Properties of the Body of a Turbidity Current at Near‐Normal Conditions: 1. Effect of Bed Slope

Abstract

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.

Modeling of Breaching-Generated Turbidity Currents Using Large Eddy Simulation

“…To reduce the computational time, grid clustering was used in x-direction; a width of 2 mm was used for the cells in the first 1.5 m, while the width of the remaining cells was gradually increased with a growth rate of 1.04 with an upper limit of 5 cm. The cell dimensions in the y and z directions were kept constant with a value of 2 mm and 0.5 mm, respectively (leading to maximum ∆z + = 15 for the first velocity point located at 1 2 ∆z). The average computational time of the runs presented in this study was about 4 days.…”

“…Turbidity currents are buoyancy-driven underflows generated by the action of gravity on the density difference between a fluid-sediment mixture and the ambient fluid. The excess hydrostatic pressure within the turbidity current drives the current downstream while complicated interactions with the surrounding environment take place; it interacts with the ambient fluid at the upper boundary and with the bed at the lower boundary, producing turbulence at both boundaries [1]. Turbidity currents are vital agents of sediment transport that deliver sediment from the river mouths to deeper waters [2].…”

Properties of the Body of a Turbidity Current at Near‐Normal Conditions: 2. Effect of Settling

“…As in the companion work (Salinas et al, 2019), we perform the simulations in two stages. In the first stage, we perform a simulation of a turbidity current with a roof (denoted as "TCR"-see Cantero et al, 2009) over a bed inclined at an angle .…”

“…In the second-stage simulation the top boundary condition is enforced atz = 12 (see Figure 1b in Salinas et al, 2019), while the top of the turbidity current, which is the interface between the current and the ambient fluid above, is allowed to evolve naturally without any imposed boundary conditions. In the sections that follow, an overbar (·) denotes mean value obtained by averaging over a horizontal plane (x −̃) and over five realizations.…”

“…Our investigation in Salinas et al () is, however, restricted to the limit of very fine sediments, whose settling velocity can be ignored when compared to the friction velocity of the flow. In this limit, the only two parameters that characterize an instantaneous depth‐averaged state of the flow are the bulk Richardson and Reynolds numbers of the flow ( Ri and Re ).…”