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Define grid motion udf issue: Wing geometry breaks down with time

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Old   June 1, 2022, 11:22
Exclamation Define grid motion udf issue: Wing geometry breaks down with time
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I'm developing of CFD pipeline for a flapping wing with ANSYS Fluent, including 4 degrees of freedom: rotations about the three axes plus a spanwise twist. I've used define cg motion udf to simulate wing motion as a rigid body (i.e. without incorporating the twisting deformation) and that works just fine, but I face a problem when using the define grid motion udf to apply the twisting wing deformation. It seems that the wing becomes distorted rapidly if I use a time step smaller than 1e-5 seconds. If I use a time step equal to 1e-5 I can see the wing slowly expanding, almost doubling in size a few cycles in (see attachements). Switching to an even smaller time step, 1e-6, seems to reduce the problem to non-noticable levels, at least for the first 10 cycles or so that I've tested it for.

Does anyone have an idea why this is the case, and/or any suggestions on how to deal with the problem? I'm looking for a solution that eliminates the problem altogether rather than reduces it (as seemingly does, for example, decreasing the time step). I would greately appreciate your advice!

Many thanks!
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grid motion, time step, udf, wing deformation, wing flapping

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