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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: When a turbulent vortex is near a thin, stratified interface, the entrainment rate across the interface depends on three conventional parameters: the Richardson, Reynolds, and Schmidt numbers (Turner 1973). However, recent experiments have revealed that these parameters by themselves are not sufficient to determine the entrainment rate. For example, a horizontal jet parallel to and below the interface entrains at a rate proportional to Ri −3/2 (Schneider 1980), while the same jet, operating at identical values of these parameters but oriented to impinge vertically on the interface, entrains at a rate proportional to Ri −1/2 (Cotel 1995). Therefore, an additional parameter must be necessary to determine the entrainment rate. This paper presents the arguments for one particular candidate.
    No preview · Chapter · Jan 1996
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    ABSTRACT: A new model is proposed for the entrainment rate by vortices across stratified interfaces. In the model, different entrainment regimes are distinguished by the conventional parameters Richardson, Reynolds, and Schmidt number as well as a new parameter, the “vortex persistence”. Vortex persistence is defined as the number of rotations a vortex makes during the time it moves its own diameter with respect to the interface. It is further proposed that the concept of vortex persistence is important whenever a vortex is near any kind of surface, either stratified or solid. The model is in accord with most field and laboratory observations in a variety of stratified and bounded flows, including measurements of wall heat transfer and vortex formation in starting jets.
    No preview · Article · Sep 1996 · Applied Scientific Research
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    ABSTRACT: Using laboratory water-tank experiments, detrainment from a vertical jet rising through a density interface is explored. The experiments reveal that detrainment only occurs for a specific range of two governing parameters, the Richardson number associated with the vortices 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. Detrainment is only observed when the penetrating jet consists of fluid with two different densities.
    No preview · Article · Oct 1997 · Journal of Geophysical Research Atmospheres
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