Exploring the Biofluiddynamics of Swimming and Flight

Many organisms must move through water or air in order to survive and reproduce. Therefore both the development of these individuals and the evolution of their species are shaped by the physical interaction between organism and surrounding fluid. One characteristic of macroscopic animals moving at typical speeds is the appearance of vortices, or distinct whorls of fluid.

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These vortices are created close to the body as it is driven by the action of muscles or gravity, then are ‘shed’ to form a wake (in effect a trackway left behind in the fluid), and ultimately are dissipated as heat. It is useful to analyze fluid motion as a collection of vortices, yet the dynamics are complex: vortices interact with the moving organism, interact with each other, and evolve independently in time.

This research examines two flow phenomena that are central to the locomotory performance of certain organisms. The first of these is leading-edge vortex stability. A tornado-shaped vortex has been observed above insect wings, parallel to the leading edge. The leading edge vortex substantially augments lift and is integral to insect flight. Here I consider in detail the conditions that stabilize this flow pattern. The second dynamical phenomenon is vortex wake periodicity.

Depending on conditions, the wake structure behind a moving organism can be regular and predictable, or chaotic. Although the fluid flow is always deterministic, in the latter case its exact structure becomes hypersensitive to small disturbances. This can reduce the ‘forecast horizon’ within which fluiddynamic forces acting on the body can be reliably predicted. Here I describe the onset of chaotic vortex interactions in biologically relevant models, and consider the consequences for feedback-mediated neural control.

These studies were carried out using models that represent swimming fish, flying insects, autorotating plant seeds, and birds. The flow patterns and forces were observed using (in order of increasing realism) a two-dimensional flapping foil in a soap film tunnel, a dynamically scaled three-dimensional robotic fly and seed wing in oil, and freeze-dried swift wings in a wind tunnel. The measurements were designed and understood by means of dimensional analysis: dimensionless parameters can identify the fluid accelerations and stresses that dominate theflow; when mapped as a function of morphological and kinematic variables that produce theflow, they yield an overview of the biofluid-dynamical parameter space. Using this frameworkwe were able to show that: (1) Symmetric and periodic flapping fins and wings can produceasymmetric and chaotic vortex wakes. (2) Rotational accelerations stabilize leading edge vortices on revolving wings of insects and other organisms. (3) Stable leading edge vortices augment liftin both animal and plant flight. (4) Wing morphing in birds drastically improves glide performance.(5) Flapping insect wings are less efficient than spinning and translating insect wings.

This reverse-engineering analysis of biofluidic locomotion has furthermore helped us to forward-engineer two micro-air vehicles. We have designed, built, and flown a robotic flapper (DelFly) and a morphing model swift (RoboSwift). Clearly the formal methods and findings presented here can lead directly to novel technological products.