Entrainment and detrainment of a jet impinging on a stratified interface

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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.

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    ABSTRACT: The aim of the present paper is to understand the interaction between a rising thermal and an inversion in the atmosphere and to quantify the turbulent entrainment rate due to a thermal impingement on a stratified interface. This problem was simulated in the laboratory using a water tank. The thermal is created by releasing a small volume of buoyant fluid into a stratified environment composed of two layers of different densities. A thin interface separates the lower layer from the lighter upper layer. The entrainment of upper layer fluid into the thermal is investigated using a passive dye flow visualization technique. The entrainment rate is found to obey a Ri-3/2 power law, as predicted by Cotel and Breidenthal [1997]. The effect of simulated evaporative cooling on the entrainment of a thermal impinging on a stratified interface is also investigated experimentally. Evaporative cooling in atmospheric clouds is simulated in the laboratory using alcohol-water mixtures, so that the mixed fluid is denser than either parent parcel. This is realized in the laboratory by releasing a mixture of ethyl alcohol and ethylene glycol in an aqueous solution. It rises first through a relatively dense lower layer fluid and then impinges on a thin stratified interface, above which is a layer of relatively light fluid. The entrainment rate for values of the buoyancy reversal parameter D* between 0 and 0.5 was found to obey a Ri-3/2 power law. The entrainment rate is independent of D* between 0 and 0.5 for a range of Richardson numbers Ri from 3 to 25. This is consistent with the behavior of the buoyancy-reversing thermal in an unstratified environment observed by Johari [1992].
    Journal of Geophysical Research Atmospheres 01/2000; 105:15457-15468. · 3.44 Impact Factor