# swith between subsonic and supersonic

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 December 19, 2012, 02:32 swith between subsonic and supersonic #1 New Member   Lipo Wang Join Date: Dec 2012 Posts: 14 Rep Power: 6 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!

 December 20, 2012, 01:21 #2 New Member   Join Date: Nov 2012 Posts: 3 Rep Power: 6 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

December 20, 2012, 03:17
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Lipo Wang
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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 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

December 20, 2012, 10:45
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Quote:
 Originally Posted by nickna 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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