LES of lock-exchange compositional gravity currents: a brief review of some recent results

Abstract

Three-dimensional 3-D Large eddy simulation (LES) has become a powerful tool to investigate evolution and structure of gravity currents, especially for cases (e.g., high Reynolds number flows, flows with massive separation) where 3-D Direct numerical simulation using non-dissipative viscous solvers is computationally too expensive. In this paper we briefly review some important results obtained based on high-resolution 3-D LES of bottom-propagating compositional Boussinesq currents in lock-exchange configurations. LES was used to provide a detailed description of the structure of the current, to discuss the role of the large-scale coherent structures, and to predict the evolution of the front velocity over the different stages of the current propagation. Three main types of lock-exchange flows are considered: (1) currents with a high volume of release (HVR) and a low volume of release (LVR) propagating in a channel with a smooth horizontal bed; (2) HVR and LVR currents propagating in a horizontal channel containing a porous layer; and (3) currents propagating in a horizontal channel containing an array of bottom obstacles (2-D dunes and ribs). The simulations are performed using non-dissipative numerical algorithms and sub-grid scale models that predict a zero eddy viscosity in regions where the turbulence is negligible. Experimental data is used to validate LES predictions. LES results show that in most cases the evolution of the front velocity is consistent with that predicted based on shallow-flow theory. LES flow fields are then used to estimate important quantities (e.g., bed friction velocity, sediment entrainment capacity) that are very difficult to obtain from experiments and to understand how the structure and evolution of the current change because of the additional drag induced by obstacles present within the channel or at the channel bed. The paper also discusses how the evolution and structure of the current change as the Reynolds number is increased to values that are relevant for gravity currents encountered in geosciences and environmental engineering applications.