Wind tunnel study on natural instability of the normal force on a full-scale wind turbine blade section at Reynolds number 4.7 center dot 10(6)

Wind turbines are exposed to the turbulent wind of the atmospheric boundary layer. Consequently, the aerodynamic forces acting on the rotor blades are highly complex. To improve the understanding, a common practice is the experimental or numerical investigation of 2d (wind turbine) blade sections. In these investigations, the flow around the 2d blade section is assumed to be two-dimensional; however, 3d effects are known to occur. Therefore, we combine 2d CFD simulations and experimental investigations in a wind tunnel with a 2d wind turbine rotor blade section at full-scale (i.e., chord length c=1.25m$$ c=1.25\kern0.1em \mathrm{m} $$ and chord-based Reynolds number of Rec=4.7 center dot 106$$ R{e}_c=4.7\cdotp 1{0}<^>6 $$). In the wind tunnel, the inflow turbulence intensity is TI approximate to 1.5%$$ TI\approx 1.5\% $$. To avoid wall effects biasing the results, the profile does not span the whole test section. The profile was equipped with two rows of pressure taps around the airfoil, close to the center, to monitor the time-resolved aerodynamic response as well as the flow around the airfoil. The normal force, cp$$ {c}_p $$ curves, and the separation point are analyzed. While 2d simulations and experiments match well, in the experiments, we find natural instabilities, that is, local and temporal variations of the flow separation point at angles of attack close to the maximum lift that are not triggered externally, for example, by inflow variations.