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Engineering Notes ENGINEERING NOTES are short manuscripts describing new developments or important results of a preliminary nature. These Notes should not exceed 2500 words (where a figure or table counts as 200 words). Following informal review by the Editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).

An experimental study was conducted to investigate the behavior of the wing-tip vortex structures generated by a square-tipped, rectangular NACA0012 wing. A Stereoscopic Particle Image Velocimetry (SPIV) system was used to conducted detailed flow field measurements to elucidate the key features of the wing-tip vortex structures in the near field. One of the great advantages of the present SPIV measurements over the classical measurement technique is that the vortex wandering can be removed directly by tracking the center of the wingtip vortex in the instantaneous measurement frames. By tracking the center of the wingtip vortex, the wandering and turbulence in the vortex can be decoupled completely. This method was applied to investigate the effects of the angle of attack of the test wing and wind tunnel wall on the evolution of the wingtip vortex in the near field. In order to decouple the effects of vortex wandering, Devenport et al. (1996) suggested an analytical method to predict the wandering free velocity profile. The velocity profile predicted by the Devenport et al. (1996) method was compared with the SPIV re-centered velocity profile quantitatively.

An experimental study was conducted to explore the potential applications of compact, gearless, piezoelectric flapping wings with the wing size, stroke amplitude and flapping frequency within the range of actual insect characteristics for the development of novel insect-sized, flapping-wing-based Nano-Air-Vehicles (NAVs). Unlike most of previous studies with 2-D flapping airfoil models, a fix-rooted 3-D piezoelectric flapping wing was used in the present study with the consideration of more practical configurations usually used in NAV designs. The experimental study was conducted in a low-speed wing tunnel with the test parameters of chord length of C = 12.7mm, chord Reynolds number of Re = 1,200, flapping frequency of f = 60 Hz, reduced frequency of k = 3.5, and non-dimensional flapping amplitude at wingtip h = A/C = 1.3. The corresponding Strouhul number of the root-fixed 3-D piezoelectric flapping wing is Str = 0.30, which is within the optimal range of 0.2 < Str < 0.4 usually used by flying insects and swimming fishes. A digital particle image velocimetry (PIV) system was used to achieve phased-locked and time-averaged flow field measurements to quantify the formation and separation processes of the Leading Edge Vortex (LEV) structures on the upper and lower surfaces of the flapping wing in relation to the phase angle (i.e., the positions of the flapping wing) during upstroke and down stroke flapping cycles. The evolutions of the wake vortex structures in the chordwise cross planes at different wingspan locations of the rootfixed flapping wing were compared quantitatively to elucidate underlying physics for better understanding of the unsteady aerodynamics of the flapping flight and to explore/optimize design paradigms for the development of novel insect-sized, flapping-wing-based NAVs.