Detecting the chaotic nature of advection in complex river flows

In order to detect signatures of chaotic advection in river surface motion, surface buoys equipped with GPS were deployed in a field experiment in River Danube, Hungary. The buoys were released in the vicinity of groynes where complex mixing processes occur. A detailed analysis of the trajectories was carried out, focusing on the time evolution of the distance between buoy pairs. The analysis included the determination and comparison of local Lyapunov exponents and prediction times of finite-time hyperbolic behaviour, which is related to strong mixing. Despite of the small number of applied buoys we found evidence on Lagrangian chaos in the wake of a groyne field. In order to supplement the field data obtained by this, inherently Lagrangian, approach, experiments in a small-scale laboratory model were also carried out, in which the Lagrangian surface dynamics was detected by following the motion of numerous floaters using particle tracking velocimetry (PTV).