Résumé de la publication

Turbulent eddies from rotor to chord length scales carried along the mean wind in wind turbine wake flows cause rapid changes in speed and direction over the rotor disk. Some of these effects such as dynamic stall and its associated hysteresis are well known. Dynamic stall models are already  implemented in most of BEM solvers using the famous Beddoes-Leishman model (see e.g. [1]). However, the developed theories are generally validated against inviscid theories with thin profile approximations and/or low Reynolds number experiments using a dynamic oscillation of the blade profile which reproduces effective velocity variation effects (see. e.g. [2]). Additional effects such as gust and turbulence are generally not taken into account in these approaches. Static or dynamic grid can be added at the inlet of a test section to reproduce a universal homogeneous isotropic turbulence [3] and a gust-like perturbation can be obtained using an additional by-pass duct [4]. The present study focuses on the characterization of a new system added at the inlet of the LHEEA wind tunnel test section to reproduce a sudden deficit of the wind inflow with large turbulent scales superimposed. The range of the produced deficit duration is targeted to be from one order of magnitude faster than the gust time scale in the atmosphere, to the order of a few seconds, to allow characterisation of actuator/sensor dynamics. The facility is a standard return-circuit aerodynamic facility. The test section is 2.6m long and has a 0.25 m2 cross-section. It can operate at a maximum speed of 40 m/s, which leads, with blade chords of around 0.1m, to a chord Reynolds number of ~2.105. The new perturbation system, called later “chopper”, is a rotor with an eccentric rotating axis relatively to the test section so that, when the chopper blade is passing through the test section, it induces an abrupt deficit of the wind inflow (figure 1a). The amplitude of this deficit can be controlled through the relative displacement between the chopper and the 2D blade profile. The associated wake of the chopper blade produces turbulent structures proportional to its blade section, which can be modified. First chopper blade shapes that will be tested are rectangular plates with sharp edges and no pitch angles in order to minimize and localize the shear area. Due to the rotation of the shopper blade, the time scale of the produced turbulent structures is driven by the rotation frequency of the chopper. Also, a grid can be added in front of the chopper, for an increase of the background turbulent intensity. A preliminary characterisation shows the ability of this perturbation system to reach a velocity deficit of 8m/s in 0.2s, which is an order of magnitude faster than the gust-like perturbation reproduced by [3] who obtained a velocity deficit of 4m/s in 2s (figure 1b). All these unsteady and turbulent features will allow us to extract the actuator/sensor dynamics and will allow studying the impact of unsteady and large scale turbulent inflows on the blade aerodynamics. First evaluations of the length scales downstream of the chopper system will be provided in the oral presentation.

« This work was carried out within the framework of the WEAMEC, West Atlantic Marine Energy Community, and with funding from the Pays de la Loire Region »

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