# Mathematical expression for buoyantPressure boundary condition

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 March 18, 2013, 09:25 Mathematical expression for buoyantPressure boundary condition #1 New Member   Adam Join Date: Mar 2012 Location: St. Paul, MN Posts: 12 Rep Power: 14 Hello all, Based on the release notes, "the buoyantPressure boundary condition is applied to the pressure field, which calculates the normal gradient from the local density gradient." I am not fluent in C++ and I'm having some difficulty determining the mathematical expression associated with this condition. The source code computes the gradient as: gradient() = -rho.snGrad()*(g.value() & patch().Cf()); I think the equivalent mathematical expression for this is n · p = -(n · ρ)gh, where n is the normal vector, p is pressure, rho is density, g is gravity and h is cell length. Would anyone be able to confirm or deny this? Thanks. rajibroy likes this.

 May 24, 2013, 04:31 #3 New Member   Join Date: May 2013 Posts: 3 Rep Power: 13 Hi Adam, thanks for your explanation, itīs really helpful. But I still donīt understand how to specify this BC type. Considering the example from doxygen documentation: myPatch { type buoyantPressure; rho rho; value uniform 0; }What does the specified value refer to in this example? Does anybody know the answer? Davide

 June 25, 2013, 11:08 #4 Member   Join Date: Mar 2013 Posts: 98 Rep Power: 13 Hi, thanks for your explanations,it's very helpful. I have a question: the pressure to which refers the formula is static pressure or p_rgh? In doxygen documentation it write p and seems is the static pressure but the formula is: p_rgh= p -rgh so I think that if doxygen refers to a static pressure there is a sign error... Probably is only an unfortunatly notation and p refers to ones of the pressure elencated (p_rgh,pd or ph_rgh) Thanks