Disk wakes in nonlinear stratification

TL;DR

This study uses large eddy simulations to analyze how nonlinear stratification, especially pycnoclines, affects wake turbulence, wave trapping, and internal gravity waves generated by a moving disk at Reynolds number 5000.

Contribution

It provides new insights into wake behavior in nonlinear stratification, including effects of pycnocline positioning and nonuniform density profiles on turbulence and wave dynamics.

Findings

Nonuniform stratification increases wake turbulence compared to linear cases.

Wake defect velocity decays faster when the disk is outside the pycnocline.

Internal gravity waves and Kelvin wake waves are significantly affected by pycnocline shift.

Abstract

Nonlinearity of density stratification modulates buoyancy effects. We report results from a body-inclusive large eddy simulation (LES) of a wake in nonlinear stratification, specifically for a circular disk at diameter-based Reynolds number ($Re$) of $5000$. Five density profiles are considered; the benchmark has linear stratification and the other four have hyperbolic tangent profiles to model a pycnocline. The disk moves inside the central core of the pycnocline in two of those four cases and, in the other two cases with {a} shifted density profile, the disk moves partially/completely outside the pycnocline. The maximum buoyancy frequency ($N_{max}$) for all the profiles is the same. The first part of the study investigates the centered cases. Nonuniform stratification results in increasing wake turbulence relative to the benchmark owing to reduced suppression of turbulence production as well as the wave trapping in the pycnocline. Steady lee waves are also quantified to understand limitations of linear theory. The second part pays attention to the effect of a relative shift between the pycnocline and the disk. The wake defect velocity decays faster in the cases with a shift. The effect of disk location on the Kelvin wake waves (a family of steady waves within the pycnocline) and its modal form is obtained and explained by solving the Taylor-Goldstein equation. The family of unsteady internal gravity waves that are generated by the wake is also studied and the effect of disk shift is quantified.