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k-epsilon boundary condition in openFoam

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Old   February 27, 2013, 01:32
Default k-epsilon boundary condition in openFoam
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Amitabh
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I am trying to validate k-epsilon model on openFoam for a simple turbulent channel via the pisoFoam solver. The boundary condition being used for k and epsilon is the kqRWallFunction. While the previous forum posts suggest that kqRWallFunction simply applies a zero gradient condition for k at the wall, I'm not seeing dk/dy=0 at the wall for the solutions I am getting. In fact, for the converged solution, there seems to be a very steep gradient in k at the wall, and k(y=0) seems to be non-zero. My first grid point at the wall is at around y^+=1.

I am still not very good at reading the source code, and I found scare documentation for this. So does anyone know what exactly the kqRWallFunction wall BC means ? I'm guessing it is some sort of log-law based wall-function, and maybe my first grid point needs to be outside the viscous region.

Any insight on this would be great.

Amitabh
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Old   February 27, 2013, 02:48
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Bernhard
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You are right about your first grid cell. If you got y+ around wall, you should not use wall functions. How did you calculate y+? (See this thread: y+ and u+ values with low-Re RANS turbulence models: utility + testcase )

You can find the wall functions here:
src/turbulenceModels/incompressible/RAS/derivedFvPatchFields/wallFunctions

Of special interest is the epsilonWallFunction, have a look at epsilonWallFunctionFvPatchScalarField.C , the member function updateCoeffs (line 175 and further). Here you can see what is actually being calculated (G is the production term).
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Old   February 27, 2013, 03:40
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Thanks Bernhard, this was very helpful. The source code seems to indeed use a log-law based wall function for epsilon. I clearly need to use a larger grid size near the wall.

I am basically imposing pressure gradient in the flow, so it is then easy to calculate u_tau from dP/dx and channel half-width H. I calculate l^+ as nu/u_tau. Let me know if this does not sound right.

thanks !

Amitabh
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Old   February 27, 2013, 03:52
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You could also choose to switch to a Low-Re RANS-model, but than you may have to do some implementation yourself. It depends a bit on your needs.

I am not 100% sure about your way of calculating y+, but you can easily check it afterwards with the utilities provided in the thread I linked above. You may want to just calculate y* if you decide to coarsen your mesh.
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Old   February 27, 2013, 08:51
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I had a follow-up question. It seems to me that (for high-Re RANS) there should be a slip velocity at the wall if y+ of the first grid point is large. The slip velocity can be obtained from the log-law, i.e. U=u_tau*(log(y+)/kappa+B_i). Alternatively dU/dy=u_tau/(kappa*y) can be imposed.

For all the example problems I have seen, U=0 is imposed at the wall, which seems wrong.

Again, any insight on this would be great.

regards

Amitabh

p.s.: seems like I'll need to start writing my own boundary condition routines soon
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Old   January 9, 2014, 10:47
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Dear Amitabh,

did you finally managed with k-epsilon? I did a periodic channel flow simulation with different turbulence models. I tried low Re k-epsilon model, but did not manage to get the right velocity gradient at the wall, no matter what wall treatment I'm using. My wall+ is around 0.8.

In any case my wall gradient is under estimated, it is around 2/3 of the DNS database I'm using. If I use v2f instead, everything is fine.

Thanks,
Lilla
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Old   January 9, 2014, 11:37
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I did indeed manage to figure out the low Re k-epsilon last year -- it seemed to be working ok. I can send you the files if you can somehow send me your email, since there are upload limits here.
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Old   January 9, 2014, 11:50
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Kapa Lilla
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Thank you very much! My email is kapalilla@gmail.com
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Old   January 17, 2014, 05:50
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Hi

Is it possible for you to send me the low Re k epsilon model files too? I am trying a similar exercise in validating the model for channel flow with periodic inlet and outlet. My email id is manan.lalit@gmail.com. Thanks.

Last edited by MaLa; February 14, 2014 at 12:13.
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