Energy extraction performance of tandem flapping foil undergoing elliptical motion trajectory

The energy extraction performance of tandem flapping was numerically investigated by varying the flapping trajectory and position between the foils at a Reynolds number of 1173. A simple sinusoidal flapping trajectory is compared to a 2D simulation of tandem flapping foil following an elliptical trajectory. The simulation was carried out with NACA 0012 by varying the Strouhal number (St) between 0.2 and 0.5. The results show that with an elliptical motion trajectory pattern and an inter-foil spacing of 1.5 cm- 2cm (where cm is the mean chord length) between the foils would significantly enhance the energy extraction, whereas an unfavourable spacing between the forewing (fw) and hindwing (hw) foils results in unfavourable wake interaction, which reduces the energy extraction when compared to solo flapping foil. The highest energy extraction efficiency of 44% is achieved at St = 0.4. This research sheds new light on the creation of bio-mimetic devices by utilising a wake vortex to improve energy extraction efficiency.