Vortex-Induced Vibration of Symmetric Airfoils Used in Vertical-Axis Wind Turbines

Abstract

The present work considers a flexibly-mounted NACA 0021 airfoil, allowed to oscillate in the crossflow direction to investigate its Vortex-Induced Vibration (VIV) response at different angles of attack. This airfoil is considered, since it is one of the airfoils used in Vertical-Axis Wind Turbine (VAWT) designs. Based on the experimental results of the current work, for smaller angles of attack, α, no oscillation was observed. Oscillations started at α = 60°, with a rather small amplitude and small lock-in range, and grew in peak amplitude and width of the lock-in range through α = 90°, where VIV persisted for a wide range of reduced velocities: 1.7 < U* < 12 (where U* = U/f_nD, is the reduced velocity, U is the flow velocity, f_n is the structure’s natural frequency in otherwise still water, and D is the chord length) and reached an amplitude of A* = 1.93 (defined as A* = A/D, where A is the amplitude of oscillations and D is the chord length of the airfoil). Flow visualizations were conducted using Bubble Image Velocimetry (BIV) technique, with image pairs captured by a high-speed camera. These flow visualizations indicated 2S vortex shedding patterns in the wake of the oscillating airfoils at a frequency equal to the oscillation frequency, confirming that the observed oscillations are VIV.