Energy Harvesting from a Vortex Ring Using Highly Deformable Smart Materials
Hu, Jia Cheng
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Recently, the growing demand of compact energy harvesters for wireless sensor networks has lead to an increasing interest in exploring the energy harvesting capabilities of smart materials. Smart materials are a category of transducer that is able to convert dynamic structural deformations into electricity and vice versa. To investigate the feasibility and the potential of small-scale smart material-based harvesters to obtain energy from ambient fluid environments, interactions between a vortex ring and smart material structure are examined. Vortex rings are a class of coherent structure that is common in nature and act as a canonical representation of vortex structures. Herein, two energy harvesting configurations are considered. First, energy transfer from a passing vortex ring to a cantilevered plate, which is oriented parallel to and offset from the vortex ring's path, is modeled and optimized. The three-dimensional problem is simplified to a two-dimensional problem using a novel method that maintains the loading characteristics of the vortex ring. Two-dimensional Kirchhoff-Love plate theory and two-dimensional potential flow theory are utilized to represent the solid and fluid, respectively. The coupled fluid-structure model is solved simultaneously and validated against published experimental data. Employing this analytical model, the optimization study aims at locating the resonance frequency with respect to the change in fluid and structure parameters. The dimensionless resonance frequency is established as a specific ratio between the plate's fundamental frequency and vortex convective time-scale using a classical moving point load analysis. The result of the optimization study provides a description and empirical formulation of the shift in dimensionless frequency as a result of various fluid and structure parameter adjustments. In the second configuration, the energy harvesting potential of a vortex ring impacting an ionic polymer composite (IPMC) annulus is examined experimentally. The annulus is axis-symmetrically aligned with an incoming vortex ring that is generated by a piston/cylinder setup. The tip deflection of the annular energy harvester is measured with a laser displacement sensor, while the cross-sectional flow field is measured with particle imaging velocimetry and the electrical energy accumulated by the IPMC is estimated with the short-circuit current output. The experimental results unveiled that the annulus is first pulled by the vortex ring low pressure core, and then pushed upon impact. A secondary vortex ring is observed convecting away from the annulus. It is possibly formed out of the vortex induced vorticity at the annulus tip, while the incoming vortex ring is destroyed at impact. The experimental result is found to be in good agreement with an analytical solution.