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Old   October 29, 2012, 11:15
Question Question about convergence
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May I ask why very fine mesh always need more iterations to converge?
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Old   October 29, 2012, 11:38
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For time-explicit solvers, this can be explained quite eaisly:

The timestep is limited by the CFL-criterion, thus the CFL-number may not be greater than 1 (or 2 for some solvers).
A small cell size means small timestep to satisfy the CFL-criterion.

In a smaller time, the solution "travels" a shorter way in the computational domain. Thus the change of the solution is smaller with each timestep.
To achieve the same change from the initial condition compared to a coarse-mesh-computation, more time steps are needed.
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