[elisabet]

Hi Kyian,

As always, you should go to the sources. In this case:
OpenFOAM-1.5.x/applications/solvers/heatTransfer/buoyantSimpleFoam/UEqn.H

there, you would see that both the pressure gradient term and the body force term, which in its most general form are:

-grad(p) + rho*g

become

-fvc::grad(pd) - fvc::grad(rho)*gh

this transformation is straight forward if you consider p=pd+rho*g*h+pref

In this case, rho is obtained from the "basicThermo" object (see createFields.H), and hence, depends on the equation of state defined there. You should go to basicThermo sources to know more about it.

Good luck,

elisabet