1. Introduction

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 trailing edges.

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

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

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

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

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1. The airfoil of a serrated trailing edge has a plate shape, and the span of a rotor blade is

2. Noises are produced by the turbulent components that pass through the trailing edge with a 0� angle of attack, under the condition that noises are only present in the trailing edge. 3. The Kutta condition is met, and the flow that passes through the plate-shaped airfoil has a

4. Under the same flow conditions, the ratios between the amplitude of serrations and the

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].

Eq. (2) was created with the root-to-tip distance set at "h ≥ 0", for the case in which a serrated

ð1Þ

boundary layer thickness and the wake of 2D airfoils are constant.

based on the following preconditions:

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

infinitive.

low Mach number.

trailing edge is not used.

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 performance also only showed changes in noise performance.

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