**3. Experimental result and discussion**

This chapter describes the result of having investigated about the basic characteristic of a producing wind tunnel, and the result of having performed sound verification.

### **3.1. The fluid-dynamic characteristic and the acoustic characteristic of a producing wind tunnel**

In order to understand the performance of a producing wind tunnel, investigation of the minimum flow velocity and the maximum flow velocity was performed using the Pitot tube. The minimum flow velocity in the test section was 2.5m/s, when the number of rotations of a fan was 100min-1, and the maximum flow velocity in the test section was 35m/s when the number of rotations of a fan was 1300min-1.

In a low noise wind tunnel, it becomes important especially to suppress propagation of the op‐ eration noise of the fan. Since this wind tunnel is a suction type wind tunnel, it is necessary to make it not accept fan generating noise in a test section. Accordingly, it is important not to leak the operation sound of the fan outside a fan room. So, the noise characteristic of the around of a wind tunnel was investigated. In order to understand the quietness of the wind tunnel, the sound pressure level around the test wind tunnel when it is driven or stopped was measured. Generally, the noise when the wind tunnel is operated is divided into air flow noise, and the op‐ erating noise of the blower. It is especially important in the fluid-dynamic noise measurement to suppress the propagation of the operating noise of the blower. The wind tunnel should not accept the blower generation noise in the measurement section. It is important that the operat‐ ing sound of the blower does not leak outside the fan room. It is necessary, therefore, to under‐ stand the noise characteristics around the wind tunnel. The microphone positions for the noise measurement around the wind tunnel are shown in Fig. 4. Microphones are set up outside the fan room at a height of 1m off the ground, at measurement points (A-K). At measurement points (L1, L2) in the blower room, microphones are set up at a height of 1m, and placed a 700mm away from the electric motor and the blower outlet. Figure 5 shows the noise measure‐ ments at each measurement point when the circular cylinder is not set up in the measurement section and when the wind tunnel is in operation. The noise levels around the wind tunnel, al‐ most the same, but differ inside and outside of the fan room, and when flow velocity increases, the difference increased. The noise levels inside and outside the fan room were 26dB and 32dB, respectively, when the wind tunnel was not operating. The level of sound intensity is defined by *L*=10log10*I/I*0 (dB) (*I*0 is an intensity of the sound of the standard: 10-12 W/m2 ). Here, when the level of intensity of a sound inside the fan room is defined as *LIN*, and the level of intensity of a sound outside the fan room is defined as *LOUT*, the ratio of the level of sound-intensity *LIN/LOUT* is given by *LIN/LOUT*=10(*LIN*-*LOUT*)/10. The air flow velocity range is 5-35m/s, so the ratio of the level of sound-intensity *LIN/LOUT* becomes 155 -6760. Therefore, it is clear that the noise in the blower room is intercepted.

Fig. 4. Sound measurement points around a wind tunnel pressure level ¶ **Figure 4.** Sound measurement points around a wind tunnel pressure level Fig. 4. Sound measurement points around a wind tunnel pressure level

**3.2 Flow characteristics in the measurement section** 

**3.2 Flow characteristics in the measurement section**

ranging from 5m/s to 28m/s

¶

¶ ¶

¶ **¶** 

¶ **¶**  SPL (dB)

all noise level and frequency analyses are done using the precision sound level meter and the fast Fourier transform analyzer. The flow velocity distribution in the measurement sec‐ tion and the measurement of the disturbance intensity relative to the main flow is as follows. The I type probe of the hot-wire anemometer is inserted detaching the microphone, it traver‐ ses in a vertical direction (y direction) at 5mm intervals (the interval of traverse is 2.5mm near the wall), and the air flow velocity is measured at the microphone installation position. The frequency of the oscillating flow due to Karman vortex shedding from the circular cylin‐ der is measured as follows. The I type probe of the hot-wire anemometer is fixed in a posi‐ tion such that a clear shape of the waves can be obtained, and the output signal and frequency are using the fast Fourier transform analyzer. Here, averaging is performed ten

