Airfoil load mitigation by an active slat under turbulent inflow conditions

Kurzfassung

The presence of wind turbines in the atmospheric boundary layer causes them to be exposed to turbulent inflow conditions. The wind turbine blades experience inflow velocity fluctuations because of turbulent structures of various scales and wind gusts [1]. This results in the fluctuation of the angle of attack as perceived by the sectional airfoils. This induces fluctuating loads on the wind turbine blade, which is undesirable because of its deteriorating effect on the blade life and efficiency [2]. The massive inertia of the entire blade inhibits the common blade pitch control to react to these high frequency load fluctuations. An actively deformable leading-edge slat system was recently investigated by Neuhaus et al. [3], which aimed for airfoil load mitigation through fast reaction under sinusoidal inflow conditions .The work focused on the characterisation and estimation of the concept’s initial capabilities. It was reported that the leadingedge active slat significantly delays the stall to higher angles of attack. For a sinusoidal inflow, the active slat was able to reduce 20% of the lift force fluctuations. It was also reported that there is a dependency of the lift coefficient on the gap size between the slat and the main body of the airfoil. As it was a preliminary investigation, this property was not utilised for designing the control strategy. The present study takes the work of Neuhaus et al. [3] further by comprehensively gauging the performance of the active slat by testing it under harsh turbulent conditions. An active grid is used to create spanwise correlated turbulent inflow angle fluctuations having different intermittency levels. Figure 1 (a) shows one of the generated inflow angle time series. These fluctuations directly result in lift coefficient (CL) fluctuations (figure 1(b)). The active slat provides the ability to change the aerodynamic forces acting on the airfoil. Different open loop control strategies are designed and implemented, which leverage this property of the active slat, to reduce the fluctuating aerodynamic forces under the influence of turbulent inflow conditions. The loads on the airfoil in the controlled slat cases are compared to the case where the slat is static. For most of the cases, the control strategies are successfully able to reduce the load fluctuations. In some cases, CL standard deviation reduction of as much as 60% (figure 1(b)) are observed.