Passive Flexibility Effect on Oscillating Foil Energy Harvester

It is well known that structural flexibility enhances the performance of flapping foil propellers. There is, however, much less knowledge about the effect of deformability on the flow energy extraction capacity of flapping foils. Following recent work on an oscillating foil energy harvesting device with prescribed foil deformations (Liu et al., A Bio-Inspired Study on Tidal Energy Extraction with Flexible Flapping Wings, Bioinspiration and Biomimetics, Vol. 8, No. 3, 2013, Paper 036011), fully coupled dynamics of a flapping foil energy harvester with a passively deformable foil is investigated. Toward this end, the dynamics of a foil with a realistic internal structure (containing a rigid leading edge and a flexible trailing edge with a stiffener) in an energy harvesting regime through a fluid-structure interaction scheme is computationally studied. To examine the effect of different levels of flexibility, various materials (ranging from metals such as copper to virtual materials with arbitrary elasticity and density) for the stiffener have been tested. With the virtual materials, the effects of Young's modulus and density ratio have been studied. The simulation results show that flexibility around the trailing edge could enhance the overall energy extraction performance. For example, with a copper stiffener, an increase of 32.2% in efficiency can be reached at high reduced frequency. The performance enhancement is achievedmostly in cases with low Young's modulus and density ratio. A possible underlying mechanism is that the specific foil deformations in these cases encourage the generation and shedding of vortices from the foil leading edge, which is known to be beneficial to flow energy extraction.