Abstract—This paper presents a study of laminar to turbulent transition on a profile specifically designed for wind turbine blades, the DU91-W2-250, which belongs to a class of wind turbine dedicated airfoils, developed by Delft University of Technology. A comparison between the experimental behavior of the airfoil studied at Delft wind tunnel and the numerical predictions of the commercial CFD solver ANSYS FLUENT® has been performed. The prediction capabilities of the Spalart-Allmaras turbulence model and of the γ-θ Transitional model have been tested. A sensitivity analysis of the numerical results to the spatial domain discretization has also been performed using four different computational grids, which have been created using the mesher GAMBIT®. The comparison between experimental measurements and CFD results have allowed to determine the importance of the numerical prediction of the laminar to turbulent transition, in order not to overestimate airfoil friction drag due to a fully turbulent-regime flow computation. Keywords—CFD, wind turbine, DU91-W2-250, laminar to turbulent transition I. INTRODUCTION AND BACKGROUND ITH its 2020 goals to increase the share of renewable energy in the overall energy mix to 20% and to cut carbon emissions by 20%, the EU is leading the world in terms of renewable energy deployment, exports and promotion. Today Europe gets approximately 20% of its electricity from renewable energy sources, including 5.3% from wind Energy. That share will increase up to 2020 when, under the terms of the EU’s renewable energy directive, which sets legally binding targets for renewable energy in Europe, 34% of the EU’s total electricity consumption will come from renewable energy sources, with wind energy accounting for 14% [1]. In this scenario, the continuous quest for clean energy appears to be connected with the development of the aerodynamics of actual wind turbines, in order to achieve a growth of their performances, both for the classical horizontal-axis (HAWT) and also the vertical-axis (VAWT) concepts [2]. For the past years, it was common practice to use existing airfoil families, like the well known NACA series, for the design of wind turbine blades, however the need of furthering wind turbine technologies has led to the quest for alternatives. Marco Raciti Castelli is a Research Associate at the Department of Industrial Engineering of the University of Padua, Via Venezia 1, 35131 Padua, Italy (e-mail: marco.raciticastelli@unipd.it). Giada Grandi is a M.Sc. student in Aerospace Engineering at the University of Padua, Via Venezia 1, 35131 Padua, Italy. Ernesto Benini is Associate Professor at the Department of Industrial Engineering of the University of Padua, Via Venezia 1, 35131 Padua, Italy (e- mail: ernesto.benini@unipd.it). The airfoil analyzed in the present work is the DU91-W2- 250, which belongs to a class of wind turbine dedicated airfoils developed by Delft University of Technology. At present, DU airfoils are being used by various wind turbine manufacturers worldwide, in many different rotor blades. The design of the DU91-W2-250 airfoil followed wind tunnel tests on a 25% thick NACA airfoil from the 63-4xx series, linearly scaled from 21%. To compensate for the resulting loss in lift of the upper surface, a certain amount of lower surface aft loading was incorporated, giving DU91-W2- 250 the typical S-shape of the pressure side. This airfoil, like other 25% thick airfoils, has very high peak lift coefficient in the smooth condition and presents an acceptable performance in the rough situation, differently from classical NACA airfoils. The main features of the mid span airfoil are a good maximum lift to drag ratio and a smooth stall behavior [3] [4]. Fig.1 Comparison between the DU91-W2-250 airfoil and a 5-digit NACA airfoil Every flow causes pressure and friction on the body surface, which result in forces and moments acting on the body itself. Nowadays, thanks to advances in numerical methods and computing power, the investigation and solution of the flow field around an airfoil has become relatively simple. By performing CFD analysis on the DU91-W2-250, together with turbulence and transition modeling testing, the main purpose of the present work is to investigate its behavior, with particular attention to the laminar to turbulent transition phenomena. Lombardi et al. [5] tested the capability of a classical RANS solver of predicting the friction drag over a NACA 0012 airfoil for 0 deg angle of attack and compared CFD results with the values given by a coupled potential/boundary-layer method. The analyzed range of Reynolds numbers varied from 300,000 to 9,000,000. As a result, being the local skin friction coefficient defined as: c f = τ w / (½·ρ·c·V ∞ 2 ) (1) M. Raciti Castelli, G. Grandi and E. Benini Numerical Analysis of Laminar to Turbulent Transition on the DU91-W2-250 airfoil W World Academy of Science, Engineering and Technology International Journal of Aerospace and Mechanical Engineering Vol:6, No:3, 2012 717 International Scholarly and Scientific Research & Innovation 6(3) 2012 scholar.waset.org/1307-6892/12816 International Science Index, Aerospace and Mechanical Engineering Vol:6, No:3, 2012 waset.org/Publication/12816