A Review on Fish Swimming and Bird/Insect Flight

TL;DR

This review comprehensively covers the fluid dynamics, biomechanics, and energetics of fish swimming, bird flight, and insect flight, highlighting recent advances, theoretical models, and biological principles underlying animal locomotion.

Contribution

It synthesizes recent theoretical, experimental, and computational studies across aquatic and aerial animal locomotion, providing new insights into energy conservation and propulsion mechanisms.

Findings

Hydrodynamic viscous drag on fish is mainly due to laminar boundary layers.

Nonlinear theories improve understanding of large-amplitude propulsion.

Unsteady aerodynamics explain high lift in insect flight.

Abstract

This expository review is devoted to fish swimming and bird/insect flight. (i) The simple waving motion of an elongated flexible ribbon plate of constant width, immersed in a fluid at rest, propagating a wave distally down the plate to swim forward is first considered to provide a fundamental concept on energy conservation. It is generalized to include variations in body width and thickness, vortex shedding from appended dorsal, ventral and caudal fins to closely simulate fish swimming for which a nonlinear theory is presented for large-amplitude propulsion. (ii) For bird flight, the pioneering studies on oscillating rigid wings are briefed, followed by presenting a nonlinear unsteady theory for flexible wing with arbitrary variations in shape and trajectory with a comparative study with experiments. (iii) For insect flight, more recent advances are reviewed under aerodynamic theory and modeling, computational methods, and experiments, on forward and hovering flights with producing leading-edge vortex to give unsteady high lift. (iv) Prospects are explored on extracting intrinsic flow energy by fish and bird to gain thrust for propulsion. (v) The mechanical and biological principles are drawn together for unified studies on the energetics in deriving metabolic power for animal locomotion, leading to a surprising discovery that the hydrodynamic viscous drag on swimming fish is largely associated with laminar boundary layers, thus drawing valid and sound evidences for a resolution to the fish-swim paradox proclaimed by Gray (1936, 1968).