ofa~s ano oceans ELSEVIER Dynamics of Atmospheres and Oceans 23 (1996) 171-182 The long-time evolution of the initially turbulent wake of a sphere in a stable stratification G.R. Spedding *, F.K. Browand, A.M. Fincham Department of Aerospace Engineering, University of Southern California, Los Angeles, CA 90089-1191, USA Received 29 June 1994; revised 24 January 1995; accepted 9 March 1995 Abstract 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 success- ful 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. 1. Introduction Wakes of axisymmetric bodies in a stable stratification exhibit certain interest- ing properties that distinguish them from their non-stratified counterparts. Among the most well known of these is the propensity for the long time wake to evolve to a state that is characterised by the presence of stable patches of vertical vorticity having large horizontal length scales compared with any vertical structure, and for this apparently stable wake structure-to persist for very long times. These are the 'residual' motions referred to by Lighthill in this volume. The topic of wakes in * Corresponding author. 0377-0265/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0377-0265(95)00414-9