At some critical Reynolds number (Re), animals switch from flapping or undulatory locomotion to ciliary, flagellar, and other low Re methods of locomotion. It has been suggested that below a critical Re, flapping appendages can no longer generate enough thrust to swim or fly efficiently. Previous work from our group has shown that below a similar Re, the relative lift forces produced during flapping flight decrease while drag forces increase significantly. These transitions can be connected to a change in the behavior of the vortex wake behind the flapping appendage. For Reynolds numbers around 1 and below, organisms typically move using cilia and flagella.
APS DFD Movie on the Aerodynamics of Tiny Insect Flight
One focus in the group is to understand the aerodynamics of flight in the smallest insects. In ‘typical’ insect flight, lift is produced when a leading edge vortex is formed and remains attached to the wing, and a trailing edge vortex is formed and separates from the wing during each stroke. For tiny insect flight, neither the leading or trailing edge vortices separate from the wing during the duration of each stroke. This vortical ‘near’ symmetry is responsible for the reduced lift forces generated. It appears that very small insects have developed a trick known as ‘clap and fling’ to augment lift production by reintroducing vortical asymetry. This occurs when the wings are clapped together and peeled apart at the beginning of each downstroke. Two large leading edge vortices are formed on each wing, and no trailing edge vortices are formed initially, resulting in increased lift. There is a very large cost in terms of drag at these low Reynolds numbers, however. Very large forces are required to intially fling the wings apart, greatly reducing the aerodynamic efficiency of flight. Recent work from our group suggests that wing flexibility and wing bristles might reduce the drag forces required to clap and fling while maintaining the lift augmenting effects.
3D Immersed Boundary Simulation of Low Re Flapping Wings