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 inters titial fluid, though less than the initial bulk density of the suspension. In this case a gravity current is initiated above the underlying boundary u ntil sufficient particles have sedimented from the flow, at which time the particle-laden fluid becomes less dense than the surrounding ambient. The b uoyancy of the residual suspension reverses and it lifts off the boundary t o ascend through the ambient. Such phenomena are encountered in industrial and natural situations. This study presents a laboratory investigation of f inite volume releases of particle-laden fluid which undergo reversing buoya ncy. A simple box model theory is proposed to describe the flow and to pred ict 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 stu dies. Additionally, we investigate these flows using the shallow-water equa tions which are analyzed using asymptotic series. These reveal the structur e of the internal dynamics within the currents and predict lift-off distanc es which verify the validity of those obtained from the less rigorous box m odel. (C) 1999 American Institute of Physics. [S1070-6631(99)02309-0].