By default, Ansys Icepak uses the Boussinesq model for natural-convection flows. This model treats density as a constant value in all solved equations, except for the buoyancy term in the momentum equation:

(40–95)

where ρ₀ is the (constant) density of the flow, T₀ is the operating temperature, and β is the thermal expansion coefficient. Equation 40–95 is obtained by using the Boussinesq approximation to eliminate ρ from the buoyancy term. This approximation is accurate as long as changes in actual density are small; specifically, the Boussinesq approximation is valid when β(T−T₀)≪1