Mass Flow
The Mass flow boundary condition constrains the mass flowing into the domain across an inlet or outlet boundary. The mass flow can be specified in a number of ways.
Pointwise Mass Flux
The pointwise mass flux sets the velocity at the boundary to
where mf is the normal mass flux and ρ is the density.
Mass Flow Rate
The mass flow rate boundary condition sets the total mass flow through the inlet boundary according to
where dbc (only present in the 2D Cartesian axis system) is the boundary thickness normal to the fluid-flow domain and m is the total mass flow rate. For the outlet boundary, the mass flow rate boundary condition sets the total mass flow as
In addition to the constraint on the total flow across the boundary, the tangential velocity components are set to zero on the boundary
(3-34)
Standard Flow Rate
The standard flow rate boundary condition specifies the mass flow as a standard volumetric flow rate. The mass flow through the inlet boundary is set by the equation
where dbc (only present in the 2D component Cartesian axis system) is the boundary thickness normal to the fluid-flow domain, ρst is the standard density, and Qsv is the standard flow rate. The standard density is defined by one of the following equations:
where Mn is the mean molar mass of the fluid, Vn is the standard molar volume, pst is the standard pressure, R is the universal molar gas constant, and Tst is the standard temperature. For the outlet boundary, the mass flow rate is set by the equation
Equation 3-34 or Equation 3-35 is also enforced for compressible and incompressible flow, respectively, ensuring that the normal component of the viscous stress and the tangential component of the velocity are zero at the boundary.