1. Introduction
Recently, the total installed capacity of wind energy over the world has been continuously increasing. Wind energy supplies the second largest capacity among different renewable energy sources in the most recent ten years [
1]. As the demand and necessity for improving wind turbine performance increase, this study investigates the effective blade design for better performance of the wind turbine, mainly focused on the airfoil shape. Since the requirement of a rotating horizontal axis wind turbine rotor is different from the horizontal flying airplane wing, the need for the specific design of wind turbine airfoil has risen. The roughness sensitivity, higher gliding ratio, and structural stability on the root part with the tip part’s aerodynamic effectiveness are found to be some of several requirements of the airfoil in a wind turbine [
2].
The National Renewable Energy Laboratory (NREL) designed several airfoils for both stall-regulated and pitch control wind turbines incorporated with the Solar Energy Research Institute (SERI) [
3]. The team of Delft University of Technologyhas generated 15–40% thickness airfoils dedicated to the wind turbine [
3,
4]. The Risø national laboratory designed the thin-shaped airfoil for higher aerodynamic efficiency [
5,
6]. To make the airfoil suitable for the wind turbine application, the researchers have used various optimization strategies. The gradient-based optimization method searches for the solution by the convergence in the local solution of the function with the given derivatives. The example of airfoil design with the gradient method is in reference [
7]. However, because of the flexibility of the design variable settings, stochastic methods such as genetic algorithms (GAs) have been used frequently for the airfoil design [
8]. Chen used GA and artificial intelligence methods for increasing the lift to drag ratio of the airfoils [65]. The multi-objective GA is also applied to find the airfoil design for two objective functions, the maximum lift, and maximum gliding ratio by Chen [
8]. Single and multi-objective genetic algorithms with the artificial neural network are used for novel airfoil design by Ribeiro with Computational Fluid Dynamics (CFD) coupling [
9,
10]. As the final optimum shape of the airfoil is from the vast numbers of candidate airfoil designs within the group called population, the GA optimization method is advantageous. The candidate designs in the population before the convergence can be saved and investigated for the other performance parameters. It is more useful for the current design research, as there is no need to devise an equation for the gradient-based optimization derivatives [
7].
The researchers have designed the wind turbine airfoils from the 2D flow calculations before the concept of 3D rotational augmentation [
11], mainly based on the blade element momentum (BEM) theory and the flow solver XFOIL [
12,
13]. However, it is essential to consider the 3D rotational flow situation in the airfoil design because the rotor’s lift coefficient shows a significant difference between 2D and 3D calculations. The rotation of the rotor is observed to increase the lift coefficient of the blade, especially in the inboard section, for example, by Himmelskamp [
13], Ronsten [
11], and Bruining [
14].
In recent years, wind turbine size has increased to meet the increasing demand for clean energy supply. The enlarged wind turbine size can also be found in offshore applications with relevant research [
15,
16]. Like these, there is a need for thick airfoils with excellent structural stability [
17]. However, the aerodynamic performance decreases as the blades take on thicker airfoils. A thinner airfoil leads the effective lifting of the blade while a thicker airfoil ensures structural strength. Therefore, the inverse relationship between the thickness and the aerodynamic performance, and the proportional relationship between the thickness and structural stability are the inevitable trade-off in the blade airfoil design. The structural stability and the excellent aerodynamic performance are two requirements to be satisfied in the airfoil of the large size rotors. As the rotor’s root is more exposed to the rotational flow, it is necessary to consider the airfoil’s design at the inboard part of the rotor with the 3D rotational augmentation [
18].
The angle of attack is the angle formed by the chord of the airfoil and the direction of the relative incoming wind [
19]. As the complex rotational flow at the root section of the blade at the higher angle of attack with stall condition causes the secondary flow and the centrifugal force, a delay of flow separation happens at the part of the blade root. Moreover, the inertial forces from the rotation and the Coriolis force reduce the stall area at the blade sections in the rotational flow [
20,
21]. Therefore, the aerodynamics on the root part reduces the drag and increases the lift. Thus, the researchers have corrected the prediction models to supplement the absent physical description of the BEM theory as compensation.
Snell [
22,
23,
24] derived a foundation for relating the 3D rotational augmentation with the ratio of the chord to the radius. This knowledge influenced several available empirical correction models in later years. An example of such correction was made, for instance, by Hansen and Chaviaropoulos. The semi-empirical correction law can be applied to correct the load coefficients. In general, the wind turbine has generated power in the 3D rotation 15–20% higher than in the 2D calculation [
25]. Kim and Bangga [
26] tried the airfoil shape comparison on different 3D rotational augmentation impacts.
