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(d) Mass-flow boundary conditions

Just as enthalpy is the dependent variable which affects energy sources, concentration that which affects chemical-species sources, and velocity that which affects momentum sources, so is pressure the variable which, in PHOENICS, affects mass sources. Information about the inflow and outflow of mass to the domain is therefore conveyed to PHOENICS by way of PATCH and COVAL statements pertaining to pressure.

The physical notion is that fluid is forced into the domain because some external pressure (the prescribed 'value') exceeds the pressure which obtains in the cell. If the 'coefficient' is very small, the external pressure must be very large; then variations such as are likely to occur in the cell pressure, associated with the velocity fields within the domain, will have little effect. If the 'coefficient' is very large, by contrast, such variations will be effective; therefore the magnitude and even the sign of the flow rate will be hard to determine in advance. Typically, the inflow to a domain is represented by a low-coefficient pressure boundary condition, whereas an outflow is represented by a high- coefficient one; but variants are numerous.

The mass flow source added to the continuity equation is:

where Cp and Vp are the coefficient and value for pressure, ie the C and V of the COVAL for P1. The value of the pressure at the nodal point P is denoted by pP.