Flow-induced bending of disks settling in water*

Interactions between flexible bodies and the surrounding fluid medium are ubiquitous in nature. Examples range from the great trash island of plastic bags in the Pacific Ocean to the swimming of jellyfish. To study such interactions, a widely used model system is the settling of a body under the influence of gravity. Here, we investigate the bending of a thin, freely-falling elastic disk in water as a function of its relative flexibility. The parameters of the disk are fixed to produce a zig-zag, fluttering motion. Non-monotonous changes in the fluttering amplitude are observed as a function of increasing flexibility. We show that, as the disk flutters, it experiences out-of-plane bending that dynamically switches between the leading edge and the sides of the disk. These observations differ from those of rectangular fluttering wings in which only the leading edge deflects.  These findings suggest that flexibility alters the nature of the fluid-disk interactions in a non-trivial manner, that could be potentially exploited for various applications in engineering and bio-related problems.