very little force can be said to result from the thruster developing higher ambient pressure on one side of the vessel relative to the other. But that "very little" difference is still NONZERO and, significantly, it has the SAME SIGN as that of the blade thrust.
The Feynman sprinkler, for obvious reasons, develops perhaps HALF of that pressure difference in the best case. We might say that the entire ambient environment is at a common pressure, but in the area close to the vent, the pressure is lower. If you put your finger over the vent, you can easily feel it being drawn toward the vent. That's a rough measure of the available motive force in this negative pressure scenario.
The fluid in that region has mass and therefore resists being accelerated. The various resulting force vectors in the neighborhood cancel except for the component along the axis of the vent.
In short, this force may be modest but it is NONZERO, and it has the SAME SIGN as the flow through the duct, which is inwards in the case of a Feynman sprinkler. A broadly conical vent will tend to contain this negative pressure and direct its force more in line with the vent axis. It will still be more diffuse, therefore less directed and effectively weaker, relative to what is possible with a positive pressure through the vent.
But if you imagine making the vent into a diffuser, you can see how easily the positive pressure scenario can be weakened as well, until the two scenarios become quite closely comparable.
How does this apply in a situation where you suck on a straw? You are creating a pressure differential between your mouth and the water, at which point the water travels up the straw to enter your mouth thus balancing the differential. I would consider that a pull.
Unless you're talking ferrofluid and magnets.
Ohh wait is it becasue the pressure of the world outside the straw is now greater than the pressure in your mouth and it pushes the water up the straw?
@@brianthibodeau2960 yeah, when you arent sucking the air pressure inside the straw and outside are the same. once you start sucking, theres less air pressing down on the liquid inside the straw vs outside, so the air outside is able to push the liquid up the straw to try and equalize the pressure. if you had a straw going all the way to space (so just a tall straw with a vacuum inside it) it would only be able to push the liquid up a certain distance before the weight of the water in the straw is too much for the atmosphere to keep lifting. so you could put a tube from the ocean to space and it wouldnt drain the ocean
When water is spit out it all goes one direction (due to its momentum inside pipe) but when sucked in, it comes in from almost every direction (except for the direction of the pipe) since its initial momentum is close to zero. This is why “put-put” boats work.
But they need to make it unnecessary complicated
The simplest sprinklers are composed of hoses with holes punctured at regular intervals.
Great presentation and delivery of the material. Only 4 minutes in but I appreciate the thought and craftsmanship that has been invested in communicating this problem.
It doesn't feel like this is really answering the question. I do not think the original hypothetical was supposed to consider the effect of the internal cavity of the sprinkler. That seems like the bearing resistance issue the experiment was trying to solve for.
We establish initially that the normal sprinkler rotates because of the force of water going through the tubes, without considering what happens when the water first comes into the central cavity. So, to ask what happens when water is sucked out, it doesn't seem like we should be looking at what happens when the water enters that central cavity.
What would happen if the water was sucked all the way out of the sprinkler system (for example, all the way to the side basin) rather than into the central cavity where the flows press into each other? Does that make a difference.
What if instead of the tubes ending pointing towards each other they were offset slightly to cancel this effect or were taken and pointed 90deg down, probably no cavity effect and no motion.
When I try and think about it in simplistic terms, if you have a tank of fluid with no rotation and when it leaves the sprinkler, the exiting water has no rotation, I would have thought there would be no net change in rotational momentum and therefore no overall torque??
Seems like what they have here is an experimental quirk and haven’t answered the question
7:55 "Timmy, close the window" "Oh, sorry dad."
6:40 - yes. Sucking and blowing can be the same thing.
However. Context is really important
A vacuum cleaner can be made to suck or blow, however the suction is very local and directed into the head, but blowing is always at a distance, and the effects are much more random, which is why I object to council road and park maintenance operatives using fossil-fuel driven leaf blowers to scatter the leaves in a general direction, before being picked up by other means. If they had vacuum cleaners, the leaves would be sucked into receptacles on site or by hoses connected directly to their vehicle's leaf collector directly.
