[1] Turbidity currents exhibit fascinating physics as their sustained propagation depends on a tight interplay between the suspended sediments and turbulence. If resuspension dominates over deposition the intensity of the flow will increase, while if deposition dominates the flow turbulence can be completely damped inducing rapid settling of sediments and, eventually, flow extinction. This work explores the phenomenon whereby turbulence in a dilute turbidity current with non-cohesive sediments is abruptly extinguished owing to increased suspended sediment stratification. Three parameters control the flow dynamics: Reynolds number (Re t ), Richardson number (Ri t ) and sediment settling velocity (Ṽ z ). The condition for complete turbulence suppression can be expressed as a critical value for Ri tṼ z . Based on simulations, limited experiments and limited field data, the critical value appears to have a logarithmic dependence on Re t . Citation: Cantero, M. I., M. Shringarpure, and S. Balachandar (2012), Towards a universal criteria for turbulence suppression in dilute turbidity currents with non-cohesive sediments, Geophys.

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.

We study the body of turbidity currents in normal flow conditions by means of highly resolved direct numerical simulations of a homogeneous model. We focus on turbidity currents where the net amount of sediment is held fixed. We consider the sediment to be fine enough that their settling effect is neglected, and in the companion work we consider the effect of settling velocity. We consider five different shear Richardson numbers from 5 to 80. Under normal condition, basal drag and entrainment at the interface precisely balance the momentum supplied to the current from the excess weight of the sediment. The normal flow properties of a turbidity current can be fully characterized in terms of bulk Richardson number Ri and bulk Reynolds number Re. The velocity, concentration, and turbulent kinetic energy profiles take a self-similar shape when the current is at near-normal conditions. We observe the flow to display supercritical features for Ri ⪅ 0.4 and to display subcritical features for Ri ⪆ 0.7. From the behavior at intermediate Richardson numbers it appears that the transition between subcritical to supercritical behavior is not sharp. We observe good agreement between experimental and simulation results in both regimes. The entrainment coefficient as a function of bulk Richardson number at normal condition is in good agreement with the empirical relation and with available experimental results. We present a simple model for drag coefficient as a function of bulk Reynolds and Richardson numbers.

Long-running gravity currents are flows that are submerged beneath a deep layer of quiescent fluid and they travel over long distances along inclined or horizontal surfaces. They are driven by the density difference between the current and the clear ambient fluid above. In this work we present results on highly resolved direct numerical simulations of turbid underflows that involve nearly 1 billion degrees of freedom. We assess the effect of bed slope on the flow statistics. We explore the turbulence dynamics of the interface in the classical sub- and supercritical regimes. We investigate the structure of interfacial turbulence and its relation to the turbulence statistic. A transcritical regime is identified where intermittent cascading interfacial instabilities appear. We investigate how departure from the self-sustaining equilibrium state may be the mechanism responsible for this cyclic evolution of the transcritical regime.

Turbidity currents are sediment-laden flows that move over a sloping surface. These flows are driven by the density difference between the sediment-laden current and the clear ambient fluid above. Here we focus on studying the process of ambient fluid entrainment at the interface between the current and the sediment-free ambient fluid and obtain a closure model for the entrainment coefficient. In particular, we extend the classical dependence of entrainment coefficient on the bulk Richardson number, by including the effect of sediment settling velocity. We focus our attention to turbidity currents under normal flow condition, where the depth-averaged streamwise velocity U is nearly a constant. We employ direct numerical simulations of temporally evolving turbidity currents. Similarly, we study the dependence of basal drag distribution on Richardson number and settling velocity. In addition, we study the turbulence structures of the flow and its relation to turbulence production.