# Application of Simultaneous Symmetric and Cambered Airfoils in Novel Vertical Axis Wind Turbines
## 1 Introduction
Further, it has been observed by [PERSON] and [PERSON] that thicker airfoils usually have a lower power coefficient than thinner ones [26]. [PERSON] et al. tried to maximize the generated torque by changing the NACA 4-series airfoil parameters, including maximum thickness, maximum camber, and position of maximum camber; they could enhance the power efficiency of VAWTs up to 6% compared with NACA0015 [27]. Another intrinsic parameter of VAWTs is the blade's angle of attack, which can affect the performance of VAWTs substantially. Hence, the use of pitch angle for changing the angle of attack is a solution to performance improvement. The pitch angle may be fixed or variable during rotation; the optimum fixed pitch angle can improve power production by more than 5%, reported by [PERSON] et al. [28]. On the other hand, [PERSON] et al. showed that the sinusoidal variable pitch angle causes a higher average power coefficient than the fixed ones [3]. Further, [PERSON] et al. tried to use an intelligent controlling system based on an artificial neural network for controlling the pitch angle during rotation. It was found that this active controlling system can improve power production by more than 25% in comparison with a fixed-pitch angle [24]. Apart from the effects of the airfoil and its angle of attack on the VAWT power coefficient, wind turbines with a higher H/D fraction can improve the power coefficient [18]. On the other hand, although higher solidity negatively influences the power coefficient, the higher Reynolds number can positively affect the power coefficient [29]. Innovative ideas for boosting power production include use of guide vanes for VAWTs to elevate the wind velocity [30]; with this technology, [PERSON] et al. boosted the power coefficient by approximately 30-35% [31]. Given the different geometric characteristics of guide vanes, [PERSON] et al. observed that power production does not depend on the number of vanes [32]. The impact of guide vane angles on the power coefficient has been analyzed, indicating that the angles influence power production significantly [33]. [PERSON] et al. used a blade with a wave-like trailing edge and positively affected the power coefficient [34]. [PERSON] and [PERSON] used a wave-like profile for the leading edge instead of the trailing edge. They proved that the power coefficient might be improved by 18.7% [35]. Apart from modifying trailing and leading edge profiles, [PERSON] et al. stated that a little gurney flap at the airfoil trailing edge could improve the power coefficient by about 23% [36]. Another innovative work was carried out by [PERSON] et al. for power coefficient improvement by using deformable blades, in which their shapes would change as a result of wind pressure [37]. On the other hand, this flexibility may have a negative aspect at a high rotational speed due to unfavorable shape changes [38]. Dynamic stall is an intrinsic specification of VAWTs. This phenomenon has a crucial impact on the power production of VAWTs [39]. Dynamic stall can be simulated by oscillating an airfoil for different ranges of the angle of attack with significant impacts on aerodynamic loads [40; 41; 42]. An airfoil under dynamic stall phenomena deforms the wake of the airfoil, which influences the performance of the downstream airfoils in the rotor of a VAWT [43]. Dynamic stall results in the airfoil lift coefficient increasing up to 1.5 times compared to the airfoil lift coefficient in normal conditions [44]. Dynamic stall in VAWTs occurs due to a large angle of attack variation during turbine rotation. The angle of attack variation domain is larger in lower TSR. Therefore, dynamic stall phenomena usually happen in lower TSRs [45]. Controlling dynamic stall and flow separation results in power elevation, where the use of a plasma actuator on the leading edge is one way of attaining this [46]. Apart from plasma actuators, helical blades and leading edge serration can resolve flow separation and torque fluctuations due to dynamic stall.