12-year-old me: "he he he"
It's obvious that it wouldn't spin if the liquid is drawn uniformally from the system (which can be achieved through inner arrangement of the sprinkler) because there is no net change in the angular momentum of the water.
Basically the way it spins depends on how the water is leaving the system, not how it enters. Same as with regular sprinkler.
Edit:
The answer given in the video is only correct if you want to know what forces do sprinkler arms contribute and ignore everything else. Which is not quite the same as the original question
Interesting and informative. I got it right at the pause, but I knew that I didn't know enough for sound reasoning. I was quite amazed at the answer, because so many of the components I'd thought of were present. Great video.
People saying that the impact of the particles on the bend will balance the suction have forgotten about 2 things: 1) that the bend is angled, only a component of the incident force would counter act the "suction" and more importantly 2) the particle would bunce off the bend and then hit the other side of the tube, cancelling out the tangential force on the apparatus.
I'm guessing a simpler explanation is - particles are moved by a pressure difference guiding them, and the impacts on the walls are too small to matter.
So, if you added a 90 degree bend pointing up to the suction area then all rotation should stop. Right?
And you could change the direction of rotation by changing the angle the pipes enter the central chamber, right?
And the direction of rotation actually wouldnt be affected by the external angle of the pipes, assuming the vortices still formed in the same manner, right?
Maybe there is no sprinkler??
The paper proved there's at least one, as one of the authors 😄
12:59 if you don't want a 13 minute history of sprinklers.
It seems like you could make it spin whichever direction you wanted by changing the direction of the inlets into the central chamber to something other than oppositional. If they had built the central chamber so the inlet pipes pointed up or down instead of oppositional or left or right you'd achieve a different result by modifying the way vortices form or don't form.
They did all this work to basically prove nothing because the design of the system simply shifts the "blowing" effect from external to internal.
0:35 I think the sprinkler is likely to spin into the suction, but perhaps slower than it normally would given the water resistance
dear god I tired. I love the subject, your voice is decent, but I only got 2:11 in before I got sick of flashing to your face. I don't care about your face , it isn't a sprinkler or Feynman's face. I don't care about the room you're in. So i stopped watching and skipped to reading the paper, because that at least has the sense to make it about the physics and not their faces
Physicist: Let's try to theorize about how the sprinkler will spin underwater.
Engineer: Let's put a sprinkler underwater, hook up a pump and see what happens.
As someone who usually doesn't have a problem with scientific concepts, fluid dynamics makes my brain hurt.
My father has tried this when I was a kid. My older brother was a 7th grader. They were siphoning from the second floor to the ground floor through the bathroom window.Their result was it does not move. If moved backward it stops quickly, if they jolt it forward it slows down over a much longer time. This must have been in 1971 in a very rural area, so I bet a lot of other people must have tried it without ever becoming public with their result.
Theory, in a non-Newtonian fluid the sprinkler spins backward as the energy of the small holes sucking it in causes it to harden making it more like the sprinkler is spinning around to catch fluid while in a superfluid it would spin forwards as the fluid rushing in towards the sprinkler and combined with the sucking force it would spin forwards. In water it should stay stationary while it travels into the sprinkler
Gonna actually watch the video now to see how wrong I am
What excellent experimental and apparatus design. You can tell the researchers really loved what they were doing.
If guys what to skip all jumbos, there you go 13:17
the best and most mind-blowing video I ever saw in my entire life and yet, it has a three percent dislike ratio?!
I've seen terrible videos where cooked up explanations with no scientific basis had only a tenth of the dislikes this video got.
