Fluid-particle interaction and generation of coherent structures over permeable beds: An experimental analysis

We study the dynamics of the oscillatory bottom boundary layer (BBL) that develops at a porous bed under the action of propagating water waves (also known as "ventilated BBL"). In particular, experimental tests of a ventilated BBL generated over a permeable bed made of regular plastic spheres are analysed. With the purpose of characterizing typical dynamics of nearshore waves evolving over dissipative seabeds, we focus on one specific forcing condition, characterized by large nonlinearities and vorticity generation, and expect that such dynamics be qualitatively similar also for other wave regimes. Particle tracking enables an accurate definition of the hydrodynamics generated over the porous medium with specific focus on Eulerian velocity and vorticity fields and on Lagrangian particle trajectories. Nearbed velocity components are strongly influenced by the bed configuration (sphere caps) and are characterized by small-scale oscillations due to the presence of coherent structures over the interstices. Counter-rotating vortex sheets are generated and evolve during the entire wave period; their thickness grows rapidly when flow injection from the bed occurs, while it remains constant under the influence of in-bed flow suction. The evolution of near-bed particles is influenced by strain or vortex-dominated regions. When the intensity of the vortices just outside the BBL is large (generally after flow inversion), the momentum associated to the rotation of the lower layer of vortices in the BBL is comparable with that of the passive tracers, hence the particles are captured and their original trajectories are modified, jumping to a different layer. On the contrary when strain regions are dominant, particles are restricted in the same vorticity layer until sweep or ejection turbulent processes are observed and anomalous particle transport occurs from lower to higher vortex layers and vice versa. The latter occurs during the entire wave cycle, becoming the dominant process, while the former occurs before flow inversion and close to the porous bed. This result reveals that nearbed small-scale phenomena are weakly influenced by the wave mean flow. A detailed description of fluid suction and injection is proposed in terms of the mean flow dynamics (at wave scale), while the actual inflow/outflow of particles at the bed is seen to depend on local, small-scale flow properties. Suction and injection are generated during positive and negative water surface elevations and either squeeze or expand the flow downward/upward. The suction/injection perturbations contribute to the triggering of sweep and ejection events. Suction is mainly concentrated very close to the bed, injection is rapidly transported above the BBL, but the highest turbulence occurs in correspondence of suction events.