Dynamics of complete turbulence suppression in turbidity currents driven by monodisperse suspensions of sediment

Abstract

Turbidity currents derive their motion from the excess density imposed by suspended sediments. The settling tendency of sediments is countered by flow turbulence, which expends energy to keep them in suspension. This interaction leads to downward increasing concentration of suspended sediments (stable stratification) in the flow. Thus in a turbidity current sediments play the dual role of sustaining turbulence by driving the flow and damping turbulence due to stable stratification. By means of direct numerical simulations, it has been shown previously that stratification above a threshold can substantially reduce turbulence and possibly extinguish it. This study expands the simplified model by Cantero et al. (J. Geophys. Res., vol.114, 2009a, C03008), and puts forth a proposition that explains the mechanism of complete turbulence suppression due to suspended sediments. In our simulations it is observed that suspensions of larger sediments lead to stronger stratification and, above a threshold size, induce an abrupt transition in the flow to complete turbulence suppression. It has been widely accepted that hairpin and quasi-streamwise vortices are key to sustaining turbulence in wall-bounded flows, and that only vortices of sufficiently strong intensity can spawn the next generation of vortices. This auto-generation mechanism keeps the flow populated with hairpin and quasi-streamwise vortical structures and thus sustains turbulence. From statistical analysis of Reynolds stress events and visualization of flow structures, it is observed that settling sediments damp the Reynolds stress events (Q2 events), which means a reduction in both the strength and spatial distribution of vortical structures. Beyond the threshold sediment size, the existing vortical structures in the flow are damped to an extent where they lose their ability to regenerate the subsequent generation of turbulent vortical structures, which ultimately leads to complete turbulence suppression.