Non-linear piezoelectric fluidic energy harvesters: The mutual interaction of two oscillating cylinders

The presence of a bluff body upstream of a cantilever beam promotes persistent, aero-elastic vibrations of the beam. Vortex-induced vibration in an array of two mutually interacting bluff bodies in such configurations undergoing two-degrees of freedom transverse oscillation has not been investigated before. In the present work, we have studied experimentally, the unsteady response of an array of two similar rigid cylinders, positioned side-by-side in reference to the freestream velocity, each one mounted on the upstream end of an elastic cantilever beam. By fitting the beams with piezoelectric layers, these configurations are converted to piezoelectric fluid energy harvesters (PFEH) that can extract small amounts of energy from the flow. Comparing the performance of linear (L-PFEH), non-linear (NL-PFEH), and a non-linear array (NLA-PFEH) of harvesters show that NLA-PFEH has the widest broadband operating velocity range and the greatest generated power followed by NL-PFEH and then L-PFEH. The maximum electric power output of NLA-PFEH was similar to 1000% greater than for NL-PFEH with a corresponding similar to 250% increase in the operating velocity range. Different cylinder configurations reveal the presence of hysteresis in the behavior of NLA-PFEH when the distance between the cylinders (so-called cylinder gap to diameter ratio), G/D < 0.5. At large distances from each other (G/D >= 4), the two cylinders behave like independent, isolated harvester units with rather weak mutual interaction.