rather than p_rgh which introduces an imbalance between the pressure and
buoyancy forces. The relaxation factor for p_rgh specified in fvSolution is
used to relax the net force as the intent is to relax the pressure and this
provides convenient usage and backwards-compatibility.
Optional relaxation for the thermodynamic pressure p is also available for case
this provides convergence benefit for steady cases by relaxing the pressure work
term is the energy equation.
Note these changes only relate to the operation of the isothermalFluid solver
module for buoyant cases.
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