{"references": ["European Wind Energy Association, EU energy policy after 2020,\nwww.ewea.org, 2011.", "M. Raciti Castelli, G. Grandi and E. Benini, \"Numerical Analysis of the\nPerformance of the DU91-W2-250 Airfoil for Straight-Bladed Vertical-\nAxis Wind Turbine Application\", submitted to ICAFM 2012:\nInternational Conference on Advances in Fluid Mechanics, Florence\n(Italy), February 28-29, 2012.", "W.A. Timmer, W. A. and R. P. J. O. M. van Rooij, \"Summary of the\ndelft university wind turbine dedicated airfoils\", AIAA-2003-0352.", "W.A. Timmer, W. A. and R. P. J. O. M. van Rooij, \"Design of airfoils\nfor wind turbine blades\", DUWIND, section Wind Energy, Faculty\nCiTG, 03 May, 2004.", "G. Lombardi, M. V. Salvetti and D. Pinelli, \"Numerical Evaluation of\nAirfoil Friction Drag\", J. Aircraft, Vol. 37, No. 2, pp. 354-356, 2000.", "Y. Lian and W. Shai, \"Laminar-Turbulent Transition of a Low Reynolds\nNumber Rigid or Flexible Airfoil\", AIAA Journal, Vol. 45, No. 7, July\n2007, pp. 1501-1513.", "F. R. Menter, R. B. Langrty, S. R. Likki, Y. B. Suzen, P. G. Huang and\nS. V\u00f6lker, \"A Correlation-Based Transition Model Using Local\nVariables - Part I: Model Formulation\", Journal of Turbomachinery,\nVolume 128, Issue 3, pp. 413-422, 2006.", "F. R. Menter, R. B. Langrty, S. R. Likki, Y. B. Suzen, P. G. Huang and\nS. V\u00f6lker, \"A Correlation-Based Transition Model Using Local\nVariables - Part II: Test Cases and Industrial Applications\", Journal of\nTurbomachinery, Volume 128, Issue 3, pp. 423-434, 2006.", "E. Benini and R. Ponza, \"Laminar to Turbulent Boundary Layer\nTransition Investigation on a Supercritical Airfoil Using the \u256c\u2502-\u256c\u00a9\nTransitional Model\", 40th Fluid Dynamics Conference and Exhibit, 28\nJune - 1 July 2010, Chicago, Illinois, AIAA 2010-4289.\n[10] S. Hosseinverdi and M. Boroomand, \"Prediction of Laminar-Turbulent\nTransitional Flow over Single and Two-Element Airfoils\", 40th Fluid\nDynamics Conference and Exhibit, 28 June - 1 July 2010, Chicago,\nIllinois, AIAA 2010-4290.\n[11] L. Yuhong and L. Congming, \"A numerical simulation of flow around a\nwind turbine airfoil based on transition model\", WNEC 2009: World\nNon-Grid-Connected Wind Power and Energy Conference, Nanjing\n(China), 24-26 Sept. 2009.\n[12] W.A. Timmer, W. A. and R. P. J. O. M. van Rooij, Aifoil DU91-W2-\n250 Coordinates and Measurements in Delft University 1.25x1.80 m\nLow-speed Wind tunnel, data provided by direct contact from the\nauthors.\n[13] ANSYS FLUENT 12.0-12.1 Documentation, Release 12.1 \u252c\u00ae ANSYS,\nInc. 2009-10-01.\n[14] J. Johansen, \"Prediction of Laminar/Turbulent Transition in Airfoil\nFlows\", Ris\u251c\u00a9 National Laboratory, Roskilde, Denmark, May 1997,\nRis\u251c\u00a9-R-987(EN).\n[15] M. Raciti Castelli, F. Garbo, E. Benini, \"Numerical investigation of\nlaminar to turbulent boundary layer transition on a NACA 0012 airfoil\nfor vertical-axis wind turbine applications\", Wind Engineering, Vol. 35,\nNo. 6, 2011, pp. 661-686."]}

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.