Experimental Investigation with PIV on Characteristics of Shedding Vortices in Wake of Flapping Wings

Qualitative and quantitative studies on the vortices structure, motion path and energy in the near-field wake of flapping wings were conducted through dye-flow tests and particle image velocimetry(PIV) in tank. Results indicate that two kinds of horn-shaped vortices structures exist during flapping under insufficient or sufficient spanwise flow. Anticlockwise and clockwise vortices arise alternately during downward and upward flapping, whose traces are approximately symmetrical. The elevation angle of symmetry-axis is slightly larger than the attack angle of flapping wings. The vorticity of vortex core reaches its maximum at the top or bottom point, while the maximum circulation occurs at 1/5~2/5 period before the top or bottom point. The average circulations of both vortices during half period of producing them ascend with the increase of reduced frequency. At small reduced frequency, the average circulation during downward flapping is larger than that during upward flapping. Contrary result is observed in case of large reduced frequency. Flapping amplitude significantly influence the vortex intensities, e.g. when reduced frequency is 2~2.5, the average circulation of vortices with an amplitude of ±40° is about twice that of amplitude of ±30°, and the influence grows with the increase of reduced frequency.