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 stratified fluid reveal some startling similarities and differences when compared with unstratified, 3D wakes. Predicted decay rates stemming from 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 component is almost or exactly zero. On the other hand, the mean wake defect velocity is almost one order of magnitude higher than in the unstratified case. This is due to the increased coherence and organisation of 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 applications.