# question on the physical meaning of p_rgh

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 September 26, 2014, 03:46 question on the physical meaning of p_rgh #1 Senior Member   Hua Zen Join Date: Mar 2009 Posts: 117 Rep Power: 10 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.

 September 26, 2014, 04:55 #2 Senior Member   Pablo Higuera Join Date: Jan 2011 Location: Singapore Posts: 359 Rep Power: 10 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 jherb likes this.

 September 26, 2014, 21:18 #3 Senior Member   Hua Zen Join Date: Mar 2009 Posts: 117 Rep Power: 10 Thanks Pablo for your reply. It helps me a lot.

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