Particle dynamics due to interaction between a breaking-induced vortex and a nearbed vortex

A single regular wave, travelling over a submerged abrupt discontinuity, is able to generate a pair of counter-rotating vortices. A nearbed vortex is generated by the flow separation that occurs at the bed, while a surface vortex can be generated by either a direct (co-rotating vortex) or a backward (counter-rotating vortex) breaking. Starting from recent laboratory test results, which showed the influence of the wave nonlinearity on the interaction between the counter-rotating vortices and led to the identification of three different regimes, the present work illustrates the main findings obtained from the optical analysis of the flow field induced by three waves, each belonging to a specific nonlinear regime. Specifically, for each test, synthetic particles have been seeded within the domain, with the aim to obtain long-lasting trajectories driven by the Eulerian flow field obtained through the Particle Tracking Velocimetry (PTV) analysis, to be studied by means of single-particle and multi-particle statistics. Both absolute and relative statistics confirm that a ballistic regime exist just after the particle release at each location of the domain. At larger times, the absolute statistics suggest a subdiffusive regime both within the vortices and between such areas (in correspondence of the breaking-induced jet), followed by a superdiffusive regime, dominated by rotation and particle release. Differently, the relative diffusivity suggests the occurrence of a superdiffusive regime at t>TL, corresponding to enstrophy cascade and exponential growth, followed by a Richardson regime and then by an oscillatory behavior, during which particles are periodically trapped and released by vortices.