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How to get the real pressure if I need it

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Old   March 7, 2011, 13:58
Default How to get the real pressure if I need it
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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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Old   March 7, 2011, 21:08
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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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Old   March 7, 2011, 21:20
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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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Old   March 7, 2011, 23:01
Default How fine the mesh should be?
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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
So, the questions are these,
  1. how fine the mesh should be, that I can have a reliable forceCoeff?
  2. Is this bcoz this is a laminar case? What will happen in a turbulence case.




Last Edited:

Is this because singularities in the corners?
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Last edited by lakeat; March 10, 2011 at 16:31.
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Old   March 10, 2011, 16:18
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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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Old   March 10, 2011, 21:18
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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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