Encyclopaedia Index

### (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.
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