Mixing in lock-release gravity currents

Abstract

The density front bounding a gravity current is usually highly unstable and this leads to mixing between the fluid of the gravity current and that of the surroundings. This mixing plays an important part in the dynamics of gravity currents and determines how fluid properties are transported and distributed by the flow. In this paper we describe a new experimental technique to determine the density structure of gravity currents, and present some results for the lock-release case. We find that substantial mixing occurs in the early stages of the evolution of lock-release gravity currents and that this mixing leads to the formation of a complex internal density structure. Mixed fluid is produced across the concentration range and we examine the mixing rates. By contrasting three experiments with different lock aspect ratios, we examine the effect of the aspect ratio of the release on the subsequent evolution of the flow. Gravity currents are produced when fluid of one density is released into fluid of a different density at a horizontal boundary. The variation in buoyancy force along the boundary, which is caused by the density difference, produces a horizontal pressure gradient which drives a flow along the boundary. Typically, this flow has the form of a penetrating current bounded downstream by a strong frontal region, called the 'head', and a shallower following flow, called the 'tail' (Simpson, 1987). The simplest situation, and the one we shall consider in this paper, is the lock-release case. This consists of two fluids of different densities contained in a rectangular channel, initially at rest and separated by a vertical barrier. When the