The evolution and decay of
initially-turbulent wakes in a stably-stratified
environment

Geoff Spedding, University
of Southern California

Turbulence in both natural and engineering applications is frequently intermittent in space and time, and decays under the influence of the background stratification. The particular case of the wake generated by a towed sphere can be used as a canonical reference case, and it has long been known that the late wake is characterized by large coherent structures --the 'pancake eddies'. This is true for a broad range of internal Froude number, up to O(10) (where increasing Froude number denotes a decrease in the importance of buoyancy forces).

Recent experimental work shows that the persistent, stable vortex structures emerge from the initial turbulence even at Froude numbers above 200. The apparent lack of dependence on Froude number is at first surprising, but can be understood in terms of the time history. Eventually, regardless of the initial energy in the turbulent motions, the buoyancy forces will become important, and the stratified flow will be qualitatively and quantitatively different from its counterpart in a homogeneous fluid.

These wake characteristics will be described, and will lead to quite general conclusions concerning the evolution of stratified turbulence. Since most of the data are in isopycnal planes, details of the vertical structure must be inferred with some care. Nevertheless, certain of these vertical plane measurements can be used to explain interactions of the wake with preexisting background motions that are more representative of real oceans and atmospheres.

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