[silvan]

Dear foamers,

I wish you a sunny and happy day! :-)

I am trying to understand the way OpenFOAM calculates the pressure force via the function "forces". This topic has already been enlightened partially by the following post:
http://www.cfd-online.com/Forums/ope...tml#post183147
Unfortunately, it is only for incompressible solvers. Now, I would like to extend my understanding to compressible flows.

I found the lines of code in forces.c where e.g. the pressure force is being calculated:

891 vectorField fN
892 (
893 rho(p)*Sfb[patchI]*(p.boundaryField()[patchI] - pRef)
894 );

This is the same formula for incompressible and compressible solvers. In the incompressible case, rho(p) is the density specified by rhoInf in the dict fvSolution. In compressible cases rho(p) is 1. This can be found in the source code. So far, everything is clear.
I then tried to understand the role of pRef. I came that far (incomp+comp):
if one of the BC for p are fixedValue then pRef does not come into effect. But, when I don't specify a fixedValue the solver takes the pRefValue defined in fvSolution.
Please correct me, if I am wrong, so far.

My questions are:
1) Why is pRef subtracted? In my understanding it would make more sense to add it to p...
2) Why is pRef in the formula used in the compressible case? Isn't p already in absolute readings? Please enlighten me.
3) How does the direction come into play (it is defined as a vectorField...)? I guess that information has to be in Sfb. Please help me to understand this.

I am already looking forward to your answers. Sorry, for stealing your time on such a sunny day... :-)

Thank you,
Silvan

[louvel]

hey, i think i know the answer:

if we have a closed body, eg. an airfoil. In this case reference pressure does not matter because it will cancel out: sum of face normals on a closed surface is zero. That takes care of all closed bodies, cars, planes etc.
so when we calculate the force of a closed body, openfoam use the pressure difference p-pref ,because pref is the same everywhere, it makes no contribution to the forces.

[silvan]

good morning,
sorry, no answer yet on all questions...
if anyone could point me to the correct answers, I would be very happy! :-)

[louvel]

Originally Posted by silvan

good morning,
sorry, no answer yet on all questions...
if anyone could point me to the correct answers, I would be very happy! :-)

do you think it could be like this:

we need delta P to calculate the pressure forces.
so we can get the delta P by subtracting pRef....
vectorField pf(Sfb[patchi]*(p.boundaryField()[patchi] - pRef));