Pressure boundary_collocated

George

Hi all! I have one question: How exactly can I implement an inlet pressure boundary condition for a colocated grid for compressible flow(subsonic inflow).The grid line I=1 is where I set the boundary conditions.When for this line I have P=1 bar and P'=0.0 and I find Uw from the discretised contuity equation,I compute -dP/dx like this: dP/dx=(P(1,j)-P(2,J))/dx , the pressure in the computational domain is higher than the specified boundary pressure, which shouldn't be.When on the other hand I comute -dP/dx like this: dP/dx=(P(1,J)-(P(2,J)+P(3,J))/2)/dx , the result is again the same. When finaly I set the boundary pressure in line I=2 and there I set P=1 bar and P'=0.0 I find again the same results, independently of how I compute -dP/dx there. I try to implement the precedure that are in Peric book,but with no luck. Any suggestions?