Flow Induced Motions (FIMs) of a single, rigid, circular cylinder with end-springs are investigated for Reynolds number 30,000≤Re≤120,000 with mass ratio, damping, and stiffness as parameters. Selective roughness is applied to enhance FIM and increase the hydrokinetic energy captured by the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter at higher Reynolds numbers. The second generation of virtual spring-damping system Vck, recently developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded computer-controlled change of viscous-damping and spring-stiffness for fast and precise oscillator modeling. Experimental results for amplitude response, frequency response, energy harvesting, and efficiency are presented and discussed. All experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The oscillator can harness energy from flows as slow as 0.3946 m/s with no upper limit. (2) Increasing the spring stiffness, shifts the VIV synchronization range to higher flow velocities, resulting in reduced gap between VIV and galloping, where the harnessed power drops. (3) In galloping, the harnessed power increases with the mass ratio.

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Flow Induced Motions (FIMs) of rigid circular cylinders, and particularly VIV (Vortex Induced Vibrations) and galloping, are induced by alternating lift. The VIVACE (VIV for Aquatic Clean Energy) Converter uses single or multiple cylinders, in tandem, on elastic end-supports, in synergistic FIM, to convert MHK energy to electricity. Selectively distributed surface roughness is applied to enhance FIM and increase efficiency. In this paper, two cylinders are used in tandem with center-to-center spacing of 1.57, 2.0 and 2.57 diameters, harnessing damping ratio 0.00< <0.24, for Reynolds number 30,000 Re 120,000. The virtual spring-damping system V ck in the Marine Renewable Energy Laboratory (MRELab) enables embedded computer-controlled change of viscous-damping and spring-stiffness for fast and mathematically correct oscillator realization, without including the hydrodynamic force in the closed control loop. Experimental results for oscillatory response, energy harvesting, and efficiency are presented and the envelope of optimal power is derived. All the experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) For the tested cylinder spacing, two cylinders harness power is between 2.56 and 13.49 times the power of a single cylinder, the efficiency of two cylinders is between 2.0 and 6.68 of a single cylinder. (2) The MHK power harnessed by the upstream cylinder is increased by up to 100%, affected by the downstream cylinder. (3) The MHK power harnessed by the downstream cylinder and its FIM are affected to a lesser extent by the interaction. (4) VIVACE can harness energy from flows as slow as 0.4m/s with no upper limit in flow velocity. (5) Close spacing and high spring stiffness yield highest harnessed power. (6) The optimal harnessed power shifts to softer springs as spacing increases.