When multiple species are involved in the solidification/melting process, the density of the liquid pool varies with temperature as well as species composition. In the presence of a gravitational field, buoyancy will be induced by two mechanisms:
Thermal buoyancy: is the gravitational force due to the variation of density with temperature.
Solutal buoyancy: is the gravitational force due to the variation of density with the change in the species composition of the melt. In multi-component solidification problems, the solutes are continuously rejected near the liquid solid interface, leading to the enrichment of the liquid pool with the solutes. This enrichment of the solutes causes a concentration gradient in the immediate vicinity of the solid-liquid interface, and consequently a density gradient.
Modeling of thermal and solutal buoyancy is often important for the accurate prediction of the overall solidification behavior. The flow due to thermal buoyancy tends to promote mixing and smooths out temperature gradients. Therefore, excluding this term can lead to inaccurate predictions of solidification time. Solutal buoyancy moves the enriched liquid away from the liquid-solid interface and replaces it with the far field nominal composition liquid. Excluding solutal buoyancy can lead to an over prediction of segregation patterns for multi-component materials.
For multi-component solidification problems, the buoyancy induced flows are modeled in Ansys Fluent using the Boussinesq approach. The thermal buoyancy is calculated as described in Natural Convection and Buoyancy-Driven Flows Theory. The body forces due to solutal buoyancy are calculated using a similar approach:
 | (15–24) |
where 
is
the mass fraction of the 
th solute in the liquid phase and 
is the reference mass
fraction of the 
th species. 
is the solutal expansion coefficient of the 
th species and 
is the total number of solute species, 
is the gravity vector, and 
is the reference density.
The total body force is the sum of the solutal and thermal buoyancy, which can have similar or opposite signs, and their relative importance can change significantly during the course of the simulation.
Refer to Modeling Thermal and Solutal Buoyancy in the User's Guide to learn how to include thermal and solutal buoyancy.