Evolution of the self-similar diffusion test for Gaussian and selfsimilar initial conditions. At t = 1.40 s, the profile lies in the χ t >> 1 regime and both initial profiles show a self-similar diffused profile, albeit for different ϕ₀ . The solutions are run with the explicit method for Δt = 10⁻⁵ s. We note that different scales are used for the colour maps between t = 0 s and t = 1.4 s.