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Old   February 5, 2000, 09:52
Default implementing boundary conditions
  #1
Andreas Abdon
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Hello everybody!

I'm implementing boundary conditions on a user scalar equation in user fortran subroutine USRSRC. I want to assign the scalar a value in the near wall cells. The manual recommends the following procedure:

SP=-c SU=c*value AM=0 for all faces

where c is "set equal to approximately the maximum flux in the CV.. .. to stop potential errors due to the Rhie-Chow terms." (quote). I have tried this approach and also different constant values of c and found that it may distort the solution seriously, especially when the flux (velocity) is close to zero. Does anybody have a recommendation how to proceed? Is a large (1.0E10) value of c "safe" to use?

Greetings

Andreas Abdon

Div. Heat Transfer Lund Institute of Technology Box 118 22100 Lund Sweden
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Old   March 14, 2000, 14:00
Default Re: implementing boundary conditions
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Andreas Abdon
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I'm stuck!

** Does ANYBODY have any info on this matter, please help! **

Even if you haven't come across these problems when implementing boundary conditions I would be grateful if you let me know how you did it. I have traced the problem to one of my equations, it's a diffusion eq. with NO CONVECTION. I have tried different c values and the high ones (like 1e10) gives a reasonable solution but slows down convergence seriously. A low value gives a very good convergence rate (final residual levels as for c=1e10) but the solution is not very good.

Andreas (...nervous breakdown coming soon)
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Old   March 15, 2000, 14:20
Default Re: implementing boundary conditions
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Andreas Abdon
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----> I FOUND IT! <------

Don't use AM(NCELL,0,IPHASE) when putting AM=0 for all six facial components! You have to loop over each individual face: AM(NCELL,1,IPHASE)=0, AM(NCELL,2,IPHASE)=0,AM(NCELL,3,IPHASE)=0, and so on....

Doing this, the solution is the same for c=1.0 nd c=1.0e10, i.e. dependence of c value is eliminated.

Cheers

Andreas
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