The age of a wake

Abstract

This study attempts to quantify the decay rates of stratified wakes in active oceanic environments, characterized by the presence of intermittent turbulence and double-diffusive convection. Of particular interest is the possibility of utilizing standard oceanographic microstructure measurements as a means of wake identification and analysis. The investigation is based on a series of direct numerical simulations of wakes produced by a sphere uniformly propagating in stratified two-component fluids. We examine and compare the evolution of wakes in fluid systems that are (i) initially quiescent, (ii) double-diffusively unstable, and (iii) contain preexisting turbulence. The model diagnostics are focused primarily on the dissipation of turbulent kinetic energy (ε) and thermal variance (χ). The analysis of decay patterns of ε and χ indicates that microstructure generated by an object of D = 0.6 m in diameter moving at the speed of U = 0.02 m/s could be detected, using modern high-resolution profiling instruments, for 0.5–0.7 h. The detection period depends on environmental conditions; convective overturns are shown to be particularly effective in terms of dispersion of microscale wake signatures. The extrapolation of model results to objects of ∼10 m in diameter propagating with speeds of ∼10 m/s suggests that the microstructure-based wake detection is feasible for at least 4 h after the object’s passage through the monitored areas. The overall conclusion from our study is that the measurement of microscale signatures of turbulent wakes could represent a viable method for hydrodynamic detection of propagating submersibles.