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Podcast: Fluids and Flocks

Birds often travel in flocks, and fish in schools — but can physics help us understand why?


Image Credit: Becker, A.D. et al.

From a fluid dynamics perspective, there are actually a few very good reasons.  On today’s podcast, we catch up with Dr. Hassan Masoud, Assistant Professor in the Department of Mechanical Engineering at the University of Nevada, Reno, and co-author of a recent paper on the subject.  Armed with a circular tank containing flappable “wings” (right), computer simulations, and a mathematica model, the researchers set out to discover how members of a flock or school interact with the fluid flow around them to find stable and power-saving modes of mass transit.  As a bird flies or a fish swims, it generates vortical flows that stick around long enough for the next flyer or swimmer to encounter them.  This self-interaction between the flappers and the fluid medium through which they move dictates the spatial arrangements that are most effective for group locomotion.

Depending on the spacing between members of the group, a particular formation can be stable and easy to maintain — or not. In fact, the team found that in some cases, even with the exact same kinematics (frequency and amplitude of flapping), two different group arrangements are stable.  In the "slow mode," the flock or school can save a lot of power by traveling slower than a single, isolated flapper, whereas the "fast mode" still saves a little power for the group when they travel at a higher speed.  All of the nuance of animal behavior aside, it seems fluid dynamics by itself can go a long way toward explaining why birds of a feather flock together.

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