SEMINAR Department of Aerospace Engineering Droplet dynamics and the decay of turbulence Wednesday, April 8, 2015, 4:00 p.m. | HRBB 702 Gregory Bewley Group Leader Max Planck Institute for Dynamics & Self-Organization [email protected] ABSTRACT Fluid turbulence is fascinating in part because it seems naturally resistant to organization. With modern instrumentation, we have great control over the properties of turbulence generated in the laboratory and great resolution in the measurement of the properties. I describe experiments motivated by two issues: the rate of kinetic energy consumed by turbulence and the rate of sedimentation of droplets in a multiphase turbulent flow. In a wind-tunnel experiment that reached higher Reynolds numbers than ever before and covered more than two orders of magnitude in the Reynolds number, we measured the decay rate of turbulence with unprecedented precision. Surprisingly we do not generally know how quickly turbulence decays, though the process underlies general turbulence phenomena and modeling. What we found in this experiment is that the Reynolds number played no role in setting the decay rate when it was high enough. A separate experiment mimics an atmospheric cloud and combines techniques for precisely controlling turbulence within the cloud with techniques that track in three dimensions multiple cloud droplets at very small scales. We found that nonlinear effects probably cause droplets to fall on average more slowly through turbulence than through still air. Conversely, droplets of the right size resonate with the turbulence and fall on average more quickly. Yet the original, simple, questions remain unanswered: what controls the rate at which turbulence dissipates and how quickly droplets settle out of a turbulent cloud? BIO Gregory Bewley is a group leader at the Max Planck Institute for Dynamics and Self-Organization, where he performs experiments to uncover generic aspects of turbulence, both in its intrinsic properties and in its role in environmental settings. In clouds, turbulence causes droplets to collide. In superfluid helium, it causes quantized vortices to reconnect. Left to itself, the turbulence slowly dissipates and disappears. He has contributed to making these phenomena accessible experimentally through novel interpretation of experimental data and the invention of new devices and techniques. Greg received his bachelor’s degree from the mechanical engineering department at Cornell University in 2000. He was awarded a PhD from Yale University in 2006 for discovering how to observe quantized vortex dynamics experimentally. He continued this work at the University of Maryland before joining the MPI-DS in 2007.