You should be familiar with buoyancy in single phase flows. For details, see Buoyancy.

In addition to the buoyancy forces that can exist in single phase flows, the difference in density between phases produces a buoyancy force in multiphase flows (including particle tracking). For this reason, buoyancy is almost always important in multiphase flows.

For information on modeling low-speed compressible multiphase flow, see Buoyancy and Pressure.

For multiphase flows, it can be important to correctly set the buoyancy reference density. For a flow containing a continuous phase and a dilute dispersed phase, you should set the buoyancy reference density to that of the continuous phase. This is because the pressure gradient is nearly hydrostatic, so the reference density of the continuous phase cancels out buoyancy and pressure gradients in the momentum equation.

This is much the same as in single phase, where any non-buoyant calculation implicitly assumes a buoyancy reference density of the fluid, thereby avoiding potential roundoff problems. For non-dilute cases (which include all free surface cases), all terms can be equally important for each fluid, so roundoff errors will be introduced for one of the fluids if there is a significant difference in density. You should choose the density of the lighter fluid because this gives an intuitive interpretation of pressure (that is, constant in the light fluid and hydrostatic in the heavier fluid). This simplifies pressure initial conditions, pressure boundary conditions and force calculations in postprocessing.