Vortex shedding lock-in due to pitching oscillation of a wind turbine blade section at high angles of attack

Abstract

The unsteady flow around a pitching two-dimensional airfoil section (NREL S809) has been simulated using unsteady RANS with the transition SST turbulence model. This geometry is chosen to represent a wind turbine blade in a standstill configuration. The Reynolds number is based on a chord length of 1 m. A prescribed sinusoidal pitching motion has been applied at a fixed amplitude of for a range of high angles of attack . At these incidences, the airfoil will behave more like a bluff body and may experience periodic vortex shedding. It is well known that, in bluff body flows, oscillations can lead to a lock-in (lock-in) of the vortex shedding frequency, , with the body’s motion frequency, . In order to investigate the susceptibility of airfoil to lock-in, the frequency ratio () has been varied around . The lock-in region boundaries have been proposed, and an analysis of the effect of the oscillation amplitude has been conducted. The lock-in map obtained suggests that, for the vibration amplitude considered, the risk of vortex-induced vibration is more significant in the regions of and , i.e., for shallower characteristic lengths. Finally, a lumped parameter wake oscillator model has been proposed for pitching airfoils. This simple model is in qualitative agreement with the CFD results.