I am trying to create reversed flow in a sudden expansion geometry case. In FLUENT when the outlet is locally supersonic it extrapolates the data in the interior of the geometry and creates new values at the outlet. What can I do to force the outlet boundary conditions to be enforced so that I can create an upstream interaction in the isolator region before the sudden expansion? Is there a udf that needs to be created? My inlet boundary condition is supersonic. Any help will be appreciated.

I have solved the problem. I simply initialized the case from the outlet and everything has worked out.

Hi Stephen,

For a supersonic fully compressible flow, outlet boundary conditions should be ignored just like Fluent is doing otherwise your problem will be illposed.

The treatment of compressible flows is dictated by the theory of characteristice. You can find a discussion of this in "Numerical Computational of Internal and External Flows" Volume 2 (chapter 16 I think) by Charles Hirsch.