Flapping wings propelling in a fluid

Sophie Ramananarivo

Applied Mathematics Laboratory

Courant Institute, NYU




Birds and fish both move in a fluid by flapping their wings or fins. Through such cyclic movements, the animal imparts momentum to the surrounding flow and creates a jet that propels it forward. The unsteady flow structures generated, and hence the strength of the jet, are very sensitive to the shape of the wings. This means that, by tuning the wing geometry carefully, propulsive performance can be improved. The following question then arises: what would a fast-flying wing look like? Inspired by the natural mechanism of evolution, we address this search for "good shapes" through a combined experimental and numerical process mimicking natural selection.

In the second part, we consider experimental observations of two swimmers moving independently in close proximity. The tandem pair, otherwise free to assume an arbitrary relative position and forward speed, is shown to spontaneously aggregate and form stable arrangements. Our results suggest that fluid-mediated interaction alone could be sufficient for coordinated motion to emerge in an assembly of individuals, such as bird flocks or fish schools.