"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!!!!
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.
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.
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.
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!?
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.
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
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.
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.
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.
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..
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.
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 !
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.
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.
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?
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
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
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?
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.
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'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
Me, skipping randomly on work through the video: 7:37 _"[...] Interestingly here due to our slightly imprecise use of language when we describe sucking and blowing [...]"_ With a humor stuck stil in puberty this line without context humours me a little.
With this same effect you can use a small (phone) speaker to blow out a candle. A simple voice coil speaker essentially continuesly switches between blowing and sucking as the diaphragm moves. The Action Lab demonstrated this nicely in a video some time ago.
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?
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.
It turns the same direction in both modes because of the momentum change that occurs in the bent tubes. The method of impulse and momentum requires that the change in velocity pushes against the outside radius of the arms.
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.
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.
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 the mass difference exiting or entering tube is a crucial reason for the rotation of a sprinkler other that the obvious other reasons and that changes the outcome of suction. It's just amazing they didn't use something else to compare the problem to
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.
The impulse of the molecules points towards the intake first and up at the end. There are no molecules sucked in from one direction, so the vectors don't exactly add up to zero, the molecules going directly into the tube have nothing to cancel them out. If you want it to go faster just flare out the inlet. I kind of think it's obvy but I'm also very smart.
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.
It would be interesting to see them tweak the angle at which the tubes join the center to either balance or accentuate the difference in vortex sizes… just to show that it affects the turning rate. It would also be interesting to move the be de of the tubes further from the center to allow the difference in fluid velocity across the cross-section of the tube to dissipate, which would be expected to lessen the effect of the turning of the sprinkler. Alternatively, they could insert some kind of mixing vane inside the tubes to “scramble” the various velocities.
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.
This experiment raises another question in me. If you were to extend the arm pipes inside the hub 90° upwards, would the sprinkler not rotate and instead move deeper into the water? The suction tube that removes the water from the sprinkler is mechanically decoupled from the hub so it should not cause any back-pressure and with the feynman sprinkler effectively being an inside-out regular sprinkler, I would expect my modified setup to simply sink.
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.
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!
The way I see it, when you pump water out a sprinkler, it has momentum as it exists, giving ratational thrust. But conversly as water enters its coming from all kinds of different directions and so the force cancels out, and it stays relatively still, though it may move as its waterlogged initially.
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 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.
the particles being sucked into the sprinkler reminded me of a blackhole's event horizon. the moment of rapid execration looked like half a sphere, not cone shaped. I only wondered because the concept of a lower pressure drawing the tube forwardm even though the particles are acting like there is excess pressure, flowing away from the vacuum.
Basically with suction there are many small forces at play with none really dominant, so as an engineer it would be most reasonable to say that without further details most sprinkles in suction mode would not rotate. The setup in the paper is a very specific very low friction device.
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!
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 think the difference between sucking and blowing is ultimately key here in terms of the magnitude being much smaller. My first instinct is to think in terms of conservation of angular momentum (though often even if I can get an answer through conservation laws I then like to see whether I can understand my answer in terms of forces).
Because the reason that the tube is rotated has to do with the air being pushed against one side of the sprinkler, I believe that it will rotate the same direction. The air which enters the tube will still have to change direction, which can only happen through interaction with the walls of the tubes, which the front would still have an outsized effect.
Well, now that we can see into it, the results are kind of _intuitive_, the "reverse" sprinkler is just a sprinkler, that sprinkles the water "inwards". So, while in the normal sprinkler, the spinning direction depends mostly on the outlet's direction, and the inlet is only a minor force, here the roles are reversed. The placement and direction of what would be the inlet of a normal sprinkler is the main force. In this experiment that direction is 90°, revealing the very small force that the outlet's direction imparts on the system. You could probably angle the inlet of the arms to make the reverse sprinkler spin in any direction, irrespective of how the normal sprinkler spins.
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.
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!
