Author(s): Marco Mazzuoli; Aman G. Kidanemariam; Francesco Battista
Linked Author(s):
Keywords: Lagrangian transport; Wave boundary layer; Ripples; Microplastics; Stratification
Abstract: The processes underlying the dispersal of solid particles caused by non-breaking progressive wind-waves approaching the coast over shallow bottom are investigated by numerical means. In these conditions, waves strongly interact with the bottom possibly causing the mixing and the transport of the bottom material such as inert sediments, nutrients and organic pollutants (PCB, PAH, plastic particles), as well as biological material (algae, pathogens), which can threaten the human health. Let us consider monochromatic wind-waves propagating over a horizontal bottom. The irrotational velocity field can be numerically computed by solving the nonlinear wave problem, which however does not account for the effects of the bottom boundary layer, hereinafter referred to as wave boundary layer (WBL), where the flow is dominated by viscous stresses and turbulence may appear. At the leading order of approximation, the WBL is driven by harmonic oscillations of the pressure gradient (oscillatory boundary layer, OBL). The OBL was used as a prototype flow to study the dynamics of bedload sediment particles, but may misrepresent the dynamics of suspended and floating particles. In fact, a wavewards steady current arises in the WBL, even in the presence of bottom roughness or ripples. Vittori & Blondeaux showed that a steady streaming appeared in the OBL over a wavy bottom, which was associated with the chaotic shedding of vortices from the waviness crests. Little is known about ripple and turbulence effects on the WBL (thickness, energy dissipation) and on the Lagrangian properties of the wave flow (particle dispersion, preferential patterns). In the present contribution, these properties are explored for the first time by means of direct numerical simulation (DNS). To this aim, the trajectories of both passive tracers and inertial particles are computed. Moreover, the dynamics of inertial particles are compared with those obtained within a turbulent open-channel flow (OCF) under stable stratification. In such a case, the system is characterised by five dimensionless parameters: the Reynolds number, the particle Froude number, the Richardson number, the Stokes number, and the particle-to-fluid density ratio. The present study focuses on varying the last three numbers to assess the influence of stratification on particle dynamics.
Year: 2025