(1)

Conservation of momentum of a viscous fluid flow with a gravitational acceleration in the

(2)

*Re* is the Reynolds number, and *Fr* the Froude number, .
By introducing a new variable for the pressure, which includes the both the
static and the hydrostatic component of the pressure,

(3)

the equation of momentum, Eq.(2), can be written as

(4)

The Freud number will now enter explicity in the boundary condition instead of the governing equations.