# Calculating Lift and Drag Coefficients: All 0?

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 October 22, 2016, 13:03 Calculating Lift and Drag Coefficients: All 0? #1 New Member   Peter Join Date: Sep 2016 Posts: 13 Rep Power: 9 Hi, I'm trying to calculate lift and drag coefficients on the propeller test case (in pimpleDyMFoam). I seem to have gotten it working with no errors, but forces.dat just contains 0s. Here is the file: Code: ```# Forces # CofR : (2.500000e-01 0.000000e+00 0.000000e+00) # Time forces(pressure viscous porous) moment(pressure viscous porous) 0.000333333 ((0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00)) ((0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00))``` Here's the config file (or whatever it's called): Code: ```forces { type forces; functionObjectLibs ("libforces.so"); outputControl timeStep; outputInterval 1; patches ( “propellerStem1.*” ); pName p; UName U; rhoName rhoInf; log true; CofR (0.25 0 0); rhoInf 1; } forceCoeffs { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( “propellerStem1.*” ); pName p; UName U; rhoName rhoInf; log true; liftDir (0 0 1); dragDir (-1 0 0); CofR (0.25 0 0); pitchAxis (0 1 0); magUInf 2.00; rhoInf 1000; lRef 1; Aref 1; }``` What makes it all 0? Am I measuring from the wrong place? Should I somehow measure the whole propeller? How would I do that? Thanks for your help!

October 30, 2016, 12:51
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 Originally Posted by APCoding Hi, I'm trying to calculate lift and drag coefficients on the propeller test case (in pimpleDyMFoam). I seem to have gotten it working with no errors, but forces.dat just contains 0s. Here is the file: Code: ```# Forces # CofR : (2.500000e-01 0.000000e+00 0.000000e+00) # Time forces(pressure viscous porous) moment(pressure viscous porous) 0.000333333 ((0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00)) ((0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00) (0.000000e+00 0.000000e+00 0.000000e+00))``` Here's the config file (or whatever it's called): Code: ```forces { type forces; functionObjectLibs ("libforces.so"); outputControl timeStep; outputInterval 1; patches ( “propellerStem1.*” ); pName p; UName U; rhoName rhoInf; log true; CofR (0.25 0 0); rhoInf 1; } forceCoeffs { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( “propellerStem1.*” ); pName p; UName U; rhoName rhoInf; log true; liftDir (0 0 1); dragDir (-1 0 0); CofR (0.25 0 0); pitchAxis (0 1 0); magUInf 2.00; rhoInf 1000; lRef 1; Aref 1; }``` What makes it all 0? Am I measuring from the wrong place? Should I somehow measure the whole propeller? How would I do that? Thanks for your help!
You need to stick with one density. In the quoted script, you defined rhoInf 1 and then you defined it again below but rhoInf 1000. Have you noticed that?

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