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Tree-inspired piezoelectric energy harvesting
William B. Hobbs a, David L. Hu a,b,Ã a b

School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA School of Biology, Georgia Institute of Technology, Atlanta, GA 30332, USA

a r t i c l e i n f o
Article history: Received 23 July 2010 Accepted 6 August 2011 Keywords: Vortex induced vibrations Piezoelectric energy harvesting Tandem cylinders

abstract
We design and test micro-watt energy-harvesters inspired by tree trunks swaying in the wind. A uniform flow vibrates a linear array of four cylinders affixed to piezoelectric energy transducers. Particular attention is paid to measuring the energy generated as a function of cylinder spacing, flow speed, and relative position of the cylinder within the array. Peak power is generated using cylinder center-to-center spacings of 3.3 diameters and flow speeds in which the vortex shedding frequency is 1.6 times the natural frequency of the cylinders. Using these flow speeds and spacings, the power generated by downstream cylinders can exceed that of leading cylinders by more than an order of magnitude. We visualize the flow in this system by studying the behavior of a dynamically matched flowing soap film with imbedded styrofoam disks. Our qualitative visualizations suggest that peak energy harvesting occurs under conditions in which vortices have fully detached from the leading cylinder. & 2011 Elsevier Ltd. All rights reserved.

1. Introduction Energy harvesting is the process by which ambient energy is captured from external sources (thermal, wind, solar and hydrodynamic). One way to harvest energy is through the use of piezoelectricity, the ability of certain materials such as bone (Fukada and Yasuda, 1957), wood (Fukada, 1955) and ceramics to

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