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Large residual distributed at uniform flow region

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Old   August 16, 2015, 23:10
Question Large residual distributed at uniform flow region
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Meimei Wang
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As you can see in the attached pictures, the region with vortex doesnít have large residual over there but the region with quite uniform flow is distributed with large residual which causes convergence problem. Could anyone explain thatís why and how to solve this problem?
Attached Images
File Type: jpg residual1.jpg (97.6 KB, 8 views)
File Type: jpg residual2.jpg (97.5 KB, 7 views)
File Type: jpg velocity2.jpg (46.3 KB, 6 views)
File Type: jpg velocity1.jpg (65.2 KB, 5 views)
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Old   August 17, 2015, 02:54
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You have very high aspect ratio elements in the wake region. It is always going to have a hard time resolving the wake when the elements are that long and skinny. You will need to reduce the aspect ratio of those elements.
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Old   August 19, 2015, 00:45
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Quote:
Originally Posted by ghorrocks View Post
You have very high aspect ratio elements in the wake region. It is always going to have a hard time resolving the wake when the elements are that long and skinny. You will need to reduce the aspect ratio of those elements.
I improve the aspect ratio over there but convergence doesnít become any better. I think coarsening the grids over the vortex region at the trailing edge is the general way to improve the convergence for this kind of problem (of course not too coarse).
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Old   August 19, 2015, 01:31
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Refining the mesh results in less numerical dissipation which means the inherent instabilities in shear layers, wakes and similar features become more difficult to resolve. So yes, I would expect convergence to get more difficult as mesh size gets smaller.

Be careful coarsening the grid as you will be loosing accuracy by doing it. Make sure it is not a critical part of the flow, and do a sensitivity analysis on it to be sure.
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