Wouldnt the potential direction of rotation depend on the design of the sprinkler and angle at which the arms come into the center of the sprinkler as welk as the internal design of the sprinkler? Another thing that may have an effect, though I'm not sure how or what the effect would be (or whybit would necessarily effect it) would be to set up the sprinkler like a normal sprinkler with the water coming into it from below as if you had set the sprinkler on the bottom of a pool or in a lake. The feed line could still be used as a siphon for suction it would just need to be ran across the bottom of the tank and then up the wall of the tank instead of straight up and out from the center of the sprinkler.
But the rotation caused by the turbulence inside the sprinkler doesn't dismisses that it may also rotate this way due to the low pressure at the tip of the exit holes as we can observe in the PIV. Can't the rotation be caused by both effects?
Also in 14:14 you can see that the suction of the water causes the water on the outer walls of the tubes is flowing towards the opening, applying friction that might cause a rotation (or cancel the counter clockwise rotation) clockwise. Every single effect of these should be looked into, every effect we can try to calculate it's contribution to the rotation.
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
What an absolutely great explanatory video! I am a lowly engineer with a MS in fluids--and I could so completely understand this! Seriously, thank you!
The vortices are the result of an interesting flaw in the setup, if you eliminate them, the centrifugal forces caused by the flow through the tubes on their own probably won't be enough force to move the whole system.
I wouldn't be surprised if there was some centrifugal effect from the water being pulled towards the centre while the pump was spinning. In theory, this should provide additional torque in the direction of the spin. While it wouldn't be able to provide the initial impulse, it could magnify any spin that would otherwise occur. However the shape of a sprinkler to maximise the centrifugal effect would likely minimise the vortices caused by the water curving.
This was surprisingly interesting. As others have pointed out in the comments, the cause seems to depend on the design of the inner cavity which is ultimately not defined in the question. For example, you could put a bend on the inner tube entrances to have it spin in any direction, or zero out the vortices. Or, you could bend the inner tubes 90 deg so they are pointed out of the screen, and eliminate the collision and vortices. I think the answer to the original question as intended then is that the bent outter arms do not impart any rotation in suction; any resulting rotation is dependent on how the water leaves the tubes at the inner cavity. As is often the case, in my experience, the answer to difficult questions is often dependent on factors not defined in the question. Or, am I interpreting the results incorrectly?
NOTHING is defined in the question other than which way will an 'ordinary' sprinkler turn. . What all his pointless babbling FAILS to clearly indicate is that their combined and correct conclusion is that the explanation at time 5:41to 6:09 is the correct physics and the bent tube creates NO sucking ‘thrust’. That’s why they analyzed and settled on the center hub’s flow as dominant. If you carefully account for the true physics of the way fluid curves around from all directions going INTO the end of the tube, you see that force can be considerable, just as it is the dominant effect in creating lift above a wing - as Gustave Eiffel, the Eiffel Tower designer, determined in his aerodynamics testing.
Anyone who has ever used a suction pump to empty a tank will tell you that sucking is not the opposite of blowing. Afterall, the intake pipe doesn't race around the inside of the tank in a frantic attempt to attack or rush at the water. I was expecting a small motion, perhaps a jolt as the liquid inside the immersed sprinkler first started to move. How other fources played a role was where I was becoming interested. Viscosity of the liquid the sprinkler is immersed in for example. It's surely higher than the air the thing usually operates in. Doesn't take a lot of friction to completely balls up something like a free spinning low speed rotor.
Isn't it just the water pressure from outside against the tubes? On the side with the hole the water inside can be pushed away, but on the other side it pushes against the solid tube.
I don't have the math to back it up, but it would seem that there would also be an effect from higher pressure on the opposite side of the pipe as the intake as well as friction on the sides of the pipe as the fluid that's in contact with it runs toward the end of the nozzle before undergoing a 180 degree change in momentum which in itself would have to impart force somewhere.
So it depends a lot on the alignment and angle of those inside openings. Presumably, it wouldn’t hard to direct them slightly to I evade or the other cause whatever rotation you want.
