Texas Tech University
PRESIDENTS’ DISTINGUISHED LECTURE SERIES
IN ENGNEERING, SCIENCE, AND MEDICINE
James J. Riley, NAE
PACCAR Professor of Engineering, University of Washington
On the Dynamics of Turbulent/Non-Turbulent Interfaces
in Stably-Stratified Fluids
Abstract
Turbulence in stably-stratified fluids is usually localized, such as when an internal wave breaks down into turbulence, the
mixing layer of the ocean, a stably atmospheric boundary layer, and many other circumstances. Of considerable interest is
the behavior of the turbulence near the interface between the turbulent and non-turbulent regions.
In stably-stratified flows internal waves propagate from the turbulent to the nonturbulent region. These waves, which do not have the usual properties of
turbulence, are rotational. We extend the classical definition of turbulence by
including potential vorticity in its definition; potential vorticity is a quantity which
does not exist in an internal wave region, but which is large in a turbulent
region. We then extend the methodology of identifying a turbulent region in a
stably-stratified fluid, carry out direct simulations of a turbulent wake in stable
stratification, and perform in-depth analysis of the region near the turbulent/nonturbulent interface.
We find, for example, that the thickness of the interface scales with the
Kolmogorov scale, similar to what is found for non-stratified flows. In addition, we
find that there is a region within the wake, but close to the interface, which has
strong properties of internal waves whereas, in the deep interior of the wake,
internal waves appear to be less important. We find that there is a very significant
flux of energy across the interface due to the radiation of internal waves; the rate of
loss of energy in the wake due to internal wave radiation is a large fraction of the
rate of loss due to kinetic energy dissipation. Finally, we examine the wake from a
number of perspectives, including the enstrophy and potential enstrophy, the use of
the rotational/divergent velocity decomposition, the Ozmidov and Kolmogorov
length scales, and the buoyancy Reynolds number (the Gibson number).
Bio:
James J. Riley received his PhD from the Johns Hopkins University in 1972, having worked under the guidance of Stanley
Corrsin. He joined the University of Washington in 1983, where he is now a Professor in the Department of Mechanical
Engineering, and an Adjunct Professor in both the Departments of Applied Mathematics and of Aeronautics and
Astronautics. While on sabbatical at the Joseph Fourier University in Grenoble, France, Professor Riley occupied the
Visiting Chair in Industrial Mathematics. More recently he was a Senior Fellow at the Newton Institute for the
Mathematical Sciences at Cambridge University. Professor Riley’s research interests have included particle dispersion in
turbulent flows, waves and turbulence in stably-stratified and in rotating fluids, boundary layer and shear layer
transition, fluid/compliant surface interactions, and chemically-reacting turbulent flows. Professor Riley is a member of
the National Academy of Engineering, and of the Washington State Academy of Sciences. Among other things he is a
Fellow of the American Physical Society and of the American Society of Mechanical Engineers. He is an associate editor
of the Journal of Fluid Mechanics and of the Journal of Turbulence, and until recently was a member of the editorial
boards of the Annual Review of Fluid Mechanics and of the Applied Mechanics Reviews.
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Seminar type: Presidents’ Distinguished Lecture Series in Engineering,
Science, and Medicine
Where: Livermore 101, When: 10-19-2015, 2:00-3:00 PM