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Old   November 21, 2005, 07:50
Default Turbulence Models and external flow.
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Dear CFD users,

I am simulating a symmetrical spoiler 1200mm above the ground at a speed of 150 km/hr. The finite volume model comprises of 2.1 million cells.

To solve for a solution, I have used two turbulence models. The S-A turbulence model and the RnG K-Epsilon model. The problem is that the S-A turbulence model struggles to solve and after 80 to 90 iterations it complains about turbulence viscosity being limited to (a figure). When I look at the contours of velocity magnitude I see a huge red spot being formed behind the spolier, I have no idea whats happening there!

However, the RnG k-Epsilon model does not behave this way.

I will be grateful if any one can suggest whats happening in the S-A turbulence model case. I have checked for cells skewness, the inlet and outlet boundary conditions and every thing seems fine.

Thanks a lot in advance.

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Old   November 21, 2005, 20:32
Default Re: Turbulence Models and external flow.
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Hi Alan, Since u r getting turbulence velocity warning..probably the problem would be with mesh...check the skewness which should be within 0.8. Also check the boundary conditions of turbulence parameters. Also all turbulence models are not universal..check out with other turbulence models as well.

hope this helps. bharat

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Old   November 22, 2005, 05:34
Default Re: Turbulence Models and external flow.
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S-A is a one equation model its sutible for high velocity flow you can use K-epsilon
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Old   November 22, 2005, 05:46
Default Re: Turbulence Models and external flow.
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Hi Bharat,

Yes, you are right I will play with differernt mesh size this time! I have used the RnG K-Epsilon model and it does not struggle at all, well it does not give me a converge solution as well but the S-A turbulence has struggled so far.

The problem seems to lie on the symmetry plane for some reason. A huge velocity region is building up and causes the lift and drag values not to converge.

Any ways thanks a lot for the advice. I hope remeshing would work!

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