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Center for Nonlinear Studies |
Accurate estimation of small scale diapycnal (irreversible) turbulent mixing is essential for understanding the dynamics of density stratified geophysical flows such as in the oceans and the atmosphere. For example, the small scale fluxes due to mixing impacts the meridional overturning circulation, global climate, ocean heat budget, ocean productivity etc. The effect of small scales is generally parameterized in large scale ocean and climate models in terms of eddy diffusivities. However, despite the prevalence of a number of studies on this topic, a universal parameterization of diapycnal mixing remains elusive due to a number of challenges. In this presentation, I will discuss my work on improved parameterizations of diapycnal mixing in oceanic flows. In this work, we studied this interesting problem through an integration of theoretical knowledge with observational and high resolution numerical simulation data. First, analyses of field data are carried out that are obtained from vertical microstructure profilers. This study suggests that there are probably a number of ways to make inferences on oceanic mixing from measured microstructure data, each subjected to its own specific set of assumptions. A key finding is that the traditional canonical value of constant mixing efficiency does not necessarily hold in different parts of the ocean. Also, the commonly used parameter for the parameterization of mixing are ambiguous and not universal, hence, robust parameterizations based on physical insights are essential. Furthermore, high resolution direct numerical simulations (DNS) are performed that provide an avenue to investigate small scale physics, as DNS resolves all scales of turbulent motions. Based on physical scaling arguments some new insights for inferring diapycnal diffusivity and mixing efficiency are provided and the fidelity is tested with DNS data. The agreement between the proposed formulations and exact mixing using DNS data is remarkable and highlights the strength of the proposed new parameterizations in its ability to predict turbulent mixing in stably stratified geophysical flows. The study also suggest that turbulent Froude number is the most dynamic parameter that governs small scale mixing in stratified flows. However, turbulent Froude number is not readily measurable quantity using current state-of-the-art instruments. With scaling arguments and using high resolution DNS data, it is shown that the turbulent Froude number can be inferred from measurable length scales. This finding will be useful in oceanography for inferring the state of stratified turbulence, i.e. whether the flow is in a strongly stratified regime, or in a weakly stratified regime and thereby facilitate the use of appropriate parameterizations in a universal manner.
Bio: Amrapalli Garanaik has completed her PhD in Civil and Environmental Engineering from Colorado State University in the summer of 2018. She obtained her Master degree from IIT Kanpur, India and Bachelor degree from VSSUT Burla, India, both in Civil Engineering. Her research interests are stratified turbulence, environmental/geophysical fluid dynamics, turbulent mixing, turbulence modeling and numerical simulations
Host: Mark Petersen