Question about convergence
 

Hi

May I ask why very fine mesh always need more iterations to converge?

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.