This chapter describes the result of having investigated about the basic characteristic of a

**3.1. The fluid-dynamic characteristic and the acoustic characteristic of a producing wind**

In order to understand the performance of a producing wind tunnel, investigation of the minimum flow velocity and the maximum flow velocity was performed using the Pitot tube. The minimum flow velocity in the test section was 2.5m/s, when the number of rotations of a fan was 100min-1, and the maximum flow velocity in the test section was 35m/s when the

In a low noise wind tunnel, it becomes important especially to suppress propagation of the op‐ eration noise of the fan. Since this wind tunnel is a suction type wind tunnel, it is necessary to make it not accept fan generating noise in a test section. Accordingly, it is important not to leak the operation sound of the fan outside a fan room. So, the noise characteristic of the around of a wind tunnel was investigated. In order to understand the quietness of the wind tunnel, the sound pressure level around the test wind tunnel when it is driven or stopped was measured. Generally, the noise when the wind tunnel is operated is divided into air flow noise, and the op‐ erating noise of the blower. It is especially important in the fluid-dynamic noise measurement to suppress the propagation of the operating noise of the blower. The wind tunnel should not accept the blower generation noise in the measurement section. It is important that the operat‐ ing sound of the blower does not leak outside the fan room. It is necessary, therefore, to under‐ stand the noise characteristics around the wind tunnel. The microphone positions for the noise measurement around the wind tunnel are shown in Fig. 4. Microphones are set up outside the fan room at a height of 1m off the ground, at measurement points (A-K). At measurement points (L1, L2) in the blower room, microphones are set up at a height of 1m, and placed a 700mm away from the electric motor and the blower outlet. Figure 5 shows the noise measure‐ ments at each measurement point when the circular cylinder is not set up in the measurement

producing wind tunnel, and the result of having performed sound verification.

times in the frequency analysis.

**tunnel**

**3. Experimental result and discussion**

152 Wind Tunnel Designs and Their Diverse Engineering Applications

number of rotations of a fan was 1300min-1.

Fig. 5. Sound pressure level around a wind tunnel with a flow velocity in the test section ranging from 5m/s to 28m/s Fig. 5. Sound pressure level around a wind tunnel with a flow velocity in the test section **Figure 5.** Sound pressure level around a wind tunnel with a flow velocity in the test section ranging from 5m/s to 28m/s

Flow characteristics in the measurement section where the sound absorbing wall (fibered glass wall) had been used were investigated. The hot-wire probe was inserted in the microphone's installation position; it traversed in a vertical direction (y direction), and the velocity and disturbance intensity were measured. Figure 6 shows the velocity distribution and the disturbance intensity when the air flow velocity is *U*=28m/s. The velocity distribution (� symbol) for an acrylic wall is plotted for comparison. Here, the abscissa is a measurement position. The center of width in the vertical direction of the measurement section is assumed to be zero points; the upper side is assumed to be + mark, and the lower

Flow characteristics in the measurement section where the sound absorbing wall (fibered glass wall) had been used were investigated. The hot-wire probe was inserted in the microphone's installation position; it traversed in a vertical direction (y direction), and the velocity and disturbance intensity were measured. Figure 6 shows the velocity distribution and the disturbance intensity when the air flow velocity is *U*=28m/s. The velocity distribution (△ symbol) for an acrylic wall is plotted for comparison. Here, the abscissa is a measurement position. The center of width in the vertical direction of the measurement section is assumed to be zero points; the upper side is assumed to be + mark, and the lower