I assumed the sprinkler arms wouldn't suck themselves forwards because it reminded me of those spinning "perpetual motion machines" that just reach equilibrium and don't move at all, despite being weighted towards one side in theory.
It's amazing to me that the obvious in this is invisible. lowered pressure at the mouth, and not on the rest of the tube. That very small difference, across the area of the mouth tangential to the diameter, is the push. The momentum transferred to the meeting flows comes from that. If the liquid were frictionless, as the spinner speed rose, the two flows would meet more and more perfectly head-on.
I kind of already knew that it would be in the opposite direction because the water is attracted to the nozzles ends of the sprinkler tubes and so are the sprinklers to so like a weird sort-of gravity so they would attract rotating the sprinkler forwards. (Aka. In the direction that all the nozzles bend towards).
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!
Heh, yeah, it's tempting to think of the 'suction' as coming equally from all directions, but that isn't at all the case. The easiest way to see this is just to consider the area behind the opening that is occupied by the pipe itself and the inflowing water. That itself is an asymmetry, though it does get _partially_ compensated for by inflow wrapping around the opening. However... by default, not all that much gets pulled from behind and to the sides. Even in a system _designed_ to do that (the intake lips of a jet engine's cowling, the 'trumpets' of a short straight-pipe intake on a race car) _most_ of the incoming air comes from in front of the opening, because it takes energy for the air to 'turn the corner' to come from behind.
not only that but the friction with the surrounding fluid shedding off while spreading water outward for faster turning also pushing against the surrounding water for more force but the backwards sucking way also working like a sail cupping the wind or in this case water to slow it's momentum and that doesn't even include your internal liquid flywheel yet😅❤
Both a great problem and a great video. I love the experimental results that reveal the forces in this subtle problem. Often, I see people get bogged down in the math of fluid dynamics and skip the intuitive. I also see explanations of why airplane wings lift by resorting to Bernoulli's Principle, but airplanes can fly straight and level for large distances upside down. Why? Momentum.When you understand that the ailerons change the direction of airflow it's easy to understand Newton's Third Law in action this case. BTW, seams on baseballs and dimples on goofballs both drag the air to change its direction, causing the baseball to curve and the golf ball to lift. In the case of the Feynman Sprinkler, it's a lot more subtle and, thus, very interesting.
i think eliminating the vortices on the central cavity would not eliminate the asymetry on the forces aplied at the center of the aparatus. the cavities are there to show us whats happening after the streams interact with eachother, since the change in speed of the water already happens inside the tubes themselfs.
It wasn't until you showed the diagram of the forces within the submerged sprinkler system that I realized my confusion. Technically the thing moving the sprinkler is less about thrust (though that does play a part) and more about the water within the curve colliding with the tubes. If you think of the system in reverse the radial and tangential forces trade but nothing more really happens so (I think) it should spin as normal.
An important concept is the _friction_ of water particles against the internal tube linings. This friction delivers a force causing the sprinkler to rotate in reverse while sucking. This is the same concept that I would teach Elon Musk: instead of using only 33 boosters on the starship rocket, use thousands of mini- or micro-boosters instead. The _friction_ of the jet exiting the nozel creates an additional lift that in turn increases the rocket's efficiency drastically.
In a perfect frictionless world there is no increase in entropy so the system is reversible and blowing while going forward in time is equivalent to sucking going backwards in time so clearly when sucking it should spin in reverse (or forward, the other way in time)
It would be interesting to systematically vary the internal and external diameters of the nozzles to isolate the effect of the low pressure areas in the water intake region. Also, using a narrow diameter T-junction (or two split pipes) in the middle to remove the impact of the vortices. They may only be relevant with the large diameter central outlet cavity.
The hub effect, if any is minimal. Yes, the mechanical design should have been simpler. They would have avoided the essentially faulty conclusion about the cause of the reverse rotation.
It wouldn't be the vortexes themselves as they are just a symptom of the forces. The actual motive force would be the flow gradient exiting the tubes.(which could be enhanched if they were offset from true radials. Such an offset may also slightly increase rotation under normal forward flow.
