A slug of dye injected into a moving fluid will spread out because of diffusion and because of convection by the fluid motion. Under many circumstances the spreading is very close to diffusion under a mean fluid motion with an enhanced diffusivity - the dispersion coefficient. This effect also occurs in unsteady separated flows even when there is no underlying molecular diffusion. One way to study dispersion is by integrating the time and space dependent velocities to obtain particle trajectories and then to average the rate of longitudinal spreading of the trajectories of a "slug" of starting particles, see vortex wave devices, the experimental part of which has been published, see Hwu, Sobey & Bellhouse 1996 . A second paper covering details and results of simulations has also appeared, see Hwu, Sobey & Bellhouse, 1997.

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