Passive flow control of vortex-induced vibrations of a low mass ratio circular cylinder oscillating in two degrees-of-freedom

Offshore wind turbine towers are often placed vertically before installation at ports. At certain wind speeds, these towers are subject to vortex-induced vibrations and can lose up to a third of their fatigue live within a few days. So far, helical strakes have primarily been used in the upper part of the towers. These have so far proven to be not effective enough to suppress vortex-induced vibrations. The aim of this study was to develop a passive flow control method for offshore wind turbine towers and to experimentally investigate its efficiency. Experiments were performed in a circulating water tank to investigate using fin plates to suppress vortexinduced vibration of fully submerged flexible cylinders in flows at Reynolds numbers, referred to the cylinder diameter, between 1.4 x 104 and 8.0 x 104. The corresponding non-dimensional reduced velocities (normalized via the free stream velocity, the excitation frequency, and the diameter of the cylinder) were between 1.1 and 6.1. The cylinders' of aspect ratio 18.8 had a mass ratio of 0.535. This mass ratio, referred to the mass of an oscillating cylinder normalized by the mass of displaced fluid, was below the critical mass ratio of 0.545. Each cylinder consisted of a polyvinyl chloride pipe mounted as a cantilever beam on a six degrees-of-freedom piezoelectric transducer. The top end of the cylinders was free to oscillate in the streamwise and lateral directions. An accelerometer was placed near their tops to measure accelerations and, via twofold integration, to retrieve top-end cylinder motions. A Laser Doppler Velocimetry system was employed to measure free stream velocities at the inlet of the water channel and wake velocities behind the cylinder. The wake velocities were used to calculate shedding vortex frequencies. Dynamic forces and displacements were also measured. Results demonstrated that fin plates attached to the cylinders mitigated their vibrations considerably. Optimized fin plate arrangements reduced amplitudes of streamwise and lateral vibration by 81 and 75%, respectively, and amplitudes of lateral forces acting on the cylinders by between 40 and 75%. Low uncertainty estimates of measured values substantiated the reliability of the results.