The characterization of wing-wing vortex interactions of a tandem flapping wing configuration and its relationship to the phase angle and wing spacing.

A number of flying insects make use of tandem wing configurations, suggesting that such a setup may have potential advantages over a single wing at low Reynolds numbers. Dragonflies, which are fast and highly maneuverable, demonstrate well, the potential performance of such a configuration. In a tandem wing configuration, the hindwing often operates in the wake of the forewing and, hence, its performance is affected by the vortices shed by the forewing. Changes in the phase angle between the flapping motions of the fore and hind wings, as well as the spacing between them, can affect the resulting vortex/wing and vortex/vortex interactions. In this thesis flapping wings in a tandem configuration were simulated using an incompressible Navier-Stokes solver on composite overlapping grids. Harmonic single frequency sinusoidal oscillations consisting of combined pitch and plunge motions were used for the flapping wing kinematics at a Strouhal number of 0.3. Different wing spacings ranging from 0.1 chords to 1 chord were tested at three different phase angles, 0°, 90° and 180°. It was found that changes in the spacing and phase angle affected the timing of the interaction between the vortex shed from the forewing and the hindwing. Such an interaction affects the LEV formation on the hindwing and results in changes to the aerodynamic force production and efficiencies of the hindwing. It is also observed that changing the phase angle has a similar effect as changing the spacing. The results further show that at different spacings the peak force generation occurs at different phase angles, as do the peak efficiencies. The aerodynamics of the hindwing was also compared in detail to a single wing, with the same geometry and undergoing the same flapping kinematics, to determine the effect of vortex shedding from the forewing on the hindwing, as well as how the phase angle affects the interaction. The average lift, thrust and power coefficients and the average efficiency of the fore and hind wings were compared to a single wing to determine how the tandem wing interaction affects performance.