On the visualisation of flow structures downstream of fluttering piezoelectric energy harvesters in a tandem configuration

A qualitative and quantitative investigation is carried out on two tandem, identical piezoelectric energy harvesters fluttering in a parallel, smooth flow. Previous reports revealed up to a 40% increase in power output from the downstream harvester at a certain position, though the reasons are still unknown. Here, two orthogonally positioned high-speed cameras are synchronised in time and deployed to capture smoke flow visualisation data simultaneously, in the vicinity of the harvesters. In addition, the image data from the two cameras is synchronised with real-time voltage data from the energy harvesters, permitting comparison between the image and voltage data. New results and findings are presented: (1) the occurrence of maximum voltage for these harvester types approximately coincides with the point of maximum PVDF tip velocity in the flutter cycle, (2) the Leading Edge Vortex (LEV) shed by the upstream harvester does not influence the downstream harvester, and (3) the horseshoe vortices generated by the upstream harvester act to increase the maximum PVDF tip velocity of the downstream harvester, which amplifies the power output.