# forcesCoeffs - how to use relative pressure instead of absolute pressure

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 December 10, 2015, 11:51 forcesCoeffs - how to use relative pressure instead of absolute pressure #1 Member   Join Date: Dec 2015 Posts: 74 Rep Power: 10 Hi, In controlDict I would like to use the relative pressure instead of the absolute pressure for the calculation of CD CL ecc. I'm using the compressibile solver rhoSimplecFoam, and in the calculation of forces the absolute pressure is used. I noticed the absolute pressure is also used in the calculation of force coefficents. I would like to use the relative pressure (so Prel =Pabs-Pref) in these calculations. Is it possible to specify Pref? Thanks in advance, WhiteW Code: ``` forces { type forces; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches (wall_body); pName p; UName U; rhoName rhoInf; log true; rhoInf 1.16; CofR (0 0 0); pitchAxis (0 -1 0); } forcesCoeffs { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( wall_body ); pName p; UName U; log true; rhoInf 1.16; CofR (0.98460 0 0.35195); pitchAxis (0 -1 0); liftDir (0 0 1); dragDir (1 0 0); magUInf 44.85; lRef 0.3034; Aref 0.578; }```

 December 12, 2015, 07:26 #2 Member   Join Date: Dec 2015 Posts: 74 Rep Power: 10 Hi, anyIdea? I can calculate the corect forces with Paraview using the filters: ExtractSurface GenerateSurfaceNormal Calculator my reference pressure is 97956 Pa, so: (p - 97956) * Normals IntegrateVariables I get the correct forces on the body. If I use the formula (p) * Normals in the calculator, I get the output forces of OpenFoam(that uses the absolute pressure) . Is there a way to do this operation inside openfoam? Thanks, WhiteW

 December 22, 2015, 12:40 #3 Member   Join Date: Dec 2015 Posts: 74 Rep Power: 10 Hi , is it possible to correct the evaluation of forces, using the relative pressure, only in the post-processing phase?