I'm somewhat unsure what happened here.
okay, the only critique I can find it that it never explains why the turbulent low pressure zone in front of the nozzle apparently cancel out with the bending momentum of the laminar flow inside the tube, which I thought would be more powerful and therefore make it spin backwards.
okay, there's one other thing that I'm missing. to me, this all looked like as if minor construction differences formed those inconsistencies; meaning that a different setup could cause it to spin into the opposite direction. it's not clear why always those two corners would take the "upper hand."
so, it could dive deeper into this topic, but the main takeaway is that hardly anyone, including some of the greatest minds, had internal vortices on their radar to ultimately consider.
Chapeau! Great edit, clear explanations and interesting topic. Don't know why you didn't popped in my feed before but you gained a sub!
0:49 Angular momentum conservation. The water has none. So sprinkler must spin so that water exits purely radially. In a pool it's the opposite. Water enters tips of the sprinkler from all directions, with pressure forces on inside and outside of the tube in balance. No force on the tube, no spinning.
Thank you for your charismatic presentation and the thorough content. I appreciate the illustrative visuals and all the effort you put into your videos. It's impressive how you manage to honor the hundreds of man-hours that scientists dedicate to their research throughout the years. Your work truly brings their contributions to life!
This is a fairly simple problem. First, we just need to simplify it a bit. Suppose instead of dealing with a submerged sprinkler, we are dealing with a submerged box that has a front side and a back side. The front side keeps getting the water there pumped out through a large hole so water keeps being taken away from that face as fast as surrounding water can replace the water that gets removed, so you effectively have zero pressure on the front side of the box. However, you still have the normal amount of water pressure on the back side. You then have pressure and thus force on the back side and none on the front side. That makes the box move forward. If we make this box be a nozzle of a sprinkler, then we are essentially just attaching the left or right side of the box to a stick that keeps the box going around in circles instead of going in a straight line, so the sprinkler head turns, moving backwards. Changing the shapes involved also is irrelevant so long as the nozzle direction stays the same relative to the sprinkler pivot point because the water pressure physics stay the same overall. This should not be a difficult physics problem.
I used to have an aquarium. When doing water changes you suck the air out of a hose you put in the water. If i remember correctly, this moved the hose "forward" in the direction of the opening first but then when the water hits the backside it basically bounces back, then little to no motion at all.. but it's not free-spinning like the sprinkler anyways. So it depends on which force is stronger then. The forward force of sucking in the water, the backwards force of it hitting the backside of the tube/sprinkler, or if they are the same strength it would not move at all. Dunno if this isnt an oversimplification, but i would assume the reverse to happen as if we run it normally.. it spinning the other way around as if we propelled water from it.
Edit: Wow that was a cool explanation, and the green particle demonstration looked brilliant aswell!
"Bad laser safety" + "most pressing problems" topped by "winner of nominative determinism" = gladly subscribed
My initial conclusion when hearing the problem was that it wouldn’t spin for the same intuitive reasons that explained why the force from sucking in fluid is much much weaker than expelling fluid.
Now I also made an assumption that those tubes that went into the sprinkler housing, didn’t just terminate immediately into an empty cavity where vortices can form. I assumed the tubes would bend downwards.
If the tubes did bend downwards once inside the housing, would the sprinkler rotate at all in this case?
it'll obviously propell itself outside the water and colonize Mars
i have a three-arm rotating sprinkler that has one behavior we haven't ever managed to explain: at low flux with the faucet barely on the sprinkler has a threshold to overcome after which it spins; as the flux is increased the sprinkler spins faster. So far this makes sense. What baffles us is that if the flux is increased from zero to maximum rapidly, the sprinkler doesn't turn at all, it just sits there spraying three streams of water -- while if the flux is increased to maximum slowly the sprinkler just keeps rotating, which shows that the flux is not the issue.
I'm guessing it's because of static friction and the slight delay between the water pressure reaching the sealed bearing vs the nozzles on the arms. There may also be some leaking water but only at lower pressures, and that could affect the static friction.
Best video of the year. It was something we discussed for a few weeks in the 1990s. I'm sure my colleagues will remember. Obviously we had no idea at the time. Well lots of ideas, no consensus and none of them correct in retrospect.
