The experimental characterisation of dynamic stall of the FFA-W3-211 wind turbine airfoil

Abstract. In this work, an experimental campaign was carried out to determine for the first time both the static and dynamic aerodynamic properties of the FFA-W3-211 airfoil. This airfoil is widely used in the wind energy community as part of IEA reference wind turbine designs but is lacking experimental data for design, simulation tool validation and dynamic stall modelling purposes. The airfoil model was designed and manufactured to be tested in the Low Speed, Low Turbulence wind tunnel of the TU Delft. The airfoil was tested statically for Reynolds numbers from Re_c = 5 x 10⁵ to Re_c = 3.5 x 10⁶ and dynamically for up to Re_c = 2 x 10⁶, covering the steady, unsteady and highly unsteady aerodynamic behaviour. Data were acquired through pressure measurements at the surface of the airfoil and in the wake, as well as by the use of thermal cameras. The static results highlight a strong dependence of the lift and drag polars on the Reynolds number and indicate the presence of laminar separation bubbles for the lowest static Reynolds number regimes. Therefore, two distinct regimes can be identified for the static data between which a fundamental change in flow behaviour is observed. The dynamic behaviour was studied for the positive, negative and linear regions of the polar. The positive region is governed by the lack of a leading-edge vortex. This is in contrast to the negative region of the polars where the effects of a vortex close to the leading edge dominate. The sensitivity of results to reduced frequency, amplitude, and Reynolds number is discussed.