Unsteady high-lift mechanisms from heaving flat plate simulations

Flapping flight of insects, bats, and birds exploit high-lift mechanisms during a wing-beat, for example, delayed stall, wake recapture, or fling and clap. This has inspired engineers to attempt to reproduce these efficient aerodynamic mechanisms in micro-air vehicles.

Previously, engineering researchers have been focusing on the unsteady effects of dynamic stall for rotary-wing aircraft applications, and more recently, they have investigated wind turbines, both of which are within a Reynolds number regime of 10⁶. There are also many computational and experimental works relating to unsteady flapping motion of insect flight, whose Reynolds number is in the range of O (10²-10³). Delayed stall phenomena has been of particular interest where a leading edge vortex on the upper edge of the wing is responsible for lift throughout the downstroke. Leading edge vortex and other large flow structures have shown that traditional quasi-steady flow analysis relying on inviscid flow theory has not been capturing the correct physics.

Professors Jennifer Franck and Kenneth Breuer at Brown University in the United States focused on an intermediate Reynolds number regime O (10⁴-10⁵). for which there has been fewer publications on flapping flight as well as the aerodynamics of larger vertebrates such as bats. The authors adopted a flat plate computational model and a time-resolved large-eddy simulation, thus providing an insight into the unsteady high-lift mechanisms of flapping flight. Their research work is published in International Journal of Heat and Fluid Flow.

The authors performed simulations of a heaving flat plate at 1°, 5°, 9°, 13°, 18° attack angles, and computed the mean and phase-averaged lift as well as the resulting flow fields. The proposed study was based on an investigation where the lift increased as a result of a transition from a steady to flapping mode. The main objective of the simulated heaving plate model was to capture the two-dimensional lift-enhancement effects resulting from the leading edge separation at a transitional Reynolds number of 40,000.

Jennifer Franck and Kenneth Breuer characterized the results of the simulations by three regimes. They observed that for heaving at an attack angle of 1°, representing the first regime, the lift was 6.9% more than that recorded by a traditional quasi-steady model. For the transitional regime, represented by attack angles of 5-13°, they observed a lift enhancement of 18-24% more than the quasi-steady prediction and about 5-17% more than the corresponding flow over a stationary airfoil.

Considering that the static stall starts at an angle of attack of 9°, these plunging flows extended into the attached flow regime and then maintained lift within the stall regime owing to coherent leading as well as trailing edge vortices. The lift enhancement could be attributed to the reduction in pressure on the suction side of the foil during downstroke. A comprehensive analysis of the vortex formation together with the pressure contours along the surface over a heaving cycle offered more details of the mechanism of lift enhancement.