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in kepsilon wall function approach high Re turbulence models: question of velocity 

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June 27, 2013, 04:04 
in kepsilon wall function approach high Re turbulence models: question of velocity

#1 
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Roman Thiele
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Hej,
I looked through the different wall function for kepsilon turbulence models, however I feel I am missing something. In Wilcox (Turbulence modeling for CFD 2006) as well as many other publications, in wall functions the velocity seems to be set in the first grid point. In OpenFOAM, the following properties are changed with wall functions in the example of the kepsilon turbulence model,
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July 2, 2013, 05:18 
took a while but here is a solution to the problem

#2 
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Roman Thiele
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Hej,
I think I found the solution to this. I found it in Pope, Turbulent Flow, 2000 under the way that wall functions are created. The original question was why the velocity is not set by any of the wall function. The answer is, that wall functions are apparently more robust by setting the shear stress and not the velocity, which makes sense when looking at separation points and other nasty flow phenomena, in which the velocity can become essentially 0. The way this is for the kepsilon model done is nutkWallFunction
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July 2, 2013, 13:22 

#3 
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Mohsen KiaMansouri
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Thanks roman
it was very helpful.
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July 2, 2013, 19:02 

#4 
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Ehsan
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hi
Whould you explain more and Why stage 4 is done?
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July 3, 2013, 04:26 

#5 
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Roman Thiele
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Step 4 is introduced in order to have a more stable solver. Instead of setting the velocity in the first point away from the wall, a force (the wall shear force) is introduced in the equation. This way, the solver won't have any 0 velocities introduced by accident (think of stagnation points), this could destabilize the solver as you would be introducing 0's into the matrix.
The easiest way to introduce this source term into the equation is by modifying the turbulent viscosity, see Pope, Turbulent Flows, 2000, pp 442443 and Versteeg and Malalasekera, An introduction to computational fluid dynamics: The finite volume method, pp. 276277
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March 15, 2015, 22:12 

#6  
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Huang Xianbei
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Quote:
Thank you for your explain. I have a question about the wall function for nonlinear models. As the nonlinear model can't be depicted by a simple turbulent viscosity, how can we set the shear stress? say we obtained the shear stress as in your form: u_shear * U_p,real / U_p,nominal, so we should set the Reynolds shear stress by u_shear * U_p,real / U_p,nominal  nu*dU/dy, here, y represents the wallnormal direction. However, the nonlinear Reynolds stress is not symmetric, it has 6 shear stress components, which one should be modified? I'm confused by this problem for some days. I'm looking forward for your reply. Thank you Xianbei 

Tags 
turbulence model, velocity, wall function 
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