THE LONG-TIME EVOLUTION OF THE INITIALLY TURBULENT WAKE OF A SPHERE IN A STABLE STRATIFICATION

Experiments on late wakes (Nt > 20) of towed spheres in a stably strat ified fluid reveal some startling similarities and differences when co mpared with unstratified, 3D wakes. Predicted decay rates stemming fro m 3D, turbulent wake studies are unexpectedly successful in accounting for the decay in fluctuating horizontal velocity components and their spatial gradients, even at late times when the vertical velocity comp onent is almost or exactly zero. On the other hand, the mean wake defe ct velocity is almost one order of magnitude higher than in the unstra tified case. This is due to the increased coherence and organisation o f the patches of vertical vorticity, which are stable, and persist for very long times. A correct accounting for this type of wake structure will be essential in modelling efforts for certain practical ocean ap plications.