| Oceanic turbulent wakes can arise in the lee of seamounts and other topographic discontinuities, at the shelf-break, and from the motion of ships, submarines and mobile ocean creatures, the latter ranging in size from plankton to whales. As such, stratified wakes can contribute to ocean mixing affecting local stratification, the distribution of nutrients, dissolved oxygen, and CO2, thus affecting bio-feeding patterns, primary productivity, and global ocean balances.Wakes in stratified fluids embody many of the fundamental processes inherent to stratified flow dynamics that are of interest to this symposium, including: stratified mixing and collapse, internal wave generation, and the appearance of coherent, late-wake vertical vorticity. While all of these fundamental processes were known over twenty years ago, the review by Lin and Pao (Ann. Rev. Fluid Mech. 1979, 11: 317-38), focused on wake turbulence and collapse, giving only passing reference to late-wake eddies. The presence of the wake-generated internal wavefield was not mentioned. Since that time, laboratory and numerical studies have considerably furthered our understanding of these processes.The processes governing the evolution of stratified wakes have been found to be a function of the evolution time scale, which is related to the inherent period of oscillation of the ambient water column, the Brunt-Väsäla frequency. At early times the wake does not ‘feel’ the stratification and thus the wake behaves as though stratification were not present. At intermediate times there exists a ‘non-equilibrium regime’ where the wake collapses and potential energy is converted into kinetic energy. At late times the stratification causes a cessation of vertical motion and a quasi-two dimensional, horizontal motion persists. While wake turbulence results from the turbulent boundary layer on the body, the sources of the internal wavefield are more diverse. Internal waves are generated by displacement of the ambient fluid by the body itself, the overall collapse of the wake, and small-scale turbulent fluctuations within the wake. This review will discuss the processes extant in each of these wake flow regimes, covering advances during the past twenty years, including some recent, hitherto unpublished findings. Recent results include studies on the coupling of the wake flows to the internal wave field at low Froude numbers, where this coupling is quite substantial, and on the nature of the three-dimensional density distributions in the early wake, which pose the initial conditions that drive the intermediate and late wake dynamics (and provide appropriate initial conditions for numerical simulations). Outstanding issues regarding our understanding of the evolution of stratified wakes will also be discussed. |
|
|