Figure 2

Pe = 50, for the same porous medium as in Fig. 1: (a) snapshot of the concentration profile at t = 0.1 × test; (b) the corresponding cumulative concentration distribution in the x direction, c(x), at t = 0.1 × test; (c) snapshot of the concentration profile at t = 0.3 × test; (d) c(x) at t = 0.3 × test. test roughly denotes the time for a solute particle with average velocity to reach the boundary (see Methods for its precise definition).