Gravity currents propagating over fixed beds of spherical particles

We investigate full-depth, saline lock-exchange gravity currents propagating over densely packed layers of spherical particles by laboratory experiments and direct numerical simulations. Our objective is to explore how the speed and composition of the current is influenced by this macro-rough bed.

Particle Tracking Velocimetry was employed to measure flow fields along the centerline of the flume while light attenuation was used to measure the spanwise averaged density field. Numerically, we solve the three-dimensional Navier-Stokes equations in the Boussinesq approximation, with an immersed boundary method to fully resolve the flow around each particle. The no-flux condition for the salinity at the particle surface is implemented by a volume of fluid approach. We vary the number of particle layers in the bed as well as the current height. For an increasing ratio of particle to current size, the current mixes more strongly with the bed fluid, so that it becomes progressively more diluted and its propagation speed is reduced. Simulations furthermore show that the dense fluid in the gravity current tail triggers a Rayleigh-Taylor instability within the porous bed.