Effect of Bottom Boundary on VIV for Energy Harnessing at $8×103<Re<1.5×105$

The concept of extracting energy from ocean/river currents using vortex induced vibration was introduced at the OMAE2006 Conference. The vortex induced vibration aquatic clean energy (VIVACE) converter, implementing this concept, was designed and model tested; VIV amplitudes of two diameters were achieved for Reynolds numbers around $105$ even for currents as slow as 1.6 kn. To harness energy using VIV, high damping was added. VIV amplitude of 1.3 diameters was maintained while extracting energy at a rate of $PVIVACE=0.22×0.5×pU3DL$ at 1.6 kn. Strong dependence of VIV on Reynolds number was proven for the first time due to the range of Reynolds numbers achieved at the Low-Turbulence Free Surface Water (LTFSW) Channel of the University of Michigan. In this paper, proximity of VIVACE cylinders in VIV to a bottom boundary is studied in consideration of its impact on VIV, potential loss of harnessable energy, and effect on soft sediments. VIV tests are performed in the LTFSW Channel spanning the following ranges of parameters: $Re∊[8×103–1.5×105]$⁠, $m∗∊[1.0–3.14]$⁠, $U∊[0.35–1.15m/s]$⁠, $L/D∊[6–36]$⁠, closest distance to bottom boundary $(G/D)∊[4−0.1]$⁠, and $m∗ζ∊[0.14–0.26]$⁠. Test results show strong impact for gap to diameter ratio of $G/D<3$ on VIV, amplitude of VIV, range of synchronization, onset of synchronization, frequency of oscillation, hysteresis at the onset of synchronization, and hysteresis at the end of synchronization.