Direct numerical simulations of spanwise slope-induced turbidity currents in a fine sediment-laden steady turbulent channel: Role of suspended sediment concentration and settling velocity

摘要

Wave- and current-supported turbidity currents (WCSTCs) constitute a subclass of turbidity currents that ubiquitously participate in shaping the marine geomorphology. Rather than the turbulence generated by its forward motion, WCSTCs require boundary-layer turbulence generated by the surface waves and/or currents that are parallel or normal to the shore to keep the sediments suspended. This study focuses on the characteristics of WCSTCs due to shore parallel current only. Such flows can be approximated as particle-laden channel flows with a mild spanwise slope. Due to the spanwise slope, the submerged weight of the suspended sediments creates spanwise force and thus a spanwise current. The resultant flow is affected by two competing mechanisms: (i) additional turbulence generation by the spanwise turbidity current and (ii) turbulence suppression due to suspended sediment-induced stable density stratification. In this study, the role of sediment settling velocity and concentration on these competing processes is investigated by direct numerical simulations. The results of the conducted simulations suggest that turbulence production due to spanwise current is a quadratic function of the suspended sediment concentration, whereas buoyancy dissipation due to sediment-induced stably density stratification linearly varies with the suspended sediment concentration. Turbulent energy distribution among the fluctuating velocity components also suggests that spanwise velocity fluctuations rely less on the turbulence production due to streamwise current. Therefore, a high-concentration turbidity with fine sediment content tends to evolve into a self-sustaining turbidity current, whereas the low-concentration turbidity composed of relatively larger sediments require the current-induced turbulence to sustain its motion. Published by AIP Publishing.