Federico Alzamora Previtali


Numerically having a CFL value of 1 means that the flow will propagate by one cell length (delta x) at any given time step. The most conservative approach is then to take your maximum velocity and minimum cell length in setting your delta t. This ensures that the flow never goes beyond one cell, at any given time step, which is good practice if you have infinite resources but may be over restrictive. Hence, in practice it may not be practical to have CFL <= 1 everywhere.

Hence, you should aim for CFL = 1 in the regions of interest for your purpose, and decide on the mesh size and time step to resolve the relevant  physics (space and time scales) of your flow.