Experimental characterization of dynamic stall of the FFA-W3-211 wind turbine airfoil

Abstract. In this work, an experimental campaign was carried out to determine both the static and the 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 modeling purposes. The airfoil model was designed and manufactured for testing in the low-speed, low-turbulence wind tunnel at TU Delft. The airfoil was tested statically for Reynolds numbers ranging from Rec=5×105 to Rec=3.5×106 and dynamically for up to Rec=2×106, encompassing steady, unsteady and highly unsteady aerodynamic behavior. Data were acquired through pressure measurements at the surface of the airfoil and in the wake, as well as by using thermal cameras. The static results highlighted a strong dependence of the lift and drag polars on the Reynolds number and a change in trends around Rec=2×106. The suspected presence of laminar separation bubbles for the lower Reynolds numbers could explain this fundamental change in flow behavior. The dynamic behavior was studied at high positive angles of attack, high negative angles of attack and within the linear region of the polar around the zero-lift angle. 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 appearing close to the leading edge dominate. The sensitivity of the results to reduced frequency, amplitude and Reynolds number is discussed. Overall, for the FFA-W3-211 airfoil, it is recommended to use experimental data of Rec=2×106 or above to capture the correct physical (static and dynamic) trends relevant for larger wind turbine blades. For dynamic stall model tuning, it may also be important to consider the significant change in behavior between positive and negative stall angles.