The surface reflection of the internal wave field emitted by a localized submerged stratified turbulent source

Peter Diamessis, School of Civil and Environmental Engineering, Cornell University

Friday, April 8, 2016
1:30 p.m.

Hopeman 224

The stratified turbulent wake of a towed sphere is a canonical model for geophysical and naval wakes. The coherent vortical structures that form in the intermediate-to-late wake, the stratified turbulence that occurs embedded within them and the associated radiation of internal gravity waves are topics of active ongoing research. This presentation focuses on the surface manifestation of the internal waves (IWs) radiated by the stratified wake of a towed sphere. Results from  Large Eddy Simulations are examined over a range of sphere-based Reynolds number, Re, and Froude number, Fr, values in an idealized linear stratification. Extending to a non-dimensional time of Nt≈300, where N is the buoyancy frequency, the IW field’s horizontal wavelength and wave period, are computed at the sea surface through wavelet transforms of the corresponding horizontal divergence signals. The power laws underlying the temporal decay of the mean observable wavelength suggest that wave dispersion is the dominant process in the far-field, and not nonlinear effects, as predicted by a Re/Fr-independent linear propagation model.  This finding suggests that the most energetic waves observed at the free surface originate from the early-time wake and its adjustment to buoyancy posing open questions to how efficient later-time strongly stratified turbulence may be in radiating energetic IWs. The presentation concludes with a discussion of the local enrichment ratio of model surfactants and its linear increase with IW steepness such that an empirically proposed remote visibility threshold is exceeded at operationally relevant values of Fr. In a similar context, the observed local divergence in lateral mass transport, measured by the Lagrangian drifts of the ocean surface tracer particles is also discussed.