Abstract

I will discuss two problems in fluid dynamics: the collective locomotion of flying animals, and the interactions between a thin extensible membrane and inviscid fluid flow. First, I will present a model of formation flight, viewing the group as a material whose properties arise from the flow-mediated interactions among its members. This aerodynamic model explains how flapping flyers produce vortex wakes and how they are influenced by the wakes of others. Long in-line arrays show that the group behaves as a soft, excitable “crystal” with regularly ordered member “atoms” whose positioning is susceptible to deformations and dynamical instabilities. Second, I will talk about rectangular membranes with deflections that have significant spanwise nonuniformity and that shed a trailing vortex-sheet wake when immersed in a three-dimensional inviscid fluid flow. Examples include textile fabric, rubber, or the skin of swimming and flying animals.