Interface-resolved direct numerical simulations of sediment transport in a turbulent oscillatory boundary layer

TL;DR

This study uses interface-resolved direct numerical simulations to analyze sediment transport in turbulent oscillatory boundary layers, revealing the influence of turbulence and flow acceleration on sediment movement.

Contribution

It introduces a numerical approach that fully resolves flow around moving sediment particles and evaluates sediment transport under different turbulence regimes.

Findings

Turbulence appears intermittently at low Reynolds numbers and is persistent at high Reynolds numbers.

Sediment transport rates are influenced by flow acceleration, especially at low Shields parameters.

Turbulent eddies significantly affect sediment dynamics in oscillatory boundary layers.

Abstract

The flow within an oscillatory boundary layer, which approximates the flow generated by propagating sea waves of small amplitude close to the bottom, is simulated numerically by integrating Navier-Stokes and continuity equations. The bottom is made up of spherical particles, free to move, which mimic sediment grains. The approach allows to fully-resolve the flow around the particles and to evaluate the forces and torques that the fluid exerts on their surface. Then, the dynamics of sediments is explicitly computed by means of Newton-Euler equations. For the smallest value of the flow Reynolds number presently simulated, the flow regime turns out to fall in the intermittently turbulent regime such that turbulence appears when the free stream velocity is close to its largest values but the flow recovers a laminar like behaviour during the remaining phases of the cycle. For the largest value of the Reynolds number turbulence is significant almost during the whole flow cycle. The evaluation of the sediment transport rate allows to estimate the reliability of the empirical predictors commonly used to estimate the amount of sediments transported by the sea waves. For large values of the Shields parameter, the sediment flow rate during the accelerating phases does not differ from that observed during the decelerating phases. However, for relatively small values of the Shields parameter, the amount of moving particles depends not only on the bottom shear stress but also on flow acceleration. Moreover, the numerical results provide information on the role that turbulent eddies have on sediment dynamics.