Laminarisation in Flows of Concentrated Benthic Suspensions

Abstract

In quiescent water bodies that form part of estuarine or coastal water systems, such as harbours and access channels, fine sediments may deposit to form concentrated benthic suspensions (CBS). The flow velocities in these mud layers may become low, and the viscosities may become large, so that the Reynolds number decreases below a critical value and a transition from turbulent to laminar flow, designated herein as laminarisation, takes place. In this report laminarisation of CBS, modelled as a Newtonian fluid, is simulated using the Prandtl mixing-length turbulence model supplemented with a modified Van Driest model of low-Reynolds-number flow, and a transition criterion based on the concept of a critical value of the turbulence Reynolds number. This criterion is calibrated with experimental data obtained from the literature. The model is shown to reproduce the vertical distribution of the mean velocity at low Reynolds numbers quite well, Larninarisation in a slowly decelerating layer of CBS is found to take place at a Reynolds number of 1020, where the Reynolds number is defined as the product of mean velocity and layer depth divided by the kinematic viscosity. The inability of mixing-length models of reproducing memory effects, which actually do occur in laminarising flows, and effects of non-Newtonian CBS properties are briefly discussed.