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-   -   swith between subsonic and supersonic (http://www.cfd-online.com/Forums/main/110721-swith-between-subsonic-supersonic.html)

 nickna December 19, 2012 02:32

swith between subsonic and supersonic

Hi, all
The following question related to supersonic simulations always puzzles me:
we say that the boundary conditions for supersonic and subsonic are different. This means that at every time step on the boundaries we need to judge the local Mach number to see if the local flow is supersonic or subsonic and then apply the corresponding different B.C.s?
Thanks a lot!

 leebro61 December 20, 2012 01:21

Yes, for a supersonic flow the (u+c) and (u-c) characteristics are both positive, so disturbances from the interior of the domain do not affect the inlet boundary condition. For exit boundary conditions the opposite logic applies. Hope this helps :)

 nickna December 20, 2012 03:17

Dear Leebro. Thanks for your reply. However this is not my question. More specifically, for instance even for the exist, part is supersonic and part is subsonic. For this case how to impose B.C.s? I mean shall we judge every grid point on the exit to see the local Mach and use different B.C. at different grid points on the exit boundary? Hope to get help from you.

Quote:
 Originally Posted by leebro61 (Post 398361) Yes, for a supersonic flow the (u+c) and (u-c) characteristics are both positive, so disturbances from the interior of the domain do not affect the inlet boundary condition. For exit boundary conditions the opposite logic applies. Hope this helps :)

 leebro61 December 20, 2012 10:45

Quote:
 Originally Posted by nickna (Post 398374) Dear Leebro. Thanks for your reply. However this is not my question. More specifically, for instance even for the exist, part is supersonic and part is subsonic. For this case how to impose B.C.s? I mean shall we judge every grid point on the exit to see the local Mach and use different B.C. at different grid points on the exit boundary? Hope to get help from you.
Sorry if I misunderstood! Yes, I think you want to be checking each time you apply the boundary conditions and applying subsonic/super sonic conditions based on local Mach Number.

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