# Dynamic Aeroelastic Response of Stall-Controlled Wind Turbine Rotors in Turbulent Wind Conditions
## 1 Introduction  
Renewable energy production, especially wind power, has been growing over time to become an important source for providing for electricity and fuel demands. Technological advances have made wind the fastest-growing renewable energy technology in electricity generation systems. Following the current global trend of the wind turbines to be commissioned and the studies conducted by the European Wind Energy Association (EWEA), the next generation of state-of-the-art turbines will have a generating capacity of 20 MW with rotor diameters of 250 m or larger [1]. This systematic increase in rotor size is prompted by economies-of-scale factors, thereby resulting in a continuously decreasing cost per kWh generated. However, such large rotors have larger masses associated with them and necessitate studies in order to better understand their dynamics. This has led to numerous efforts on the development of suitable control approaches [2; 3; 4; 5]. [PERSON] et al. [6] provided details on some common load control methodologies. Among the ones that have been studied and implemented in the context of utility-scale wind turbines are stall control [7], pitch control [8], active stall control [9], and passive control using aeroelastic devices [10]. In all cases, wind turbine control systems have to deal with the challenges of an intermittent and difficult-to-predict source of energy. Wind, by its very nature, varies with place and time, making energy conversion difficult. This calls for the development of improved technologies that efficiently harness energy from the available wind resources, and advanced control systems are a key research aspect of wind turbine technology [11]. Such systems are needed to maximize the output power at low wind speeds, limit rotorpower at wind speeds above the nominal value, and reduce fluctuating loads on the turbine blades that may compromise their long-term operational life [12]. That is, the basic objective is to maximize the rotor's power production for wind speeds below the nominal level and limit it for wind speeds above the nominal level, keeping it as close as possible to the nominal power of the drive train in order to avoid exceeding the nominal power of the shafts, bearings, gear box, generators, and power electronics. Among the several families of control methods mentioned above, pitch and stall control have been by far the most commonly used in modern utility-scale machines. Both are based on altering the aerodynamic characteristics of the blade sections in order to control the forces that produce the rotor's torque, power, thrust, and deformation. By rotating the blades around their root, pitch-control systems allow us to smoothly change the aerodynamic forces acting on the blade by altering the angle of attack on the airfoil blade sections. By changing the tangential projection of the aerodynamic force (which is the one contributing to the driving torque), pitch systems can smoothly control the rotor's output power. In stall-control turbines, the blades are rigidly attached to the hub. Thus, the setting angle of every airfoil section along the blade span is fixed. In the classic fixed-speed stall-control method, the rotational speed of the rotor is kept constant at its nominal value, and the geometry of the blades is designed in such a way that, as the wind speed increases above the nominal value, the angle of attack of the flow incoming onto the blade's airfoil sections increases, and a larger portion of the blade span enters into stall. The stall condition is characterized by the separation of the boundary layer on the rear-upper surface of an airfoil section at high angles of attack. Separation disrupts the flow circulation and alters the pressure distribution, drastically reducing lift and increasing drag. The overall result is that the portions of the blade span that enter into stall stop contributing to the driving torque, limiting the rotor's power production. The stall transition is not intended to occur simultaneously all over the span.