Hydrokinetic energy conversion by two rough tandem-cylinders in flow induced motions: Effect of spacing and stiffness

Highlights

The effects of tandem spacing, spring stiffness, and damping on power harness by two circular cylinders with passive turbulence control are studied experimentally.

The Vck based oscillator enables embedded computer-controlled change of viscous-damping and spring-stiffness for precise oscillator modeling and fast parametric testing.

Amplitude response, frequency response, harnessed power, and efficiency are presented vs. flow velocity with spring stiffness, damping, and spacing as parameters.

In the galloping range, two cylinders in synergistic flow induced motion can produce more power than the same cylinders in isolation.

All the experiments were conducted in the TrSL3 (20,000<Re<300,000) flow regime.

Abstract

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<ζ < 0.24, for Reynolds number 30,000 ≤ Re ≤ 120,000. The virtual spring-damping system Vck 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 0.4 m/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.

Keywords

  • Hydrokinetic energy harnessing;
  • Virtual spring-damping;
  • Flow induced motions;
  • Vortex induced vibrations;
  • Galloping;
  • Distributed roughness

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