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Cd calculation when velocity is varying with time

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Old   September 14, 2015, 04:25
Default Cd calculation when velocity is varying with time
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DongHo
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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
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Old   September 29, 2015, 19:36
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Davi Barreira
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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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