Entrainment and detrainment of a jet impinging on a stratified interface
ABSTRACT Thesis (Ph. D.)--University of Washington, 1995 The entrainment rate and ratio of a vertical jet impinging on a stratified interface are measured in water tank experiments. The lateral vortex, formed at the sides of the impingement dome, is largely responsible for the entrainment and the mixing of upper fluid into the lower layer. At low Richardson number, both the entrainment rate and ratio (the ratio of upper to lower fluid in the mixed fluid) decline with increasing Richardson number, approximately as the inverse square root. At a Richardson number of about ten and Reynolds number of 2500, the entrainment rate suddenly drops to a much lower value, which is constant for larger values of Richardson number at that constant Reynolds number. From these experiments, a model is proposed for the entrainment rate of vortices near a stratified interface. In the model, a new parameter, the "vortex persistence", distinguishes between different entrainment regimes. Vortex persistence is defined as the number of rotations a vortex makes during the time it moves its own diameter. This new model of stratified entrainment is in accord with most observations in a variety of flows over a wide range of parameter values. To further test this model, pulsed jet experiments were done to look at the transition from a persistent to a non-persistent regime.The opposite of entrainment is detrainment. It was observed in the laboratory that detrainment from a vertical jet rising through a density interface only occurs for a specific range of two governing parameters, the Richardson number and the normalized interface height. The shape of the detrainment domain is explained with simple physical arguments involving velocity gradients, baroclinic torques between the pure unmixed fluid and the environment, and baroclinic torques between the mixed fluid and the surrounding fluid.