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(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:

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.