Vortex and Wake-Induced Vibrations of Vertical-Axis Wind Turbine Blades

We present a series of experimental results to investigate vortex and wake-induced vibrations of an airfoil used in vertical-axis wind turbines (VAWTs). The experiments were conducted in both a re-circulating water tunnel and wind tunnel to study the response of a flexibly-mounted NACA 0021 airfoil, which is used in some designs of VAWTs. The airfoil was free to oscillate in the crossflow direction, and tests were conducted in a Reynolds number range of 600<Re<13,300 and reduced velocity range of 0.6<U*<13. The amplitudes of oscillations and flow forces acting on the airfoil were measured over the entire possible range of angles of attack, 0°<α<180°. In the VIV case, the airfoil was observed to oscillate in the range of 60°<α<130°, where α=90° exhibited the widest lock-in range and peak amplitude. For all cases where oscillations were observed, the oscillation frequency remained close to the structure's natural frequency, defining a lock-in range. Flow visualization tests were also conducted to study the changes in vortex shedding patterns, especially between symmetrical angles of attacks about α=90°.