1. Introduction

The airfoil is the geometrically shaped structure for mechanical force generation from the relative movement between the airfoil and surrounding airflow of the airfoil structures [1]. For wind turbines, the airfoil shape of the blades influences the turbine power production. The lifting efficiency of the blades determines the effectiveness of rotor rotation to cause productive energy conversion from wind kinetics to rotor rotation, which leads to higher electricity generation from the drive unit.

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

From the late eighteenth century, the curving surface geometry was discovered to be advantageous for lifting efficiency in windmill by Smeaton [2]. In the 1880s, Lilienthal discovered the specific shape from the bird's wings, which inspired the airplane invention by Wright brothers [3]. The research of Prandtl and Tietjens revealed the benefit of the thick airfoil through their mathematical skills and wind tunnel tests in 1917 [4]. The US National Advisory Committee for Aeronautics (NACA)-generated airfoil groups, "NACA airfoil families" in the 1930s, and the series have been widely used even in these days [5]. In contradiction to the mathematical methods to calculate the pressure distribution of airfoil, Jacobs proposed the airfoil design which causes the desired pressure distribution. The laminar flow of airfoil was expanded to cause higher L/D ratio and smaller drag [6]. Later, different types of airfoils for various airplane design and off-design requirements were continuously designed.

The first wind turbine blades were also designed by the airfoils from aeronautic applications. However, in the 1980s, the airfoils specially dedicated for wind turbines were begun to be made due to the defects of aeronautic airfoils applied in a wind turbine. The sensitivity roughness effect on the leading edge arose to be the required element for wind turbine airfoil. The airfoil series for stall-regulated, variable-pitch control wind turbine was developed by NREL in 1984, incorporated with SERI and Airfoils [7]. The wind turbine-dedicated airfoils with the thickness from 15 to 40% of the chord were also made by the team of the Delft University of Technology with the design objective of low sensitivity to roughness, Gurney flaps, and trailing-edge wedge consideration [8]. The airfoils from Risø were designed to have high aerodynamic efficiency and slender blade shape [9]. The airfoil design using numerical optimization for tip region of the blades was researched by Grasso [10]. As mentioned in these studies, the higher aerodynamic efficiency, insensitivity to roughness effect, structural stability and smooth post-stall exhibition, etc., are required for wind turbine airfoil design. To accomplish these objectives, boundary layer consideration of the wind turbine airfoil can be advantageous as it was proven from the laminar airfoil by Jacobs [6].

The boundary layer of the airfoil is exerted by additional pressure generated by the curvature shape of airfoil compared to the constant pressure on boundary layer made of the plate with zero incidences. The pressure distribution on the edge of the boundary layer is same with the pressure distribution on the wall in the plate. However, due to streamline curvature of airfoil surface, the pressure gradients and compensation for the centrifugal force of the streamline flow are generated inside the boundary layer. Furthermore, the transition point of the boundary layer on the airfoil is determined by the outer flow and its pressure difference generated by the curvature shape of the surface [11].

To generate the airfoil shape which has the advantage for pressure distribution in the boundary layer and transition points, genetic algorithm (GA) optimization was used in this study. As all airfoils are designed for higher aerodynamic performance, GA objective functions therefore had two objectives—higher transition points of the larger laminar boundary layer and higher gliding ratio (GR). The airfoil S809 of NREL airfoil series for the wind turbine was chosen as a reference. The shape of insensitiveness to the roughness effect of the airfoil S809 could be maintained in the optimized airfoil. The final evaluations of turbine performance were done with the sample of stall-regulated wind turbine of NREL phase VI, which consists of the same airfoil-type composition [12].

The B-spline parameterization was used for the airfoil description, and the y points of the spline were considered to be the variables. The values of boundary layer parameters and GR of the airfoil were calculated by the flow solver XFOIL. The power performances of turbine unit with blades of the optimized airfoil were calculated by using blade element method (BEM) of the software QBlade. The CFD simulations from OpenFOAM® were performed to visualize the improved aerodynamic aspects.

Section 2 explains the GA airfoil optimization method, Section 3 presents the aerodynamic and boundary layer results of the optimized airfoils with improved power production of turbine unit, Section 4 visualizes the airflow of the optimized airfoil with the reference, and Section 5 concludes this chapter.
