[nickna]

Dear all,
A basic problem, which I thought should be quite simple, puzzles me a long time...
For compressible simulations (still subsonic, Mach is about 0.7) of flow passing a square block, the mesh is Cartesian and the scheme is MacCormack. I tried different boundary conditions of the pressure on the block surface, for instance: extrapolation from two internal points (as said in Anderson's book), zero gradient pressure (i.e. p_surface=p_1), or from the continuity eq. to update pressure etc. But none of them can work satisfactorily and finally the density runs to negative around the corner.
Could anybody give me suggestions?
Many thanks.