Aerodynamics of dragonfly flight based on lift measurements and flow field analysis.

摘要

The flight of dragonfly demonstrates an important feature of varying phase differences between forewing and hindwing stroke cycles. A majority of dragonfly species employ an inclined stroke plane and benefit from drag-based lift mechanism. In the dissertation, I investigated the aerodynamic effects of forewing-hindwing phase differences by testing a pair of dynamically scaled robotic dragonfly wing models. The results showed that for hovering flight, in-phase flight enhanced lift force on the forewing by 17%; antiphase reduced the lift generation on the hindwing, but it was beneficial to vibration suppression and power efficiency. The results may explain the behavior of the dragonfly that in-phase is commonly used in acceleration mode and antiphase is commonly observed in hovering mode. Wing-wing interaction in forward flight was always beneficial for forewing lift while detrimental for hindwing lift; the hindwing lift was slightly reduced when phase was 0∼90° and significantly reduced by up to 60% with 270° phase. This result explains why dragonflies employ 50∼100° during forward flight, but 270° is never favored.;I further qualitatively investigated the wing-wing interaction mechanism using the Digital Particle Image Velocimetry (DPIV) system, and found that a large downwash flow was generated by forewing motion, which was responsible for lift reduction of the hindwing. The downwash passed through the dorsal side of the forewing, which coincided with the hindwing stroke area. On the other hand, an upwash generated by hindwing motion enhanced the forewing lift. The upwash was proved to be a result of hindwing leading-edge vortex (LEV). I summarized that dragonflies alter the phase differences to control timing of the occurrence of flow interaction to achieve certain aerodynamic effects.;To investigate the correlations between aerodynamic forces and flow field, two approaches were attempted to predict lift by analyzing flow field from aspects of velocity and vorticity, respectively. In the velocity approach, lift was calculated by applying momentum theorem to a controlled volume that enclosed wing model and the results matched lift measurements well. Particularly, the sectional lift predictions on the 9th and 10th sections provide a close match to force measurements too. The vorticity method calculated lift by integrating the circulation bound to the wing model based on Kutta-Joukowski theorem. Nevertheless, the predictions showed a 1/8 cycle delay compared with measurements and the mismatch between the measurements and predictions from circulation method were persistent. In addition, the circulation lift from LEV was already above the magnitude of measured lift, implying that the LEV may not contribute to lift generation in the way that previous studies suggested. The results from this dissertation may bring challenges to the conventional conclusions regarding circulation lift and LEV lift enhancement in flapping flight aerodynamics.