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I started to learn OpenFOAM from the Cavity tutorial which is provided at the web-site. When experimenting with different Reynolds numbers, in section "2.1.8.2 Running the code", tutorial says to rerun the solver because "It is sensible to increase the solution time". But when I done this I couldn't find any kind of differences between the flow in the cavity with the low (0.2) and the high (0.6) Courant number.

How do I know if I need to rerun the simulation?

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Due to the under-resolved boundary layer near the lid, the velocity in the adjacent cells is significantly lower than the lid. This section is showing you a trick to make the code run faster while still being stable. Increasing the Courant number would normally make the method unstable, but since the velocity in all interior cells is significantly less than the velocity of the boundary layer, you can get away with increasing the time step.

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