2. Theoretical background of trailing edge serration

#### 2.1. Noise reduction mechanism of trailing edge serrations

The noise reduction mechanism of trailing edge serrations was introduced by Howe based on an experiment on trailing edge noises in a 2D airfoil [1]. Figure 1 shows the shape-related variables of Estimation Method to Achieve a Noise Reduction Effect of Airfoil with a Serrated Trailing Edge for Wind… http://dx.doi.org/10.5772/intechopen.73608 123

Figure 1. Turbulent flow over a serrated trailing edge [1].

1. Introduction

122 Stability Control and Reliable Performance of Wind Turbines

trailing edges.

Studies on serrated trailing edges have been conducted in order to find solutions to noise problems in the wind turbine rotor field. In 1991, Howe [1] suggested a theory to predict noise reduction in a 2D airfoil with a serrated trailing edge. He defined the variables depending on the shapes of the edges and identified differences in noise reduction depending on various aspect ratios of serrated trailing edges. Based on Howe's serrated trailing edge theory, Braun [2] confirmed in 1999 that noise produced by a wind turbine rotor could be reduced by the application of a serrated trailing edge with a diameter of 16 m, and observed that changes in the frequency domains of noises occurred depending on the installation angles of serrated

In 2009, Gruber and Joseph [3] reported the noise reduction effect and the boundary layer thickness of a serrated trailing edge on a 2D airfoil by designing a 2D airfoil based on Howe's theory. Oerlemans et al. [4] applied a serrated trailing edge to a 2.3 MW wind turbine rotor to

From 2010 to the current, Phillip [5] has been conducting a wind tunnel test on a sawtoothshaped trailing edge and a slit-shaped trailing edge, and has confirmed the noise reduction effect and predicted noise changes based on Howe's theory. Michel Roger [6] carried out a wind tunnel test on five types of blush-shaped trailing edges. Based on the cross-section of the 2D airfoil of NACA65(12)-1, changes in the frequency components caused by variations in wake and flow were recorded using a hot-wire anemometer. In 2012, Dennis Y.C. Leung [7] applied a trailing edge with a 0 angle of attack to a 2D airfoil, and confirmed noise reduction

As mentioned above, the theoretical background of the previous studies was based on Howe's theory, but among the experiments involving wind tunnel tests and the actual wind turbine rotor tests, none showed improvements in both aerodynamic performance and noise performance simultaneously. Also, the studies on the noise reduction effect of serrated trailing edges only confirmed the noise reduction effects, while most of the studies on aerodynamic perfor-

This study aimed to examine changes in both aerodynamic performance and noise reduction by applying serrated trailing edges to 2D airfoils in a wind tunnel experiment. Also, this study proposes a prediction model for noise reduction effects with the use of serrated trailing edges,

The noise reduction mechanism of trailing edge serrations was introduced by Howe based on an experiment on trailing edge noises in a 2D airfoil [1]. Figure 1 shows the shape-related variables of

observe noise changes, and used the beam-forming method to confirm noise changes.

effects owing to changes in the aspect ratio of serrated trailing edges.

based on the experimental results obtained from the wind tunnel test.

2. Theoretical background of trailing edge serration

2.1. Noise reduction mechanism of trailing edge serrations

mance also only showed changes in noise performance.

serrated trailing edges applied to 2D airfoils in this study. The noise reduction effects of the serrated trailing edges were defined with the following criteria: span-wise wavelength (λs), amplitude of serrations (h), incline angle (θ), main stream velocity (U), and acoustic frequency (ω).

Howe's theory of the noise reduction effect induced by the use of trailing edge serrations is based on the following preconditions:


Based on the above preconditions, Howe [1] proposed an equation for noise reduction accomplished by serrated trailing edges. Eq. (1) is a function which defines the noise reduction effect (Ѱ(ω)) induced by trailing edge serrations with boundary layer thickness (σ), span-wise wavelength (λ), and blade-tip clearance based on the turbulent fluctuation frequency (ω) [1].

$$\Psi(w) = \left(1 + \frac{1}{2}\epsilon \left.\frac{\partial}{\partial \epsilon}\right\vert \not{p}\left(\frac{w\delta}{U\_{\varepsilon}}, \frac{h}{\lambda}, \frac{h}{\delta}; \epsilon\right) \tag{1}$$

Eq. (2) was created with the root-to-tip distance set at "h ≥ 0", for the case in which a serrated trailing edge is not used.

$$\Psi\_0(w) = (w\delta/U\_c)^2/[(w\delta/U\_c)^2 + \epsilon^2]^2\tag{2}$$

be adjusted depending on the experimental conditions. The aerodynamic performance of the experimental models was measured during the experiment by installing 2EA of three-axis balance. The aerodynamic performance and noise performance were simultaneously measured by installing each three-axis balance in the upper and lower part of the wind tunnel test section and by fixing both tips of the experimental model. The noise performance test was conducted with a wind velocity of about 30 m/s (RE = 700,000) and a Reynolds number in consideration of the downscaling of the rotor radius to 75% of actual size. Noises were measured with a total of seven microphones, which were installed 1750 and 1830 mm away from the trailing edge of the blade. In the 2D airfoil experiment, Figure 5 shows the total of 7 types of serrated trailing edges were tested together with airfoils; basic information about the shapes of serrated trailing edges is described in Table 1. The wind tunnel test on 2D airfoils was conducted in an open experimental section in order to measure the aerodynamic performance and noise performance at the same time. In this case, precise calibration of the open experimental section was necessary to measure aerodynamic performance. Wind tunnel tests in open experimental sections are often subject to a simultaneous occurrence of flow stream line curvature and down-wash deflection phenomena, which rarely happens in free-air conditions. These phenomena resulted in a decrease in the angles of attack and the lift curve slope of the 2D airfoils, and caused drag-changing shapes. The calibration methods suggested by Brooks & Marcolini [11]

Estimation Method to Achieve a Noise Reduction Effect of Airfoil with a Serrated Trailing Edge for Wind…

http://dx.doi.org/10.5772/intechopen.73608

125

Figure 2. Configuration of wind tunnel test stand for airfoil experiment.

The noise reduction effects of serrated trailing edges are produced by changes in the noise components of the turbulent boundary layers of 2D airfoils. At this time, the turbulence-induced frequency is assumed to meet the condition, and the changes in the turbulent boundary layer of serrated trailing edges resulted in the reduction of noises greater than the minimum. Also, under the same boundary layer conditions, the varying noise reduction effects depend on the different aspect ratios of serrated trailing edges [1, 8].

However, when a plate-shaped serrated trailing edge was applied to a 2D airfoil in an actual experiment, it is impossible to meet the preconditions proposed by Howe, which entail an experimental environment with a 0� angle of attack and a constant ratio between the blade-tip clearance and the boundary layer thickness of serrated trailing edges.

In this study, the authors confirmed the preconditions proposed by Howe for the noise reduction effect brought about by the use of serrated trailing edges, and identified the factors restricting the noise reduction effects when a serrated trailing edge is applied to a 2D airfoil in a wind tunnel test. Also, the study proposed a model that predicts airfoil self-noises. In addition, the study utilized the wind tunnel test results to review the validity of the noise prediction empirical model for 2D airfoil self-noises, which was introduced by Brooks.