This to me makes perfect sense. And there's a clear issue with the question being asked here that makes it seemingly difficult to answer. When you talk about how a sprinkler rotates, you have to consider that a sprinkler in general takes a single, already directional thing (in this case fluid) and pushes it through something stationary with openings that are all facing in a direction that is optimized for spinning the sprinkler head. So the single directional "fluid" in this case is forced to change direction in 3 or more areas by "running it into" the curves of the outlets of the sprinkler head all at the same time and with the same directional change while forcing it out of the only exit(s). Then the moving fluid runs into a stationary surrounding, in this case the sprinkler head itself, and also a whole bunch of surrounding stationary fluid. (moving water with velocity hits water without velocity, and the water pushes back on the moving water, causing the tube it is coming from to move in the opposite direction of the water's velocity, meaning you are simply changing the direction)
This question is so much easier to answer when you look at it from the other side and remove the bearing and the hose supplying it with water, lets make it EVEN easier and lets say the sprinkler is instead a simple single opening end that goes into 3 nozzles that come out at an angle, 90 degrees from the inlet, then rotated 30 degrees to angle them to spin it, a single plastic part. If you were to attach a big syringe full of water (maybe with a solenoid attached to push the plunger arm in or pull it out) directly to the single inlet and had pushed the plunger inward, squeezing the water and thus pushing it through the inlet, and then up through the nozzles it will try to spin the whole system. And, here's the fun part, if you take that syringe off the inlet, then attach 3 syringes to the 3 nozzles, then PUSH the water into the 3 nozzles [which forces it out the single inlet], it would LIFT the base of the inlet up like a rocket from the water exiting the single outlet.
So with that in mind, now reverse it from push to pull. If you attached 3 syringes to the 3 nozzles and pulled water through it, it wont try to spin, it would just pull water through the single inlet because of the vacuum in the syringe tubes trying to pull inward. So if you pull fluid through the 3 outlets and it doesn't spin, then if you pull fluid by using the single syringe on the inlet, it still doesn't spin. The fact is, that the structure would crush itself before it could move because of the vacuum on the inside of the system. With a vacuum strong enough, it would collapse the walls of the sprinkler, and to the extreme, eventually crush it so hard, it would eventually become a black hole. So the question you are asking, we do not have an answer for. You are REALLY really asking: is there any velocity at the absolute center of a black hole.
Back to reality though, the issue is you are really comparing Vacuum vs Thrust. Thrust is pushing, vacuum is pulling. The same thing applies here, if you are "pulling" you are creating negative "pressure" at the outlets of the sprinkler head, not *creating* "velocity" in the opposite direction. And so you are not *changing* the direction of a velocity that already exists. When you pull from the inlet, you create negative pressure within the sprinkler head, so the water is simply in the way of the inward force that is trying to pull on the inside of the sprinkler head, so it moves it out of the way, and the only place it can go is that new low pressure zone you just created by the inlet. And it just so happens that with that negative pressure at the 3 outlets, you pull the water in from all directions at the same time at all 3 outlets, creating a self cancelling system.
OK I'll play ball and engage because you showed me an interesting problem :)
My hypothesis at the start of the video is that the sprinkle-sucker will rotate counter to its above-water counterpart. I visualized the forces of a space ship to arrive at this answer. The water jet of a sprinker has essentially the same properties as a rocket. It's just a jet of water instead of a jet of fire.
So, the inverse seems to be the most likely outcome, since we have inverted the forces at play.
Loved the vid, had my brain going the entire time, and the way you summarized dense research was so helpful ty!
My first thought before watching the video is that it would not spin at all. There's some conflicting forces and resistance that I think would prevent it from moving. The water sucked in would push against the internal walls of the arm which would try to turn it forward, but the low pressure from the suction would be pulling it backwards, and then there's the resistance from being submerged in water. Edit: After watching, the answer for spinning backwards was much more complex and way cooler than I expected.
8:00 Wait a sec, did you use buttered side down footage??? Awesome!!!
couldn't see any sources in the description. wonder if they had permission...
