Reversing buoyancy of particle-driven gravity currents

Particle-laden flows exhibit reversing buoyancy behavior if the density of the ambient through which they propagate is greater than that of the interstitial fluid, though less than the initial bulk density of the suspension. In this case a gravity current is initiated above the underlying boundary until sufficient particles have sedimented from the flow, at which time the particle-laden fluid becomes less dense than the surrounding ambient. The buoyancy of the residual suspension reverses and it lifts off the boundary to ascend through the ambient. Such phenomena are encountered in industrial and natural situations. This study presents a laboratory investigation of finite volume releases of particle-laden fluid which undergo reversing buoyancy. A simple box model theory is proposed to describe the flow and to predict the distance from the source at which lift-off occurs. The predictions of the model agree well with both our experiments and those of previous studies. Additionally, we investigate these flows using the shallow-water equations which are analyzed using asymptotic series. These reveal the structure of the internal dynamics within the currents and predict lift-off distances which verify the validity of those obtained from the less rigorous box model.