(a) The general idea
Setting up a flow-simulating computation involves specifying
boundary conditions, a task needing careful thought. In
particular, it involves the specification of convective and
diffusive fluxes at surfaces bounding the domain.
PHOENICS has a flexible procedure for this, involving the
following five points:
- PHOENICS always treats a boundary condition as a source of
the entity in question (mass, momentum, energy, chemical
species, turbulence energy, etc). It therefore does NOT
insert boundary values directly.
- Since sources are inserted at the centres of cells, not
at their walls, "boundary conditions" are not truly inserted
at boundaries. Of course, near-boundary cells can be made small
enough for the shift of location to be unimportant; but PHOENICS
also has other ways of effecting what is want.
- PHOENICS accepts specifications of sources (and therefore of
boundary conditions) in terms of a 'coefficient' (C) and a
'value' (V). The source for variable j determined by Cj and
Vj is then calculated from:
Cj * (Vj - jP),
where jP is the value of j at node P, ie the in-cell value of j.
- The PHOENICS SATELLITE accepts specifications of the C and V
quantities through a PHOENICS Input Language (ie PIL) command
- Where (ie to which cells) the boundary condition is to be
applied is conveyed to PHOENICS by way of another command,
The COVAL and PATCH commands form part of the PHOENICS Input
Language, described elsewhere in the Encylopaedia; and examples
of boundary conditions specified with their aid will be found
in the PHOENICS Input Library. Here it is necessary only to
explain how EARTH reacts to the COVAL specification.