@@Jerkal yeah, that would be a nice touch, or at least add the name of the source in the corner while the footage is being shown. I think because it's a few seconds clip he might haven't bothered
What a great video and explanation in simple terms.
Repetition of the experiment needs to occur, with changes to the sprinkler design.
You need to use the same sprinkler under water as you would use out of water for consistency.
Perhaps use magnetic frictionless bearings to eliminate any friction. Also the internal cavity where the arms extend from needs to be redesigned to eliminate internal vortexes. Perhaps extend the spinning arms directly down to where the water enters sprinkler , making sure that there are no internal spaces for water to accumulate above the bearing position..
However with a conventional water sprinkler that you would use to irrigate your grass or lawn operating under water with the pump working in reverse. The sprinkler head does in fact work in reverse as proved. Why it does so is a different problem.
If the sprinkler is redesigned and used to irrigate grass and doesn’t work as the one shown wouldn’t then have you proved anything anyway?
I love science.
I was thinking it won't move because suction isn't the reverse of blowing and intake is slow compared to repellent and it's just intaking inside the same material.
But then of course, slight inconsistancies in fluent motion causing a slight spinning angle. It's so simple yet so complicated.
Ok for the start of the video challenge I've got a few potential ideas for arguments based around the sprinkler. First it will help to think about when the pump is on in the usual case as a rocket equation (ie assuming the hydrostatic pressure of the water the sprinkler is submerged in isn't too high, it should function as normal submerged, because it is still ejecting material)
1) Argument from equilibrium state. Consider the lack of presence of a pumping force at all with the pump submerged. We know that the pump does not spin. By pure stochastic happenstance we expect some water molecules to move from the tubing, through the sprinkler and out of the end. The effect of this is indistinguishable from jets coming out of the sprinkler only at smaller scale. We know since there is nothing doing net work the sprinkler should not accelerate into spinning so there must exist a counteracting torque. Since any material exiting contributes to the jet torque, we must conclude that stochastic motion into the sprinkler constitutes the counter-torque. Therefore the spin generated from backpumping should be expected to be in reverse direction. Were this not to be the case and both caused acceleration in the same direction we would see spontaneously induced rotational motion with no work having been done.
2) Argument from net momentum change. Really we just need to consider what's happening at the nozzle, since only motion perpendicular to the radial direction is relevant. In the driven case the sprinkler takes water initial moving (approximately) radially, then accelerates it to be perpendicular to the radial direction (or with a component that is). Equal and opposite reaction force means at the nozzle the sprinkler must experience a force in the other direction. Across all the arms this leads to rotational motion. Water heading the opposite direction would require that the net effect be the opposite, again leading to reverse direction of spin.
So I'm tentatively in favour of reverse direction of spin, let's see how wrong I am.
I was taught that there are 5 possible methods to any kinematics problem. This one is readily solved with "conservation of angular momentum". Angular momentum is imparted to the water drawn into the sprinkler. The key question is where is this angular momentum shed. If it's also shed within the sprinkler then it won't spin (except initially when turned on). If it's shed outside the sprinkler then it will spin in reverse, as shown. Doesn't seem super hard. Of course, without "conservation of angular momentum", it becomes one of the hardest problems imaginable.
his Path integral formulation is quite remarkable, i never really understood the math of quantum mechanics, but his idea makes it understandable
Since there are left and right openings causing four vortices, two big ones are form by collision of two inward jets. two inward jets are formed from left and right pipes (2 direction).
The question to ask are
1. what happens if it is 3 direction or 4 direction sprinkles for these experiment? Doesn't it stops?
2. does the vortices being indirectly related to the mirrored direction of the bend pipes? If so can't we create a specific bend pipe that stops spinning 2 direction?
If the three pipes are still concentric, then you'll get three jets meeting at offset angles, which probably will still cause spin. That said, minor changes in the geometry of any part of the system could cause significant change in rotation speed, in its direction and even in whether it happens at all. You can almost certainly get a three-tube model to spin in reverse, in the same direction as when sprinkling or even to not spin, depending on how you build it.
