Turbulent wakes in a non-uniformly stratified environment

Abstract

This study explores the behavior of turbulent wakes generated by a sphere propagating with constant speed in a non-uniformly stratified fluid. The investigation is based on a series of high-resolution direct numerical simulations in which the background stratification is systematically varied. We consider one linear and three nonlinear density profiles and discover that even modest, spatially localized non-uniformities of stratification can profoundly influence the wake dynamics, structure, and evolution. The analysis of microstructure signatures shows that wakes in non-uniformly stratified fluids tend to be more spread horizontally, and internal waves are much stronger than in linear stratification. Simulations performed with Gaussian perturbations are characterized by a vertically asymmetric energy distribution, which is attributed to internal wave reflections from low-gradient regions. Using microstructure decay rates, we estimate the effective persistence period of wakes, showing that it substantially increases with the increasing Froude number. We also find that wakes persist much longer in high-gradient profiles, whereas weak local gradients can substantially reduce the wake longevity.