Encyclopaedia Index

(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:

  1. 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.
  2. 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.
  3. 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.

  4. The PHOENICS SATELLITE accepts specifications of the C and V quantities through a PHOENICS Input Language (ie PIL) command named COVAL.
  5. Where (ie to which cells) the boundary condition is to be applied is conveyed to PHOENICS by way of another command, named PATCH. 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.