I did a very simple experiment on this years ago to determine what happened.
I cut the top off a 1.5L plastic sparkling water bottle to create a simple tube and drilled a 3mm hole in one side flank of each of the 6 flukes formed at the bottom of the bottle so that when filled with water it streamed out roughly tangentially. When this bottle was floated in water and then filled internally to a level higher than external the water exited the holes and the bottle rotated as expected. When the empty bottle was weighted and put back into water such that the outside level was higher than inside then water flowed into the bottle which then rotated in the opposite direction. The bottle needed a weight at the bottom for stability.
No expense was spared with my method!
That leaves a couple of design to look at to verify the experiment.
1) Make the tubes perfectly straight.
2) Angle their direction entering the center portion.
Thank you. “Experimental design” questions answered that occurred to me as you were presenting the facts, possible solutions and attempted proofs. Very clearly demonstrated and well explained for a visual, life long learner.
I ain’t smart but I’m at 0:45 seconds and figure I’d give it my shot. I imagine it’s backwards from a combination of the lower pressure in the tube, the higher pressure on the outside and the adhesive water pulling other water around the surface of the outside of the nozzle. Now let’s find out how dumb and wrong I am!
Dang I was totally wrong. I definitely didn’t expect that! Pretty neat!
In the end, the answer is: it depends!
You can design the sprinkler to move in any direction or speed, by adjusting the inner parts.
Interesting.
While sucking air or fluid, air molecules are going in the pipe from everywhere, except the pipes cross section. That assymetry is driving the motion. I hope someone thought of this simple explanation and discarded it in favour of a fancy complex one, just to insult occam and his razor.
I think you're pretty close. ;-)
That video of a sprinkler leisurely rotating backwards is great. Final proof and maybe even lil lighthearted flex xD
Just taking a guess here before getting to any answers - I’m betting it has to do with the center of the sprinkler which is a non-factor to this problem when pushing water out through the sprinkler since it is full of water which applies a mostly uniform distribution of forces in all directions aside from the outward flows, but when pulling water in this cavity would become a much more significant factor in the dynamics of how the system spins..
there needs to be used a different kind of suspended particles that dont change the physical properties of water
Put the sprinkler in a pressurized tank and pump water “backward” through the sprinkler that is vented to atmospheric pressure. This is to increase the operating differential pressure between the outlet and inlet of the sprinkler.
A sprinkler operates on a differential between the hose and atmospheric pressures. Hose pressure (pushing water out) can be 3, 4, or 5 bar… there is no limitation on pressure. But there is a limitation on inlet pressure of a pump (suction, sucking water in): -1 bar gage (neglecting cavitation). The maximum differential pressure of a “reverse sprinkler” is less than 1 bar. But if the sprinkler is submerged in a pressurized tank (and vented to atmosphere), then the differential pressure can be set to any level: 3, 5, 10 bar or more. Viscous drag and density significantly decrease rotational speed of a submerged sprinkler. Increasing the differential pressure overcomes these losses and any observable effects will become more distinct at higher operating differential pressures.
Perhaps, but it isn't necessary. The lowered "suction" pressure is adequate to measure the effects.
This video is correct though it is explained a bit poorly in places.
I'm convinced if you shift the inner tube's angle, you could get the reverse sprinkler to spin in any direction you want.
Wow! As a former fluid physics student, this was a lot of fun to see if my intuition would hold up. My gut predicted counterrotation, and giving it some thought I predicted a net rotation in the water to impart angular momentum. It was probably a lucky guess though!
One of the things that I noticed about the options given is that none of them considered the difference in mass that air has versus water. The folks predicting that it would stay still were the closest, but I didn't notice any mention that the water particles that are pushing their way back into the tube are pressing against a pipe that's been backstopped against pressurized water. I'm only at 8:45, so we'll see what they conclude, but I think that's likely why there was the difference when it's water being sucked rather than air being sucked in Feynman's experiment.
