Encyclopaedia Index

### (e) Convective and diffusive sources for j

When mass enters a cell from outside the flow domain (but not when it leaves), the values of all the dependent variables pertaining to the inflowing fluid need to be prescribed. The correct effect is achieved by specifying these values as the V's in the correspondi COVAL specification. The C's represent the coefficients of the diffu fluxes from the boundary to the nodes in the PATCH. The mass-flow ra are specified through the pressure boundary condition as explained a

The convective and diffusive fluxes of j represented by the above mentioned settings are given by the following expressions for the corresponding source of j:

Cp(Vp - pP)(Vj - jP) + Cj(Vj - jP) ,

for Vp > pP, ie inflow, and,

Cj (Vj - jP),

for Vp %lt; pP, ie outflow.

The omission of the convective term for outflow is an expression of the upwind principle; and it is analogous to the practice used for the interior coefficient aE (see equations (2.4-7) and (2.4-8)).

To summarize, the task is to specify Cp and Vp for pressure P1, and to specify Cj and Vj for the variable j. In this way, the convective- and diffusive-fluxes at boundary surfaces are specified.

Very often the diffusive contribution is zero, ie,

Cj = 0,

which may also be represented in COVAL by substitution of the word ONLYMS (for only mass transfer) in its third argument.

Further information on boundary-condition implementation is provided under the COVAL entry in this encylopaedia.

The entries for PATCH, COVAL and SOURCE should be inspected for further information, with special attention to the significance which attaches to the use of the arguments FIXVAL, FIXFLU, ONLYMS, FIXP, OPPVAL and SAME.

The keyword commands INLET, OUTLET, WALL and VALUE can also be used to set the commonly-used boundary conditions which correspond to their names.

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