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Please help: Multiphase flow boundary condition

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Old   August 27, 2004, 10:11
Default Please help: Multiphase flow boundary condition
Alex Costa
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Dear group, I am trying to understand the following PIL statement: PATCH(TOP,CELL,NX,NX,1,1,1,1,1,LSTEP) COVAL(TOP,P1,1.0,0.0);COVAL(TOP,P2,RHO2,0.0)

Let me describe my understanding: It is applied in just one cell (WHY?) During all the time steps. Applies for mass flow boundary conditions for phase 1 and phase 2. For phase 1, it sets an extra term in the pressure equation equal to T 1 (0 - P1) For phase 2, it sets an extra term equal to T RHO2 (0 - P2) All right, But what is the physical meaning of these two bondary conditions?

Futhermore, After I compile the q1 the above statement is replaced by: PATCH (TOP ,CELL ,1,1,0,0,0,0,1,10) COVAL (TOP ,R1 , 1.000000E+00, 0.000000E+00) COVAL (TOP ,R2 , 1.000000E+03, 0.000000E+00) Does it mean R2 is set to 0 and adds -T 1 R1 to the R1 equation? What is the relationship with the previous statement for pressure and again what is the physical meaning?

Any interpretation about these statements are very welcome.

Alex Costa
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Old   September 1, 2004, 13:37
Default Re: Please help: Multiphase flow boundary conditio
Mick Hughes
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Hi Alex, I'm not an expert on two phase flows, I may be able to shed some light on your questions. Firstly library case W771. I just tried this case but I made changes as it did not converge straight out of the box.

Firstly, shorten the run time or increase the number of run time discrtisations.

Secondly, This is an old library case so I used the photon text viewer to look at the results, this needs the case to be at least 2d, so I made 2 cells in the Y-direction and stored(V1,V2), FIINIT(v1,v2)=0.

Dont forget to change the "gravity" and "top" patches from ...1,1,... to ...1,ny,....

thirdly, add CSG1='a' and IDISPA=1 to drop out intermediate PHI files.

Hopefully, the case should now run and give some physically realistic result.

About the following boundary condition : PATCH(TOP,CELL,NX,NX,1,ny,1,1,1,LSTEP) COVAL(TOP,P1,1.0,0.0);COVAL(TOP,P2,RHO2,0.0)

Density is used to multiply the coefficient so as in cell mass fractions are used C:\phoenics\D_POLIS\D_LECS\IPSA\IPSA.HTM

However, this cell is not an outlet BC, I think what is happening here is that pressure is being fixed so that a unique solution can be found. Just like flow in a closed box needs a pressure relief cell, this example is essentailly a closed box, so we need to fix the pressure somewhere to zero. This we do for both phases. The "CELL" multiplier is therefore fine for this situation.

Just some extra words, the pressure correction equation is essentially solving a Laplacian type equation. To guarantee a unique solution to this type of equation the value has to be fixed in one cell at least. For flow simulations it is not the absolute value of pressure that drives the flow field, but the pressure differences, so this is generally OK.

Hope this helps some.

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Old   September 1, 2004, 17:21
Default Re: Please help: Multiphase flow boundary conditio
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P.S. I increased LSWEEP when running the above mentioned case
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Old   September 2, 2004, 02:24
Default Re: Please help: Multiphase flow boundary conditio
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Welcome back, Mick!
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