# Unsteady or noisy force history

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 February 20, 2008, 20:46 Hi everyone, running icoFoa #1 Member   Patrick Bourdin Join Date: Mar 2009 Posts: 40 Rep Power: 8 Hi everyone, running icoFoam on a 2D, corrugated, airfoil-like geometry, I obtained the following history for the vertical force: Freestream Re is 10^4. linearUpwind cellLimited leastSquares 1 was used for U, Laplacian schemes are linear corrected (mean non-Orthogonality of the mesh is 5 deg), Euler was used as temporal discretization. Max Co was always around 0.6-ish. Plotting the contours of p or U at different times does not highlight any noticeable unsteadiness. So, what do I observe, a truly unsteady signal or simply numerical noise (the oscillation is about 3% of the mean value). It's still there whether I reduce or increase the time step by one order of magnitude. I ran simpleFoam, starting from the latest time computed by icoFoam (t=100s), I still observe those oscillations but the magnitude of the mean force is 7% higher than with icoFoam. I used the laminarForceAndTorque library from Hrv to compute the forces.

 February 21, 2008, 05:07 Hi Patrick For a circular c #2 Senior Member   Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Deltares, Delft, The Netherlands Posts: 1,607 Rep Power: 25 Hi Patrick For a circular cylinder exposed to a uniform flow, the r.m.s. value of the lift coefficient is different from zero and a function of Re, [1]. This might be the same thing you are observing. Try replacing your geometry with a cylinder and see if you get an value in the order of magnitude of 0.4 in the r.m.s value of the lift coefficient for Re_D=1e4. Best regards, Niels [1] Hydrodynamics Around Cylindrical Structures, B. Mutlu Sumer and Jørgen Fredsøe, Advanced Series on Coastal Enginnering, World Scientific, 1999 __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request.

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