Imagine a square cavity with dimensions (LxL), featuring two openings: one at its top end and another at its bottom end. Air circulates around the cavity; from left to right in the upper region and inversely in the lower region, at a velocity (U_ref)​.
Using the numerical methods I have learned at the CTTC - UPC, the following video simulates the non-dimensional velocity field, independent of grid and time step, of this problem from static initial conditions to a quasi-steady state, for a Reynolds number of 1e04.