The trend towards ever larger rotor diameters of wind turbines not only leads to an increase in cost efficiency per kilowatt hour, but also, as a result of increased blade flexibility, to an increased susceptibility of the rotor blades to vibrations during start-up, normal operation and emergency stops. Under unsteady flow conditions, such as atmospheric turbulence, wind gusts and strong directional and spatial variations in wind shear, the blades may experience dynamic stall. Depending on the instantaneous angle of attack, the extent of stalling, and the strength of the dynamic stall vortex, a light or deep stall of the blades can occur. The latter is characterized by a strong hysteresis of the loads as well as a reduced damping, which not only leads to strong aerodynamic forces and moments on the blades and hub, but also to aeroelastic instability problems. Hence, the investigation of unsteady aerodynamics to predict blade loads during dynamic stall is crucial for the design and operation phases of wind turbines.
To determine the unsteady loads on wind turbine blades during light and deep dynamic stall, experimental studies are carried out at the Institute of Aeroelasticity both in the transonic wind tunnel (DNW-TWG) within the project "ELBA" and in the High Pressure wind tunnel (DNW-HDG) within the project "Aeroelastic Modelling". In both projects an identical wind blade profile (NREL S809) has been selected to determine both the influence of the Mach number in TWG (M > 0.3, Rec = 106) and the influence of the Reynolds number in HDG (M < 0.1, Rec = 107) on the aeroelastic behaviour. By using pitch test rigs that have been specially designed and manufactured for each wind tunnel, various influencing parameters, such as the angle of attack and the pitch frequency and amplitude, can be varied as function of the Mach number and Reynolds number. Their influences on the unsteady aerodynamics can be obtained by means of unsteady pressure and acceleration sensors in the model, piezoelectric force and moment balances in the test rig, and a wake rake in the wind tunnel.
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