How to get the real pressure if I need it
 

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)
Attachment 6770Attachment 6771

If so, im quite confused, then how could I calculate the real pressure? If I really need it.

Thanks

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.

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.

How fine the mesh should be?
 

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.

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?
:confused::confused::confused::confused::confused: :confused::confused::confused::confused::confused:

:confused::confused:To calculate real pressure, Pls help and see if this is correct?:confused::confused:

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?

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.