Synergy of Multiple Cylinders in Flow Induced Motion for Hydrokinetic Energy Harnessing.

Abstract

Vortex Induced Vibrations for Aquatic Clean Energy (VIVACE) Converter is a converter of marine hydro-kinetic (MHK) energy invented in the Marine Renewable Energy Lab (MRELab). It harnesses MHK from ocean currents. Using passive turbulence control, VIVACE maximizes and utilizes flow induced motion in the form of vortex induced vibration (VIV) or interference/proximity/wake/soft/hard galloping. MRELab has achieved back-to-back VIV and galloping for a single cylinder with passive turbulence control thus more than doubling the range of synchronization of flow induced motion. The goal of this research is to maximize the synergy of multiple cylinders in flow induced motion (FIM) for MHK energy harnessing in order to increase the power volume density. In order to achieve this goal, the effects of tandem spacing, staggering, passive turbulence control, mass ratio, spring stiffness, damping, and number of cylinders in FIM of multiple cylinders are studied experimentally. 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 flow regimes. The following observations and conclusions are drawn: (a) By introducing PTC, all cylinders (2,3,4 tandem configurations) achieve galloping increasing the range of FIM synchronization with high amplitudes 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 stiffness (d) Higher mass ratio (m*) results in higher MHK 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 MHK energy than the sum of the energy harnessed by each cylinder acting in isolation.