In this scenario a multiphase model is not necessary. The air is modeled as Boussinesq, so the operating density set in the ‘operating conditions’ panel does apply in the air zone. Thus, the model will converge well if the operating density is set to the density of oil, since only in the oil zone will this operating density be used. If the operating density is set to air density, or is calculated automatically by Fluent, the model will not converge, since it will be using a light operating density in a zone with a heavy fluid. This unique scenario is different from natural convection problems which use an actual multiphase model, in which we typically specify the operating density as the density of the lighter phase.