This is NOT the Feynman sprinkler "finally solved". This is just an explanation of the behaviour in one particular set up. The REAL answer is that out of the 3 options shown at 6:26, the "stationary" people were right (for an ideal theoretical sprinkler). That's why there is no decisive direction of motion in most experiments. A quick look at the Wikipedia will tell you this but you skipped over the main solution in order to make your storyline work. You introduced those 3 options and then proceeded to give absolutely no reason for disregarding them and then acted like it was explained when you introduce the new source of torque. I would've loved a video explaining why the forwards and backwards forces exactly cancel out because I actually don't understand it and would like to.
A retroactive parallel-construction explanation for why I might have happened to decide on the correct direction of spinning: Conservation of angular momentum + water being sucked in with a bias towards the inlet side = the sprinkler experiences a torque opposite to the angular momentum imparted to the water in order to get sucked in. No matter what else happens inside the sprinkler. If the water leaving the sprinkler through the central tube carries no angular momentum, then the conservation law insists that the sprinkler has to gain angular momentum opposite to that of the water just outside the sprinkler boundary.
They should have put a small added friction where velocity is higher near the internal exit, and see if the bias can be reversed and hence rotate in opposite direction. Proving it is indeed the internal mechanism.
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. 👍🖖
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.
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
0:47 build it, test it, problem solved. And don't make the system so weak it explodes.
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
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
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!?
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'?
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.
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
"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?
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.
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.
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?
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?
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.
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 !
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
What a fascinating result. Fluid dynamics - elegantly simple rules that often defy expert intuition.
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.
That answer is actually really fascinating, and it just goes to show you that it's not always outside, but what's inside that really counts
i had a feeling the fan topic was gonna be brought up and lo and behold, 6:42 comes up
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.
Next experiment: stop those vortexes from forming within the center of the hub
That's what I thought.
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?
7:55 "Timmy, close the window" "Oh, sorry dad."
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
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
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
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"
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.
his Path integral formulation is quite remarkable, i never really understood the math of quantum mechanics, but his idea makes it understandable
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
Me, skipping randomly on work through the video:
7:37 _"[...] Interestingly here due to our slightly imprecise use of language when we describe sucking and blowing [...]"_
With a humor stuck stil in puberty this line without context humours me a little.
They also called the simulation PIV...
With this same effect you can use a small (phone) speaker to blow out a candle. A simple voice coil speaker essentially continuesly switches between blowing and sucking as the diaphragm moves. The Action Lab demonstrated this nicely in a video some time ago.
8:00 Wait a sec, did you use battered side down footage??? Awesome!!!
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?
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.
It turns the same direction in both modes because of the momentum change that occurs in the bent tubes. The method of impulse and momentum requires that the change in velocity pushes against the outside radius of the arms.
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.
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.
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 the mass difference exiting or entering tube is a crucial reason for the rotation of a sprinkler other that the obvious other reasons and that changes the outcome of suction. It's just amazing they didn't use something else to compare the problem to
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.
The impulse of the molecules points towards the intake first and up at the end. There are no molecules sucked in from one direction, so the vectors don't exactly add up to zero, the molecules going directly into the tube have nothing to cancel them out. If you want it to go faster just flare out the inlet. I kind of think it's obvy but I'm also very smart.
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.
It would be interesting to see them tweak the angle at which the tubes join the center to either balance or accentuate the difference in vortex sizes… just to show that it affects the turning rate. It would also be interesting to move the be de of the tubes further from the center to allow the difference in fluid velocity across the cross-section of the tube to dissipate, which would be expected to lessen the effect of the turning of the sprinkler. Alternatively, they could insert some kind of mixing vane inside the tubes to “scramble” the various velocities.
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.
This experiment raises another question in me. If you were to extend the arm pipes inside the hub 90° upwards, would the sprinkler not rotate and instead move deeper into the water? The suction tube that removes the water from the sprinkler is mechanically decoupled from the hub so it should not cause any back-pressure and with the feynman sprinkler effectively being an inside-out regular sprinkler, I would expect my modified setup to simply sink.
