komegaSST model

stix · March 1, 2013, 02:32

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

PLD · March 4, 2013, 07:22

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,

stix · March 4, 2013, 07:31

Quote:

Originally Posted by PLD

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?

PLD · March 4, 2013, 08:26

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.

stix · March 4, 2013, 09:12

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?

PLD · March 4, 2013, 09:30

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 :
http://www.cfd-online.com/Forums/ope...-openfoam.html

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

March 1, 2013, 02:32	komegaSST model	#1
stix New Member Join Date: Feb 2013 Posts: 7 Rep Power: 13	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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March 4, 2013, 07:22		#2
PLD New Member Pierre-Luc Join Date: Oct 2011 Location: UK Posts: 16 Rep Power: 15	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,

March 4, 2013, 08:26		#4
PLD New Member Pierre-Luc Join Date: Oct 2011 Location: UK Posts: 16 Rep Power: 15	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.

March 4, 2013, 09:12		#5
stix New Member Join Date: Feb 2013 Posts: 7 Rep Power: 13	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?

March 4, 2013, 09:30		#6
PLD New Member Pierre-Luc Join Date: Oct 2011 Location: UK Posts: 16 Rep Power: 15	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 : http://www.cfd-online.com/Forums/ope...-openfoam.html Let me know if your results are better with y+ = 1.