Encyclopaedia Index

### (c) Special cases

When the boundary condition dictates that the value of jP should
be fixed, what happens is that V is set to the desired j-value,
and C is set to a very large number; for then the equation reduces
as follows:

jP = f( ... + V x C , ... + C ) = V

because VxC is much larger than all other terms in the numerator,
and C is much larger than all other terms in the denominator.
When the boundary condition is of the fixed-flux kind, by contrast,
C must be given a very small value, so that it is negligible in
comparison with the other terms in the denominator; then V must
be chosen so that the product V x C equals the desired flux.

In general, any kind of boundary condition can be expressed in
this way; for C and V need not be constants; and they may be
recalculated in GROUND, for each grid node and time instant,
as the computation proceeds.

wbs