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subsonic inlet-exit boundary conditions

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Old   July 7, 2005, 16:05
Default subsonic inlet-exit boundary conditions
  #1
Cfd Student
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Dear CFD-Gurus, I need info/paper explaining the details regarding the implementation of subsonic inlet/outlet BC. Any help is greatly appreciated. Best regards.

CFD Student
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Old   July 7, 2005, 23:19
Default Re: subsonic inlet-exit boundary conditions
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sylvain
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Subsonic flow means that 2characteristic lines are positive and one is negative (a, u+a, u-a). Consequently you need 2 upstream informations and one upstream:

AT the inlet: the 2 upstream information are for example u=uinfinity and T =Tinfinity. The downstream information needs to be extrapolated

At the outlet: 2 extrapolated information: for example du/dx=0 at the outlet and dT/dx=0 at the outlet and 1 downstream information for example P=P backstream

If you don;t understand review the theory of characteristics.

Sylvain
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Old   July 8, 2005, 19:36
Default Re: subsonic inlet-exit boundary conditions
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Svetlana Sushchikh
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For addition to previous reply:

you can read how to inplement BC for example at

Culbert B. Laney " Computation GAsdynamic" , Cambridge Univ. Press, first 1998.

Sveta
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Old   July 10, 2005, 01:28
Default Re: subsonic inlet-exit boundary conditions
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Thanks a lot for all...

CFD Student.
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Old   July 13, 2005, 01:45
Default Re: subsonic inlet-exit boundary conditions
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Dominic
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Hi,

How is it in Multi-D ?
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Old   July 26, 2005, 07:14
Default Re: subsonic inlet-exit boundary conditions
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enis
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hi slyvian,

what did you mean by "P=P backstream" at the outlet. should P_backstream be a certain number? or P_{i+1}=P_i ?(as in continuative boundary condition).

thanks.

enis
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Old   July 26, 2005, 10:16
Default Re: subsonic inlet-exit boundary conditions
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sylvain
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When the flow is subsonic, you have two characteristic with a positiv slope and one with a negative slope. (assuming a velocity being positive). Hence, one of the boundary condition needs to be specified outside of the domain. Let's say you want to specify the backstream pressure as your outside of the domain condition. Then you can specify P=Pbackstream.

If you are considering an external flow then Pbackstream=Patmospheric (remember that along a characteristic line the pressure is constant). If you consider an internal flow then it depends on the type of problem I guess. Hope it helps. If it sounds obscure to you check out the theory of characteristics, and how the eigenvectors and eigenvalues are to be calculated and how they relate with CFD problems

Sylvain
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