#### **3.2. Flow characteristics in the measurement section**

Flow characteristics in the measurement section where the sound absorbing wall (fibered glass wall) had been used were investigated. The hot-wire probe was inserted in the micro‐ phone's installation position; it traversed in a vertical direction (y direction), and the velocity and disturbance intensity were measured. Figure 6 shows the velocity distribution and the disturbance intensity when the air flow velocity is *U*=28m/s. The velocity distribution (△ symbol) for an acrylic wall is plotted for comparison. Here, the abscissa is a measurement position. The center of width in the vertical direction of the measurement section is assumed to be zero points; the upper side is assumed to be + mark, and the lower side is assumed to be - mark. The measurement position is made dimensionless by width *B*=376mm in the ver‐ tical direction of the measurement section. The ordinate shows the velocity distribution and the disturbance intensity, respectively. The symmetry of a flow was good to the center and turbulence intensity was less than 0.5% in the range which is maintaining the equality of a flow. The disturbance intensity to keep the uniformity of the flow was within 0.5%. Here, if uniform flow velocity *U*=28m/s and the measurement point *x*=400mm are calculated by Prantl's exact solution (*δ*=0.22(*ν/Ux*) 0.167*x*), the thickness of the boundary layer is 9.6mm. The thickness of the boundary layer as in Fig. 6 is 10mm for the acrylic wall, and this agrees with the value from Prantl's expression. However, the thickness of the boundary layer above the sound absorbing wall was about 28mm, and was about three times that in Prantl's expres‐ sion because the sound-absorbing wall was made of fibered glass with a rough surface. It was clarified that the wall was necessary to obtain a wide measurement section and thus im‐ prove the uniformity of the flow. side is assumed to be - mark. The measurement position is made dimensionless by width *B*=376mm in the vertical direction of the measurement section. The ordinate shows the velocity distribution and the disturbance intensity, respectively. The symmetry of a flow was good to the center and turbulence intensity was less than 0.5% in the range which is maintaining the equality of a flow. The disturbance intensity to keep the uniformity of the flow was within 0.5%. Here, if uniform flow velocity *U*=28m/s and the measurement point *x*=400mm are calculated by Prantl's exact solution (*δ*=0.22(*ν/Ux*) 0.167*x*), the thickness of the boundary layer is 9.6mm. The thickness of the boundary layer as in Fig. 6 is 10mm for the acrylic wall, and this agrees with the value from Prantl's expression. However, the thickness of the boundary layer above the sound absorbing wall was about 28mm, and was about three times that in Prantl's expression because the sound-absorbing wall was made of fibered glass with a rough surface. It was clarified that the wall was necessary to obtain a

wide measurement section and thus improve the uniformity of the flow.

¶

**¶** 

Fig. 6. Flow velocity distribution and turbulence intensity in the test (measurement) section at a main flow velocity of *U*=28m/s ¶ **Figure 6.** Flow velocity distribution and turbulence intensity in the test (measurement) section at a main flow velocity of *U*=28m/s

#### **3.3 The relation between sound source and measurement position**  It is known that the fluid-dynamic noise made from the circular cylinder placed into the air **3.3. The relation between sound source and measurement position**

flow is a dipole sound. Since there is single directivity also in a microphone, it is important to understand the influence on the measurement result by the spatial relationship of a sound source and its microphone. Figure 7 shows the measurement result of the sound pressure level when varying the distance *x* between centers of a circular cylinder and a microphone in the range from 5mm to 95mm. Here, the airflow velocity in a test section was *U*=28m/s It is known that the fluid-dynamic noise made from the circular cylinder placed into the air flow is a dipole sound. Since there is single directivity also in a microphone, it is important to understand the influence on the measurement result by the spatial relationship of a sound

> and the diameter of circular cylinder was 20mm. It is understood that the measured sound pressure level is almost the same. So, in measurement of acoustic frequency, distance between centers of circular cylinder and microphone was set to 50mm. Here, it is expected that the pressure fluctuation of a short-distance field is included in the sound pressure

