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Old   April 29, 2009, 08:09
Unhappy Unable to achieve a steady flow
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I'm trying to obtain a steady flow solution for flow in a convergent-divergent duct using a RANS calculation with the SA turbulence model. The Mach numbers in the duct varies from 0.03 to 1.1.

I'm not using wall functions in my simulations and I use a very fine grid spacing near the wall (with the first grid node well within a y+ =1). I know that you should avoid discontinuities in grid spacing and the ratio should be below 1.3. However, I do obtain a steady flow when the ratio between one grid spacing to the next was left as 2. But when I used a grid spacing of 1.3 I fail to get a 3-order-of-magnitude drop in the residuals and never achieve a steady flow.

I do not understand why this might be the case (surely when I have a ratio of 2 it should fail?) and I can't find any other problem with my setup.
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Old   April 29, 2009, 11:22
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"Better" RANS grids (as in more refined) usually lead to the resolving of energy-carrying scales of motion in the flow. This manifests itself as an inherent unsteadiness in the solution and the residuals will not drop to a seemingly converged value.

My guess is that there is some separation in the diverging part of your duct. Try to find out where the maximum change in flow residual is occurring and check the flow field in this area to look for signs of separation or vortex shedding.
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