Weekly Seminar: Fall 2012
Speaker: Gregory Bewley (Max Planck Institute, Germany)
Title: "Experimental Explorations in Turbulence"
Date: Friday, September 21, 2012
Time: 11:00 a.m.
Location: Gilman Hall 50 (Marjorie M. Fisher Hall)
Fluid turbulence is fascinating in part because it is chaotic and therefore seems naturally resistant to organization. I present a series of experiments in turbulence motivated by two issues: the decay rate of freely decaying turbulence and the collision rate of droplets in a turbulent flow. The approach to these issues led, in ways that I will explain, to several discoveries that are interesting in their own right, and which capture generic features of turbulence. For example, small particles in liquid helium stick to a particularly simple kind of vortex, a quantized vortex. This adhesion of the particles to the vortices made observation of quantized vortex dynamics, including vortex reconnection, possible for the first time. A new kind of active grid demonstrates in a wind tunnel that long-range correlations between velocity fluctuations can be dynamically imprinted into turbulence. I will also show how the anisotropy in these correlations is connected to the anisotropy in the amplitudes of the velocity fluctuations. These findings may point a way toward understanding the decay rate of turbulence. Finally, an experiment that mimics cloud conditions shows that droplets experience the so-called "sling effect." The sling effect causes droplets to break free from the turbulence and to fly toward and through each other. This new mechanism by which turbulence can increase the collision rate of the droplets may explain the formation of rain drops in clouds. Yet the original, simple, questions remain unanswered: what is the decay rate of turbulence, and what is the collision rate of droplets in a turbulent flow?
Gregory Bewley is a group leader at the Max Planck Institute for Dynamics and Self-Organization, where he performs experiments to uncover the generic aspects of turbulence, and to quantify the deviations from these generic aspects that arise in real flows. The experiments include a large pressurized wind tunnel that reaches higher Reynolds numbers under controlled conditions than previously possible, a classic wind tunnel fitted with a new kind of active grid, and a new jet facility to study the Reynolds number dependence of turbulence statistics. In addition, he studies droplet dynamics in turbulence with two "soccer ball" facilities that he invented. Gregory received his bachelor's degree in mechanical engineering from Cornell University. He was awarded a PhD from Yale University for observing inertial waves in rotating turbulence using liquid heluim, and for discovering how to visualize quantum vortex dynamics in superfluid helium, while under the supervision of professor K.R. Sreenivasan. He continued these experimental investigations as a postdoctoral associate at the University of Maryland, before joining the Max Planck Institute.