The Gravity model permits the inclusion of the buoyancy source terms in the momentum equations when using the Segregated Flow model or the Coupled Flow model. For variable-density flows using the Ideal Gas model or the Polynomial Density model, the buoyancy source term is included in the momentum equation without recourse to modeling. For Constant Density flows, the Boussinesq Model is used to approximate the buoyancy source term. The formulation of these buoyancy source terms is presented below.