where κ =k ρCP
is thermal diffusivity and x is the minimum grid spacing. This limitation means that the number of time steps increases as the square of the decrease in the grid spacing, which can be quite inconvenient for high-resolution thermal models. If larger time steps are indeed employed, numerical oscillations occur that increase with the number of time steps (Fig. 10.2, see also program example Explicit_implicit_1D.m). The implicit finite-difference discretisation is given by (Fig. 10.3)