# 2d NS supersonic BC

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 September 2, 1999, 13:36 2d NS supersonic BC #1 thampy Guest   Posts: n/a 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

 September 2, 1999, 13:58 Re: 2d NS supersonic BC #2 John C. Chien Guest   Posts: n/a (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.

 September 2, 1999, 14:26 Re: 2d NS supersonic BC #3 stephane baralon Guest   Posts: n/a 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.

 September 2, 1999, 14:56 Re: 2d NS supersonic BC #4 Hongjun Li Guest   Posts: n/a 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.

 September 3, 1999, 11:44 Re: 2d NS supersonic BC #5 clifford bradford Guest   Posts: n/a 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

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