Xinjiang Xiang | University of Southern California (original) (raw)
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In a stable background density gradient, initially turbulent flows eventually evolve into a state... more In a stable background density gradient, initially turbulent flows eventually evolve into a state dominated by low Froude number dynamics and frequently also contain persistent pattern information. Much empirical evidence has been gathered on these latter stages, but less on how they first got that way, and how information on the turbulence generator may potentially be encoded into the flow in a robust and long-lasting fashion. Here an experiment is described that examines the initial stages of evolution in the vertical plane of a turbulent, grid-generated wake in a stratified ambient. Refractive-index-matched fluids allow optically-based measurement of early (N t < 2) stages of the flow, even when there are strong variations in the local density gradient field. Suitably-averaged flow measures show the interplay between internal wave motions and Kelvin-Helmholtz-generated vortical modes. The vertical shear is dominant at the wake edge, and the decay of horizontal vorticity is observed to be independent of Fr. Stratified turbulence, originating from K-H instabilities, develops up to non-dimensional time N t ≈ 10, and the scale separation between Ozmidov and Kolmogorov scales is independent of Fr at higher N t. The detailed measurements in the near wake, with independent variation of both Reynolds and Froude number, while limited to one particular case, are sufficient to show that the initial turbulence in a stratified fluid is neither three-dimensional, nor universal. The search for appropriately-generalisable initial conditions may be more involved than hoped for.
In a stable background density gradient, initially turbulent flows eventually evolve into a state... more In a stable background density gradient, initially turbulent flows eventually evolve into a state dominated by low Froude number dynamics and frequently also contain persistent pattern information. Much empirical evidence has been gathered on these latter stages, but less on how they first got that way, and how information on the turbulence generator may potentially be encoded into the flow in a robust and long-lasting fashion. Here an experiment is described that examines the initial stages of evolution in the vertical plane of a turbulent, grid-generated wake in a stratified ambient. Refractive-index-matched fluids allow optically-based measurement of early (N t < 2) stages of the flow, even when there are strong variations in the local density gradient field. Suitably-averaged flow measures show the interplay between internal wave motions and Kelvin-Helmholtz-generated vortical modes. The vertical shear is dominant at the wake edge, and the decay of horizontal vorticity is observed to be independent of Fr. Stratified turbulence, originating from K-H instabilities, develops up to non-dimensional time N t ≈ 10, and the scale separation between Ozmidov and Kolmogorov scales is independent of Fr at higher N t. The detailed measurements in the near wake, with independent variation of both Reynolds and Froude number, while limited to one particular case, are sufficient to show that the initial turbulence in a stratified fluid is neither three-dimensional, nor universal. The search for appropriately-generalisable initial conditions may be more involved than hoped for.