Synergy of multiple cylinders in flow induced motion for hydrokinetic energy harvesting

Eun Soo Kim – University of Michigan, Doctoral Dissertation, 2013


Vortex Induced Vibrations for Aquatic Clean Energy (VIVACE) Converter is a converter of marine hydro-kinetic energy invented in the Marine Renewable Energy Lab (MRELab) and patented by the University of Michigan. It harnesses hydrokinetic energy from ocean/tidal/river currents. In its simplest form the VIVACE Converter is a single circular cylinder on springs with a power take-off system. Using passive turbulence control, VIVACE maximizes and utilizes flow induced motion in the form of vortex induced vibration or interference/proximity/wake/soft/hard galloping. MRELab has achieved back-to-back vortex induced vibration and galloping for a single cylinder with passive turbulence control thus more than doubling the range of synchronization of flow induced motion (FIM).
The goal of this research is to maximize the synergy of multiple cylinders in flow induced motion for hydrokinetic energy harnessing in order to increase the power volume density. In order to achieve this goal, the eff ects of tandem spacing, staggering, passive turbulence control, mass ratio, spring sti ffness, damping, and number of cylinders in flow induced motion of multiple cylinders are studied experimentally. For these experimental studies, two diff erent generations of VIVACE converters were designed and built, and at the same time the old Low Turbulent Free Surface Water (LTFSW) Channel was replaced by the new LTFSW Channel. All model tests were conducted at high Reynolds numbers in the range of 28,000<Re<120,000, which primarily covers the TrSL2 and TrSL3 ow regimes. The following observations and conclusions are drawn: (a) By introducing PTC, all cylinders (2,3,4 tandem con gurations) achieve galloping increasing the range of FIM synchronization with high amplitudes (2.2-3.49D) regardless of other parameters. (b) For tandem spacing of 1.43D, FIM of two

cylinders in tandem is distinct: galloping starts earlier, amplitude is higher for the 2nd cylinder, oscillation frequency is lower for both cylinders, and energy conversion is 60% higher. (c) For two cylinders in close tandem proximity (d/D<2.0), contrary to single cylinder studies, energy envelope points in galloping may not correspond to the highest spring stiff ness (d) Higher mass ratio (m*) results in higher marine hydrokinetic energy conversion in galloping with practically no change in the VIV region. Increase by a factor of 2.5 was measured for 0.65<m*<1.66. (e) Multiple cylinders in tandem can be in synergistic FIM in close proximity of d/D<2.0. (f) 2, 3, and 4 cylinders in synergistic FIM can harness more marine hydrokinetic energy than the sum of the energy harnessed by each cylinder acting in isolation.

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