The RevealFlight Project

The RevealFlight project aims at shedding light on the efficiency optimization mechanisms deployed by biological flyers. We will focus on birds, and in particular on migratory birds, which are known to exhibit such efficiency-seeking mechanisms at several levels, while maintaining relatively stable flight conditions, leading to impressive results.

At the level of an individual flyer, biomechanical and aerodynamic factors have long been identified but their interconnection has never been studied in details. On the one hand, the morphology and the neuro-muscular configuration of the bird can be seen as the frame and actuation layer that realize the gait of the bird. On the other hand, this gait is also meant to exploit aerodynamic phenomena at various scales: a bird indeed achieves lift through the unsteady aerodynamics of its flapping wings but also through fine-scale flow control mechanisms by means of feathers and wing compliance. At the level of the flock, it is well known that migratory birds adopt flight formations in which downstream flock members exploit the upwash regions of upstream birds' wakes resulting in efficiency gains. But the mechanisms by which they self-organize to achieve these gains are not fully understood.

Objective

The over-arching objective of the present project is to provide an improved comprehensive understanding of bird flocks flight by capturing the interplay between the physics, the bio-engineering disciplines, and the problematic of control and self-organization.

To that end, RevealFlight proposes to synthesize the flight mechanics of birds into a unified framework, combining biomechanical, sensory, aerodynamic and social interaction models, in order to reproduce the flying gaits and the interactions within a flock. This multi-disciplinary and fully coupled approach brings several advantages. It enforces consistency between the biomechanical stresses and the sustentation forces and enables the unified treatment of flight as a problem that encompasses all the physics ranging from muscle activation to flow turbulence.

Such a framework allows envisioning several advances in the understanding of flight, its metabolic cost and the interplay between actuation and aerodynamics. In particular, we want to understand the efforts developed during the gait, the role of compliance, and when the delayed stall intervenes in the wing stroke. This approach could open perspectives for the design and optimization of artificial flyers. More fundamentally, our work could also provide some insights on the scaling laws that govern flapping flight parameters across animal sizes.

Finally, this approach is brought to the scale of the flock by introducing two additional components: the resolution of the birds' wakes over long distances and the introduction of "social" or game-like dynamics. This will allow the model flyers to interact both aerodynamically and socially, and thence provide a novel perspective on the nature of the interactions needed to trigger collaborations at the group level. A particular attention will be paid to how much efficiency can be gained by simple pairwise reciprocation between flyers.

Methods and models at a glance

A neuro-mechanical model of the birds will be developed, capturing bio-inspired principles both in the wing biomechanics (e.g. structure and compliance) and in its coordinated control (through e.g. a network of coordinated oscillators). The dynamics of this model will be solved by means a multi-body solver and in turn, coupled to a massively parallel flow solver (an implementation of the Vortex Particle-Mesh method) in order to capture the bird's wake up to the scales of the flock. The study of self-organization phenomena and inter- bird interactions will begin on simple conceptual models, and be gradually extended to the comprehensive models developed during the project. It will aim at comparing the efficiency of flocks of selfish flyers with that of flocks in which collaboration takes place, whether implicitly or explicitly.