Experimental investigation of Passive Load Reduction under dynamic inflow conditions

Abstract

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

Experimental investigation on the unsteady lift of an airfoil in a sinusoidal streamwise gust

“…11 If a gust passes through such a spatially variable velocity field, its characteristics are altered. 12 All these factors which may cause deviations between theoretical predictions and practical situations are not fully understood. Hence theoretical approaches described above have to be validated experimentally for correct application.…”

“…Then the sinusoidal shape of the gust is distorted, causing variations in the amplitude of its associated velocity field which is related to the fluctuating pressure through unsteady Bernoulli's equation. 12 Thus there is a significant reduction in the fluctuating pressure difference over the front half of the airfoil, and the variation of the fluctuating pressure on the upper side is more significant, compared to the lower side. It can be observed that the distortion shows a frequency dependence.…”