# Cd calculation when velocity is varying with time

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 September 14, 2015, 04:25 Cd calculation when velocity is varying with time #1 Member   DongHo Join Date: Jul 2013 Posts: 51 Rep Power: 12 Dear. Foamers I am working on Cd calculation for the case of a changing velocity with the time using URANS with k-w SST (accelerated cylinder case). Actually, initial velocity 1 m/s and then, after 1 sec, 1 to 5 m/s, the velocity is linearly changing with a ramp profile until 2 sec, finally, 5 m/s will be kept until 6 sec. For this case, how I can set up the value of "magUInf" in a file "forceCoeffs" even though I set up the velocity profile in "U"? If the magnitude of velocity is not changing, "magUInf" is set to be initial velocity, though.. Please give me any advice if you have an experience like this. Thanks

 September 29, 2015, 19:36 #2 Member   Davi Barreira Join Date: Apr 2014 Location: Fortaleza Posts: 76 Rep Power: 12 Indeed the coefficients are calculated using a fixed velocity. You can set up a fixed velocity in the forceCoeffs dictionary (like 1m/s) and then postprocess the results with a programming language (or Excel) by recalculating the force coefficientes based on the new velocity. So: Cd = 2*D/(rho*U^2) If you put U = 1, then Cd' = 2*D/rho In your forceCoeffs.dat you have time, so you can use to calculate your velocity at a given time. U = f(t) Therefore, Cd = Cd'/(Ucalc^2)

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