The study of the relationship between an airfoil’s shape characteristics and its influence on the wind turbine performance became necessary to achieve an aerodynamically efficient and structurally stable airfoil in the blade inboard section with the dominant rotational flow. Sandia National Laboratories (SNL) inserted the blunt trailing edge airfoil in the inner region of the blade for the structural strength and aerodynamic efficiency at the same time [
27]. However, Bangga found a significant unsteady flow and trailing edge separation at the thicker trailing edge [
28].
The limit of the upper airfoil surface reduces the surface velocity around it. There is a connection between the position of the maximum thickness and the Gliding Ratio (GR). There is an impact from the nose radius from the leading edge of the airfoil on the blade performance. Moreover, the S-shaped tail of the airfoil is applied to improve the lift coefficient, although it has the possibility of turbulent separation [
17]. The subtle difference of the airfoil shape properties such as curvature, thickness, camber line, nose radius, and tail sharpness influence the flow around the surfaces. The affected flow from the various shapes of the airfoil causes different velocity and pressure distribution over the curvature.
Because of the discrepancy of the 2D governing equation in the airfoil design with the rotational flow [
12], it is necessary to research the airfoil’s sectional shape characteristics’ impact on the rotational flow. There is not much research in this field, but it has great potential to reveal the knowledge connected to the power improvement of the wind turbine. This study investigates the impacts of different airfoil shapes, which originated from the reference S809 of NREL Phase VI wind turbine [
29]. Because of the mono airfoil type blade and sufficient experiment validation data, it is useful to check the direct impact of the sectional shape change of the blade on wind turbine power performance.
This research to find specific shape characteristics of the airfoils that influence the 3D rotational augmentation effect on the power of HAWT. Moreover, it also targets visualizing details on the 2D aerodynamics of the blade’s sectional shape with different airfoils. It also illustrates the stall differences caused by the airfoil shape change in detail. The 2D flow characteristics of three airfoils, its 3D-corrected lift data, and its blade’s power performance describe the relationship between airfoil shapes and aerodynamic performance in 3D rotational flow.
Section 2 introduces the method and material used in the study,
Section 3 presents all the results, and
Section 4 explains the physics and reasons behind the results based on the given wind turbine performance theory and physics.
4. Discussion
The shape differences of the airfoil S809, S809gx, and S809r show different pressure, GR distribution, transition point, lift coefficient, power production, separation, and stall area. Commonly, the GR at the fully attached range is considered the main factor in increasing power production. Therefore, the airfoil is usually designed to have the highest angle of attack at the design point. However, the airfoil S809r, which produces the highest power output in 3D correction, shows the lowest maximum GR value in the GR-alpha graph of the 2D XFOIL calculation. It shows the airfoil S809r, which has a significant improvement in lift coefficient by 360-degree extrapolation, led its highest power production. The higher lift coefficient region is mainly from the higher angle of attack, where the airfoil S809r shows the higher GR value from XFOIL. The CFD simulations of the three blade sections show that the leading-edge roundness of the airfoil influences the pressure gradients over the upper surface, which causes the less trailing edge vortex. The rounded nose impact is more significant than the trailing edge sharpness for the separation at stall alpha.
The rounded nose and the S-shaped trailing of the S809r show higher tolerance at the stall. It is in contrast to the dramatically decreased GR values of the reference airfoil at the stall alpha region. At the higher angle of attack, the airfoil S809r has an extensive laminar boundary layer. The delayed transition from laminar to turbulent flow at the S809r with the enhancement of 3D rotational flow causes less separation and stall area of the airfoil compared to the reference. The upper surface laminar region is more influential than the lower surface boundary layer. As the more laminar boundary layer at the higher angle of attack enhanced by the rotational augmentation reduces the laminar separation bubble, the lift coefficient and tangential force from the blade with the S809r show the highest lift coefficient and tangential force.
The shape difference of airfoils makes the lift, drag coefficients, and inflow angle of attack different at the same incoming velocity. The airfoil shape with increased lift and decreased drag considered with the specific incoming flow angle make a significant increase in torque. It is under sine and cosine function of normal and tangential equations that lead the desired power production by the specially shaped blades’ airfoil. These findings make the airfoil shape with reduced separation and stall at higher alpha to achieve aerodynamic performance.
The stall and separation reductions at the airfoil S809r are related to the airfoil shape, making the higher GR at the higher angle of attack region, rather than the airfoil with the highest peak the GR graph. The pressure distribution over the upper surface generated by the more rounded leading edge and symmetric airfoil geometry is connected to the blade’s aerodynamic performance and structurally stable wind turbine power production. It gives the airfoil designer insight to increase the rotor’s stable aerodynamic efficiency with the smallest cost to change the design. Further research to find another critical area of airfoil shape which influences the 3D rotational augmentation would enhance the link between the airfoil design and its impact on HAWT’s rotor aerodynamic performance.