Problem with wall function

gregjunqua · August 23, 2011, 03:01

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.

lindner · August 23, 2011, 15:11

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?

gregjunqua · August 23, 2011, 21:15

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

lindner · August 24, 2011, 07:24

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.

gregjunqua · August 24, 2011, 20:57

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

gregjunqua · August 25, 2011, 23:32

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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August 23, 2011, 15:11		#2
lindner Member Join Date: Mar 2010 Posts: 42 Rep Power: 17	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?

August 24, 2011, 07:24		#4
lindner Member Join Date: Mar 2010 Posts: 42 Rep Power: 17	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.

August 24, 2011, 20:57		#5
gregjunqua Member Gregoire Junqua Join Date: Jun 2011 Location: China Posts: 58 Blog Entries: 1 Rep Power: 15	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

August 25, 2011, 23:32		#6
gregjunqua Member Gregoire Junqua Join Date: Jun 2011 Location: China Posts: 58 Blog Entries: 1 Rep Power: 15	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?