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timestep and mesh size vs sensor sensitivity in experimental data

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Old   November 18, 2011, 23:35
Default timestep and mesh size vs sensor sensitivity in experimental data
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Mehrdad
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I am trying to validate interaction of waves with a bridge structure. I do mesh and time step sensitivity analysis. How ever it turns out that coarser mesh results with bigger time step is closer to experimental data.
in CFD how do we usually validate our simulation? dont we need to make the mesh finer and finer and time step smaller and smaller till the result don't change?
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Old   November 19, 2011, 08:49
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That's impossible in the most cases. Just imagine a complex system like a F1 car or an underhood case etc. You can't refine the mesh to get a mesh-independent solution, that would be way too much.
Also keep in mind, CFD usually is good in predicting trends, but it's not easy to predict an absolute number. Therefore it doesn't always make sense to keep refining the mesh just to try to hit the absolute number.

Usually (but not necessarily) a finer mesh and a smaller time step gives better results. But when refining the mesh, you might change unintentionally something else with the mesh size. Maybe the ratio of the biggest to the smallest cell. Or the CFL number or whatever. Some side effect can cause the worse result, although the mesh would produce the better result. Or you just don't iterate long enough.
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