# Pressure definition in OF 2.0.1 (simpleFoam etc.)

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 September 27, 2011, 12:46 Pressure definition in OF 2.0.1 (simpleFoam etc.) #1 Member   Miles Join Date: Sep 2011 Posts: 48 Rep Power: 14 Hi, Does anyone know what is pressure in OF2.0.1 for solver like SimpleFOAM? I have read that in previous versions of the code it was in fact Pstatic-rho*g*h, than in another version it was just Pstatic/rho. What is the actual statement for OF 2.0.1? The source of my question is simple: Consider a water box. you model inlet and outlet. If the outlet is at the bottom (i.e.altitude is lower than the inlet) what BC do I define for pressure at the outlet??? If P stands for p/rho, I have to calculate the hydrostatic pressure at he bottom. If p stands fo p/rho-g*h: I just have to put P=0. Thanks for you help. Regards, Miles P.S;: I have found post on the subject but it was for older version as OF 1.7

September 28, 2011, 15:25
#2
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Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 621
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Quote:
 Originally Posted by miles_davis Hi, Does anyone know what is pressure in OF2.0.1 for solver like SimpleFOAM? I have read that in previous versions of the code it was in fact Pstatic-rho*g*h, than in another version it was just Pstatic/rho. What is the actual statement for OF 2.0.1? The source of my question is simple: Consider a water box. you model inlet and outlet. If the outlet is at the bottom (i.e.altitude is lower than the inlet) what BC do I define for pressure at the outlet??? If P stands for p/rho, I have to calculate the hydrostatic pressure at he bottom. If p stands fo p/rho-g*h: I just have to put P=0. Thanks for you help. Regards, Miles P.S;: I have found post on the subject but it was for older version as OF 1.7
Miles,

For simpleFoam it is a relative pressure related to a reference value. If you look at the creatFields.H and Peqn.H in the simpleFoam solver that might help.

Dan

 September 28, 2011, 17:16 #3 Senior Member   Vesselin Krastev Join Date: Jan 2010 Location: University of Tor Vergata, Rome Posts: 368 Rep Power: 20 You are confusing compressible and incompressible solvers. As far as I know, all the incompressible solvers in OF (like simpleFoam or pisoFoam), in all the releases (at least from the 1.6 to the 2.0.1, including the -dev/-ext ones), solve for the kinematic relative pressure, i. e. for p/rho (if rho is a constant, defining the pressure source term in the momentum equation as -1/rho*grad(p) or as -grad(p/rho) is equivalent). For the compressible solvers, things are not so established between different releases (honestly i really can't understand why), as for example in the 1.6 release a solver like rhoSimpleFoam assumes the static pressure as the dependent variable, while in the 1.7.0/1/x (and I think also in the 2.0.0/1/x family) the dependent variable is p-rho*g*h. Hope this helps V.

 September 29, 2011, 18:35 #4 Member   Miles Join Date: Sep 2011 Posts: 48 Rep Power: 14 thanks for your replies regards miles