# Wind Turbine Enhancement via Active Flow Control Implementation
## 1 Introduction  
The number of articles tackling Active Flow Control (AFC) applied to airfoils is rising sharply [1; 2; 3; 4; 5]. The AFC technology basically consists of adding or subtracting momentum to/from the main flow in order to interact with the boundary layer and delay its separation. Its main advantage over passive flow control approaches is that it is not creating a drag penalty in off-design conditions. The different AFC techniques were divided by [PERSON] and [PERSON] [6] into three categories: (1) moving body actuators [7], (2) plasma actuators [8; 9; 10; 11], and (3) fluidic actuators (FA). From these three categories, FA are the only ones capable of generating large momentum coefficients, and this is why they were chosen for the present research. In some particular FA designs, the origin of the self-sustained oscillations was recently unveiled in [12; 13; 14]. From the different FA, Zero Net Mass Flow Actuators (ZNMFA), also called synthetic jet actuators (SJA), have the advantage of not requiring external fluid supply, being particularly effective in controlling the separation of the boundary layer [15; 16; 17; 18]. The use of pulsating flow has the advantage of coupling with the boundary's layer natural instabilities and so being more energetically efficient [19; 20; 21]. In fact, in order to minimize the energy required in AFC implementations, it is needed to somehow optimize the different parameters associated with each particular AFC application; nowadays, the most common methodology employed to tune the AFC-associated parameters is a parametric optimization, although the use of optimizers is increasingly accepted [22; 23; 24]. Active Flow Control implementation in Vertical (VAWT) and Horizontal Axis Wind Turbines (HAWT) is recently gaining momentum [1; 25; 26; 27; 28; 29; 30; 31; 32], although so far, the bases to optimize the five AFC-associated parameters along the blade are not being established. Among the different devices employed in AFC applications on (WT), synthetic jets (SJ) appear to be particularly effective, as they combine the use of periodic forcing (blowing/suction) with a sufficiently large momentum coefficient \(C_{\mu}\), which is defined as \(C_{\mu}=(h\rho_{jet}(U_{i}^{2})\sin\theta)/(C\rho_{\infty}(U_{\infty}^{2}))\), where \(h\) is the jet width, \(\rho_{jet}\) and \(\rho_{\infty}\) are the jet and far field densities, respectively, \(U_{j}\) is the maximum jet velocity, \(C\) is the airfoil chord, \(U_{\infty}\) is the free-stream velocity, and \(\theta\) is the jet inclination angle with respect to the adjacent surface, a combination that appears to be particularly effective in delaying the boundary layer separation. The non-dimensional frequency associated with periodic forcing is defined as \(F^{\pm}=fC/U_{\infty}\), with \(f\) being the boundary layer natural dimensional frequency. A thorough experimental work employing SJ on wind turbine blade sections was undertaken by [25], where it was established that AFC-SJ was capable of delaying airfoil stall and proved that a significant reduction of wind turbine start-up velocity was possible. In [26; 33], SJ were employed to control the fluid flow vibration induced on turbine blades; they observed that the boundary layer reattachment obtained when applying AFC was having associated lower amplitude lift and drag dynamic coefficients and with it smaller structural vibrations. The use of SJ to enhance the performance of VAWT was experimentally addressed in [27]; they obtained a considerable reduction of the noise generated while increasing the power generated by the turbine as well as its safety. Similar experimental studies using SJ but on a VAWT S809 and a HAWT S809 airfoil were performed by [28] and [31], respectively. The former study demonstrated that lift, drag, and pitching moment hysteresis could be significantly reduced. In the later research they observed that a 10.6% power input reduction to drive the rotor was achieved when a total of 20 SJA along each blade were employed.