The gravity force is defined from the acceleration of gravity vector, g, and the density, ρ. Under usual conditions and in Cartesian coordinates with the z-axis in the vertical direction,

When gravity is considered, a volume force equal to ρg is included in the momentum equation. For example, for laminar weakly compressible flow, it reads:

Introducing a constant reference density ρref, and assuming that g is homogeneous, this equation is equivalently written:

where r is the position vector and rref is an arbitrary reference position vector.

In Equation 13-12, the gravity force is written .

When the relative pressure is used (default option) the interface dependent variable represents the relative pressure and the absolute pressure is defined as . When the pressure is used to define a boundary condition (for example when p0 defines the pressure condition at an outlet), it represents the relative pressure. Hence defining the outlet pressure as compensates for the gravity force for an ambient reference pressure of 0 Pa when the density is constant, there is no external force, and provided pref, g and r0 are defined consistently.

When the reduced pressure is used, the interface dependent variable (named p by default) represents the reduced pressure. The absolute pressure is then defined as . In this case when the pressure is used to define a boundary condition (for example to define a pressure condition at an outlet), its value corresponds to the reduced pressure. Hence, the prescribed pressure compensates for an approximate hydrostatic pressure, which is exact only when the density is constant and there is no external force.