Downstream flow structures of a fluttering piezoelectric energy harvester

The flow structures downstream of a fluttering piezoelectric energy harvester are examined, since 40% increases in power output have been noted from a downstream harvester. Smoke wire flow visualisation with a novel bi-lighting technique, and simultaneous voltage measurements are used to understand this phenomenon. The bi-lighting technique helped to confirm the out-of-plane velocity fields caused by the tilted cone vortices shed from the upstream harvester. It was speculated that the time-varying local pressure gradients induced by the propagating vortices coupled beneficially with the free-stream dynamic pressure and flutter instabilities, acting to elevate the flutter amplitude of the downstream harvester. Also, the phase lag between the harvesters showed a nonlinear relationship with the free-stream wind speed.