This paper is devoted to an experimental characterization of the collapse under buoyancy forces of the turbulent motions in the wake of a sphere in a linearly stable saline stratification. This phenomenon is part of the transition between two commonly recognized regions of the wake, the near wake and the far wake. It is shown here that the wake evolves in four distinct stages: the three-dimensional near wake, the collapse, a transition region, and finally the far wake. Hot-film measurements in the near wake and the region beyond show that initially the mean defect velocity decreases as in a nonstratified fluid, but then increases briefly before continuing to decrease again. The short period of mean defect-velocity increase or mean-flow acceleration characterizes the collapse and results in the higher levels of kinetic energy found in stratified wakes as compared to nonstratified wakes. The far-wake region of the flow was studied by a digital particle image velocimetry technique yielding decay laws for the defect velocity as well as the vertical component of vorticity for both laminar and turbulent wake regimes. The decay of these far-wake quantities was found not to depend on the regime considered, implying that the decay rates are independent of the collapse. However, it was found that the characteristic longitudinal scales do depend on the Froude number, implying a selective memory of the initial conditions in spite of collapse. (C) 2002 American Institute of Physics. [References: 24]
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