Plume Entrainment in Stratified Flows

Abstract

The work presented here consists of an experimental and numerical study of the entrainment processes occurring when a low momentum, two dimensional turbulent plume interacts with a sharp density interface. The overall behaviour depends on the relative local densities of the plume centre and the interfacial density step. For very high density gradient interfaces, the plume impinges on the interface with little direct entrainment and spreads laterally; as the density step is reduced, there is more and more direct entrainment, with lateral intrusions adjusting at their density levels. The experiments are analyzed using an array of micro-conductivity probes with a traversing mechanism that allows the evolution of the density profiles to be calculated. Visual methods are also used, and a computer-controlled video and digitizer determines the average interfacial spread and other geometrical parameters of the process. The numerical model solves, via Large Eddy Simulation (LES) the two dimensional Boussinesq equations in a box comparable with the experimental apparatus, the density differences are adjusted in a similar way as in the experiments. The global features of the flow agree with the experimental data, even if a simple turbulence parameterization is used.