In this project, we hypothesize that trailing edge flexion directly influences leading edge vorticity, and thereby the magnitude of aerodynamic forces on the flexible flapping wings. To test this hypothesis, we visualized the flows on wings of varying flexural stiffness using a custom 2D Digital Particle Image Velocimetry (DPIV) system, while simultaneously monitoring the magnitude of the aerodynamic forces. Our data show that as flexion decreases, the magnitude of the leading edge vorticity increases and enhances aerodynamic forces, thus confirming that the leading edge vortex is indeed a key feature for aerodynamic force generation in flapping flight. We found that camber influences instantaneous aerodynamic forces through modulation of the leading edge vorticity. (JRSI paper, BB paper)
Lift and drag coefficients polar plots of various wings of different flexual stiffness.
Wing-Wing Interactions in Dragonflies
Dragonflies employ different phase angles between forewing and hindwing under different flight modes. Based on the force and PIV measurementson a pair of mechanical model wings, we found that in-phase flight enhanced the forewing lift and the hindwing lift wasgenerally reduced. The total lift was only increased at in-phase phase angle. The results explains the commonly observed behavior of the dragonflies that empoy in-phase for acceleration. While the forewing generated a downwash flow which is responsible for the lift reduction on the hindwing, an upwash flow resulted from the leading edge vortex of the hindwing helps to enhance lift on the forewing. Dragonflies alter the phase differences to control timing of the flow interactions to achieve certain aerodynamic effects. (ICRA09 and submitted journal paper).
Model wings in tow tank to simulate forward flight.
Lift force coefficients vs. phase angle between forewing and handwings in forward flight.
Flow measurements of out-of-phase wingbeats in hover.
Vortex Generation and Induced Flow on Flapping Wings
Here we investigate the vorticity dynamics in flapping and revolving wings using the volumetric 3-component Velocimetry system (V3V). The measurements show a strong correlation between the spanwise component of the flow and the vorticities generated. Using the fundamental vorticity equation, we evaluated the vorticity convection, stretching and tilting in the rotating wing frame to understand the generation and evolution of vorticities. We found that vorticity generated at the leading edge are carried away by strong tangential flow into the wake, and travel downwards with the induced downwash. The figure to the left shows the vortex rings and induced flow beneath a pair of flapping wings. (EXIF paper)
School of Mechanical Engineering,
West Lafayette, IN 47907
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