# How to get the real pressure if I need it

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 March 7, 2011, 13:58 How to get the real pressure if I need it #1 Senior Member     Daniel WEI (老魏) Join Date: Mar 2009 Location: South Bend, IN, USA Posts: 688 Blog Entries: 9 Rep Power: 12 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 __________________ ~ Daniel WEI ------------- NatHaz Modeling Laboratory Department of Civil & Environmental Engineering & Earth Sciences University of Notre Dame, USA Email || My Personal CFD Blog

 March 7, 2011, 21:08 #2 Senior Member   Hua Zen Join Date: Mar 2009 Posts: 114 Rep Power: 8 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.

 March 7, 2011, 21:20 #3 Senior Member     Daniel WEI (老魏) Join Date: Mar 2009 Location: South Bend, IN, USA Posts: 688 Blog Entries: 9 Rep Power: 12 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. __________________ ~ Daniel WEI ------------- NatHaz Modeling Laboratory Department of Civil & Environmental Engineering & Earth Sciences University of Notre Dame, USA Email || My Personal CFD Blog

 March 7, 2011, 23:01 How fine the mesh should be? #4 Senior Member     Daniel WEI (老魏) Join Date: Mar 2009 Location: South Bend, IN, USA Posts: 688 Blog Entries: 9 Rep Power: 12 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``` So, the questions are these, how fine the mesh should be, that I can have a reliable forceCoeff? Is this bcoz this is a laminar case? What will happen in a turbulence case. Last Edited: Is this because singularities in the corners? __________________ ~ Daniel WEI ------------- NatHaz Modeling Laboratory Department of Civil & Environmental Engineering & Earth Sciences University of Notre Dame, USA Email || My Personal CFD Blog Last edited by lakeat; March 10, 2011 at 16:31.

 March 10, 2011, 16:18 #5 Senior Member     Daniel WEI (老魏) Join Date: Mar 2009 Location: South Bend, IN, USA Posts: 688 Blog Entries: 9 Rep Power: 12 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? __________________ ~ Daniel WEI ------------- NatHaz Modeling Laboratory Department of Civil & Environmental Engineering & Earth Sciences University of Notre Dame, USA Email || My Personal CFD Blog

 March 10, 2011, 21:18 #6 Senior Member   Hua Zen Join Date: Mar 2009 Posts: 114 Rep Power: 8 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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