The effect of the integral length scale of turbulence on a wind turbine aerofoil

The effect of free stream turbulence on a DU96w180 wind turbine aerofoil is investigated through wind tunnel experiments. Wind turbine blades experience large scale, high intensity turbulent inflow during their service life. However, the effect of turbulence is normally neglected in the assessment of their aerodynamic performance. This is normally justified based on common assumptions on the effect of the integral length scale of turbulence, which supposedly only acts in the low-frequency range of the energy spectrum, hence affecting the angle of attack instead of the aerodynamic behaviour. In this study, an experimental setup implementing passive grids is developed to vary independently turbulence intensity and integral length scale in wind tunnel testing, with a range spanning I similar to 5-15% and L(similar to)8 - 33 cm respectively. Results show that turbulence effects are not negligible even at the largest integral length scales, provided that a critical value for the turbulence intensity is achieved. Turbulence is found to increase mean and fluctuating Lift and delay separation in stalled conditions.