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First published online December 1, 2006
Journal of Experimental Biology 209, 5005-5016 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02614

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Dual leading-edge vortices on flapping wings

Yuan Lu, Gong Xin Shen^* and Guo Jun Lai

Full Flow Field Observation and Measurement, Institute of Fluid Mechanics, Beijing University of Aeronautics and Astronautics, Beijing 100083, People's Republic of China

^* Author for correspondence (e-mail: gx_shen55{at}yahoo.com.cn)

Accepted 20 October 2006

An experimental investigation was performed with two aims: (1) to clarify the existence of the dual leading-edge vortices (i.e. two vortices with the same sense of rotation located close to the leading edge above the leeward wing surface) observed on flapping wings in previous studies; (2) to study systematically the influences of kinematic and geometric parameters on such a vortical structure. Based on a scaled-up electromechanical model flapping in a water tank, the leading-edge vortex (LEV) cores were visualized via dye flow visualization, and the detailed sub-structures of LEV were revealed through digital particle image velocimetry (DPIV) with high spatial resolution. Five wing aspect ratios (AR) (1.3, 3.5, 5.8, 7.5 and 10), eight mid-stroke angles of attack (_m) (10-80°), and six Reynolds numbers (Re) (160-3200) were examined. In addition, the well-studied case of the fruit fly Drosophila was re-examined.

The results confirm for the first time the existence of dual LEVs on flapping wings. The sectional flow structure resembles the dual-vortex observed on non-slender delta wings. Insensitive to AR, a dual LEV system such as this could be created when _m and Re reached certain high levels. The primary vortex was attached to the wing, while at the outer wing the minor vortex shed, generating a same-sense vortex behind.

Key words: flapping wing, hovering, flow separation, leading-edge vortex (LEV), electromechanical model, flow visualization, digital particle image velocimetry (DPIV)

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