source and its microphone. Figure 7 shows the measurement result of the sound pressure level when varying the distance *x* between centers of a circular cylinder and a microphone in the range from 5mm to 95mm. Here, the airflow velocity in a test section was *U*=28m/s and the diameter of circular cylinder was 20mm. It is understood that the measured sound pressure level is almost the same. So, in measurement of acoustic frequency, distance be‐ tween centers of circular cylinder and microphone was set to 50mm. Here, it is expected that the pressure fluctuation of a short-distance field is included in the sound pressure which will have been measured if the measurement position of sound is generally near from a sound source (circular cylinder). In this study, the distance between the circular cylinder and the microphone was narrower compared with the device arrangement for an ordinary sound measurement. Because the noise measurement of the flow in the fluid machine such as the gas turbines and jet engines is assumed, and the measurement of the fluid-dynamic sound caused by the flow around the object such as the supports and umbones installed in the pipeline and the duct is assumed, it becomes such arrangement. Therefore, the influence of the near field appears to be strong, making a quantitative evaluation of the sound pres‐ sure level more difficult. Resolving this is a clear challenge for future studies. The relation‐ ship between the position *rc* by which the pressure fluctuation of a short-distance field can be disregarded now, and the minimum frequency *f* is given in 20log(2*πfrc*/*a*) >=10 dB (*a* is acoustic velocity) (Iida, 1996). Distance *rc* between the circular cylinder and the microphone becomes 188mm-211mm because the range of center-to-center spacing *x* between the circu‐ lar cylinder and the microphone in this experiment is 5mm-95mm. The obtained lower criti‐ cal frequency *f* becomes 910Hz-812Hz. When the center-to-center spacing is assumed to be 50mm, distance *rc* between two points becomes 194.5mm. The lower critical frequency *f* at that time is 880Hz. which will have been measured if the measurement position of sound is generally near from a sound source (circular cylinder). In this study, the distance between the circular cylinder and the microphone was narrower compared with the device arrangement for an ordinary sound measurement. Because the noise measurement of the flow in the fluid machine such as the gas turbines and jet engines is assumed, and the measurement of the fluid-dynamic sound caused by the flow around the object such as the supports and umbones installed in the pipeline and the duct is assumed, it becomes such arrangement. Therefore, the influence of the near field appears to be strong, making a quantitative evaluation of the sound pressure level more difficult. Resolving this is a clear challenge for future studies. The relationship between the position *rc* by which the pressure fluctuation of a short-distance field can be disregarded now, and the minimum frequency *f* is given in 20log(2*̟frc*/*a*) >=10 dB (*a* is acoustic velocity) (Iida, 1996). Distance *rc* between the circular cylinder and the microphone becomes 188mm-211mm because the range of center-to-center spacing *x* between the circular cylinder and the microphone in this experiment is 5mm-95mm. The obtained lower critical frequency *f* becomes 910Hz-812Hz. When the center-to-center spacing is assumed to be 50mm, distance *rc* between two points becomes 194.5mm. The lower critical frequency *f* at that time is 880Hz.

**3.2. Flow characteristics in the measurement section**

154 Wind Tunnel Designs and Their Diverse Engineering Applications

prove the uniformity of the flow.

0.2

at a main flow velocity of *U*=28m/s

0.4

0.6

*U/Umax*

0.8

1.0

¶

¶ **¶** 

of *U*=28m/s

Flow characteristics in the measurement section where the sound absorbing wall (fibered glass wall) had been used were investigated. The hot-wire probe was inserted in the micro‐ phone's installation position; it traversed in a vertical direction (y direction), and the velocity and disturbance intensity were measured. Figure 6 shows the velocity distribution and the disturbance intensity when the air flow velocity is *U*=28m/s. The velocity distribution (△ symbol) for an acrylic wall is plotted for comparison. Here, the abscissa is a measurement position. The center of width in the vertical direction of the measurement section is assumed to be zero points; the upper side is assumed to be + mark, and the lower side is assumed to be - mark. The measurement position is made dimensionless by width *B*=376mm in the ver‐ tical direction of the measurement section. The ordinate shows the velocity distribution and the disturbance intensity, respectively. The symmetry of a flow was good to the center and turbulence intensity was less than 0.5% in the range which is maintaining the equality of a flow. The disturbance intensity to keep the uniformity of the flow was within 0.5%. Here, if uniform flow velocity *U*=28m/s and the measurement point *x*=400mm are calculated by Prantl's exact solution (*δ*=0.22(*ν/Ux*) 0.167*x*), the thickness of the boundary layer is 9.6mm. The thickness of the boundary layer as in Fig. 6 is 10mm for the acrylic wall, and this agrees with the value from Prantl's expression. However, the thickness of the boundary layer above the sound absorbing wall was about 28mm, and was about three times that in Prantl's expres‐ sion because the sound-absorbing wall was made of fibered glass with a rough surface. It was clarified that the wall was necessary to obtain a wide measurement section and thus im‐

