Numerical Investigation of Energy Extraction in a Tandem Flapping Wing Configuration

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 design. In this paper, a tandem-wing flapping configuration is simulated at a Reynolds number of 10,000 using an incompressible Navier-Stokes solver and an overlapping grid method. The flapping motion consists of a simple sinusoidal pitch and plunge motion with a spacing of one chord length between both wings. The arrangement was tested at a Strouhal number of 0.3 for three different phase angles: 0, 90, and 180 deg. The aerodynamics of the hindwing was 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 hindwings were compared with a single wing to determine how the tandem-wing interaction affects performance. The results show that adjusting the phase angle allows the tandem wing to change the flight mode. At 0 deg phase lag, the tandem wing produces high thrust at high propulsive efficiency, but low lift efficiency. Switching to 90/180 deg phase lag decreases the thrust production and propulsive efficiency but greatly increases the lift efficiency. At 90/180 deg, the power coefficient is much lower than at 0 deg, due to the hindwing extracting energy from the wake of the forewing.