3. Experimental apparatus and procedures

#### 3.1. 2-Dimension airfoil noise test

This study conducted a wind tunnel test on 2D airfoils in a closed test section with a size of 1.25 � 1.25 m by using the subsonic anechoic wind tunnel test facility at Chungnam National University. The wind tunnel experimental apparatus was set up to measure aerodynamic performance and noise reduction, as seen in Figure 2. To assess the aerodynamic performance of 2D airfoils, a three-axis balance was used, whereas a pressure scanner system with 50 pressure holes and 50 channels was installed in order to measure the pressure distribution. Also, a microphone system with a total of seven channels was installed to measure noises, as shown in Figure 3. A wind tunnel experiment was measured in the low-speed semi-anechoic wind tunnel at Chungnam National University. The volume of the anechoic chamber was 211.9 m<sup>3</sup> and the tapered anechoic chambers had a cut-off frequency of 150 Hz [9].

Figure 4 shows the 2D cross-section of the experimental models with a downscaled radius 75% as large as the size of an actual rotor. The rotor blade had a chord length of 0.35 m and a span of 1.249 m, and it was made of 60-class aluminum in order to minimize the possible structural vibrations and noises of the experimental models during the wind tunnel test. The model was installed with a structure supporting both ends (by fixing the upper and lower blades) and on a turn table with 360� rotation, and the wind tunnel velocity and airfoil angles of attack could 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.

ð2Þ

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

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

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

This study conducted a wind tunnel test on 2D airfoils in a closed test section with a size of 1.25 � 1.25 m by using the subsonic anechoic wind tunnel test facility at Chungnam National University. The wind tunnel experimental apparatus was set up to measure aerodynamic performance and noise reduction, as seen in Figure 2. To assess the aerodynamic performance of 2D airfoils, a three-axis balance was used, whereas a pressure scanner system with 50 pressure holes and 50 channels was installed in order to measure the pressure distribution. Also, a microphone system with a total of seven channels was installed to measure noises, as shown in Figure 3. A wind tunnel experiment was measured in the low-speed semi-anechoic wind tunnel at Chungnam National University. The volume of the anechoic chamber was

Figure 4 shows the 2D cross-section of the experimental models with a downscaled radius 75% as large as the size of an actual rotor. The rotor blade had a chord length of 0.35 m and a span of 1.249 m, and it was made of 60-class aluminum in order to minimize the possible structural vibrations and noises of the experimental models during the wind tunnel test. The model was installed with a structure supporting both ends (by fixing the upper and lower blades) and on a turn table with 360� rotation, and the wind tunnel velocity and airfoil angles of attack could

211.9 m<sup>3</sup> and the tapered anechoic chambers had a cut-off frequency of 150 Hz [9].

prediction empirical model for 2D airfoil self-noises, which was introduced by Brooks.

clearance and the boundary layer thickness of serrated trailing edges.

aspect ratios of serrated trailing edges [1, 8].

124 Stability Control and Reliable Performance of Wind Turbines

3. Experimental apparatus and procedures

3.1. 2-Dimension airfoil noise test

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]

Figure 3. Layout of the microphone array for the 2D airfoil noise test in the open-jet wind tunnel.

and by Garner et al. [12] were applied; tunnel height and chord length were used as major variables in the two calibration methods. The aerodynamics performance experiment results of the 2D airfoil have been introduced by Ryi & Choi [8].

ð3Þ

127

As seen in Eq. (3), the study compared differences in noise between the plane-shaped trailing edge and the serrated trailing edge within the same frequency domain. As seen in Figure 7, the results confirmed a noise reduction effect in the frequency range of about 500 Hz–20 kHz.

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

If a serrated trailing edge is attached to a 2D airfoil, a noise reduction effect is produced. However, in such a case, the basic preconditions suggested by Howe's theory are not satisfied, according to the results of previous experiments. This study assumed that the Howe's precondition amplitude of serrations and the boundary layer thickness of 2D airfoils are constant, and this is not satisfied. For this reason, the study set up the experimental apparatus to measure the wake of 2D airfoils, as seen in Figure 8, and used a single axis hot-wire anemometer to measure the wake in the y/C direction (span-wise) and in the z/C direction (wake-wise) of serrated trailing

Figure 9 shows the results of the experiments conducted with eight types of airfoils. Because of the shapes of the serrated trailing edges, measurements were taken at 240 mm. In the case of 2D airfoils with rectangular plate trailing edges, there were no changes in frequency in the span-wise direction depending on the shapes of the serrations, and there were no abnormal phenomena. However, this study assumes distinctive frequency patterns depending on the shapes of serrations and the peak and valley positions. Therefore, the study demonstrated

3.3. Experiment on wake characteristics of 2D airfoils with serrated trailing edges

edges as well as 8 different types of airfoils.

