Characterization of a new perturbation system for gust generation: The Chopper

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.