Cavity with lower flow in opposite directions at Re = 4000: velocity field

Imagine a square cavity with dimensions (LxL), with an opening at the bottom. The surrounding flow circulates in opposite directions, meeting in the middle area of the domain at a speed (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, starting at static initial conditions, for a Reynolds number of 4000.