side is assumed to be - mark. The measurement position is made dimensionless by width *B*=376mm in the vertical direction of the measurement section. The ordinate shows the velocity distribution and the disturbance intensity, respectively. The symmetry of a flow was good to the center and turbulence intensity was less than 0.5% in the range which is maintaining the equality of a flow. The disturbance intensity to keep the uniformity of the flow was within 0.5%. Here, if uniform flow velocity *U*=28m/s and the measurement point *x*=400mm are calculated by Prantl's exact solution (*δ*=0.22(*ν/Ux*) 0.167*x*), the thickness of the boundary layer is 9.6mm. The thickness of the boundary layer as in Fig. 6 is 10mm for the acrylic wall, and this agrees with the value from Prantl's expression. However, the thickness of the boundary layer above the sound absorbing wall was about 28mm, and was about three times that in Prantl's expression because the sound-absorbing wall was made of fibered glass with a rough surface. It was clarified that the wall was necessary to obtain a

<sup>0</sup> 0.1 0.2 0.3 0.4 0.5 0.0

Fig. 6. Flow velocity distribution and turbulence intensity in the test (measurement) section

**Figure 6.** Flow velocity distribution and turbulence intensity in the test (measurement) section at a main flow velocity

It is known that the fluid-dynamic noise made from the circular cylinder placed into the air flow is a dipole sound. Since there is single directivity also in a microphone, it is important to understand the influence on the measurement result by the spatial relationship of a sound source and its microphone. Figure 7 shows the measurement result of the sound pressure level when varying the distance *x* between centers of a circular cylinder and a microphone in the range from 5mm to 95mm. Here, the airflow velocity in a test section was *U*=28m/s and the diameter of circular cylinder was 20mm. It is understood that the measured sound pressure level is almost the same. So, in measurement of acoustic frequency, distance between centers of circular cylinder and microphone was set to 50mm. Here, it is expected that the pressure fluctuation of a short-distance field is included in the sound pressure

It is known that the fluid-dynamic noise made from the circular cylinder placed into the air flow is a dipole sound. Since there is single directivity also in a microphone, it is important to understand the influence on the measurement result by the spatial relationship of a sound

± ± ± ±±

*Y/B*

0

5

10

*(u ) /Umax %*

*2 0.5* ¶

¶ **¶** 

15

wide measurement section and thus improve the uniformity of the flow.

■ Glasswool wall, velocity △ Acrylic plate wall, velocity 口 Glasswool wall, turbulence

■ Fibered glass wall, velocity △ Acrylic plate wall, velocity □ Fibered glass wall, turbulence

**3.3 The relation between sound source and measurement position** 

**3.3. The relation between sound source and measurement position**

The back ground noise with acoustic half-free space of a test section was measured by making airflow velocity in a test section into *U*=28m/s. The circular cylinder of various diameters was installed in the test section, and frequency of a fluid-dynamic noise (acoustic frequency) was measured. Figure 8 shows the results of the acoustic frequency analysis with back ground noise (B.G.N.) in the measurement section and with a circular cylinder 20mm in diameter. The abscissa is frequency *f*, and the ordinate is the sound pressure level *SPL* in the figure. When the circular cylinder is set up in the measurement part, a peak at one big sound pressure level is obtained. At this time, the Strouhal number *S* (= *f d* /*U*) calculated from the frequency *f* (=275Hz) and air flow velocity *U* (=28m/s) is *S*=0.2. It is considered that the microphone measures the acoustic frequency from the fluid oscillation based on Karman vortex shedding. The frequency of the oscillating flow behind the circular cylinder

