Fluid-structure interaction (FSI) is routinely used for resolving design related vibration issues of flexible structures, for example, long-span bridges, tall buildings, airplanes (wings or tails), wind turbine and helicopter blades. In the past few years, numerous studies have used FSI as a tool to study fluid-based energy harvesting from solar, wind, and ocean. Due to high capital investment and maintenance cost, the conventional land-based wind turbines are not suitable for small-scale wind energy harvesting at low wind speeds, and due to acoustic and aesthetic issues these turbines cannot be deployed in proximity of city centers or on building roof tops or backyards, where energy is in high demand. Therefore, an alternate mechanism that exploits the various aeroelastic phenomena including vortex-induced vibration, buffeting, galloping, flutter, etc., to cause large-amplitude response in specific structural shapes has been explored by some for tapping the renewable wind energy source in the low-wind speed regime. This study explores the feasibility of a flutter-driven or flutter-induced vibration (FIV) wind energy harvester that uses rigid-body motions of section models to harvest wind energy at low wind speeds. Section models are rigid-models with 1-3 degree-of-freedom (DOF) that faithfully represent the geometry of the cross-section of a structure over a finite length, with end plates to simulate a 2D-flow. The objective of this study was to explore the parameters that will influence the performance of the FIV wind energy harvester which is assessed by the low magnitude of flutter speed and large magnitude of vibration amplitude at or near flutter.
There are various cross-sections of section models that are prone to vibration at a given wind speed along a particular DOF. Rectangular sections of aspect ratio (AR) less than 2 usually have low flutter speeds in the vertical DOF and ‘H’-shaped sections, used in old long-span bridges like Tacoma Narrows, have low flutter speeds in the torsional DOF. A hybrid section model formed by combining these two sections is therefore expected to be more sensitive to a coupled vertical-torsional flutter at low wind speeds. Such hybrid cross-sections are ideal for a 2-DOF (vertical and torsional) FIV wind energy harvester. Wind tunnel experiments were performed with geometrically scaled models to determine the flutter speed and average wind energy capture for various section models in 1-DOF and 2-DOF motions. For a comparative study, rectangular section models (AR=1.5 and AR=1) in 1-DOF (vertical or torsional) and 2-DOF (vertical and torsional), and H-shaped section models in torsional DOF were tested. The best rectangular section in the vertical DOF and best H-shaped section in the torsional DOF with respect to low flutter speed and high vertical or torsional amplitudes of vibration are combined to form a hybrid section and tested in 2-DOF (vertical and torsional) and compared with the best rectangular section and best H-shaped section for flutter speed and average wind energy available at flutter. In order to amplify the performance of the devised FIV wind energy harvester, a parametric study of the flutter mechanism is conducted in terms of mass ratio, frequency ratio (2-DOF), location of the pivot point and choice of DOF(s) to use. This study also developed a method to extract the rational function coefficients, used in time-domain flutter analysis numerically, from flutter derivatives of the specific sections used here that will enable a parametric study of the FIV wind energy harvester in the future to optimize its design and wind energy performance of its scaled-up versions for commercial usage. An estimate of the maximum power that can be captured using a linear generator from a scaled-up version of a FIV wind energy harvester that uses a rectangular section of AR=1.5 (0.60 m width x 0.40 m depth and 26.7 kg mass) suspended inside a duct in vertical DOF was made as 100 watts at 2.5 m/s (with a 8:1 inlet area contraction), which is promising
