Lock-exchange gravity currents with a high volume of release propagating over a periodic array of obstacles

Abstract

Large eddy simulations are employed to investigate the structure and evolution of a bottom-propagating compositional gravity current in a rectangular horizontal plane channel containing a series of identical large-scale obstacles (dunes and square ribs) at the channel bottom. Simulation results show that below a certain value of the additional drag force per unit streamwise length induced by the bottom obstacles (low drag cases), the gravity current propagating over an array of obstacles transitions to a regime where the average front velocity is close to constant. Past its initial stages, the total kinetic energy, Ek, increases in time proportional to t1/3, where t is the time since release. This behaviour is similar to the slumping phase observed for currents propagating over a flat bed, with the exception that in the latter case the temporal increase of Ek during the later stages of the slumping phase is much faster (Ek ~ t). Simulation results also show that above certain value of the drag force per unit streamwise length induced by the obstacles (high drag cases), the slumping phase can be very short. In this case, similar to currents propagating in a porous medium, the current transitions to a drag-dominated regime in which the front velocity decays proportionally to tβ, with β = −0.28, once the discharge of lock fluid at the position of the lock gate becomes close to constant in time.