Figure 4. Baseline airfoil test model [10].

#### 3.2. Noise measurement results of 2D airfoils with trailing edge serrations

Figure 6 shows the differences in noise levels between a 2D airfoil with a rectangular plateshape trailing edge and a 2D airfoil with serrated trailing edges. A noise value greater than zero indicates the presence of a noise reduction effect. A noise value smaller than zero indicates the absence of a noise reduction effect. It was observed that the noise effect increased with an increase in the angle of attack, and the maximum noise reduction reached about 3 dB.

These experimental results confirmed that the use of serrated trailing edges can improve aerodynamic performance and noise performance. This study intended to confirm the noise reduction effect of serrated trailing edges by applying trailing edge serrations to an existing rotor system.

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 127

Figure 4. Baseline airfoil test model [10].

and by Garner et al. [12] were applied; tunnel height and chord length were used as major variables in the two calibration methods. The aerodynamics performance experiment results of

Figure 6 shows the differences in noise levels between a 2D airfoil with a rectangular plateshape trailing edge and a 2D airfoil with serrated trailing edges. A noise value greater than zero indicates the presence of a noise reduction effect. A noise value smaller than zero indicates the absence of a noise reduction effect. It was observed that the noise effect increased with an increase in the angle of attack, and the maximum noise reduction reached

These experimental results confirmed that the use of serrated trailing edges can improve aerodynamic performance and noise performance. This study intended to confirm the noise reduction effect of serrated trailing edges by applying trailing edge serrations to an existing

3.2. Noise measurement results of 2D airfoils with trailing edge serrations

Figure 3. Layout of the microphone array for the 2D airfoil noise test in the open-jet wind tunnel.

the 2D airfoil have been introduced by Ryi & Choi [8].

126 Stability Control and Reliable Performance of Wind Turbines

about 3 dB.

rotor system.

$$L\_{\overrightarrow{differenos}}(dB) = L\_{baselines} \, (dB) - L\_{serrated \, TE} \, (dB) \tag{3}$$

As seen in Eq. (3), the study compared differences in noise between the plane-shaped trailing edge and the serrated trailing edge within the same frequency domain. As seen in Figure 7, the results confirmed a noise reduction effect in the frequency range of about 500 Hz–20 kHz.

#### 3.3. Experiment on wake characteristics of 2D airfoils with serrated trailing edges

If a serrated trailing edge is attached to a 2D airfoil, a noise reduction effect is produced. However, in such a case, the basic preconditions suggested by Howe's theory are not satisfied, according to the results of previous experiments. This study assumed that the Howe's precondition amplitude of serrations and the boundary layer thickness of 2D airfoils are constant, and this is not satisfied. For this reason, the study set up the experimental apparatus to measure the wake of 2D airfoils, as seen in Figure 8, and used a single axis hot-wire anemometer to measure the wake in the y/C direction (span-wise) and in the z/C direction (wake-wise) of serrated trailing edges as well as 8 different types of airfoils.

Figure 9 shows the results of the experiments conducted with eight types of airfoils. Because of the shapes of the serrated trailing edges, measurements were taken at 240 mm. In the case of 2D airfoils with rectangular plate trailing edges, there were no changes in frequency in the span-wise direction depending on the shapes of the serrations, and there were no abnormal phenomena. However, this study assumes distinctive frequency patterns depending on the shapes of serrations and the peak and valley positions. Therefore, the study demonstrated

differences in the components of vortex shedding frequency in the wake of the trailing edge when serrated trailing edges were used, under the same operating conditions as for 2D airfoils.

