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July 5, 2013, 16:29 |
forceCoeffs and rhoInf dependency
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#1 |
Member
Florian
Join Date: Nov 2009
Posts: 59
Rep Power: 16 |
Hello,
I have an incompressible simpleFoam solution of a flow field. I use forceCoeffs to calculate Cd etc. on some patches. Code:
coeffsCar { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 10; patches $carPatches; pName p; UName U; rhoName rhoInf; log true; rhoInf 3; liftDir (0 0 1); dragDir (1 0 0); pitchAxis (0 1 0); CofR (0.56725 0 -0.1274); magUInf 45; lRef 1.5; Aref 0.4; Is there some bug in OF 2.1.1 or a bug in my understanding of physics? Force on a patch is F=p*A, drag coefficient is Cd = F / (0.5 rhoInf U^2). Why is there no influence of rho? Thanks! |
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July 24, 2013, 13:51 |
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#2 |
Senior Member
Joachim
Join Date: Mar 2012
Location: Paris, France
Posts: 145
Rep Power: 15 |
Hi Florian,
if the solver is incompressible, the pressure computed by OpenFOAM is p/rho and not p. Hence, you don't need to know the density to get your coefficient (just divide your pressure force by 0.5*Ue^2 A). You can write anything you want for the density in forceCoeffs, it is not read when computing the coefficients. Regards, Joachim |
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July 25, 2013, 10:04 |
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#3 |
Member
Florian
Join Date: Nov 2009
Posts: 59
Rep Power: 16 |
Yeah, that became clear after posting this.
But I'm still riddled why OF requires setting a value of rhoInf. In the source code, as far as I can tell, it's determined by units if p' = p/rho (incompressible) or p' = p. Regards... |
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August 9, 2018, 03:55 |
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#4 |
New Member
zein elserfy
Join Date: May 2018
Posts: 25
Rep Power: 7 |
hi Joachim,
I want to ask you how to calculate the value of Cd and Cl ? by calculating it by dividing pressure force by (0.5*v*v*A),so this mean that results of openfoam for cd and cl are not correct ? |
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