Effect of aspect ratio on hydrokinetic energy harnessing using cylinders in VIV

Smooth, rigid, circular cylinders on elastic support in flow perpendicular to their axis undergo vortex-induced vibrations (VIV) over a broad range of velocities. VIV converts hydrokinetic energy to mechanical in the oscillating cylinder. Tip-flow introduces three dimensional effects reducing the effective length of the cylinder which provides the transverse lift force to induce oscillations and consequently the energy in the oscillator. In this study, we investigate experimentally the effect of the cylinder aspect ratio on hydrokinetic energy harnessing. Experiments are conducted in the Reynolds number range 15,000 < Re < 80,000 falling in two different flow regimes: TrSL2 (Transition Shear Layer 2: 1000 < Re < 40,000) and TrSL3 (40,000 < Re < 300,000). Converted power and maximum system efficiencies are calculated from experiments conducted in the recirculation channel of the Flow Induced Motions Laboratory, Istanbul Technical University (ITU FIMLab). It was found that the end-zones of the cylinder, which do not induce lift due to tip flow, are more dominant in lower aspect ratio cylinders. More power can be captured from TrSL3 flows due to higher shear-flow momentum. Higher efficiency in power conversion is achieved in TrSL2.