PHYSICS OF FLUIDS, vol.29, no.3, 2017 (SCI-Expanded)
Planar, Boussinesq, compositional gravity currents formed by the release of a fixed volume of heavier fluid from a closed lock and advancing on an inclined no-slip bottom surface in a reservoir with a horizontal free surface are investigated based on 3-D large eddy simulation. The initial region containing the lock fluid has a rectangular shape. Simulations are conducted for bottom slope angles, theta, between 0 degrees and 60 degrees. The running length of the inclined bottom was sufficiently long to allow a detailed study of the evolution, front dynamics and structure of the current during the latter stages of the deceleration phase (front velocity reduces with time). Results show that currents advancing over inclined surfaces with theta > 10 degrees are characterized by the formation of an intensified mixed vortex (IMV) at the back of the head. The IMV forms faster and its coherence, circulation, and size increase monotonically with increasing bottom slope angle. The paper discusses how the buoyancy in the head varies with varying bottom slope angle and with time. In particular, for theta >= 30 degrees, the current reaches a regime where the total buoyancy of the head and IMV is close to a constant and the value of this constant increases with increasing theta. During this regime, the head mainly loses buoyancy to the IMV. For theta >= 40 degrees, a close to linear decay of the head buoyancy with time is observed during the later stages of this regime. Simulation results show that, while for relatively small bottom slope angles most of the sediment is entrained beneath the head, for theta > 20 degrees the IMV has a much larger capacity to entrain the sediment compared to the head region past the initial stages of the propagation of the current. This means that sediment entrainment patterns of currents propagating over highly inclined surfaces are qualitatively very different from the widely studied case of currents propagating over horizontal surfaces. The paper also discusses the different regimes observed in the temporal evolution of the front velocity and the applicability of theoretical models derived based on the data obtained for relatively small bottom slope angles and a relatively short evolution of the current to describe the evolution of currents propagating over large bottom slope angles and/or at large times after the start of the deceleration phase. While it is found that mixing increases monotonically with increasing theta, the largest total kinetic energy for a given front position is observed for theta = 30 degrees-40 degrees. Results also show that the largest magnitude of the bed friction velocity is induced for theta = 30 degrees-40 degrees, which means that the currents with the largest capacity to entrain sediment are those with the largest rate of increase of total kinetic energy with the propagation distance. Published by AIP Publishing.