Energetics and mixing in stratified turbulent flows
type: CreativeWork
description: Turbulent mixing has an important influence on many physical processes in the ocean. For example, the upwelling of water through the stably stratified ocean interior, needed to close the global circulation, is only possible through the enhanced vertical mixing resulting from turbulence. The transport of heat, carbon and nutrients is also affected by this mixing. Understanding its spatial and temporal intermittency is therefore vital for determining the regional distribution of these important tracers. Away from boundaries, turbulence in the ocean is commonly attributed to the breaking of internal waves. When an internal wave grows to a large amplitude, it can break due to convective instabilities or shear instabilities. A fundamental understanding of the turbulent transition arising from these instabilities is useful for both interpreting observations and predicting 'hotspots' of turbulence in the field. Focusing solely on canonical instabilities however neglects the wide range of complex flows occurring at small scales in the ocean. Interactions with mean flows or with the background internal wave field can alter the properties of turbulence and the ensuing mixing. In this thesis, we use numerical simulations to investigate three problems aimed at enhancing our understanding of mixing in such stratified flows. We first investigate the development of Kelvin-Helmholtz instability in a stratified shear layer. We quantify the maximum energy extracted from the mean flow by the instability as a function of the Richardson number Ri. By applying forcing to the governing equations, we are able to extend our simulations up to the marginal stability threshold of Ri=1/4, and show that the maximum energy decreases to a small but non-zero value as this threshold is approached. Our next study focuses on mixing in stratified turbulence that is kept in a statistically steady state through large-scale forcing. We compare flows forced by vortical modes with those forced by internal g…
abstract: Turbulent mixing has an important influence on many physical processes in the ocean. For example, the upwelling of water through the stably stratified ocean interior, needed to close the global circulation, is only possible through the enhanced vertical mixing resulting from turbulence. The transport of heat, carbon and nutrients is also affected by this mixing. Understanding its spatial and temporal intermittency is therefore vital for determining the regional distribution of these important tracers. Away from boundaries, turbulence in the ocean is commonly attributed to the breaking of internal waves. When an internal wave grows to a large amplitude, it can break due to convective instabilities or shear instabilities. A fundamental understanding of the turbulent transition arising from these instabilities is useful for both interpreting observations and predicting 'hotspots' of turbulence in the field. Focusing solely on canonical instabilities however neglects the wide range of complex flows occurring at small scales in the ocean. Interactions with mean flows or with the background internal wave field can alter the properties of turbulence and the ensuing mixing. In this thesis, we use numerical simulations to investigate three problems aimed at enhancing our understanding of mixing in such stratified flows. We first investigate the development of Kelvin-Helmholtz instability in a stratified shear layer. We quantify the maximum energy extracted from the mean flow by the instability as a function of the Richardson number Ri. By applying forcing to the governing equations, we are able to extend our simulations up to the marginal stability threshold of Ri=1/4, and show that the maximum energy decreases to a small but non-zero value as this threshold is approached. Our next study focuses on mixing in stratified turbulence that is kept in a statistically steady state through large-scale forcing. We compare flows forced by vortical modes with those forced by internal g…
author: Howland, Christopher James
language: eng
keywords: Turbulence, Fluid Mechanics, Physical Oceanography
license: https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f87d709f-f0f8-441b-8ed4-911404850edd/download
license: https://creativecommons.org/licenses/by-nc-sa/4.0/
date published: 2020-11-28
date modified: 2026-07-22T22:24:01Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.58686