How to design a 2D active grid for dynamic inflow modulation

Wind turbines operate under constantly changing turbulent inflow conditions. In the rotating system, wind gusts lead to variations in the angle of attack at local blade segments resulting in dynamic effects such as dynamic stall. Such highly non-linear effects are known to produce a significant overshoot in the lift and thus an increase in loads acting on the wind turbine, leading to long-term fatigue. To better understand these effects, it is essential to perform experiments under defined conditions on 2D airfoil segments in the wind tunnel. In this study, a so-called 2D active grid is presented which allows to generate local inflow conditions with defined fluctuations of the angle of incidence (AoI) in wind tunnel experiments. The focus of the investigations is on sinusoidal variations of AoI with high amplitudes generated by different grid configurations. By changing the AoI dynamic phenomena can be induced without the need to move the object under investigation. Inertial effects during force measurements and a changing shadow casting due to a moving airfoil in particle image velocimetry measurements do not appear. Additional variations in the longitudinal velocity component are another aspect in the presented work. Such longitudinal gusts can be combined with AoI variations in arbitrary phase. This can be used to mimic various inflow situations such as yaw or tower shadow effects on wind turbines.