วีดีโอ

Hello, this case was created solely for tutorial purposes and does not have any corresponding experimental data. Some estimations about the validity of the results can be gathered by solving analytical equations for the 1D heat transfer through the heated body.

In the momentum equation gravity is added as a body force multiplied by the density. If you have constant density then this force is applied equally everywhere and therefore does not impact the flow. In other words, for forced convection in a pipe of constant density you will see no effect from gravity and it can therefore be neglected in the calculation.

@@cfdkareem Thanks for you respond. If I would like to have the gravity effect present even though my density is constant. Is it possible?

@abdullaal-tarmoom3437 yes, under general options you can check density and set the direction and magnitude. For earth gravity it will be -9.81 m/s^2 in the Y direction.

@@cfdkareem this is what i did for your case, I clicked on the gravity option and choose it to be -9.81 in the Y direction. The results stayed the same. So how can I make the gravity effect present in this case? much appreciated

@abdullaal-tarmoom3437 as stated before, you will see no effect from gravity unless you have variable density for the fluid material.

Hello Moustafa, you do not need to change this option in Space Claim. The final definition for the body will be done in Fluent and you can change between solid and fluid. In meshing you can also add a named selection to the body and give it a name such as "domain_fluid". This will tell Fluent that this is a fluid domain and will assign it correctly when the mesh is loaded. Again, not necessary since we can switch it in Fluent, but nice for workflow efficiency.

@@cfdkareem Thank you for your quick responce. I am waiting for your upcoming videos, you are doing a great job. 👍

Hello, unfortunately I cannot provide a version of Ansys other than the student license. What issues are you having with the student license that makes it insufficient for your model?

Hey Josue, 3D VOF models are very computationally intensive so if you can reduce your model to a 2D axis symmetric or 2D planer it will be much simpler. I have a video coming out soon on the deposition and solidification of a metal droplet on a solid substrate. It may help give you some direction for your model!

Definitely! However, you will still have to provide a wall thickness and material for the calculation. Simple thermal contact resistance, shown in the video, models the layer as zero thickness which only allows heat conduction normal to the surface. Shell conduction will allow heat transfer both normal and planer to the wall. Shell conduction will also account for the additional thermal mass which can be useful if you have an interface layer of significant thickness.

@@cfdkareem thank you sir😊

Yes! If you check out the short on my channel it shows you where to download it and some of the limitations of the student license. Let me know if you have any issues!

@@cfdkareem got it. Can you do a tutorial of a heat sink with a cooling fan in front of it?

I have a tutorial on my channel for forced convection through a heat sink which will give you the basics for this problem. Are you trying to model the fan explicitly?

@@cfdkareemmodel fan explicitly

@kaleb101 ok great! I'll add it to my list for future videos.

All profiles must be in SI units. Fluent always operates under the hood in SI. When you change to English in the GUI it's doing a "visual conversation" well still using SI under the hood.

Deserves way more views forgot to add!

Glad you liked it!

Hello, are you trying to change the flow from air to water? Or trying to do a multiphase simulation with air and water?

Boiling is a fairly complex simulation which will require multiphase and the boiling/condensation model. Is this what you are trying?

Hello, going from voltage to pressure requires an initial simulation that models the piezo movement. I obtained this profile from such a simulation. To model this you will have to model the whole ink chamber. You can do this with a single phase simulation and have a pressure=0 condition at the outlet. You will then input the voltage profile to drive the motion of the piezo and export the pressure generated right above the nozzle. I will add such a model to my future video list!

Glad it helped!

Glad it was helpful!

There are numerous equations for estimating the turbulent velocity profile. The most common for industrial flows is known as the Prandtl one seventh power law which takes the form: u/Umax=(y/R)^1/7. You can plug in Umax and R to solve for u over the range of y.

Our teacher said u can use k-epsilon for turbulence but with that method you can only get the first vortex others only appears for the initial iterations then dissappear.

also, with very fine mesh (i have uniform 150k grids) DES methods takes almost 2k iterations to hit 10^-3 criteria and oscillations never stops as i see.

Hello, the pressure profile is in the video description!

I used that pressure profile, I wanted to know how was that generated? Do you have any reference literature so that I can modify that pressure profile according to my operating conditions. Thank you!

@AdityaChivate this pressure profile is just an arbitrary waveform for the purpose of the tutorial unfortunately. If you look through some literature on inkjet waveform generation you may be able to copy a representative one from the literature.

@@cfdkareem Thanks for your reply. I'll check around to see if there's any existing literature that talks about this.

Hello Joaquin, near the bottom of the NASA page there are two links to the Cf and Cp data. Clicking the links will open a new tab with the data displayed in text format. You can then copy the data and paste it into excel or a text document.

Hello, in this case, with such a low Reynolds number, the viscous heating was assumed negligible. If you want to calculate viscous entropy you will have to turn that option on under viscous models > Laminar > Viscous heating, and recalculate.

@@cfdkareem Yes, I found the viscous heating option in viscous model. I found the following statement: Viscous Heating (if enabled) includes the viscous dissipation terms in the energy equation. This option is recommended when you are solving a compressible flow. Note that this option is always turned on when one of the density-based solvers is used; you will not be able to turn it off. In my case, my flow is laminar and incompressible. I am using pressure-based solver as well. In that case, how can I calculate "Viscous entropy" and Thermal Entropy"? Looking forward to your kind response. Thanking you.

Hello, the characteristic length used to calculate the Y+ is the step height, 0.01 m. Good catch on the wall distance! The one calculated on the website should have been used as the first layer height of the inflation. It looks like I forgot to paste the value!

