# 4.10.7. Mass Flux

A mass flux boundary is very similar to an open boundary, except that instead of calculating the mass flux using the specified pressure and continuity equation, the mass flux is specified by the user. When the flow is out of the domain (positive mass flux) then the conserved quantities are advected out and none of the other values specified in the open boundary condition are used. When the flow is into the domain (negative mass flux), the composition of the entrained fluid is defined by the values you set in the mass flux boundary condition (e.g. the entrained enthalpy will be calculated using the specified temperature and mass fraction in the example below).

    begin mass flux boundary condition on surface surface_4
        mass flux = 1.84
        temperature = 300
        mass_fraction He = 0.0
        mass_fraction O2 = 0.233
        mass_fraction N2 = 0.767
    end

Most specified values can be constants, in-line string functions, tabulated functions, or user subroutines. The complete list of available commands for the mass flux boundary condition and the syntax for the different options can be found in Mass Flux Boundary Condition On Surface.

Like pressure, entrained turbulence quantities can be specified directly, or using the `characteristic length` () and optional `turbulence intensity` (, 10% by default) parameters.

Note

You do not need to specify pressure on a mass flux boundary.