Experimental investigation of an oscillating tandem-wing power generator

With the rapid increase in energy demand and along with considering the environmental and sustainable aspects, clean and renewable energy is a necessity for human beings. Considering the potential possibility of energy production inspired by nature, the concept of a flapping wing power generator is investigated. In this study the feasibility and advantages of using flapping wings to generate energy are studied and documented in the literature by researchers. However, previous studies lack some aspects to develop a high performing flapping wing power generator. In the scope of this study various approaches are investigated in order to define the parameters for a flapping wing power generator for optimum performance. Firstly three dimensionality effects are studied for a single flapping wing. Secondly, flapping wing power generators operating inside solid side walls are investigated for 2D and 3D cases. And lastly, an oscillating tandem wing power generator is studied with different phase angles between fore and hindwings. In the experimental study, the flow structures and the forces acting on a finite flat plate that performs sinusoidal and non-sinusoidal pitching and plunging motions are investigated via detailed quantitative flow visualization technique with simultaneous direct force measurements. Reynolds number, pitch pivot position, pitch and plunge amplitudes are kept constant for all experiments. Two different phase angles of  = 90° and 110° between pitch and plunge are studied for single free and constrained wings. However, only  = 90° is considered during the tandem wing cases because this phase angle results in higher performance compared with 110°. Five different motion types from rapid stroke reversal (TR = 0.1) to sinusoidal reversal (TR = 0.5) are chosen as the flapping motion kinematics. Experiments are conducted in the large scale water channel located at the Trisonic laboratory in İTU. Oscillation of the airfoil is provided by servo motors controlled by computers. All particle image velocimetry (PIV) data are acquired via 2 CCD cameras by illumination of the examined flow plane with a Nd: Yag Laser. Force and torque data are acquired simultaneously by using a six axis water resistant sensor rotating with the wing. Some major results could be concluded from the experiments are described briefly in this section. Early shedding of higher strength vortices occur when the pitch reversal rate is increased. In 3D case leading edge vortex (LEV) and tip vortex interactions are observed near the tip of the finite flat plate where vortical structures shrink in size and stretch out to the wake losing their strength to roll up. As a result, three dimensionality reduces the overall efficiency as compared with 2D cases. For rapid reversals (TR = 0.1), none of the test cases were capable to produce a positive mean power coefficient. Performance of the flapping foil power generator is highly influenced by side-walls. Constrained flow has smaller effect on non-sinusoidal flapping as compared to sinusoidal flapping. The sinusoidal flapping flat plate can achieve higher increase in efficiency in constrained flow compared to the free flow. However, if the minimum distance between the trailing edge and the side wall is too small results in degraded performance on mean power generation. The results imply that there's an optimum wall distance using side-walls in flapping foil power generation that might be a way to compensate for the loss from stroke reversals at maximum and minimum plunge positions predominantly for sinusoidal flapping. In tandem configurations, the power extraction performance of the hindwing are greatly affected by its interaction with the wake of the forewing. Adjusting the phase angle between the two wings can change the mode of vortex interaction. Formation, growth and shedding process of hindwing LEV could be distributed by the shed vortex of the forewing. The timing of interaction and direction of resulted force could be controlled effectively via phase angle. As a consequence the performance loss in hindwing for tandem configuration could be reduced to minimum level and thus energy production can be maximized. Some of the cases studied exhibit constructive vortex interaction that reinforce the LEV and results in higher power production.