Lagrangian mixing in Compound Channels

In a recent study Stocchino & Brocchini (2010) have investigated the dynamics of two dimensional large-scale vortices with vertical axis evolving in a straight compound channel under quasi-uniform °ow conditions. They have showed that properties of quasi two dimensional turbulence strongly depend on the ratio rh between the main channel flow depth (h_mc) and the floodplain flow depth (h_f p). In the case of shallow flows, when the flow depth in the main channel largely exceeds the depth in the flood-plains, the macrovortices are generated owing to the vorticity production concentrated at the transition region, i.e. between the main channel and the floodplains. The source of vorticity was found to be related to the depth jump at the transition. Another fundamental property of this class of flow is that once the turbulence is fully developed, the typical size of the macrovortices is independent of the streamwise coordinate. In the present contribution, we discuss the results of a new extensive experimental campaign in terms of Lagrangian properties of the °ow. Di®erent Lagrangian measures of mixing (i.e. single and multiple particle statistics, FSLE) are introduced with the aim to characterize the mixing processes that occur in a compound channel for varying depth ratio (rh) and Froude number (Fr). The results suggest that the presence of the macrovortices at the transition region between the main channel and the °ood plains delays the establishment of fully developed conditions, increasing the decorrelation time beyond which a Brownian regime is recovered. Moreover, the growth in time of the absolute dispersion is strongly influenced by the presence of long-lived macrovortices causing a non monotonic behaviour Moreover, the dependence of the relative diffusivity on the pair separation shows different behaviours as the flow ratio rh is increased characterized by a local dynamics.