I'm at 0:50, guessing it will spin "forward", based on the water flowing into the pipes and pushing them in that direction
Dammit
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.
Spectacular exposition. Thank you.
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!
The way I see it, when you pump water out a sprinkler, it has momentum as it exists, giving ratational thrust. But conversly as water enters its coming from all kinds of different directions and so the force cancels out, and it stays relatively still, though it may move as its waterlogged initially.
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 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.
the particles being sucked into the sprinkler reminded me of a blackhole's event horizon. the moment of rapid execration looked like half a sphere, not cone shaped. I only wondered because the concept of a lower pressure drawing the tube forwardm even though the particles are acting like there is excess pressure, flowing away from the vacuum.
Basically with suction there are many small forces at play with none really dominant, so as an engineer it would be most reasonable to say that without further details most sprinkles in suction mode would not rotate. The setup in the paper is a very specific very low friction device.
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!
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 think the difference between sucking and blowing is ultimately key here in terms of the magnitude being much smaller. My first instinct is to think in terms of conservation of angular momentum (though often even if I can get an answer through conservation laws I then like to see whether I can understand my answer in terms of forces).
Because the reason that the tube is rotated has to do with the air being pushed against one side of the sprinkler, I believe that it will rotate the same direction. The air which enters the tube will still have to change direction, which can only happen through interaction with the walls of the tubes, which the front would still have an outsized effect.
Well, now that we can see into it, the results are kind of _intuitive_, the "reverse" sprinkler is just a sprinkler, that sprinkles the water "inwards". So, while in the normal sprinkler, the spinning direction depends mostly on the outlet's direction, and the inlet is only a minor force, here the roles are reversed.
The placement and direction of what would be the inlet of a normal sprinkler is the main force. In this experiment that direction is 90°, revealing the very small force that the outlet's direction imparts on the system. You could probably angle the inlet of the arms to make the reverse sprinkler spin in any direction, irrespective of how the normal sprinkler spins.
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.
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!
Wouldnt the potential direction of rotation depend on the design of the sprinkler and angle at which the arms come into the center of the sprinkler as welk as the internal design of the sprinkler? Another thing that may have an effect, though I'm not sure how or what the effect would be (or whybit would necessarily effect it) would be to set up the sprinkler like a normal sprinkler with the water coming into it from below as if you had set the sprinkler on the bottom of a pool or in a lake. The feed line could still be used as a siphon for suction it would just need to be ran across the bottom of the tank and then up the wall of the tank instead of straight up and out from the center of the sprinkler.
But the rotation caused by the turbulence inside the sprinkler doesn't dismisses that it may also rotate this way due to the low pressure at the tip of the exit holes as we can observe in the PIV.
Can't the rotation be caused by both effects?
Also in 14:14 you can see that the suction of the water causes the water on the outer walls of the tubes is flowing towards the opening, applying friction that might cause a rotation (or cancel the counter clockwise rotation) clockwise.
Every single effect of these should be looked into, every effect we can try to calculate it's contribution to the rotation.
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 an absolutely great explanatory video! I am a lowly engineer with a MS in fluids--and I could so completely understand this! Seriously, thank you!
The vortices are the result of an interesting flaw in the setup, if you eliminate them, the centrifugal forces caused by the flow through the tubes on their own probably won't be enough force to move the whole system.
I wouldn't be surprised if there was some centrifugal effect from the water being pulled towards the centre while the pump was spinning. In theory, this should provide additional torque in the direction of the spin. While it wouldn't be able to provide the initial impulse, it could magnify any spin that would otherwise occur. However the shape of a sprinkler to maximise the centrifugal effect would likely minimise the vortices caused by the water curving.
This was surprisingly interesting.
As others have pointed out in the comments, the cause seems to depend on the design of the inner cavity which is ultimately not defined in the question.
