Enhanced understanding on incipient sedimentmotion and sediment suspension through oscillating-grid turbulence experiments

Abstract

Sediment dynamics are usually described in terms of the studies developed under a steady uniform flow, where the hydrodynamic forces are taken those pertaining to the mean time-averaged flow speed. However, the inherent turbulence plays an important role and should be considered implicitly in describing the complexity of turbulence effects in geophysical phenomena. This paper reviews the implementation of isolated turbulence, generated by oscillating grid on two important sediment transport phenomena, i.e., incipient sediment motion and suspension. The generated quasi-isotropic, laterally homogenous turbulence (that is, at a distance further away from the grid) permits an in-depth investigation of the effect of turbulent fluctuations and brings new insights in understanding both phenomena. The critical Shields profile for the incipient sediment motion characterized using the second order of turbulence statistics is qualitatively similar to the Shields curve obtained under a steady uniform flow. In the suspension of particles, there is a two-way interaction between sediment and turbulence. High concentration of suspended particles changes the turbulence structure and the presence of coherent vortices changes the particle settling velocity, which subsequently alters the concentration within the suspension layer. The studies of turbulence on incipient sediment motion and particle suspension provide a better understanding of the underlying physics of sediment behavior at the near-bed region.