Kelvin-Helmholtz turbulence in complex environments

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This project focuses on mixing events that occur in sheared flows close to boundaries and in other complex environments. For example, shear-driven mixing events near the ocean surface flux heat and CO2 into the ocean interior. Previous studies of shear-driven mixing events have assumed that the event is isolated in space and time, i.e. it begins with smooth, laminar flow and is located far from any boundary or other significant flow structure that could impact its evolution. This research will address the effects of spatially proximate influences such as the ocean surface, the bottom, or another nearby mixing event. The main tool will be a sequence of direct numerical simulations of sheared flows with either a boundary or another flow structure close enough to influence the mixing. Results will be used to guide parameterization of turbulent fluxes near the ocean surface and in deep ocean currents such as dense overflows. The results will be useful in understanding dense bottom currents, surface mixed layers and in parameterizing the resulting turbulence. The project will support a postdoc and a graduate student. Both of these junior scientists will learn and use advanced numerical simulation skills. In addition, both will have the opportunity to take part in research cruises to observe shear-driven turbulence.The Kelvin-Helmholtz (KH) ansatz is a common and useful model for ocean turbulence. It is often used to describe turbulence near boundaries, e.g., in near-bottom gravity currents or at the base of a surface mixed layer. While linear theory tells us that boundary proximity can affect the basic length and time scales of KH billows, little is known of the effect on the nonlinear evolution and subsequent mixing. For example, KH billows with lengths up to 300m have been observed within 50m of the surface in the equatorial Pacific. In these cases, boundary proximity is likely to have a large effect on the resulting mixing and on air-sea property transports. In isolation, KH instability becomes turbulent via a well- explored sequence of secondary instabilities. This sequence is sensitive to the details of the initial shear flow, and it has a great influence on the amount of mixing that is ultimately achieved. It is hypothesized that a nearby boundary will alter the sequence of secondary instabilities and thereby change the amount of mixing. Even far from boundaries, e.g. in the thermocline, mixing events occur sporadically and may therefore feel the influence of other, nearby events. Using the petascale machines at UCAR, a sequence of direct simulations will be made of KH events in the presence of boundaries and other flow features. Results will be used to interpret observations of shear-driven mixing events near the surface in the equatorial oceans and to broaden our understanding of the KH instability mechanism as it operates throughout the ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.