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July 28, 2012, 15:09 
Questions about buoyantPimpleFoam and rhoPimpleFoam

#1 
Senior Member

Hi Foamers.
I'm new to openFOAM. I have some problems understanding the descriptions of two solvers which is mentioned in user's guide. as you can see in this link: http://www.openfoam.com/features/standardsolvers.php The descriptions for buoyantPimpleFoam and rhoPimpleFoam are as follows: buoyantPimpleFoam: Transient solver for buoyant, turbulent flow of compressible fluids for ventilation and heattransfer. rhoPimpleFoam: Transient solver for laminar or turbulent flow of compressible fluids for HVAC and similar applications. 1) My first question is that isn't it wrong to say "Compressible Fluids"? as i know we do not have any term as compressible fluids, instead we have "Compressible Flow". so what does it mean? does it mean that these solvers are developed for solving flows which their Mach number is above 0.3? and so we can not solve incompressible flows? 2) My next question follows the first. We do know that must of the HVAC applications take place in low velocities and so in low Mach numbers. Mostly below 0.3. So we can treat them as incompressible flows. If buoyantPimpleFoam and rhoPimpleFoam are designed for compressible flows, so how is it possible for them to be a good choice for HVAC applications? 

July 28, 2012, 21:20 

#2  
Senior Member

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I hope this explanation will help you. Regards, Awais 

July 29, 2012, 04:25 

#3  
Senior Member

Thank you very much for your answer Awais. it helps alot ~
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2) As i said my case is incompressible and so density remains constant. As you know these two solvers, also solve for "rho" while running, but in some cases like in this case when i use these solvers there is no iterations for "rho" and its always 0. Like this: diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 Does it mean that solver has detected that my case is incompressible and so it doesn't solve for "rho"? if there is a condition like this, where is it checked in source of the program? of course while running there is an output for rhoPimpleFoam in terminal which shows rho max/min, like this: rho max/min : 1.186703471 0.9931701431 But there is nothing like this in buoyantPimpleFoam. Quote:
Regards, Mojtaba Last edited by Mojtaba.a; July 29, 2012 at 04:58. 

July 30, 2012, 08:31 

#4 
Senior Member

Answer to your first question: If I remember there is some tutorial about icoFoam which actually goes step by step about the implementation of energy equation in icoFoam. It is up to you, how you want to approach the problem. If incompressible solution suits you go for it.
Answer to you second question: If you look into the fvSolution in system directory you will find actually rho is not solved (or calculated) using an iterative method. I think diag suggests that row is computed directly from a diagonal Matrix (but that is just a guess), I am not sure about the implementation. Also I see that difference between your rhomin and rhomax is significant (greater that 5% relative to rhomax). If that's what your case is reporting (after significant convergence) than may be you should check your settings again, you really might need a compressible flow solution to your problem. Regards, Awais 

July 30, 2012, 14:59 

#5  
Senior Member

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"rho.*" { solver PCG; preconditioner DIC; tolerance 0; relTol 0; } can you explain more about diagonal matrix? I have now idea about it. 

July 31, 2012, 15:08 

#6  
Senior Member

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Regards, Awais 

August 1, 2012, 04:50 

#7  
Senior Member

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Thanks again for your answers. 

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