Vortex-Induced Vibration of symmetric airfoils used in Vertical-Axis Wind Turbines

Abstract

We present an experimental study on a flexibly-mounted NACA 0021 airfoil, allowed to oscillate in the crossflow direction to investigate its Vortex-Induced Vibration (VIV) response at varying angles of attack, α, in the range of 0°≤α≤180°. 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, oscillations are observed for angles of attack in the range of 60°≤α≤130°. The peak amplitude, A∗=1.93, and the largest lock-in range, 1.7<U∗<12, are observed at α=90∘. For angles of attack for which oscillations are observed, the frequency ratio remains close to unity, implying that lock-in has occurred. The wake and crossflow force magnitudes and frequency contents for the airfoil at angles of attack symmetric about α=90∘ are compared and differences in terms of the airfoil displacement, width of the wake, force coefficient magnitudes and frequencies are shown when the leading edge of the airfoil is facing the incoming flow (0°≤α≤85°) or the trailing edge is facing the incoming flow (95°≤α≤180°). It is also shown that the observed VIV occurs within the range of reduced velocities that are expected for the full-scale VAWTs.