Fig. 7. Measurement result for sound pressure level with a directivity check **Figure 7.** Measurement result for sound pressure level with a directivity check

**3.4 Measurement result and verification of Acoustic frequency** 

#### **3.4. Measurement result and verification of Acoustic frequency**

The back ground noise with acoustic half-free space of a test section was measured by mak‐ ing airflow velocity in a test section into *U*=28m/s. The circular cylinder of various diameters was installed in the test section, and frequency of a fluid-dynamic noise (acoustic frequency) was measured. Figure 8 shows the results of the acoustic frequency analysis with back ground noise (B.G.N.) in the measurement section and with a circular cylinder 20mm in di‐ ameter. The abscissa is frequency *f*, and the ordinate is the sound pressure level *SPL* in the figure. When the circular cylinder is set up in the measurement part, a peak at one big sound pressure level is obtained. At this time, the Strouhal number *S* (= *f d* /*U*) calculated from the frequency *f* (=275Hz) and air flow velocity *U* (=28m/s) is *S*=0.2. It is considered that the microphone measures the acoustic frequency from the fluid oscillation based on Karman vortex shedding. The frequency of the oscillating flow behind the circular cylinder was measured using the hot wire anemometer for verification. Figure 9 shows the result of the frequency analysis using the microphone and the hot wire anemometer. The abscissa is fre‐ quency *f*, and the ordinate is a sound pressure level made dimensionless by the maximum value. In both measurement results, it is understood that one big peak is seen at the same frequency. Therefore, it was established that the acoustic frequency measured by the micro‐ phone was a fluid oscillating frequency based on Karman vortex shedding from the circular cylinder. This means that the fluid sound measured by making the acoustical free space can be measured in an internal flow. And, this measurement technique is considered suitable for the measurement of the fluid sound of an internal flow. was measured using the hot wire anemometer for verification. Figure 9 shows the result of the frequency analysis using the microphone and the hot wire anemometer. The abscissa is frequency *f*, and the ordinate is a sound pressure level made dimensionless by the maximum value. In both measurement results, it is understood that one big peak is seen at the same frequency. Therefore, it was established that the acoustic frequency measured by the microphone was a fluid oscillating frequency based on Karman vortex shedding from the circular cylinder. This means that the fluid sound measured by making the acoustical free space can be measured in an internal flow. And, this measurement technique is

10<sup>1</sup> 10<sup>2</sup> 10<sup>3</sup> 10<sup>4</sup>

Fig. 9. The relationship between the acoustic frequency and the fluid frequency due to

*f* Hz

considered suitable for the measurement of the fluid sound of an internal flow.

**Figure 8.** The frequency analysis of flow noise, *U*=28m/s, *d*=20mm

0.6

0.8

*PL/PLmax*

1

Fig. 8. The frequency analysis of flow noise, *U*=28m/s, *d*=20mm

Hotwire

Microphone

¶

¶ ¶

vortex shedding

¶

d=20

<sup>101</sup> <sup>102</sup> <sup>10</sup><sup>3</sup> <sup>10</sup><sup>4</sup> <sup>60</sup>

was measured using the hot wire anemometer for verification. Figure 9 shows the result of the frequency analysis using the microphone and the hot wire anemometer. The abscissa is frequency *f*, and the ordinate is a sound pressure level made dimensionless by the maximum value. In both measurement results, it is understood that one big peak is seen at the same frequency. Therefore, it was established that the acoustic frequency measured by the microphone was a fluid oscillating frequency based on Karman vortex shedding from the circular cylinder. This means that the fluid sound measured by making the acoustical free space can be measured in an internal flow. And, this measurement technique is

considered suitable for the measurement of the fluid sound of an internal flow.

B.G.N.

70

Fig. 8. The frequency analysis of flow noise, *U*=28m/s, *d*=20mm

80

90

100

*SPL* dB 110

120

¶

¶ ¶

¶

Fig. 9. The relationship between the acoustic frequency and the fluid frequency due to vortex shedding **Figure 9.** The relationship between the acoustic frequency and the fluid frequency due to vortex shedding

### **3.5. Comparison of measurement results with a blow-type wind tunnel**

**3.4. Measurement result and verification of Acoustic frequency**

156 Wind Tunnel Designs and Their Diverse Engineering Applications

the measurement of the fluid sound of an internal flow.

