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March 7, 2011, 13:58 |
How to get the real pressure if I need it
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#1 |
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Daniel WEI (老魏)
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I know its a stupid question, sorry to bring it up again.
But in a very simple case like icoFoam/cavity, when I doubled the mesh from 20*20*1 to 40*40*1, the pressure field doubled too, both the max positive pressure and max negative pressure. (See the attachments) coarse_p.jpgfine_p.jpg If so, im quite confused, then how could I calculate the real pressure? If I really need it. Thanks
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~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China |
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March 7, 2011, 21:08 |
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#2 |
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Hua Zen
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The different values of max pressure are because they are at different locations. The corner is a region with high pressure gradient, the finer resolution grid you use, the first cell closer to the corner, the higher max value you would get.
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March 7, 2011, 21:20 |
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#3 |
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Daniel WEI (老魏)
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oops, i see.. I use a threshold to replot the finer mesh case, and there's slight difference, but that is reasonable. Thanks for the hints.
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~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China |
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March 7, 2011, 23:01 |
How fine the mesh should be?
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#4 |
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Daniel WEI (老魏)
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I am more confused now, after refining the mesh, the forceCoeff (CD, CL, CM) of the movingwall of the cavity keeps changing with the refinements, im not sure whether there is a convergent point.
Code:
mesh time CD CL CM 20*20*1 : 0.5 -0.0250095 0.00127490 0.0307956 40*40*1 : 0.5 -0.0309481 0.00131934 0.0384033 80*80*1 : 0.5 -0.0368673 0.00135175 0.0460686 120*120*1 : 0.5 -0.0403592 0.00136456 0.0506198 240*240*1 : 0.5 -0.0462832 0.00137827 0.0583678
Last Edited: Is this because singularities in the corners?
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~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China Last edited by lakeat; March 10, 2011 at 16:31. |
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March 10, 2011, 16:18 |
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#5 |
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Daniel WEI (老魏)
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To calculate real pressure, Pls help and see if this is correct?
Solver: simpleFoam External Flow, but the b.c. settings as in a wind tunnel case, with outflow's constant pressure=0 (fixedValue); The case is a case flow around a circular cylinder, ** For a given Re number the Cp (pressure coeff distribution and values should remain unchanged), So we have (in simulation) Cp = (p-p0)/(0.5*rho*U*U) where, p0=0 here in our simulation, rho=1Kg/m^3, U=1m/s. in a real world - wind tunnel test, we have, Cp = (p_real-101[kPa])/(0.5*rho_air*U_real*U_real) If Cp distribution and values should remain unchanged for a given Re, whatever it is, Suppose this is in a wind tunnel, let's say in our case, U_real is 10m/s, and rho_real=rho_air=1.205Kg/m^3 So we can now calculate the p_real that we want. ??> Is this procedure correct? ??> Is rho in OpenFOAM set as 1Kg/m^3?
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~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China |
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March 10, 2011, 21:18 |
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#6 |
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Hua Zen
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simplefoam is for incompressible case. The pressure used in the calculation is actually pressure/density.
To calculate the form drag of an incompressible case, you only need the relative value of the pressure, you do not need the real value. |
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