Current Research

Vortex Pair Dynamics

Vortex pairs are model flows of both practical and fundamental significance. Such flows may occur in the wake of an aircraft and can be considerably hazardous to following aircraft. Knowledge of the rate of decay of these vortices which include the effects of atmospheric conditions such as density stratification and turbulence is critical for air traffic control. There is also fundamental interest in elementary vortex flows; the knowledge of which is prerequisite for understanding the interaction and behavior of vortices in more complex flows, e.g., turbulence. In this work, direct numerical simulations are used to investigate both two- and three-dimensional dynamics of vortex pairs in unstratified and stratified fluid. o
Our study of the three-dimensional dynamics of a counter-rotating vortex pair considers the short-wavelength elliptic instability and the effects of ambient stable stratification.

Our study of the two-dimensional dynamics of co-rotating vortex pairs identifies and characterizes the various vortex interactions and key underlying physical processes. A description of the merging process in both unstratified and stratified flows is developed. We also establish the conditions for vortex merger for the case of unequal vortices.

Turbulent Mixing and Dispersion

An important property of turbulence is its ability to disperse contaminants and mix scalar quantities. Because of its vital importance to air quality, there have been significant efforts to develop effective models of turbulent dispersion for air quality applications. One of the weaker aspects of current dispersion models is their limited ability to predict dispersion in the stable atmospheric boundary layer and there are significant uncertainties in how to model turbulent dispersion in stratified conditions. In this work, direct numerical simulations of stably stratified turbulent flows are performed to study the physics of turbulent mixing, examine existing theories for dispersion in stably-stratified flows, and determine how these theories can be generalized to account for mean shear and intermittent turbulence. Fundamental analytical studies of scalar mixing are also carried out. These efforts will provide a significant step towards modelling dispersion accurately in more complicated flows representative of the stable boundary layer.

Structure and Dynamics of Stratified Turbulent Shear Flows

Stratified sheared turbulence occurs in many environmental and engineering flows. An improved understanding of the associated physics is essential for more accurate prediction of the behavior of these flows and their mixing properties. In this work, direct numerical simulations are used to examine geometric properties and coherent structures and determine their role in the overall dynamics of the flow.

An important aspect of stratified turbulence is overturning and the associated flow structure(s). Density overturns, i.e., regions where heavy fluid resides over light fluid, are considered active sites of stirring and mixing. Full field data from direct simulations is exploited to investigate overturns and vortex structures, their spatial structure, dynamic evolution, and their significance to flow energetics and mixing. Such information contributes to our understanding of the physics of stratified turbulence and may assist in the interpretation of physical measurements, which are typically limited to one-dimensional vertical profiles.

Breaking of Internal Waves in the Ocean

The breaking of oceanic internal waves is an essential part of the deep-ocean mixing processes that contributes to the general circulation of the ocean, the exchange of heat and gases with the atmosphere, the distribution of nutrients and the dispersal of pollutants. The objective of this work is to investigate how these waves propagate, interact and evolve toward breaking and the resultant mixing of momentum, heat, and materials in a realistic ocean environment. A combination of ray tracing techniques and fully nonlinear simulations are used in the study.