70

0.6

0.8

*PL/PLmax*

1

Fig. 8. The frequency analysis of flow noise, *U*=28m/s, *d*=20mm

**Figure 8.** The frequency analysis of flow noise, *U*=28m/s, *d*=20mm

Hotwire

Microphone

80

90

100

*SPL* dB 110

120

¶

¶ ¶

vortex shedding

¶

considered suitable for the measurement of the fluid sound of an internal flow.

B.G.N.

The back ground noise with acoustic half-free space of a test section was measured by mak‐ ing airflow velocity in a test section into *U*=28m/s. The circular cylinder of various diameters was installed in the test section, and frequency of a fluid-dynamic noise (acoustic frequency) was measured. Figure 8 shows the results of the acoustic frequency analysis with back ground noise (B.G.N.) in the measurement section and with a circular cylinder 20mm in di‐ ameter. The abscissa is frequency *f*, and the ordinate is the sound pressure level *SPL* in the figure. When the circular cylinder is set up in the measurement part, a peak at one big sound pressure level is obtained. At this time, the Strouhal number *S* (= *f d* /*U*) calculated from the frequency *f* (=275Hz) and air flow velocity *U* (=28m/s) is *S*=0.2. It is considered that the microphone measures the acoustic frequency from the fluid oscillation based on Karman vortex shedding. The frequency of the oscillating flow behind the circular cylinder was measured using the hot wire anemometer for verification. Figure 9 shows the result of the frequency analysis using the microphone and the hot wire anemometer. The abscissa is fre‐ quency *f*, and the ordinate is a sound pressure level made dimensionless by the maximum value. In both measurement results, it is understood that one big peak is seen at the same frequency. Therefore, it was established that the acoustic frequency measured by the micro‐ phone was a fluid oscillating frequency based on Karman vortex shedding from the circular cylinder. This means that the fluid sound measured by making the acoustical free space can be measured in an internal flow. And, this measurement technique is considered suitable for

was measured using the hot wire anemometer for verification. Figure 9 shows the result of the frequency analysis using the microphone and the hot wire anemometer. The abscissa is frequency *f*, and the ordinate is a sound pressure level made dimensionless by the maximum value. In both measurement results, it is understood that one big peak is seen at the same frequency. Therefore, it was established that the acoustic frequency measured by the microphone was a fluid oscillating frequency based on Karman vortex shedding from the circular cylinder. This means that the fluid sound measured by making the acoustical free space can be measured in an internal flow. And, this measurement technique is

<sup>101</sup> <sup>102</sup> <sup>10</sup><sup>3</sup> <sup>10</sup><sup>4</sup> <sup>60</sup>

10<sup>1</sup> 10<sup>2</sup> 10<sup>3</sup> 10<sup>4</sup>

Fig. 9. The relationship between the acoustic frequency and the fluid frequency due to

d=20

*f* Hz

*f* Hz

Figure 10 shows the variation of the peak frequency of the sound pressure level at the time of varying a circular cylinder diameter. The back ground noise is also shown for compari‐ son. The abscissa is frequency *f* and the ordinate is a sound pressure level *SPL*. Increase of a cylinder diameter can see the tendency for a sound pressure level to increase and for peak frequency to decrease. The experimental result (Tomita et al., 1982) in the wind tunnel of a blow type with a half-opening type test section is shown in Fig. 11 for comparison with this experimental result. Although the variation of a sound pressure level or peak frequency to the variation of the diameter of the circular cylinder shows the same tendency, in each circu‐ lar cylinder, one large peak and its harmonics component are seen, and spectrum distribu‐ tion of the fluid-dynamic noise made when a circular cylinder is installed into an air current constitutes a larger sound pressure level than a back ground noise by the high frequency side which passed over the large peak. This suggests containing other sounds potential in not only the fluid-dynamic sound to be measured but also the flow noise. Therefore, it ap‐ pears that the use of a blow-type wind tunnel with a half-open measurement section is rath‐ er inconvenient for measuring a sound effect. On the other hand, the results from a sealedtype measurement section of a suction-type wind tunnel becomes a sound pressure level that only the section of the frequency of the aimed fluid-dynamic sound is big as shown in Fig. 10, and the other frequency components are the same degree of the sound pressure level as the back ground noise. This is convenient for the examination of sound effects. The suc‐ tion wind tunnel with a sealed-type measurement section can be expected to be a good measurement technique for examining sound effects.

