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Buoyancy Treatment in OpenFoam

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Old   October 29, 2017, 11:35
Default Buoyancy Treatment in OpenFoam
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From all the standard solvers that I found on OpenFoam, it seems that the buoyancy term is always treated by modifying the pressure:


-\nabla P + \rho g = -\nabla P_{rgh} - (g \cdot x) \nabla \rho

where

P_{rgh} = P - \rho g \cdot x

In OpenFoam format, this is usually written as

Code:
solve        (
            UEqn
         ==
            fvc::reconstruct
            (
                (
                  - ghf*fvc::snGrad(rho)
                  - fvc::snGrad(p_rgh)
                )*mesh.magSf()
            )
        );
Is there a particular numerical benefit for treating the buoyancy term in this fashion instead of just treating it as a source term \rho g? It seems to me this way is more complicated in the sense that you have to track two pressures (P and P_rgh), and for certain problems, creating an initial condition for P_rgh is not straightforward.
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Old   October 30, 2017, 05:31
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Knut Erik T. Giljarhus
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Yes, using the modified pressure often makes the problem easier to handle numerically, and it's easier to specify boundary conditions.
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Old   October 30, 2017, 23:51
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Quote:
Originally Posted by eric View Post
Yes, using the modified pressure often makes the problem easier to handle numerically, and it's easier to specify boundary conditions.
Do you mind elaborating on how it makes it easier to handle numerically?
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Old   October 31, 2017, 02:23
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Knut Erik T. Giljarhus
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See this post on the issue tracker from Henry Weller, where he briefly explains it:
https://bugs.openfoam.org/view.php?id=1170
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