# Flow past rotating cylinder: Problem with ForeCoeffs

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 December 14, 2013, 07:49 Flow past rotating cylinder: Problem with ForeCoeffs #1 New Member   Raphael Ouillon Join Date: Oct 2013 Posts: 3 Rep Power: 11 Hello, I am currently studying the effects of a rotating cylinder in a low Reynolds uniform flow. I need to extract the lift and drag coefficients on the cylinder and I therefore use the forcecoeffs function. The problem is that in 3D simulations, the flow displays a spanwise instability along the cylinder. Since the drag and lift are integrated on the whole cylinder, the resulting Cd and Cl are not representative of the Cd and Cl at a specific location of the cylinder. The resulting drag and lift curves are indeed very noisy and random in appearance whereas the local drag and lift curve are expected to present a clear oscillation with a fixed Strouhal number. My question is the following: Is it possible to extract the lift and drag curve at each spanwise location along the cylinder using ForceCoeffs instead of the average lift and drag? If not, is it possible to create a "line" patch at a given location and to compute the forces on this patch instead of on the whole cylinder patch? I hope my problem is clear and if not, please tell me and I'll of course provide more information. Thank you very much in advance for your help! Raphael

 December 16, 2013, 09:45 #2 Senior Member   Join Date: Dec 2010 Posts: 135 Rep Power: 14 I had this issue a few weeks ago and did solved it by postprocessing: here are my steps in short These are my paraviewsteps in short: 1)Extracted Surface 2)Generate Surface Normals 3)Calculated: cp = normals*(p2-p1); ->Pressure Drag cf=tangentials*mu*wallGradU_y -> Friction Drag (or calculated directly by foamCalc components wallGradU 4) Build Integral Drag=S(cp_X+cf_X)dA*2/rho*v*v*A Hope this helps, but there are some differences to OpenFoam Output -> could be caused by building derivates or integration failures

 Tags forcecoeffs, lift and drag, openfoam