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Layering in a Flow with Diffusively Stable Temperature and Salinity Stratification

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  • 1 Hydrosystems Laboratory, Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois
  • | 2 Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, Iowa
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Abstract

Laboratory experiments were conducted to study the formation of layers and interfaces in a fluid stratified with two scalars. Fluid with initially linear, diffusively stable temperature and salinity profiles was stirred using an arrangement of horizontally oscillating, vertical rods. Layers occurred when the density ratio, or the ratio of the contributions of temperature and salinity to the density gradient, was small, but they did not form in similar conditions of turbulence intensity and stratification strength when the density ratio was large. The difference in behavior is ascribed to differential diffusion, or the preferential transport of temperature, which occurred in all of the experiments. Eddy diffusivities were linearly proportional to εa/νN2, where εa is an averaged rate of dissipation of turbulent kinetic energy. The mixing efficiency, computed as the ratio of potential energy change to work input to the system, increased with the density ratio. As previous researchers have found, the Phillips–Posmentier mechanism describes the final layered state but not the initial, evolving states of the system.

Corresponding author address: C. R. Rehmann, Dept. of Civil, Construction, and Environmental Engineering, Iowa State University, 374 Town Engineering Bldg., Ames, IA 50011. Email: rehmann@iastate.edu

Abstract

Laboratory experiments were conducted to study the formation of layers and interfaces in a fluid stratified with two scalars. Fluid with initially linear, diffusively stable temperature and salinity profiles was stirred using an arrangement of horizontally oscillating, vertical rods. Layers occurred when the density ratio, or the ratio of the contributions of temperature and salinity to the density gradient, was small, but they did not form in similar conditions of turbulence intensity and stratification strength when the density ratio was large. The difference in behavior is ascribed to differential diffusion, or the preferential transport of temperature, which occurred in all of the experiments. Eddy diffusivities were linearly proportional to εa/νN2, where εa is an averaged rate of dissipation of turbulent kinetic energy. The mixing efficiency, computed as the ratio of potential energy change to work input to the system, increased with the density ratio. As previous researchers have found, the Phillips–Posmentier mechanism describes the final layered state but not the initial, evolving states of the system.

Corresponding author address: C. R. Rehmann, Dept. of Civil, Construction, and Environmental Engineering, Iowa State University, 374 Town Engineering Bldg., Ames, IA 50011. Email: rehmann@iastate.edu

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