That's based on the sprinkler geometry in the experiment which creates specific vortex patterns. However, those vortexes could easily be eliminated by just bending the nozzles differently to get a more laminar flow from the nozzles to the pump (or the syphon tube). The core question of the Feynman Sprinkler Problem is therefore still open: do the forces in an "ideal" sprinkler cancel out, or is there an imbalance in the flow and in the momentums (due to viscosity) which causes the sprinkler to rotate backwards regardless of its geometry?
I love that the result is
1 - what any rando would have expected
2 - due to forces that hadn't even occurred to Feynman et al.
The water pressure on the submerged sprinkler arms is equal in all directions - until the suction starts. Once the suction starts, there is a low pressure zone at the sprinkler opening and thus there is a pressure imbalance on the sprinkler arms. So it’s not the ‘suction’ that causes the sprinkler tubes to get ‘pulled’ forward causing the reverse spin; rather, it’s the unbalanced pressure on the back side of the tube that ‘pushes’ the tube foward, causing the net reverse spin.
When you said pause the video and think about it, it may seem obvious, I paused the video and it did seem obvious and I was right lol
So, basically the reason it spins backwards is the same reason the outside wheel of an axle in a turn spins faster.
The particles begin at the same point and end at the same point; however the ones on the outside of the tube are moving with more velocity. Because of the way the tubes are lined up in this experiment, the higher velocity particles are hitting the lower velocity particles from the opposite tube. This is what's creating the spin.
I assumed the sprinkler heads would pull them selves in the opposite direction of flow, but my assumption was more based the velocity change between the particles being sucked in (outside the nozzle) vs the ones inside the nozzle. Similar to how compressors work. Interesting finding for sure.
I dont get why they debated this so long when Isaac Newton gave them the answer centuries ago with conservation of momentum. Since all of the fluid starts at rest, the motion of the sprinkler is a function of the angular momentum of the flow exiting it. If there is any swirl (or net angular momentum) remaining in the central pipe as flow is sucked through, the head would move. It could be either direction due to any asymmetries specific to how the test is designed. I think the purest form would be if they had stators (flow straighteners) in the central pipe in which case the head would not move at all. This phenomenon is super common place in hovering aircraft like the F-35 or the Harrier where the inlets usually point forward but they dont move forward since their exhaust is pointed directly downward.
14:40. This shows that there can be a torque depending on how the water enters the central drum. This means if you modify the design of the tubes entering the drum, you can make it spin *either* direction, depending on how much angular momentum is acquired by the water exiting the drain. This should probably be viewed as a flaw in the experiment. If you design the drum specifically to prevent the water from acquiring any angular momentum at the drain, it will not spin. As an example, turn the tubes in the drum straight downward so that water must exit without angular momentum.
maybe the real sprinkle is friends we made along the way
The visualization of the vortices at the central hub of the sprinkler is edifying, but clearly the layout of those vortices will vary depending on the offset of the tubes? For tubes that enter the hub opposite each other, pointing straight at the axis, apparently you're getting a small torque, but for a different offset of the tubes the torque could be either amplified or reversed. So the bottom line, for me, is that the reversed sprinkler can turn in ANY direction, and it all depends on how the arms connect to the hub.
Another point worth mentioning is that (if I remember correctly) Feynman&Co weren't only looking at water sprinklers, but also at superfluid helium: the most interesting experimental setup was a sprinkler made of glass (spider-like hub with bent arms and no central channel), simply resting on a pin instead of a bearing, in a bath of liquid helium in a superfluid state. A dark spot inside of the sprinkler was heated with a laser, causing the helium inside to go from superfluid to regular fluid. The regular helium would escape the hub through the arms, causing the sprinkler to rotate, and at the same time the hub would be replenished by superfluid helium being sucked in through the arms, i.e. moving through the same channels as the "blown" helium, at the same time but in the opposite direction.