# Wind Turbine Enhancement via Active Flow Control Implementation
## 1 Introduction
Due to the high cost associated with experimental studies and the increasing trustworthiness of the computational simulations, the current tendency is to simulate the performance of any device. In this direction are going the 2D-URANS numerical simulations on VAWT performed by [1; 29]. In both cases a straight-bladed WT was investigated, although the second one was of Darriues type. Three different SJ locations were investigated in the later research, while in the former, not just the location but also the number of holes of the SJ were considered. A net power increase was obtained in both investigations; a power coefficient increase of 15.2% was obtained in [1]. A numerical investigation on the S809-HAWT airfoil aerodynamic enhancement using SJ was undertaken by [30]. Lift increase was observed for a wide range of injected flow angles. As previously performed by [1], [PERSON] et al. [32] placed the SJA at the airfoil trailing edge and numerically studied the aerodynamic characteristics improvement on a straight-bladed VAWT. They evaluated the performance of several configurations of dual synthetic jets actuators (DSJA) to improve the power capacity of the WT and observed that the power coefficient could be increased by 58.87% when the juxtaposition-type DSJA was used. One of the latest pieces of research on AFC applications to HAWT with S809 airfoils is the one performed experimentally by [PERSON] et al. [5]. Two rotors with blade aspect ratios of 7.79 and 9.74 and having 20 SJA distributed along the span of each blade were analyzed at four different turning speeds and three pitch angles. The rotor with a higher aspect ratio was found to be more aerodynamically efficient, and thanks to the use of AFC, unsteady bending and unsteady torsion could be reduced by 36.5% and 22.9%, respectively. In all the research just presented on WT, the five AFC parameters--SJ position, width, injection angle, frequency, and momentum coefficient--were chosen without performing a previous optimization, which appears to be the necessary task if minimum energy for the actuation is to be used. In fact, when aiming to optimize the different AFC parameters, the following considerations gathered from previous investigations are needed. When experimentally studying the flow control on the NACA-0025 airfoil at \(Re=10^{5}\) and AoA = \(5^{\circ}\), [PERSON] et al. [34] observed that the momentum coefficient was the primary control parameter. When analyzing the effect of jet position in controlling the boundary layer separation [35], it was observed that the maximum effectiveness was obtained when placing the jet groove close to the boundary layer separation point, either downstream or upstream. The same observations were made by [PERSON] et al. [36] and [PERSON] [37] which observed that locating the actuator near the boundary layer separation point, a lower momentum coefficient was needed to reattach the separated flow. Furthermore, [PERSON] et al. [38] observed that the required \(C_{\mu}\) to reattach the boundary layer was the lowest for excitation frequencies in the range of the vortex shedding frequency. The same observation was made by [PERSON] and [PERSON] [39] then they reported optimal actuation frequencies of \(F^{+}\) = 0.7 and 1.3, the highest one being most effective in combination with an optimum momentum coefficient of \(C_{\mu}\) = 0.0123. [PERSON] et al. [40] conducted a LES study on a NACA-0015 airfoil at \(Re\) = 8.96 \(\times\) 10\({}^{5}\) and noticed that the optimal pulsating frequency coincided with the baseline shedding frequency (\(f_{\textit{trable}}\)). This was also experimentally confirmed by [PERSON] et al. [41] and corroborated using (3D-DNS) by [PERSON] and [PERSON] [42]. Numerical studies on the NACA-23012 airfoil at \(Re\) = 2.19 \(\times\) 10\({}^{6}\) were performed by [43; 44] and concluded that maximum lift was obtained when \(F^{+}\) = 1, the jet was placed nearby the boundary layer separation point, and tangential injection/suction was observed to be particularly effective.