Numerical analysis of a tidal current generator with dual flapping wings

Flapping wings, inspired by the mechanism of birds and fish, can act as generators to harvest energy from tidal currents. The hydraulic system is simplified as a spring-damper system to establish the coupling equations relating to the wing motion and the hydrodynamic forces. To provide guidance for design of a fully flow-induced flapping wings energy harvesting system, the behaviors of both system response and energy extraction performance are analyzed using two-dimensional numerical approach. Depending on the rotary actuator radius R, and the volume ratio β between the cylinder and rotary actuator, three distinguishable behaviors are observed in the system response and energy extraction performance. At larger R and smaller β, the dual wings tend to undergo a damped reduction flapping motion because the pitching motion consumes a significant amount of energy. Both decreasing R and increasing β can reduce the energy consumption of the pitching motion, and thus allow the dual wings to achieve a sustainable flapping motion. Although an irregular response can achieve a self-sustained flapping motion, it is unfavorable owing to its unstable power output. The regular response essential for stable energy harvesting is realized over a range of coupling parameters. The energy extraction performance of the system is closely associated with β but also slightly depends on R.