In the steady state limit, flow through a composite medium (a medium with a jump discontinuity in ) is determined by a harmonic average. See Exercise 5.

For implicit finite difference methods involving the flux at time ,
it is common practice to
lag the -values by evaluating them using the **U**-values from time
.
The alternative is to
deal with a system of algebraically nonlinear difference equations using some
iterative
procedure such as Newton's method for systems of equations.