Vortex-Induced Vibration of an Airfoil Used in Vertical-Axis Wind Turbines*

In Vertical-axis wind turbines (VAWTs), when the blades are placed at high angles of attack with respect to the incoming flow, they could experience flow-induced oscillations. A series of experiments in a re-circulating water tunnel was conducted to study the possible Vortex-Induced Vibration (VIV) of a fully-submerged, flexibly-mounted NACA 0021 airfoil, which is used in some designs of VAWTs. The airfoil was free to oscillate in the crossflow direction, and the tests were conducted in a Reynolds number range of 600\textless \textit{Re}\textless 13,300 and reduced velocity range of 0.6\textless $U*$\textless 13. The amplitudes of oscillations and flow forces acting on the airfoil were measured at various angles of attack, $\alpha $, in the range of 0\textless $\alpha $\textless 90. The airfoil was observed to oscillate in the range of 60\textless $\alpha $\textless 90, where $\alpha =$90 exhibited the widest lock-in range (1.67\textless $U*$\textless 11.74) and the largest peak amplitude ($A*=$1.93 at $U*=$5.7). 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 the vortex shedding patterns.