hi,

Could anyone tell me as to what are the flow parameters that i need to specify as my exit BC when i'm simulating 2d NS supersonic flow ?

thanking you thampy

(1). In supersonic flow, the local solution depends on the upstream condition or solution. It is controlled by the characteristics network. (2). So, you can give it a rough number at eixt B.C., it is not going to affect the interior points solution at all, as long as the flow is supersonic.

If your flow is axially supersonic, then all characteristics goes out of the domain: you don't need to specify anything.

If your flow has a Y velocity component and also is not axially supersonic(M>1 but Mx<1) then you need to specify one information (one characteristic propagates inside the domain). Usually the pressure.

If your flow has a Y velocity component and also is axially supersonic, then again you don't need to specify anything.

Good point! To make it more general, you may say that if the Mach number component in the direction normal to the boundary surface is great than 1 (Mn > 1), then the exit BCs are not needed. The boundary values can be extrapolated from the upstream grid point. If Mn<1, although Mtotal is greater than 1, the exit boundary is still treated as subsonic. In that case, (if 2-D) three variables should be extrapolated from the upstream, and one variable must be specified at the exit.

the above are correct but you have to be careful if you have a boundary layer or wake exiting your domain the flow in that region will/may be subsonicin which case you'll need one exit BC. in which case the pressure is usually appropriate