FIG. 4.

Transient behavior of the effective diffusivity, D(t|L, v), for several values of the drift velocity, v, indicated near the curves, and L = 500, in dimensionless units. Other parameters, in dimensionless units, are R = l = D₀ = D_de = 1 and a = 0.1. Dashed straight lines are the corresponding values of D_eff(L, v) = D(∞|L, v), given in Eq. (4.25). The curves are drawn using Eq. (4.2) with functions f_m(t) and ${\sigma }_{t_m}^2\left(t\right)$ found using their definitions in Eqs. (3.12) and (2.9). The first two moments of the cumulative time t_m(t) entering into these definitions are obtained by numerically inverting their Laplace transforms in Eqs. (4.6) and (4.5), respectively. Panels (a) and (b) show the same curves on different time scales.