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Flow exit out of wind tunnel

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Old   February 2, 2006, 09:47
Default Flow exit out of wind tunnel
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Peta
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At the back of my wind tunnel when I look at a plot of the turbulence intensity it shows that the flow in the wind tunnel is not perfectly symmetrical, what could be the problem? could it be the setting of the vel. @ the inlet boundary codition or perhaps the exit condition?
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Old   February 3, 2006, 10:15
Default Re: Flow exit out of wind tunnel
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Jason
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Is there anything in the tunnel? Are there any Convergent or Divergent sections in your tunnel? Are you using a structured or unstructured mesh? What are you using for turbulence model?

There have been a lot of cases of asymetry in the flow due to unstructured meshes. Remember, that an unstructured mesh will diffuse the flow due to the fact that fluxes are calculated normal to the faces of the cells in your mesh, which are at odd angles to the flow when using an unstructured mesh. On the same lines, if your mesh isn't refined enough to capture the flow features, and it's only slightly asymetric, it can cause large asymetries in the results. Also, divergent sections tend to cause problems because of the adverse pressure gradient on the wall. Watch your y+ values, and pay careful attention to cell growth rate. You can also try different turbulence models. With a proper mesh, I've gotten good results using k-ep realizable. Other people I know prefer V2F for this situation, and some people have used Spallart-Almaras, but the SA model always makes me nervous because it's original design was for 2D airfoils, and anything beyond that always makes me wonder, but that's just me and many people have much more faith in the model.

Hope this helps, and good luck, Jason
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