[CFD] What Wall Functions Do I Need for Epsilon?

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Күн бұрын

[CFD] What Wall Functions Do I Need for Epsilon?
An introduction to the wall functions that are used to capture the near wall behavior of the turbulence dissipation rate (epsilon) in all mainstream CFD codes (OpenFOAM, Fluent, CFX, Star). The following topics are covered:
1) 4:03 How is the wall treatment for epsilon different to velocity and temperature?
2) 12:57 What wall functions do we use for epsilon?
3) 18:18 How do we account for cells with multiple boundary faces?
#wallfunctions #epsilon #epsilonwallfunctions
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Some useful references:
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1) ANSYS FLUENT User Manual
12.10.2 Standard Wall Functions
www.afs.enea.it/project/neptun...
2) G. Kalitzin, G. Medic, G. Iaccarino and P. Durbin
Near-wall behaviour of RANS turbulene models and implications for wall functions
Journal of Computational Physics 204 (2005) 265-291
citeseerx.ist.psu.edu/viewdoc/...
3) Moser, Kim & Mansour, Direct numerical simulation of turbulent channel flow up to Re_tau = 590. Physics of Fluids, 11(4), 1999.
cfd.spbstu.ru/agarbaruk/doc/1...
4) The complete set of statistical DNS data for turbulent channel flow:
turbulence.ices.utexas.edu/MKM...
5) J. Bredberg. On the wall boundary condition for turbulence models. Internal report 00/4, Chalmers University of Technology, Sweden, 2000.
www.tfd.chalmers.se/~lada/post...
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Disclaimer
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The methods, algorithms, equations, formulae, diagrams and explanations in this talk are for educational and demonstrative purposes only. They should never be used to analyse, design, accredit or validate real scientific / engineering / mathematical structures and flow systems. For such applications, appropriate trained, qualified and accredited (SQEP) engineers / scientists should be consulted along with the appropriate documentation, procedures and engineering standards. Furthermore, the information contained within this talk has not been verified, peer reviewed or checked in any way and is likely to contain several errors. It is therefore not appropriate to use this talk itself (or any of the algorithms, equations, formulae, diagrams and explanations contained within this talk) as an academic or technical reference. The reader should consult the original references and follow the verification and validation processes adopted by your company / institution when carrying out engineering calculations and analyses. Fluid Mechanics 101 and Dr. Aidan Wimshurst are not accountable or liable in any form for the use or misuse of the information contained in this talk beyond the specific educational and demonstrative purposes for which it was intended.

Пікірлер: 22

@santoshn8450 4 жыл бұрын

Excellent explanations Aidan.. your videos are very informative.

@diego-araya 4 жыл бұрын

Hi Aidan, this is a fantastic work!. Just wanted to point out a small typo in page 13/17, equation (10), kp should be to power of 3/2. Thank a lot!

@fluidmechanics101 4 жыл бұрын

Yikes! Yep, well spotted. I have pinned the comment so everyone can see 😄

@CoastalEng. 3 жыл бұрын

@@fluidmechanics101 When it comes to the equation (13), I think kp should be to power of 3/2. And I am unsure why there is epsilon_p in the right-hand side of equations (12) and (13). By the way, Aidan your video is so informative and nice for me. Thank you for providing this fantastic work for us.

@chrisharding 4 жыл бұрын

Well done!

@aliciapotgieter4279 4 жыл бұрын

Great content Aidan, Love your videos :)

@fluidmechanics101 4 жыл бұрын

Thanks Alicia!

@rpapa 3 жыл бұрын

Cool !!!!

@Maxakch 2 жыл бұрын

Hi Adian, thanks for video. It's looks like that you have a mistake at the 13th slide. Power of k should be 3/2 instead of 1/2.

@adityashivaappalla1384 4 жыл бұрын

I have a doubt here. For unstructured grids, the face lengths for multiple boundary faces will not be the same, always. Is it then possible to assign weights based on the area of each boundary face cell? Can you please cite any notes in this direction? Thanks for all the videos you have uploaded. Really informative!

@parasghumare8067 2 жыл бұрын

What is the epsilon_p on the RHS of equation 12 and 13?

@lastday67 Жыл бұрын

same here

@lokeshattarade2950 4 жыл бұрын

Hi Aidan.. I really appreciate your efforts for giving insights on the CFD topics, which are really hard to find at one place.. Also really well explained and documented.. I have a general question on K-espilon model. Is it suitable to use k-epsilon model in the range of y+ less than 5? I used the same model before for pressure drop caluclation in ducting system.

@fluidmechanics101 4 жыл бұрын

You sure can. Just make sure you have the low Re formulation 😊 For your application it should be fine. However, if you were looking at aerofoils/turbo machinery (were the seperation point prediction is critical) make sure to use k omega SST 👍

@lokeshattarade2950 4 жыл бұрын

@@fluidmechanics101 Can we say that k-epsilon realisable model is more better than low Re model for the application I mentioned.? Can you provide some references where I can learn more about realisable model..?

@florianschittl8079 4 жыл бұрын

Hey aidan, Very useful video (again;)) for fundamental understanding in cfd. I have one question. How did you calculate the dimensionless dissipation rate from mosers dataset? Thanks for your Time! Kind regards Florian

@fluidmechanics101 4 жыл бұрын

Yikes! I cant remember how I did this 😅 Its been a long time! I remember it being quite tricky. Il get back to you when i remember

@vivekchaurasia9807 3 жыл бұрын

Hi Aidan sir, Good Moorning Can you pls upload a videos for omega wall function?

@user-dt1hd3ye7w 3 жыл бұрын

Hi Aidan! My case is a 3D pipe flow. However , I still don’t understand where is the sub-layer in the pipe. I don't even understand the settings of the parameters. Can you make a video about 3D pipe flow instance on fluent? Thank you!

@fluidmechanics101 3 жыл бұрын

The sub layer is very close to the wall of your pipe (on all surfaces). If you plot a line normal to the wall (showing velocity) you should be able to see the sub layer in your velocity profile. If your mesh is fine enough then you should see a linear region very close to the wall where velocity varies linearly with increasing distance from the wall. However if your mesh is not very fine, this layer will be contained within the cell that is next to the wall and you won't be able to see the linear region if you plot a line normal to the wall 👍

@SoumilSahu Жыл бұрын

I've come back to this video after watching the one on wall functions for the TKE, and I have the following question: Equation 10 (in slide 13) is being used to compute epsilon_p, but this requires the prior knowledge of k_p. Do we first have to compute k_p using a wall function for TKE? Or am I missing a step somewhere? What's even more confusing to me is that computing k_p using a wall function requires the friction velocity u_tau, which itself is defined using k_p, since u_tau = sqrt (C_mu * k_p). In other words, can you clarify the order in which a CFD code uses the different wall functions?

@fluidmechanics101 Жыл бұрын

Sadly I don't think I can answer this. How different CFD codes work is hidden in their source code. The only thing we get is the equations given in the user manual. The one exception is OpenFOAM, where we can peak under the hood and see what it is doing. OpenFOAM will of course change from version to version, so the best I can offer is for you to have a look at the OpenFOAM source code in the version you are using or send an email to the customer support for the code you are using. If you are lucky, they may tell you how it works 🙃