For example, you could put a bend on the inner tube entrances to have it spin in any direction, or zero out the vortices. Or, you could bend the inner tubes 90 deg so they are pointed out of the screen, and eliminate the collision and vortices.
I think the answer to the original question as intended then is that the bent outter arms do not impart any rotation in suction; any resulting rotation is dependent on how the water leaves the tubes at the inner cavity.
As is often the case, in my experience, the answer to difficult questions is often dependent on factors not defined in the question.
Or, am I interpreting the results incorrectly?
NOTHING is defined in the question other than which way will an 'ordinary' sprinkler turn.
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What all his pointless babbling FAILS to clearly indicate is that their combined and correct conclusion is that the explanation at time 5:41to 6:09 is the correct physics and the bent tube creates NO sucking ‘thrust’.
That’s why they analyzed and settled on the center hub’s flow as dominant.
If you carefully account for the true physics of the way fluid curves around from all directions going INTO the end of the tube, you see that force can be considerable, just as it is the dominant effect in creating lift above a wing - as Gustave Eiffel, the Eiffel Tower designer, determined in his aerodynamics testing.
Anyone who has ever used a suction pump to empty a tank will tell you that sucking is not the opposite of blowing. Afterall, the intake pipe doesn't race around the inside of the tank in a frantic attempt to attack or rush at the water. I was expecting a small motion, perhaps a jolt as the liquid inside the immersed sprinkler first started to move. How other fources played a role was where I was becoming interested. Viscosity of the liquid the sprinkler is immersed in for example. It's surely higher than the air the thing usually operates in. Doesn't take a lot of friction to completely balls up something like a free spinning low speed rotor.
Isn't it just the water pressure from outside against the tubes? On the side with the hole the water inside can be pushed away, but on the other side it pushes against the solid tube.
I don't have the math to back it up, but it would seem that there would also be an effect from higher pressure on the opposite side of the pipe as the intake as well as friction on the sides of the pipe as the fluid that's in contact with it runs toward the end of the nozzle before undergoing a 180 degree change in momentum which in itself would have to impart force somewhere.
So it depends a lot on the alignment and angle of those inside openings. Presumably, it wouldn’t hard to direct them slightly to I evade or the other cause whatever rotation you want.
I assumed the sprinkler arms wouldn't suck themselves forwards because it reminded me of those spinning "perpetual motion machines" that just reach equilibrium and don't move at all, despite being weighted towards one side in theory.
It's amazing to me that the obvious in this is invisible.
lowered pressure at the mouth, and not on the rest of the tube. That very small difference, across the area of the mouth tangential to the diameter, is the push. The momentum transferred to the meeting flows comes from that. If the liquid were frictionless, as the spinner speed rose, the two flows would meet more and more perfectly head-on.
I kind of already knew that it would be in the opposite direction because the water is attracted to the nozzles ends of the sprinkler tubes and so are the sprinklers to so like a weird sort-of gravity so they would attract rotating the sprinkler forwards. (Aka. In the direction that all the nozzles bend towards).
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!
Heh, yeah, it's tempting to think of the 'suction' as coming equally from all directions, but that isn't at all the case. The easiest way to see this is just to consider the area behind the opening that is occupied by the pipe itself and the inflowing water. That itself is an asymmetry, though it does get _partially_ compensated for by inflow wrapping around the opening.
However... by default, not all that much gets pulled from behind and to the sides. Even in a system _designed_ to do that (the intake lips of a jet engine's cowling, the 'trumpets' of a short straight-pipe intake on a race car) _most_ of the incoming air comes from in front of the opening, because it takes energy for the air to 'turn the corner' to come from behind.
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.
not only that but the friction with the surrounding fluid shedding off while spreading water outward for faster turning also pushing against the surrounding water for more force but the backwards sucking way also working like a sail cupping the wind or in this case water to slow it's momentum and that doesn't even include your internal liquid flywheel yet😅❤
So a change to the internal inlet of the tube could negate the effect entirely or change the direction possibly. Neat.
