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The Department of Mechanical, Materials, and Aerospace Engineering presents its MMAE 593 seminar series featuring guest speaker Andrew Bragg, associate professor of civil and environmental engineering at Duke University. Bragg will present “What Kind of Turbulence Do Rising Bubbles Generate?” This seminar is open to the public and will take place on Wednesday, February 25, from 12:45–1:45 p.m. in room 104 of the Rettaliata Engineering Center.

Abstract

Standard Turbulent Flows (STF) are those where energy is injected into the flow at large scales, and on average cascades down to the small scales where it is dissipated. Kolmogorov developed a theory to predict the statistical moments of Eulerian velocity increments in STF which is accurate for low-order moments but is inaccurate for high-order moments due to its neglect of the phenomena of intermittency. Another interesting and important class of flows involve swarms of bubbles that generate turbulence as they rise through the liquid in which they are dispersed. A crucial difference with STF is that now the energy is injected into the flow at the small scale of the bubble, and due to this the resulting Bubble Induced Turbulence (BIT) may have properties very different from STF. Using state-of-the-art experiments (performed by collaborators at Helmholtz-Zentrum Dresden-Rossendorf) that simultaneously track the motion of finite-size deformable bubbles and tracer particles in a water column, we explore this issue in detail. We show, for the first time, conclusive experimental evidence, that below the scale of the bubble, Kolmogorov’s theory applies for low order moments of the Eulerian velocity increments even in BIT, just as for STF. We discuss how this can be reconciled with previous results in the literature that claim that in BIT one should instead observe “pseudo-turbulence”. However, when we analyze the Lagrangian velocity increments in BIT we find behavior that is radically different from STF. This difference is shown to be due to extremely intense small-scale vortices that are generated at the edges of the bubble wakes which are absent in STF. These results provide fundamental insights into the ways in which BIT flows are similar and dissimilar to the turbulent flows that have been classically studied.

Biography

Andrew Bragg’s intellectual interests and passions revolve around the desire to understand and predict the beautifully complex, enigmatic motion of turbulent flows, and their role in natural and engineered systems. The environment provides one of the richest settings motivating research on this topic, with far reaching implications for understanding atmospheric clouds, oceans, global warming, among many others. Before joining the Duke University faculty, Bragg was a postdoctoral associate in the Applied Mathematics and Plasma Physics Group at the Los Alamos National Laboratory. Prior to that, he was a postdoctoral associate in the Sibley School of Mechanical and Aerospace Engineering at Cornell University. Bragg obtained his Ph.D. in theoretical fluid dynamics from Newcastle University in England. His work has been recognized by an NSF CAREER Award and a EUROMECH Young Scientist Award.