# p_rgh in interfoam

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 October 6, 2010, 06:42 p_rgh in interfoam #1 Senior Member     ata kamyabi Join Date: Aug 2009 Location: Kerman Posts: 322 Rep Power: 8 Hi FOAMers What is the advantage of combining P and rho*g*h in one term "p_rgh" in OpenFOAM 1.7.0 comparing to OpenFOAM 1.6? Best regards Ata

 October 6, 2010, 11:56 #2 Member   Lars Kiewidt Join Date: Sep 2009 Location: Germany Posts: 54 Rep Power: 7 Have a look at Rusche's Ph.D. Thesis (pp. 155/156). The dynamic pressure is used because it simplifies the specification of the boundary conditions. For example you can use p_rgh=0 at an outlet and you dont't have to deal with the hydrostatic pressure. I think the release notes of OpenFOAM-1.7 say something about this, too. Best regards, Lars

 October 6, 2010, 15:09 p_rgh in interfoam #3 Senior Member     ata kamyabi Join Date: Aug 2009 Location: Kerman Posts: 322 Rep Power: 8 Hi Lars Thank you very much Best regards

 April 17, 2012, 07:10 tank draining with interFOAM #4 New Member     Sam Mathew Join Date: Apr 2010 Location: India Posts: 19 Rep Power: 7 Hi, I was trying a simple tank draining problem and was trying to specify the right boundary conditions. I am implementing my case in 2.1.0 and it is basically a tank with water filled up to a specific height with a small outlet port at the bottom center of the tank. At the top I gave the same boundary conditions as in the Dam_break case. I am having difficulty with specifying appropriate boundary conditions for the outlet port. I have been basically playing around with values for alpha, p_rgh and U. 1. For p_rgh = 0 (as suggested above): The water does not flow out of the domain. The other parameters were U = (0,0,0), zeroGradient and alpha = {InletOutlet}, 0. 2. For p_rgh - zeroGradient: The flow is going out but still the results are quite strange. While keeping alpha = {InletOutlet} and U - zeroGradient, the water suddenly jumps up and after a few time steps, it starts draining. It seems as if, the solver detects a high pressure at the outlet port and causes flow to occur from higher to lower pressure, but then realizes actually there is gravity acting against and then the liquid flows out. The reason I gave p_rgh to be zeroGradient is because I understood that it is the dynamic pressure and the dynamic pressure at the outlet cannot be zero but rather the normal gradient should be zero. I would be thankful for any help since I am not able to grasp yet the right implementation. In other solvers (like FLUENT, CFX), I would just specify the static pressure to be zero at the outlet with the possibility for reverse flow of air into the domain.

 April 17, 2012, 23:29 p_rgh and initial velocity specification #5 New Member     Sam Mathew Join Date: Apr 2010 Location: India Posts: 19 Rep Power: 7 Hi, I was finally able to solve the problem using the second formulation but with a finer mesh. I have another question with regard to the p_rgh formulation in OpenFOAM. If I want to specify some initial velocity in the fluid (e.g., due to rigid body motion of the tank), do I only need to specify it as the internal field in the U file or also the p_rgh file? Regards, Sam Last edited by Sam-CFD; April 23, 2012 at 00:54.

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