Sediment erosion in zero-mean-shear turbulence

Abstract

Turbulence plays an evident role in particle erosion that in many practical situations superimposes with the action of a mean flow. In this paper, the turbulence effect on particle erosion is studied under zero-mean flow conditions, by using the turbulence generated by an oscillating grid. The stirring grid is located more than two mesh size away from the particle layer. The zero-mean flow below the grid has been qualified by revisiting the k–ε model of Matsunaga et al. [Fluid Dyn. Res. 25, 147–165 (1999)]. The turbulence efficiency on the settling/resuspension of the particles is quantified for various turbulence intensities, varying the size, the nature of the particles, and their buoyancy relative to the fluid. We find that the concentrations C of eroded particles collapse fairly well onto a single trend for C ≤ 5 × 10−2, when plotted as a function of the ratio between the flux of turbulent kinetic energy at the particle bed location and the particle settling flux. Above, the concentrations saturate, thus forming a plateau. Particle erosion mechanisms have been investigated in terms of competing forces within an “impulse approach.” Horizontal drag vs friction first leads to a horizontal motion followed by a vertical motion, resulting from vertical drag and lift vs buoyancy. Particle erosion occurs when both force balances are in favor of motion for a duration of 0.1–0.3 Kolmogorov time scale.