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129

The results of the wake measurement of 2D airfoils with serrated trailing edges in the y/C direction revealed that frequency changes occurred depending on the peak and valley positions of the serrated trailing edges. Based on these experimental results, the wake in the z/C direction was measured via the mean flow distribution, depending on the locations of the serrated trailing edges in the span-wise direction. Figure 10 shows a conceptual diagram of the wake measurement experiment in the z/C direction using the same experimental devices as for the wake measurement experiment in the y/C direction. Figure 11 gives information about the axial directions of serrated trailing edges and the wake measurement locations. The tip of the serrated trailing edge was defined as the peak, while the inside of the serrated trailing edge was defined as the valley. The wake measurement experiment was conducted by defining the

Figure 12 shows the results of the wake measurements of six types of serrated trailing edges. When serrated trailing edges were attached, varying wake distributions were observed depending on the types of serrated trailing edges. Boundary layer changes of the 2D airfoils and different flow distributions in the peak and valley positions were observed. According to these results, the precondition suggested by Howe to explain the noise reduction effect of serrated trailing edges, that all the boundary layer thicknesses are constant, was not satisfied.

3.4. Measurement of serrated trailing edge wake (included information)

Figure 6. Noise level difference of baseline airfoil with serration trailing edge.

(x, y, z) axes of the tip of the serrated trailing edges as (0, 0, 0).

Figure 5. Serration trailing edge test model [10].


Table 1. Serrated trailing edge configuration.

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 129

Figure 6. Noise level difference of baseline airfoil with serration trailing edge.

Figure 5. Serration trailing edge test model [10].

128 Stability Control and Reliable Performance of Wind Turbines

Table 1. Serrated trailing edge configuration.

Serration type λ/h θ (

Baseline N/A N/A N/A N/A Rec80 N/A N/A N/A N/A Nor. Tri (narrow) 0.5 7.125 20 80 Nor. Tri (wide) 2 26.565 80 80 Skew. Tri (narrow) 0.5 14.03 20 80 Skew. Tri (wide) 2 22.5 80 80 Multi. Tri (narrow) 1 28.07 40 80 Multi. Tri (wide) 2 26.565 80 80

�) (mm) 2h (mm)

differences in the components of vortex shedding frequency in the wake of the trailing edge when serrated trailing edges were used, under the same operating conditions as for 2D airfoils.

#### 3.4. Measurement of serrated trailing edge wake (included information)

The results of the wake measurement of 2D airfoils with serrated trailing edges in the y/C direction revealed that frequency changes occurred depending on the peak and valley positions of the serrated trailing edges. Based on these experimental results, the wake in the z/C direction was measured via the mean flow distribution, depending on the locations of the serrated trailing edges in the span-wise direction. Figure 10 shows a conceptual diagram of the wake measurement experiment in the z/C direction using the same experimental devices as for the wake measurement experiment in the y/C direction. Figure 11 gives information about the axial directions of serrated trailing edges and the wake measurement locations. The tip of the serrated trailing edge was defined as the peak, while the inside of the serrated trailing edge was defined as the valley. The wake measurement experiment was conducted by defining the (x, y, z) axes of the tip of the serrated trailing edges as (0, 0, 0).

Figure 12 shows the results of the wake measurements of six types of serrated trailing edges. When serrated trailing edges were attached, varying wake distributions were observed depending on the types of serrated trailing edges. Boundary layer changes of the 2D airfoils and different flow distributions in the peak and valley positions were observed. According to these results, the precondition suggested by Howe to explain the noise reduction effect of serrated trailing edges, that all the boundary layer thicknesses are constant, was not satisfied.

Figure 9. Velocity spectral map for the plate with serrations measured in the span-wise direction: various serration plate.

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131

Figure 10. Layout of wake measurement for serration trailing edge in the z-axis direction [8].

Figure 7. Noise reduced effect for serration trailing edge.

Figure 8. Configuration of wind tunnel test stand for airfoil wake measurement system.

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 131

Figure 9. Velocity spectral map for the plate with serrations measured in the span-wise direction: various serration plate.

Figure 10. Layout of wake measurement for serration trailing edge in the z-axis direction [8].

Figure 7. Noise reduced effect for serration trailing edge.

130 Stability Control and Reliable Performance of Wind Turbines

Figure 8. Configuration of wind tunnel test stand for airfoil wake measurement system.

Figure 11. Layout of serration plate [8].
