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September 9, 2015, 08:06 
Pressure equation for compressible solver

#1 
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Akshay Kumar
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Hello!
I was trying to understand how the pressure equation(how 'p' is calculated) is formulated for a compressible flow(pressure based solver). I looked up openfoam's simpleFoam & rhoSimpleFoam to understand the differences but things are still unclear. rhoSimpleFoam fvScalarMatrix pEqn ( fvm::div(phid, p)  fvm::laplacian(rho*rAU, p) == fvOptions(psi, p, rho.name()) ); simpleFoam fvScalarMatrix pEqn ( fvm::laplacian(rAU, p) == fvc::div(phiHbyA) ); Could someone help me with these equations? Another thing I noticed in the rhoSimpleFoam solver is that they use setReference for pressure....I thought this referencing is needed only for incompressible flows that do not have any pressure boundaries so that the pressure values are not floating. pEqn.setReference(pRefCell, pRefValue); So why is this used in the compressible solver? Regards Akshay 

September 12, 2015, 06:29 
Shoutout to bruno!!

#2 
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Akshay Kumar
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Hey Bruno!!! As usual...I'm relying on you to point me in the right direction


September 12, 2015, 15:55 

#3  
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Bruno Santos
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Quick answers:
Quote:
Your question is not clear. What exactly don't you understand? Quote:
Beyond this, this pressure referencing is needed when no boundary condition defines a pressure value, e.g. if all BCs are zero gradient, then the equation would be undefined, hence the need for a reference pressure value in a specific point. 

September 18, 2015, 09:24 

#4 
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Akshay Kumar
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1. I basically wanted to know the difference between the pressure equations solved for an incompressible flow and pressure equation for a compressible flow.
2. Regarding reference pressure locations  I think some of the commercial codes use this only for incompressible solvers. Right? If yes, then why is that? 

September 19, 2015, 12:07 

#5  
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Bruno Santos
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Greetings Akshay,
Quote:
Anyway, the reason why I don't understand you question is as follows... rhoSimpleFoam Code:
fvScalarMatrix pEqn ( fvm::div(phid, p)  fvm::laplacian(rho*rAU, p) == fvOptions(psi, p, rho.name()) ); Code:
fvScalarMatrix pEqn ( fvm::div(phid, p)  fvm::laplacian(rho*rAU, p) == 0 ); Code:
fvScalarMatrix pEqn ( fvm::laplacian(rho*rAU, p) == fvm::div(phid, p) ); Code:
fvScalarMatrix pEqn ( fvm::laplacian(rAU, p) == fvc::div(phiHbyA) ); Does this answer your question? If not, then please be specific about what you want to understand . Quote:
The missing detail is probably what you're not asking: Why is the pressure relative to a reference value and not absolute? A few answers:
Bruno
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September 21, 2015, 03:03 

#6 
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Akshay Kumar
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Thanks Bruno and I apologize for not being able to hit the nail on the head with regards to my questions. Things are clear now but I have a follow up question ....
Akshay 

September 21, 2015, 12:21 

#7  
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Bruno Santos
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Quote:


September 28, 2015, 04:47 

#8 
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Akshay Kumar
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Hmm...to continue on this topic....
Interestingly, it looks like this referencing is not used for rhoPimpleFoam pressure equation but it is used for rhoSimpleFoam. What does steady or transient have to do with referencing the pressure value?? 

October 3, 2015, 09:22 

#9 
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Bruno Santos
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Hi Akshay,
From what I can deduce, there are at least two possible reasons:
Bruno
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