question on the physical meaning of p_rgh
 

Hello, I have found that p_rgh is often used in OpenFOAM.

According to Henrik thesis (4.21)

p_rgh=p-rho*g*h

"the modified pressure p_rgh is obtained by removing the hydrostatic pressure from the pressure."

I think rho*g*h is not the static pressure, as rho is not a constant, thus it should be int(rho*g)dz. So the question is what's the physical meaning of p_rgh.

Hi Lin,

p_rgh does not have any physical meaning, it is just a convenient numerical technique.

The expression is not with h, but with z:

p_rgh = p - rho*g*z

This means that if a cell is at z = 0, p_rgh = p.

Also, as you pointed out, if a cell is in water at one instant p_rgh = p - rhoWater*g*z, and if at the next time step it is in air: p_rgh = p - rhoAir*g*z, it may be induce quite a huge variation in p_rgh.

In short, p_rgh is not hydrostatic pressure because it takes into account what happens at a point, and not at all the points above. I hope this clarifies things a little.

Best,

Pablo

Thanks Pablo for your reply. It helps me a lot.