@@cfdkareemyh that was really baffling me, but thanks for clarifying

@@abhishekbenny7930 even I noticed both things. for the second point, i felt as if kareem has rounded off the value 5.2x 10-6 to 1x10-5. Though I could not understand as to why the reynolds number value appearing on the Y+ calculator did not matched to 36,000 as stated at the start of the problem. Can you advise pls. Thanks

Hey Ahmed, if you're modeling a datacenter HVAC system you often build up a series of simulations, instead of modeling each individual heat sink in each server. It starts my modeling a single server and calculating the total heat output of each server. You can then estimate the total heat output of all the servers in a rack. Each rack will be modeled as a fan boundary condition with a specific pressure jump and temperature jump. These racks are modeled inside the larger data center and the HVAC system in the room is analyzed. If this is what you are interested in I can add it to my list of future videos!

Hey James, I've added the inlet velocity profile to the video description! Copy it to a .txt file. It can then be read in as a profile for the velocity inlet.

Hello James, yes the second simulation is created using a full new model including geometry in SpaceClaim, meshing, and a new fluent setup. The second simulation is simply a 2D rectangle with a height equal to the inlet height of the primary simulation (turbulent step). Make sure you are not using a very small mesh size for this simulation. The mesh will still be fairly large, but shouldn't take more than a few minutes to create. You can also do what you mentioned, i.e. extend the inlet of the turbulent step simulation far enough to allow the flow time to become fully developed. The benefit of splitting the simulation is that your primary simulation, the turbulent step, will have a much smaller mesh size and therefore a faster calculation time.

If you have one vertex much higher than the rest of the flow it is usually one of two problems. Most likely there is a bad mesh element that is not converging well. Try remeshing and possibly refining it to see if it improves. The other potential is that the simulation is not fully converged. Try running the simulation for more iterations and see if it converges. If neither of these help them check your boundary conditions and make sure the inlet velocity/material properties are set correctly. Let me know how it works out!

@@cfdkareem I’ll give this a try - thank you for fast response :)

Great to hear! Could you describe your bugs/solutions you faced? I'll make sure to cover them in future videos.

You can use the sketch feature and select a face you want to sketch on. You can then use the rectangle tool and pull tool to create another rectangular body. You can then use this as a body of influence in meshing to refine the region.

@@cfdkareem Thanks. I'll try it.

Yes you'll want to create a line extending from the wall and plot the x-direction velocity vs y. You can then find the height where the velocity along the wall achieves 99% the free stream velocity.

@@cfdkareem Thank you, and in terms of mesh independence, how should that carried out as we have two separate domains, I mean if my parameter is the reattachment length how would I refine the first domain to export the profile then refine the second one !

Great question! In the first domain your output is the developed velocity profile. This can be calculated analytically so you could compare your simulated profile to the equation. Reducing error in the profile will give the most accurate results for the reattachment. You can then use the profile in the second and analyze the mesh sensitivity for the reattachment point. However, mesh independence mostly comes down to how much accuracy you need from the model. For me personally, in an industrial setting, I would use my engineering judgement for the first mesh, confirm the profile is acceptable, and then perform an analysis on the second, since I'm mostly interested in the reattachment and can tolerate a small amount of error. As with most modeling it's all about using your judgement, experience, and skill to determine the best way to get the information you need!

It will! You just have to make sure you extend it far enough for the flow to fully develop. It will also increase your solving time as the mesh will get much larger.

That works as well! If you have a multi-body model then the enclosure tool is more useful. For simple geometries it's just about as fast as the subtraction method!

Yes, the two meshes are identical. I would try to maintain consistent meshes when possible, but if you can't the interpolation works well. From my knowledge, Fluent always references the global coordinate system for profiles and UDFs. You will have to manually translate your input profile in reference to the global coordinate system. If you know where the profile is going to be implemented, I try and move the geometry to an ideal location in Spaceclaim to make writing the profile more convenient. For example, if I am writing an inlet profile I will put the bottom of the inlet right at the global coordinate system so the bottom edge starts at 0,0,0.

@@cfdkareem kareem, I really appreciate your answer, thank you. If you don't mind, I would like to ask you another question about DOF in fluent. In case you know the answer, the question is: I would like to now if it's possible to set magnet field in DOF of fluent to interact with magnetic wall. This magnetic field is responsible for rotate a rotor that is set as wall in fluent. The flow of fluid is responsible for increase the rotation and temperature of this wall. Also, the magnetic properties of the wall material change accordingly with the its temperature. So, at first, the inputs of my model are the inlet velocity and temperature, outlet pressure of the flow and the magnetic field that is interacting with the wall which generates rotates. Do you know if it's possible to simulate this in fluent?

@Rau379 it's definitely quite a complex problem! It would be difficult to do through the graphical user interface, but can likely be done with a UDF. The wall motion and properties can be defined using the macros DEFINE_MOTION and DEFINE_PROPERTY. There is no defined macro for defining magnetic fields that would interact with a solid boundary. You will likely have to create a custom function for the wall motion and define it under the motion macro. Check out the Fluent customization manual for more info on UDFs.

Ideally wherever you can cut the domain that would reduce mesh size and you can confidently assume the boundary condition at the point. You could also do a preliminary run with a single phase, and then export the profile right before the nozzle. It all comes down to your own engineering intuition on what will effectively capture your problem.

@@cfdkareem ok thanks a lot. I wasn't sure if I cut whole secondary phase injection (nozzle) and put that velecity profile as b.c. into a meshed face at the bottom of my test section it would simulate exactly the same condition. Like at the first time step would it model the liquid just like the real case(if I cut the whole scondary phase/nozzle) or should I provide some of the last section (after the taper) of the nozzle exit in to my whole simulation. But if the only important parameter is to reduce the computation time then it would be wise to divide it from nozzle exit and introduce whole nozzle exit velocity profile as b.c. to face mesh of nozzle exit. Thanks a lot