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-   -   pressure boundary condition in openFoam (http://www.cfd-online.com/Forums/openfoam-pre-processing/116127-pressure-boundary-condition-openfoam.html)

s.m April 13, 2013 11:04

pressure boundary condition in openFoam
 
hi foamers,
i am working on 2D multi elemet airfoil. i use simpleFoam,and k-epsilon turbulence model.
i have a question about the boundary condition and fvsolution:
1- can any one tell me, the pressure that we set for pressure boundary condition at inlet is absolute for gauge pressure?
2- what is difference between pReffValue and pRefCell in fvSolution in openFoam?
thanks for any one who can help me :)
Regards

Lieven April 13, 2013 20:06

Dear s.m,

1. I'm not familiar with the term gave pressure but what I can tell you that might help you is that the absolute pressure in an incompressible fluid simulation is not important. The fluid properties do not depend on it (as opposed to compressible flow). So whether you set the inlet to 0 Pa, 10 Pa, or 1e5 Pa will not influence your velocity field, nor will it influence your pressure gradients. It will not even influence the colors you see in paraFoam, just the value related to them.

2. pReffValue (e.g. 0 Pa) is the value of the pressure you set in pRefCell (e.g. cell 1000). These values are used when you only set Neumann boundary conditions for the pressure field. If you set a pressure at the inlet or outlet however, you just need to provide a value for these parameters but they will not be used.

Hope this helps...

Cheers,

L

s.m April 14, 2013 04:10

Quote:

Originally Posted by Lieven (Post 420345)
Dear s.m,

1. I'm not familiar with the term gave pressure but what I can tell you that might help you is that the absolute pressure in an incompressible fluid simulation is not important. The fluid properties do not depend on it (as opposed to compressible flow). So whether you set the inlet to 0 Pa, 10 Pa, or 1e5 Pa will not influence your velocity field, nor will it influence your pressure gradients. It will not even influence the colors you see in paraFoam, just the value related to them.

2. pReffValue (e.g. 0 Pa) is the value of the pressure you set in pRefCell (e.g. cell 1000). These values are used when you only set Neumann boundary conditions for the pressure field. If you set a pressure at the inlet or outlet however, you just need to provide a value for these parameters but they will not be used.

Hope this helps...

Cheers,

L


hi Lieven
thanks a lot, i mean gauge pressure, sorry for bad English :)

s.m April 15, 2013 02:54

the value for pressure and rhoInf
 
Dear Lieven
when i want solve incompressible turbulent flow around airfoil using simpleFoam solver in openFoam,
i need to initialize pressure at 0 folder, my question is that the value that i should set for pressure at outlet is absolute(10e5 pa) or gauge(0 pa)?
what about initializing pressure for solving compressible turbulent flow around airfoil?
another question i have is about the value for "rhoInf" in "forceCoeffs" function at controlDict folder, we should set it to 1 or 1000 ?
i am confused, please help me understand them,thanks a lot:)

Lieven April 15, 2013 03:17

Dear s.m.

For initializing the pressure, you have a few options but the easiest is a uniform value equal to the fixedValue at the inlet or outlet. I usually take an outlet pressure of 0 Pa. Depending on the flow, this could be a bad initial estimate and the solver will crash shortly after starting the simulation. If this is the case, you might want to try to run potentialFoam first.

Related to the compressible flow, I haven't worked with it so I can't help you from experience. However, my guess is that the pressure and the density should be the absolute values that you expect. Setting the initial conditions should be similar to the incompressible case (but in terms of absolute values).

Cheers,

L

immortality April 15, 2013 03:28

hi
rhoInf is about 1.25 if you have air


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