Water tunnel study of a cantilever flexible plate in the wake of a square cylinder
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TECHNICAL PAPER
Water tunnel study of a cantilever flexible plate in the wake of a square cylinder Emmanuel Binyet1 • Chih-Yung Huang1 • Jen-Yuan Chang1 Received: 15 December 2019 / Accepted: 25 May 2020 Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The vorticity in the wake flow behind bluff bodies can be used to generate undulating motion of polymeric flexible plates. When the plates carry piezoelectric cells, the strain energy resulting from flapping can be converted into electricity. The undulating motion of cantilevered plates in the wake of a bluff body has been described in a few studies; less information was given on the flow pattern and fewer on the influence of the geometric parameters. In this study, a polyethylene terephthalate thin plate was clamped to a square cylinder and placed in a water tunnel and the Reynolds number based on the square cylinder was set from around 1500 to 20,000. The plate deflection and related flow pattern were captured at both 5000 and 500 frames per second using two different high speed cameras. Modal decomposition, strain energy and flapping power calculation were carried out from the deflection data. The flow vectors were computed using the particle image velocimetry software PIVlab. Strain energy is maximized when higher order modeshapes occur which is only the case for longer plates because of the lower natural frequencies resulting in an increased plate-wake interaction. The plate width is an important parameter as wider plates witness strain energy wastage owing to torsion and plates that are too narrow undergo out of plane bending. Longer plates yield greater power output as long as the bending pattern is kept two-dimensional. The strain nodes have a more significant shift in positions along the beam for longer plates. This may result in significant charge cancellation within the piezoelectric cells.
1 Introduction With recent advances in piezoelectric polymeric materials, more concepts for harvesting energy from vibrations have emerged. Such as a flow induced vibration harvester composed by an undulating flexible thin plate with piezoelectric cells placed behind a wake generating bluff body. A square cylinder is the most basic type of bluff body for oscillatory wake generation as flow separation occurs at the leading edges resulting in alternating vortices being shed in the wake. This creates a sinusoidal pressure wave that would generate an oscillatory motion of the flexible plate. In the present study, we chose a slender square cylinder (height to diameter ratio exceeding 5) to be the vortex generating bluff body to allow us to simplify the study into a quasi-two dimensional fluid–structure interaction problem. The flow structures can be schematized as in Fig. 1 where periodic vortex shedding & Jen-Yuan Chang [email protected] 1
Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
occurs over the whole span except close to the cylinder top and b
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