An Experimental Study of Unsteady Vortex Structures in the Wake of a Piezoelectric Flapping Wing

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.