Abstract. The aerodynamic behaviour of a wind turbine airfoil has been measured in a dedicated 2D wind tunnel test at the DNW High Pressure Wind Tunnel in Gottingen (HDG), Germany. The tests have been performed on the DU00W212 airfoil at different Reynolds numbers: 3, 6, 9, 12 and 15 million, and at low Mach numbers (below 0.1). Both clean and tripped conditions of the airfoil have been measured. An analysis of the impact of a wide Reynolds number variation over the aerodynamic characteristics of this airfoil has been performed.

Wind turbine sizes have grown continuously in the past. That is why the questions such as how much the wind turbine size can grow and what are the aspects that limit the growth come from have to be answered. In order to analyze these questions for the rotor of such a wind turbine, the preliminary design of a 20MW wind turbine rotor has been performed within EU 6th Framework Project UPWIND. Although the rotational speed of the wind turbine is as low as around 6 rpm, due to the growth in size very high Reynolds number values of up to 25 million have been reached. The effects of very high Reynolds numbers on the aerodynamic characteristics of the airfoils are investigated and their influences on the rotor design and performance are discussed. As a result, up to 20% reduction in the chord can be achieved due to the increase in the C l and decrease in C d in high Reynolds numbers. Since the change in the Reynolds numbers influences the rotor design in many respects, more detailed investigations and especially wind tunnel tests of the airfoils for very high Reynolds numbers are required in order to have reliable future large wind turbines. Nomenclature= Mach number P = Power r = local radius Re = Reynolds number U = Free stream wind velocity y + = dimensionless wall distance λ = tip speed ratio υ = kinematic viscosity

An EERA (European Energy Research Alliance) consortium started an ambitious EU FP7 project AVATAR (AdVanced Aerodynamic Tools of lArge Rotors) in November 2013. The project lasts 4 years and is carried out in a consortium with 11 research institutes and two industry partners. The motivation for the AVATAR project lies in the fact that future 10 to 20 MW turbine design model analysis will importantly violate known validity limits of today's aerodynamic and aero-elastic models in aspects like compressibility and Reynolds number effects, laminar/turbulent transition and separation effects, all in combination with a much more complex fluid-structure interaction. Further complications enter by the possible use of active or passive flow devices. AVATAR's main aim is then to develop enhancements for aerodynamic and aero-elastic models suitable for large (10MW+) wind turbines analysis. The turbine modelling improvements will be demonstrated on a new 10MW reference turbine design model description. The first results from the AVATAR project are presented in this paper.

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