Both a great problem and a great video. I love the experimental results that reveal the forces in this subtle problem. Often, I see people get bogged down in the math of fluid dynamics and skip the intuitive. I also see explanations of why airplane wings lift by resorting to Bernoulli's Principle, but airplanes can fly straight and level for large distances upside down. Why? Momentum.When you understand that the ailerons change the direction of airflow it's easy to understand Newton's Third Law in action this case. BTW, seams on baseballs and dimples on goofballs both drag the air to change its direction, causing the baseball to curve and the golf ball to lift. In the case of the Feynman Sprinkler, it's a lot more subtle and, thus, very interesting.
People often get confused if there is more than one contribution to an effect.
i think eliminating the vortices on the central cavity would not eliminate the asymetry on the forces aplied at the center of the aparatus. the cavities are there to show us whats happening after the streams interact with eachother, since the change in speed of the water already happens inside the tubes themselfs.
It wasn't until you showed the diagram of the forces within the submerged sprinkler system that I realized my confusion. Technically the thing moving the sprinkler is less about thrust (though that does play a part) and more about the water within the curve colliding with the tubes. If you think of the system in reverse the radial and tangential forces trade but nothing more really happens so (I think) it should spin as normal.
An important concept is the _friction_ of water particles against the internal tube linings. This friction delivers a force causing the sprinkler to rotate in reverse while sucking.
This is the same concept that I would teach Elon Musk: instead of using only 33 boosters on the starship rocket, use thousands of mini- or micro-boosters instead. The _friction_ of the jet exiting the nozel creates an additional lift that in turn increases the rocket's efficiency drastically.
In a perfect frictionless world there is no increase in entropy so the system is reversible and blowing while going forward in time is equivalent to sucking going backwards in time so clearly when sucking it should spin in reverse (or forward, the other way in time)
I actually guessed correctly it would spin backwards, but slowly. Purely on intuition.
Because the story is a fun one...I'm starting to get the feeling that every story involving Feynman is a fun one.
“Nominee for nominal determinism” wins the subscription lol
It would be interesting to systematically vary the internal and external diameters of the nozzles to isolate the effect of the low pressure areas in the water intake region.
Also, using a narrow diameter T-junction (or two split pipes) in the middle to remove the impact of the vortices. They may only be relevant with the large diameter central outlet cavity.
The hub effect, if any is minimal. Yes, the mechanical design should have been simpler. They would have avoided the essentially faulty conclusion about the cause of the reverse rotation.
It wouldn't be the vortexes themselves as they are just a symptom of the forces. The actual motive force would be the flow gradient exiting the tubes.(which could be enhanched if they were offset from true radials. Such an offset may also slightly increase rotation under normal forward flow.
This is NOT the Feynman sprinkler "finally solved". This is just an explanation of the behaviour in one particular set up.
The REAL answer is that out of the 3 options shown at 6:26, the "stationary" people were right (for an ideal theoretical sprinkler). That's why there is no decisive direction of motion in most experiments. A quick look at the Wikipedia will tell you this but you skipped over the main solution in order to make your storyline work. You introduced those 3 options and then proceeded to give absolutely no reason for disregarding them and then acted like it was explained when you introduce the new source of torque.
I would've loved a video explaining why the forwards and backwards forces exactly cancel out because I actually don't understand it and would like to.
A retroactive parallel-construction explanation for why I might have happened to decide on the correct direction of spinning:
Conservation of angular momentum + water being sucked in with a bias towards the inlet side = the sprinkler experiences a torque opposite to the angular momentum imparted to the water in order to get sucked in. No matter what else happens inside the sprinkler. If the water leaving the sprinkler through the central tube carries no angular momentum, then the conservation law insists that the sprinkler has to gain angular momentum opposite to that of the water just outside the sprinkler boundary.
Time symmetry or causal symmetry would indicate if it goes forward in forward causation and will go backwards with backwards causation.
They should have put a small added friction where velocity is higher near the internal exit, and see if the bias can be reversed and hence rotate in opposite direction. Proving it is indeed the internal mechanism.