The motion of a buoyant cloud along an incline in the presence of boundary mixing

The effects of boundary mixing on the transport of a buoyant cloud along an incline were investigated using a laboratory experiment. A fixed volume of a negatively buoyant substance (a suspension of aluminium particles or a saline solution) was introduced onto a rough inclined plane, which was submerged in homogeneous water and could make planar oscillations so as to produce turbulence. At weak turbulence intensities, the buoyant input flowed down the slope as a well-defined gravity current. As the intensity of turbulence became stronger, however, the buoyant input diffused away into the interior of the fluid. The frontal velocity of the gravity current, the density structure and the growth of the thickness of the buoyant cloud were measured, and their dependencies on the intensity of turbulence at the boundary were determined. A criterion which predicts the transition from gravity-current-dominated transport to turbulent-diffusion-dominated transport was found. Scaling arguments were developed to explain the experimental results.