[marupio]

Correct me if I'm wrong, but:

Should be:
p_static = rho_ref * g * h

The code uses:
p_static = rho_local * g * h

This assumes that rho_local is the same all the way up the fluid column, which it is not. A fluid element with low density (locally) will have an erroneously low p_static.

A reference density really is needed.

[marupio]

Strike my last.

The hydrostatic pressure definition is different in buoyantFoam.

I didn't realize how the buoyancy momentum source related to the way hydrostatic pressure is defined. In general:

Sm = grad( p_hyd ) + rho g