Characterization of a new perturbation system for gust generation: The Chopper
Creators: name: Neunaber, Ingrid
name: Braud, Caroline
name: Soulier, Antoine
name: Aubrun, Sandrine
Publisher: name: Zenodo
Year: 2019
Subjects: turbulence, wind tunnel, gust

description: Turbulent eddies from rotor to chord length scales carried along the mean wind in wind turbine wake flows cause rapid<br> changes in speed and direction over the rotor disk. Some of these effects such as dynamic stall and its associated<br> hysteresis are well known. Dynamic stall models are already implemented in most of BEM solvers using the famous<br> Beddoes-Leishman model (see e.g. [1]). However, the developed theories are generally validated against inviscid<br> theories with thin profile approximations and/or low Reynolds number experiments using a dynamic oscillation of the<br> blade profile which reproduces effective velocity variation effects (see. e.g. [2]). Additional effects such as gust and<br> turbulence are generally not taken into account in these approaches. Static or dynamic grid can be added at the inlet<br> of a test section to reproduce a universal homogeneous isotropic turbulence [3] and a gust-like perturbation can be<br> obtained using an additional by-pass duct [4]. The present study focuses on the characterization of a new system<br> added at the inlet of the LHEEA wind tunnel test section to reproduce a sudden deficit of the wind inflow with large<br> turbulent scales superimposed. The range of the produced deficit duration is targeted to be from one order of<br> magnitude faster than the gust time scale in the atmosphere, to the order of a few seconds, to allow characterisation<br> of actuator/sensor dynamics. The facility is a standard return-circuit aerodynamic facility. The test section is 2.6m long<br> 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<br> 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<br> through the test section, it induces an abrupt deficit of the wind inflow (figure 1a). The amplitude of this deficit can be<br> controlled through the relative displacement between the chopper and the 2D blade profile. The associated wake of<br> the chopper blade produces turbulent structures proportional to its blade section, which can be modified. First<br> chopper blade shapes that will be tested are rectangular plates with sharp edges and no pitch angles in order to<br> minimize and localize the shear area. Due to the rotation of the shopper blade, the time scale of the produced<br> turbulent structures is driven by the rotation frequency of the chopper. Also, a grid can be added in front of the<br> chopper, for an increase of the background turbulent intensity. A preliminary characterisation shows the ability of this<br> perturbation system to reach a velocity deficit of 8m/s in 0.2s, which is an order of magnitude faster than the gust-like<br> perturbation reproduced by [3] who obtained a velocity deficit of 4m/s in 2s (figure 1b). All these unsteady and<br> turbulent features will allow us to extract the actuator/sensor dynamics and will allow studying the impact of unsteady<br> and large scale turbulent inflows on the blade aerodynamics. First evaluations of the length scales downstream of the<br> chopper system will be provided in the oral presentation.
description: {"references": ["[1] J.W. Larsen, S.R.K. Nielsen, S.Krenk, (2007), Journal of Fluids and Structures, Vol. 23, issue 7, pp. 959-982", "[2] V. Jaunet &amp; C. Braud, Renewable Energy, 126 (2018) 65\u201378.", "[3] N. Reinke, T. Homeyer, M. Hoelling, J. Peinke (2017) arXiv:1703.00721v1.", "[4] P.P. Sarkar and F.L. Haan, (2008) International Review of Aerospace Engineering (IREASE), Vol 1, issue 1,pp. 26-37."]}