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In the present work, various data processing methods for laminar to turbulent transition detection on airfoils are assessed based on experimental data. For this purpose, NACA 63-418 airfoil profile with surface microphones flush mounted both on the suction and the pressure side is used in the wind tunnel experiments. Reynolds numbers are changed in the range from 1.6 · 10 6 to 6 · 10 6 for various angles of attack. In this way, the transition behaviour of the airfoil is characterized.Time signals and spectrogram of the data, chordwise pressure level changes, and first moments are investigated. Standard deviations of pressure are observed to exhibit a peak at transition and slightly decreases for the fully turbulent flow staying always higher than the laminar flow. The most robust method to detect the transition location is found to be the characteristic frequency approach by spectral moments provided that the inflow turbulence is low. The numerical results are in agreement with experimental results on the pressure side where natural transition occurs.However, the transition due to surface irregularity and microphone placement occurs on the suction side. This cannot be predicted by the numerical tools. The Tollmien-Schlichting wave frequencies and neutral curve points are determined from the experiments. The analysis shows that the most common curve length of the transition process is around 15% to 20%, and it can vary between 0% to 30 % of the chord. Increasing the Reynolds number leads to an earlier transition position closer to leading edge at both upper and lower surfaces. KEYWORDSboundary layer, laminar-turbulent transition, transition detection, wind tunnel experiments, wind turbine airfoil INTRODUCTIONAs the size of modern wind turbines is steadily growing, high Reynolds number experimental data for the wind turbine airfoils and blades are needed for verification and improvement of the aerodynamic prediction tools in the design process. In order to contribute to the development of the airfoil design and to accurately predict airfoil characteristics, the determination of the laminar-turbulent transition point is very important. 1Power curves, which are the indication of the electrical power output at different incoming wind speeds, are highly affected by the lift to drag ratio of the aerodynamic profiles of wind turbines. However, the maximum lift coefficient must be limited due to structural load constraints, which makes drag force one of the major parameters to be studied. 2 Skin friction drag of a laminar boundary layer is significantly lower than that of a turbulent boundary layer. Therefore, by the detection of the transition location from laminar to turbulent flow, it is possible to have an improved frictional drag estimation. Moreover, transition location control mechanisms can be applied to obtain higher lift to drag ratio airfoils.It was hypothesized by Reynolds that the transition is a result of instabilities in the laminar flow, which was further developed by Rayleigh. 3On a given body, starting f...

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Abstract. Laminar-turbulent transition behaviour of a wind turbine blade section is investigated in this study by means of field experiments and 3-D computational fluid dynamics (CFD) rotor simulations. The power spectral density (PSD) integrals of the pressure fluctuations obtained from the high frequency microphones mounted on a blade section are analyzed to detect laminar-turbulent transition locations from the experiments. The atmospheric boundary layer (ABL) velocities and the turbulence intensities (T.I.) measured from the field experiments are used to create several inflow scenarios for the CFD simulations. Results from the natural and the bypass transition models of the in-house CFD EllipSys code are compared with the experiments. It is seen that the bypass transition model results fit well with experiments at the azimuthal positions where the turbine is under wake and high turbulence, while the results from other cases show agreement with the natural transition model. Furthermore, the influence of inflow turbulence, wake of an upstream turbine and angle of attack (AOA) on the transition behaviour is investigated through the field experiments. On the pressure side of the blade section, at high AOA values and wake conditions, variation of the transition location covers up to 44 % of the chord during one revolution, while for the no wake cases and lower AOA values, variation occurs along a region that covers only 5 % of the chord. The effect of the inflow turbulence on the effective angle of attack as well as its direct effect on transition is observed. Transition locations for the wind tunnel conditions and field experiments are compared together with 2D and 3D CFD simulations. In contrast to the suction side, significant difference in the transition locations is observed between wind tunnel and field experiments on the pressure side for the same airfoil geometry. It is seen that the natural and bypass transition models of EllipSys3D can be used for transition prediction of a wind turbine blade section for high Reynolds number flows by applying various inflow scenarios separately to cover the whole range of atmospheric occurrences.