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Old   August 23, 2011, 03:01
Default Problem with wall function
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Gregoire Junqua
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Dear Openfoamer.

I am a new in modelling, I want to study turbulence over a plane plate, but now
I am dealing a problem with the wall function.

Normally i should have a beautiful log law function but I got the following wind profile showed in the following picture.

My boundary are as follow

Inlet
pressure zerogradient U fixed value k fixedvalue epsilon fixedvalue

Outlet
pressure fixedvalue U zerogradient k zerogradient epsilon zerogradient

Wall
pressure zerogradient U fixedvalue k kqrwallfunction epsilon epsilonwallfunctionn

The case consist of a box
inlet is in the left
outlet is on the top and on the right
wall is on the bottom
the case is in 2D.

If someone could give me an advice it would be nice
Thanks in advance.
Attached Images
File Type: jpg windProfile.jpg (14.5 KB, 45 views)
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Old   August 23, 2011, 15:11
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Are the front and back walls with the "empty" boundary condition?
And what is the size of your mesh? Can you post the values of y+ (i think the command is yPlusRas.
Standart k-epsilon turbulence mode?
What about the convergence? Did it pass the 1e-5?
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Old   August 23, 2011, 21:15
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Gregoire Junqua
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Hi
The front and back got empty conditions. and yes i use a k-epsilon model
The profile of yplus is in the atached file.
I had done several test with several model, it show that it's the boundary conditions are the problem
I tried Pisofoam, simplefoam, with the both condition LES RAS
but the given answer is the same.
For the convergeance
i don't knbow how to get it. I give you a sample of a time step perhaps it s inside.
Quote:
Time = 10.9215

Courant Number mean: 0.729365 max: 1.54273
DILUPBiCG: Solving for Ux, Initial residual = 3.69415e-05, Final residual = 6.31557e-10, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000546025, Final residual = 9.49463e-09, No Iterations 1
DICPCG: Solving for p, Initial residual = 5.72492e-05, Final residual = 4.1219e-06, No Iterations 2
time step continuity errors : sum local = 7.61108e-13, global = 3.79935e-14, cumulative = 1.00392e-07
DICPCG: Solving for p, Initial residual = 2.02581e-05, Final residual = 9.95151e-07, No Iterations 56
time step continuity errors : sum local = 1.83755e-13, global = -5.35126e-15, cumulative = 1.00392e-07
DILUPBiCG: Solving for k, Initial residual = 9.97577e-06, Final residual = 9.97577e-06, No Iterations 0
I still searching why i got that problem
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File Type: png windprofileyplus.png (6.7 KB, 27 views)
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Old   August 24, 2011, 07:24
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Hi,
is the left y axis the y+ value? Try increasing the size of the mesh near the walls and get a value of y+ between 30 and 300, your mesh seems to small for the k-epsilon model.
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Old   August 24, 2011, 20:57
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Gregoire Junqua
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Hi,
No the wind is on the left the input is an uniform 10m/s wind. on the right is the yplus.
I will today increase the mesh size near the wall... I hope it would work.
Thanks for the advice
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Old   August 25, 2011, 23:32
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Gregoire Junqua
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Hi,
I found that it the wall function or some misunderstood of this experiment because i tried different mesh size, I try the dnsFoam
And all give the same answer.
So i think the configuration I gave is wrong
If somebody know how to have a beautiful loglaw please tell me...
perhaps the input should be a loglaw too?
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