Energy harvesting by a purely passive flapping foil from shear flows

By using a two-dimensional numerical approach, we investigate the response of a purely passive flapping foil flow energy harvester in a linear shear flow. Our focus is to prove that in such conditions the device is still capable of undergoing periodically oscillatory responses induced by flow-induced instability as it does in uniform incoming flows. Our simulations show that this regular and predictable response, essential for reliable energy harvesting, is achieved over a range of geometric and mechanical parameters when the shear rate is relatively small. It disappears when the shear rate is large. Other responses, such as tumbling motions and irregular motions, are also identified and mapped in the parametric space. The performance of the system, characterized by its energy harvesting efficiency (i.e., the percentage of the incoming flow energy flux captured by the system), is found to be comparable to that in the uniform-flow case.