Gravity
Definition
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:
(3-24)
Introducing a constant reference density ρref, and assuming that g is homogeneous, this equation is equivalently written:
(3-25)
where r is the position vector and rref is an arbitrary reference position vector.
From this equation, it is convenient to define the reduced pressure which accounts for the hydrostatic pressure,
(3-26)
In Equation 3-26, the gravity force is written (ρ − ρref)g.
Incompressible Flow
For incompressible flow the fluid density is assumed to be constant. Hence it is natural to define the reference density, ρref, such that ρ = ρref = ρ (Τref,pref ) which makes it possible to simplify Equation 3-26:
In some cases, even when the flow is modeled as incompressible, buoyancy should be accounted for. Using the approximation of the gravity force based on the thermal expansion coefficient (which is relevant in this case: since the density changes are small, the first order approximation is reasonably accurate), it is possible to rewrite the momentum equation with constant density and a buoyancy force:
Weakly Compressible Flow
For weakly compressible flow the assumption is that the density depends only on the temperature. In particular, the pressure dependency of the density is neglected, and the density is evaluated at the reference pressure: ρ = ρ(Τpref). With the relative pressure as dependent variable, Equation 3-24 is used
With the reduced pressure as dependent variable, Equation 3-26 is used:
Compressible Flow
For compressible flow the density may depend on any other variable, in particular on the temperature and the pressure. Similar equations are used as for the Weakly Compressible Flow case except that the density is not evaluated at the reference pressure.
Nonisothermal Flow Coupling
For consistency, when the Nonisothermal Flow coupling is active, the assumptions made for the single phase flow interface are also made in the heat transfer interface:
Pressure Formulation
When the relative pressure is used (default option) the interface dependent variable represents the relative pressure and the absolute pressure is defined as pA = pref + p. 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 phydro,approx = −ρrefg ⋅ (r − rref) 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 pA = pref − ρrefg ⋅ (r − rref) + p. 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, phydro,approx = −ρrefg ⋅ (r − rref), which is exact only when the density is constant and there is no external force.
Pressure Boundary Condition
For an immobile fluid the momentum equation simplifies to ∇ ⋅ (pI) =  F+ ρg or
depending on the pressure formulation.
For incompressible flow, assuming there are no external forces, this leads respectively to p = − ρrefg ⋅ (r − rref) + p0 or .
For weakly compressible flow and compressible flow, since the density varies, there is no corresponding explicit expression. We have
and .
In practice, these integrals can be problematic to evaluate. Hence, whenever possible, it is recommended to locate the pressure boundary in a region where the approximate definition of the hydrostatic pressure is applicable, or to define a boundary that is perpendicular to the gravity vector.
If it is not possible and if the pressure conditions cannot be determined, you can use a no viscous stress condition (available in the Open Boundary feature).