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Old   March 1, 2013, 02:32
Default komegaSST model
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Hi!

I am new to OpenFOAM and I have to use OpenFOAM for my diploma thesis. I want to compare the results of OpenFOAM with simple examples, which can be calculated by hand. My first example is a straight pipe flow and i already have problems. My geometry is a 1mx1m quadratic tunnel with a length of 20m. I defined all boundary conditions (inlet 5 m/s, outlet pressure=0, wall ). The flow is turbulent and i use the komegaSST model for the turbulence. i simulated the flow with different mesh resolutions (various y+). The y+ values i were 30, 50, 100, 150.

Now the problem is that i calculated the pressure loss analytically and the OpenFOAM results aren't even near the exact results. They are 30% (and for high y+ values even more) away from the exact results.

Could anybody please help me with my problem?

Thanks, Daniel
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Old   March 4, 2013, 07:22
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Hi,

You should try with y+ = 1 and Wall Functions :
- kqRWallFunction for k
- omegaWallFunction for omega
- nutLowReWallFunction for nut.

I think this is the best way to use the komegaSST turbulence model.

Regards,
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Old   March 4, 2013, 07:31
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Quote:
Originally Posted by PLD View Post
Hi,

You should try with y+ = 1 and Wall Functions :
- kqRWallFunction for k
- omegaWallFunction for omega
- nutLowReWallFunction for nut.

I think this is the best way to use the komegaSST turbulence model.

Regards,
Thank you,

but i have a question. I am really new to OpenFOAM. How do i define Wall Functions?
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Old   March 4, 2013, 08:26
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In the 0 directory you have files for k, omega and nut. You define the wall functions for your walls in these files. You should read the user guide for more information.
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Old   March 4, 2013, 09:12
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Ah ok thank you very much.

But my Re-Number is pretty high (10^5). I don't think that low Re function is correct?
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Old   March 4, 2013, 09:30
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Actually, when you use a y+ = 1, you use (or at least you should) a Low-Re formulation of the turbulence model. In OF, the difference between Wall functions and the Low-Re formulation is not very clear. You should read this thread for a better understanding :
Wall treatment with OpenFOAM

Let me know if your results are better with y+ = 1.
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