The unsteady flow around a stationary two-dimensional wind turbine blade section (NREL S809) has been simulated using unsteady RANS with the SST turbulence model at Re = 10 6 and high angles of attack. Vortex shedding frequency non-dimensionalised by the projected area of the blade is in the range 0.11 < St < 0.16. The fluctuating coefficients at the harmonics of the fundamental vortex shedding frequency for lift, drag and pitching moment are presented. The lift force and the pitching moment (calculated with respect to the midchord) are dominated by the fundamental vortex shedding frequency, but significant contributions at the higher harmonics are also present. For the drag the second harmonic is as significant as the fundamental frequency, with the third and fourth harmonics contributing to a lesser extent. From the results, it is inferred that the camber of the airfoil does not substantially affect the force coefficients, suggesting that the results presented may be generally applicable to turbine blades of different geometry.

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 Re = 10 6 based on a chord length of 1 m. A prescribed sinusoidal pitching motion has been applied at a fixed amplitude of 7°for a range of high angles of attack 30°< α < 150°. 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, f v , with the body's motion frequency, f p . In order to investigate the susceptibility of airfoil to lock-in, the frequency ratio r (r = f p /f v0 ) has been varied around r = 1. 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 α ≈ 40°and α ≈ 140°, 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.

The unsteady, incompressible flow around a translating two-dimensional wind turbine blade section (NREL S809) in the stream-wise direction has been simulated using unsteady RANS with the transition SST turbulence model. The Reynolds number is Re = 106 referred to a chord length of 1 m. A prescribed sinusoidal stream-wise motion has been applied at a fixed amplitude of 0.25 m for a range of high angles of attack [30° < α < 150°]. At these incidences, the airfoil will behave more like a bluff body and may experience periodic vortex shedding. It is well known that oscillations can lead to a synchronization (lock-in) of the vortex shedding frequency, fv, with the body’s motion frequency, fs, in bluff body flows. In order to investigate the susceptibility of the wind turbine blade section to lock-in, a parametric study has been conducted varying the frequency ratio r, (r = fs/fv0), in a range around r = 1 and r = 0.5. The lock-in region boundaries have been proposed and an analysis of the effect of the oscillation amplitude has been conducted. The synchronization map obtained suggests that, for the vibration amplitude considered, the risk of vortex-induced vibration is more significant in the regions of α = 35° and α = 145°. Furthermore, it has been found that for some stream-wise amplitudes, increasing the oscillation amplitude, lock-in appears to be unexpectedly suppressed in the vicinity of r = 1.