Two transfer functions for the unsteady lift response of an airfoil under attached flow conditions are experimentally investigated: the Theodorsen function for an airfoil oscillating in a constant free stream and the Sears function for a steady airfoil encountering a sinusoidal vertical gust. A two-dimensional airfoil with a Clark Y profile is submitted to two different unsteady excitations of distinct frequencies: a pitching oscillation around the leading edge and a sinusoidal vertical gust. The reduced frequency of the perturbation is in the range of $0.025<k<0.3$ and the Reynolds number of the undisturbed flow is in the range of $120\,000<\mathit{Re}<300\,000$. While the Theodorsen function is found to be a good estimator for the unsteady lift at moderate mean angles of attack, the Sears function does not capture the experimental transfer functions in frequency dependence or in limiting values. A second-order model provided by Atassi (J. Fluid Mech., vol. 141, 1984, pp. 109–122) agrees well with the experimental transfer function.

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Wind turbines often operate in highly turbulent conditions where the angle of attack can change significantly. The resulting aerodynamic load fluctuations are transmitted from the blades to the drive train and tower. These unsteady loads increase fatigue, which decreases lifetime and limits the upscaling of turbines. State-of-the-art pitch mechanisms are designed to alleviate load fluctuations in the order of minutes to hours but are too slow to account for high frequency fluctuations due to turbulence. Several new and faster load reduction mechanisms are currently under investigation. Passive systems which use the aeroelastic response usually only act on the entire rotor blade (e.g. bend-twist coupling). Active systems use sensors and actuators to adapt the aerodynamic properties of the blade to the inflow conditions and are usually installed in the outer regions of the blade, where load fluctuations have the largest impact on the system, but these systems involve complicated control schemes. At the Technische Universität Darmstadt, a load reduction mechanism has been developed which combines the advantages of both concepts: the robust nature of a passive system and the flexible installation possibilities of an active load reduction system. A twodimensional airfoil equipped with this concept has been investigated experimentally. Stationary experiments have been performed first to demonstrate the concept. Since gusts are non-stationary by definition, further investigations under unsteady inflow conditions were then conducted. Experiments were performed at the University of Oldenburg in an active grid wind tunnel, which offers the possibility to generate repeatable dynamic inflow conditions. The results indicate that the self-adapting camber airfoil successfully reduces load fluctuations over a wide range of operational parameters. NomenclatureA = amplitude of angle of attack (AoA) oscillation α = total AoA α m = mean AoA c = chord length Δp = pressure difference between suction and pressure side at xDP f = frequency of AoA oscillation 1 Doctoral 2 γ = trailing edge (TE) angle k = reduced frequency kθ = torsional stiffness of leading edge (LE) flap L = lift L adaptive = lift of adaptive camber airfoil L dyn = lift under dynamic inflow L rigid = lift of rigid airfoil L stat = lift under steady inflow D = drag LR dyn = dynamic load reduction factor LE mean = mean load enhancement factor M 0 = pre-camber moment u = magnitude of velocity xDP = pressure tap non dimensional chord wise position xLE = LE non dimensional chord wise position xTE = TE non dimensional chord wise position

A new passive sectional approach to alleviate gust loads on wind turbines—the Adaptive Camber Concept— is introduced. The concept entails fluid‐structure interaction, where flow conditions at the leading edge affect the airfoil shape and vice versa. A two‐dimensional airfoil equipped with the Adaptive Camber Concept is experimentally investigated under steady and unsteady conditions in a wind tunnel. Under steady conditions, the adaptive camber airfoil de‐cambers gradually with increasing angle of attack, yielding a lift curve with declined slope. Unsteady angle of attack fluctuations of various reduced frequencies are generated by means of an active grid. Under unsteady conditions, the adaptive camber airfoil is found to alleviate up to 60% of the fluctuating loads, while generating higher mean lift compared to a rigid reference airfoil.

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