For a supersonic outlet, the flow is not influenced at all by downstream conditions. All values of dependent variables, with the exception of radiation intensity, are extrapolated from upstream, and the constant gradient constraint is not imposed. The specification of a supersonic outlet boundary condition, therefore, requires no accompanying values unless a Thermal Radiation model is used. For details, see Thermal Radiation.

Ansys CFX will behave unpredictably if part or all of fluid exiting the region assigned a supersonic outlet is subsonic. The CFX-Solver might fail to converge or diverge in this situation. To correct this, you should move the outlet to a location where the flow is supersonic over the entire boundary, or move it to a subsonic location and specify a different outlet boundary condition.