A Numerical Study of a Stably-Stratified Mixing Layer
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In density stratification oceans,frequent activities of internal waves cause directly damaging influence on safety and running of ocean engineering structures.Both academic significance and engineering value exist in the research of interaction of internal waves with structures in stratified fluid.This paper introduces the research trends of interaction of internal waves with structures in stratified fluid by means of classifying the structures and the future trends of this issue are proposed.
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If a fluid's potential density decreases continuously with height then it can support internal waves that, like interfacial waves, move up and down due to buoyancy forces but which are not confined to an interface: they can move vertically through the fluid. This chapter focuses upon the dynamics of small-amplitude internal waves in uniformly stratified, stationary fluid. It also examines some effects of shear in non-uniform stratification, with a more general treatment given in Chapter 6.
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Abstract. The problem on internal waves in a weakly stratified two-layer fluid is studied semi-analytically. We discuss the 2.5-layer fluid flows with exponential stratification of both layers. The long-wave model describing travelling waves is constructed by means of a scaling procedure with a small Boussinesq parameter. It is demonstrated that solitary-wave regimes can be affected by the Kelvin–Helmholtz instability arising due to interfacial velocity shear in upstream flow.
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The present wind tunnel experiment verified streamwise development of internal gravity waves from their spontaneous generation to collapse into turbulence. Strongly stably-stratified mixing layer was realized in which the maximum local temperature gradient reached about 1 100 K/m and the local Richardson number exceeded 0.25. Simultaneous measurements were made on instantaneous temperature and velocity fluctuations in the mixing layer. Energy density levels of quasiperiodic components of temperature and velocity fluctuations below the Brunt-Vaisala frequency increased rapidly downstream and the internal gravity waves having frequency components of 1.0 Hz, 1.8 Hz, 2.8 Hz were confirmed to develop in the streamwise direction. The phase difference between the vertical component of velocity fluctuation and temperature fluctuation approached -π/2. The constitution of fine structure of the internal gravity waves swung in time as they grew downstream. The energy of their low-frequency components were transported toward the higher frequency components through their nonlinear interaction and the internal gravity waves collapsed into turbulence locally in the mixing layer. Then, the stably stratified flow field was rapidly broken down.
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An internal soliton in two-layered stratified fluids has been investigated by means of flow visualization. The relationship between the phase speed of soliton and its amplitude has been obtained experimentally. It has been also found that steady waves form behind the internal soliton traveling alongthe interface of a stratified fluid. The waves are of similar characteristics tothe lee waves which are observed behind a mountain in stratified atmosphericflow.
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When ocean's internal tidal waves “beach” at underwater topography, they transform from more or less linear into highly nonlinear waves that can break with generation of vigorous turbulent mixing. Although most mixing occurs in the half hour around a steep (bottom-)front leading the upslope moving internal tide phase, relatively large mixing also occurs some distance of several tens of meters off the bottom, just prior to the downslope moving internal tide phase and initiated by high-frequency “small-scale” internal waves. Details of this off-bottom small-scale mixing in a stratified natural environment and some of its variability per tidal period are presented here in two case studies using high-resolution temperature observations (61 sensors at 1 m intervals; <10−3 °C precision) at a 969 m deep site south of New Zealand. The observations shed some light on stratified turbulence that is generated in a relatively thick (∼30 m) weakly stratified layer and in the strongly stratified interfaces above and below. The interfacial internal waves generate turbulence with largest dissipation rate and temperature variance at the edge of the upper interface and the weakly stratified layer. When these waves steep nonlinearly, immediate moderate turbulence generation is observed below, throughout the weakly stratified layer. Largest turbulence is generated by 25 m high asymmetric Holmboe overturns.
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