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PRESENTATION: Universal Vortex Formation Time of Flapping Flight

The profound impact of lift force on human travel is mirrored by its vital role in animal locomotion. A recurring feature in animal flight and swimming is a roll-up of free shear layers into swirling vortical structures. Biological flyers periodically flap their appendages to generate aerodynamic forces. Extensive studies have made significant progress in explaining the physics behind their propulsion in cruising by developing scaling laws of their flight kinematics. Notably Strouhal number (St; ratio of flapping frequency times stroke amplitude to cruising speed) has been found to fall in a narrow range for animal cruising flights. However, St exhibits strong correlation to flight conditions; as such, its universality has been confined to preferred flight conditions. Since the leading-edge vortices (LEV) on flapping appendages generate the majority of propulsive forces, here we take the perspective of LEV circulation maximization, which generalizes the dimensionless vortex formation time to flapping flight. The generalized vortex formation time scales the duration of vorticity injection with the rate of total vorticity growth inside the LEV and the maximum vorticity allowed inside it. By comparing the new scaling with St of previously reported animal cruising flights of 28 species, we show that the generalized vortex formation time is consistent across different animals and cruising locomotion, independent of flight conditions. This finding advances the fundamental principles underlying the complex wing kinematics of biological flyers, pointing to potentially a strong evolutionary selection pressure for animals to maximize the LEV circulation. Moreover, the generalized vortex formation time provides unifying framework for understanding bio-locomotion.

PRESENTER: Dr. Chris Roh received his BS in Biological Engineering and Computational Biology from Cornell University in 2012 and his M.S. and Ph.D. degrees in Aeronautics from the California Institute of Technology in 2013 and 2017, respectively. After completing a one-year postdoctoral fellowship at Caltech, he worked as a Research Engineer and Lecturer at Caltech before returning to Cornell University as an Assistant Professor in the Biological and Environmental Engineering Department. At Cornell, Dr. Roh explores the fascinating ways insects interact with their fluid environment and how they use fluids as a medium for communication. His research program has since expanded to include plants, studying how fluid flows influence ecological and agricultural systems. Dr. Roh is a recipient of the NSF-GRFP award, the Richard B. Chapman Memorial Best Hydrodynamic Research Award, Cornell CALS Early Career Faculty Teaching Award, and the NSF-CAREER award.

FACULTY HOST: Dr. Deema Totah, Mechanical Engineering Graduate Seminar, Mechanical Engineering Department, University of Iowa