Entrainment in turbulent gravity currents

Abstract

LABORATORY gravity currents are frequently used to model a range of environmental and industrial flows¹. The manner in which these flows become diluted with distance by the surrounding fluid has important implications for turbidity currents², pyroclastic flows^3,4, avalanches⁵, accidental dense gas releases⁶, fire propagation⁷ and emission of industrial pollutants. Here we present an experimental technique for quantifying the entrainment of ambient fluid into the head of a gravity current propagating along a horizontal surface. The technique relies on the neutralization of an alkaline current by entrainment of acidic ambient fluid, and is visualized by using a pH indicator. Dimensional analysis indicates that the proportion of ambient fluid entrained into a gravity current head depends only on the initial volume of the current and distance from the release point, and is independent of the initial value of the density difference. This result is confirmed by the experimental data, which also show that little dilution of the head takes place during the slumping phase^8,9. Thereafter the dilution increases with the downstream distance, in quantitative agreement with the predictions of a theoretical model which evaluates the volume of entrained fluid. We apply the results to show that sediment slumps of initially high sediment concentrations will become dilute turbidity currents owing to entrainment of sea water before they have propagated extensively over the floors of ocean basins.