CFD basics
 

Hi,

Can someone please help me explaining: Why grids with high number of cells (3 millions) require very small timestep size? and How could I determine that how many number of iteration per time step is required for solution to converge?

Assuming you are talking about an explicit time marching method, it is because of the CFL condition: http://en.wikipedia.org/wiki/Courant...Lewy_condition.