April 8, 2015 Gregory Bewley, Group Leader, Max Planck Institute

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
Gregory.Bewley@ds.mpg.ed
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.