Effects of Reynolds Number and Flapping Kinematics on Hovering Aerodynamics

Motivated by our interest in micro and biological air vehicles, Navier-Stokes simulations for fluid flow around a hovering elliptic airfoil have been conducted to investigate the effects of Reynolds number, reduced frequency, and flapping kinematics on the flow structure and aerodynamics. The Reynolds number investigated ranges from 75 to 1700, and the reduced frequency from 0.36 to 2.0. Two flapping modes are studied, namely, the "water-treading" hovering mode, and the normal hovering mode. Although the delayed-stall mechanism is found to be responsible for generating the maximum lift peaks in both hovering modes, the wake-capturing mechanism is identified only in the normal hovering mode. In addition to the strong role played by the kinematics, the Reynolds number's role has also been clearly identified. In the low Reynolds number regime, 0(100), the viscosity dissipates the vortex structures quickly and leads to essentially symmetric flow structure and aerodynamics force between the forward stroke and backward strokes. At higher Reynolds numbers (300 and larger), the history effect is influential, resulting in distinctly asymmetric phenomena between the forward and backward strokes.

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