The Boussinesq number, B, is a dimensionless group representing the square root of the ratio of inertia force to gravitational force. It is represented as
where u is the fluid velocity, g is the acceleration due to gravity, L is a representative length or hydraulic diameter and RH is the hydraulic radius. The group occurs frequently in the analysis of Fluid Flow with an interface between liquid and gas (see, for example, Channel Flow and Waves in Fluids).
Boussinesq number is the same as Froude Number.