hi all , I am writing a program to solve 2_d lid driven cavity flow using spectral method(steady state) I am having good velovity profile .. but the problem is with pressure Earlier i was using continuity equation and i was getting very big negative pressure(-60, -80) and now i am using poisson equation for pressure.. ..now numbers seems to be ok.. but still all the pressure values are with a negative sign.. can anyone help me why its happening.. and what should i do ?? Thanx in advance, Deepak

First, compressible or incompressible?

If incompressible, then it's only the pressure difference that counts, so negative pressures are just relative to some gauge or base pressure. Because the Poison equations are naturally derived from the continuity AND momentum equations, they lead to a pressure change from one iteration to the next. That also gives values relative to a base.

Perhaps your results are OK? In any case, since you're calculating changes from some base pressure (perhaps your initial pressure field?), the negative values in themselves are not impossible.