Confluence of Lock-Exchange Density Currents

Abstract

Density currents represent a broad class of flows driven by gravity acting on a density difference with the ambient environment. The understanding of the underlying mechanisms of density currents and their implications on fluid, species, and sediment transport have been studied extensively by others. Although confluences occur naturally in terrestrial and submarine settings, little attention has been given to understanding the confluence of two density currents. Here, we systematically studied the unsteady flow in confluences and developed a methodology for describing the flows based on bulk properties in pre- and post-confluence density currents. Numerical simulations were conducted with experimental validation in which the effects of the initial density difference and channel depth were studied in a junction. In the junction, the currents accelerated and thickened. However, the postconfluence front velocity and thickness values were lower than the preconfluence values. The front velocity, front thickness, maximum near-bed horizontal velocity, and maximum bed shear stress are presented as functions of a Froude number. For the range of cases tested, the peak and postconfluence front velocities were not dependent on the initial conditions. Unlike the front characteristics, the bed shear stress continued to rise as the current’s combined front continued downstream. Therefore, two reestablishment lengths are presented describing the length needed for the front and, separately, the body to return to a constant-value phase.