Supersonic outlet working
 

Ansys CFX user guide mentions that the specification of a supersonic outlet boundary condition requires no accompanying values, except for the radiation intensity if radiation is modeled.

Therefore it can be gathered that all the flow quantities are extrapolated from the interior for a supersonic outlet.

And for a subsonic outlet, appropriate accompanying values have to be specified to complete the definition of the boundary condition.

Could someone, please explain such treatment of the supersonic outlet.

Further more, in Ansys Fluent there's nothing like a supersonic outlet but if the solver senses that the
flow in the vicinity of the outlet is locally supersonic then the specified flow quantities at the outlet will be ignored and
the solver extrapolates all the flow variables to the outlet from the interior.

However, in Ansys CFX the user has to necessarily choose the outlet to be supersonic so what if the flow is just locally
supersonic confined to some near vicinity of the outlet. It could be that such inference can be gathered from the results.

So could someone please clarify on handling this in Ansys CFX.

Ideally you could have the solver switch but in reality I think you will find the result will be very numerically unstable. If you have regions of subsonic and regions of supersonic flow then you probably have shock or rarefaction waves through the boundary - and this is going to be extremely difficult to converge.

So that is why the boundary has to be subsonic or supersonic. Well, at least that's my guess.

But I do not know why CFX does not automatically sense whether the whole boundary is subsonic or supersonic and modify itself accordingly. I would have thought that should be possible.