back pressure at exit for supersonic flow

BigFrank

I use Fluent 6.2 version. I am simulate the Laval nozzile. In the convergent part, it is subsonic. In the divergent flow, the flow is supersonic, when the pressure at exit is very low. Now I increase the back pressure at exit, this pressure would propagate upstream to genreate a shock wave which would be between the throat and the exit, according to the text book. However, the simulate result showed that this back pressure could not be exerted on the flow. The result of the contour showed that the the flow always supersonic flow, the shock wave is not generated. Firstly, I use the converged superonic flow in the divergent as initial condition, the pressure at exit is 7725 pa. Then, I given the "pressure-outlet" as 505325 Pa, then computing. When I plot, I find that the pressure at exit is still 7725 Pa.

Could any body comment this? How to make the back pressure on role to the flow at the exit?

Thanks in advance!