November 20
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12:00 PM
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1:00 PM
The Midwest Mechanics Seminar Series is a weekly seminar given by campus and visiting speakers on topics across the spectrum of mechanics research (solids, fluids, and dynamics). Professor Filippo Coletti is a professor at ETH Zurich.
Presentation Title: Turbulence scaling laws under wind-driven wave
Abstract: Below wind waves, a boundary layer develops with similarities and differences with respect to the classic wall-bounded turbulence. This so-called drift layer features significant shear driven by the tangential stress from the wind, but also orbital motions that periodically stretch, compress and tilt vortical structures. Under such conditions, scaling laws for the turbulent sub-surface flow are not trivial to formulate, because the assumptions behind established theories are questionable. Such relations are also difficult to test experimentally: measurements are challenging, and the mean current, orbital motions and turbulent fluctuations are not easy to disentangle. Here we use data from extensive campaigns in a wind-water tunnel where particle image velocimetry, laser induced fluorescence, and infrared thermography are used to characterize the fluid dynamics below young wind-driven waves. This allows us to test the Prandtl-von Kármán theory of the mean velocity profile and the Kolmogorov theory of the velocity structure function in the drift layer. The logarithmic profile predicted by the Prandtl-von Kármán theory is retrieved only under wave crests, as shown by phase-locked averaging of the surface-parallel velocity. In the other wave phases, the momentum is redistributed in approximate agreement with linear wave theory. The Kolmogorov theory does apply in the inertial range of the turbulent fluctuations, but only if the latter are appropriately separated from the orbital motions. The Reynolds stresses, on the other hand, are profoundly different compared to those in a solid-wall boundary layer with similar friction velocity, and can be compared with the predictions by the Craik & Leibovich theory of Langmuir circulations
Bio: Filippo Coletti is Professor of Experimental Fluid Dynamics at ETH Zurich, where he has been since 2020. Previously he was McKnight Land-Grant Professor of Aerospace Engineering & Mechanics at the University of Minnesota, which he joined in 2014. He performed his doctoral studies at the von Karman Institute for Fluid Dynamics and at the University of Stuttgart, where he obtained his PhD in 2010, and was postdoctoral fellow at Stanford University between 2011 and 2013. He received the CAREER award in 2015 from the US National Science Foundation and the Consolidator grant in 2022 from the European Research Council. He is a fellow of the American Physical Society and associate editor of Journal of Fluid Mechanics. He founded and co-organizes the Fluid Mechanics Tour of the Alps, an itinerant seminar series featuring the world’s top fluid mechanicians. His research interests focus on multiphase flows, which he studies with a range of experimental approaches and with applications to environmental, biomedical, and industrial problems.