**¶** 

¶

¶ ¶

¶

**3.5 Comparison of measurement results with a blow-type wind tunnel** 

Figure 10 shows the variation of the peak frequency of the sound pressure level at the time of varying a circular cylinder diameter. The back ground noise is also shown for comparison. The abscissa is frequency *f* and the ordinate is a sound pressure level *SPL*. Increase of a cylinder diameter can see the tendency for a sound pressure level to increase and for peak frequency to decrease. The experimental result (Tomita et al., 1982) in the wind tunnel of a blow type with a half-opening type test section is shown in Fig. 11 for comparison with this experimental result. Although the variation of a sound pressure level or peak frequency to the variation of the diameter of the circular cylinder shows the same tendency, in each circular cylinder, one large peak and its harmonics component are seen, and spectrum distribution of the fluid-dynamic noise made when a circular cylinder is installed into an air current constitutes a larger sound pressure level than a back ground noise by the high frequency side which passed over the large peak. This suggests containing other sounds potential in not only the fluid-dynamic sound to be measured but also the flow noise. Therefore, it appears that the use of a blow-type wind tunnel with a half-open measurement section is rather inconvenient for measuring a sound effect. On the other hand, the results from a sealed-type measurement section of a suction-type wind tunnel becomes a sound pressure level that only the section of the frequency of the aimed fluid-dynamic sound is big as shown in Fig. 10, and the other frequency components are the same degree of the sound pressure level as the back ground noise. This is convenient for the examination of sound effects. The suction wind tunnel with a sealed-type measurement section can be expected to

Fig. 10. The characteristics of fluid-dynamic noise, in the case of present test section **Figure 10.** The characteristics of fluid-dynamic noise, in the case of present test section

Fig. 11. Characteristics of fluid-dynamic noise, in blow-type wind tunnel (Tomita et.al., 1982) **Figure 11.** Characteristics of fluid-dynamic noise, in blow-type wind tunnel (Tomita et.al., 1982)

#### **¶ 3.6. Effect of acoustic material and sound directivity**

**3.6 Effect of acoustic material and sound directivity** 

In order to verify the effect of the sound-absorbing material (fibered glass) in the measurement section, the acoustic frequency from the circular cylinder was measured. At this time, the microphone is set up from the bell mouse to 500mm upstream side by equal height to the circular cylinder installation position. The air flow velocity of the measurement section was *U*=28m/s. As a result, the measurement result differed according to the existence of the sound-absorbing material. Figure 12 shows the measurement results for circular cylinders 20mm, 25mm, 30mm, and 40mm in diameter when upper and lower sidewalls made of an acrylic board are used. Here, the back-ground noise (B.G.N.) is shown In order to verify the effect of the sound-absorbing material (fibered glass) in the measure‐ ment section, the acoustic frequency from the circular cylinder was measured. At this time, the microphone is set up from the bell mouse to 500mm upstream side by equal height to the circular cylinder installation position. The air flow velocity of the measurement section was *U*=28m/s. As a result, the measurement result differed according to the existence of the sound-absorbing material. Figure 12 shows the measurement results for circular cylinders 20mm, 25mm, 30mm, and 40mm in diameter when upper and lower sidewalls made of an

for comparison. The sound pressure levels for the peak frequency of each circular cylinder are small, and the peak frequency is twice the value of fluid oscillating frequency, based on the Karman vortex shedding shown in Fig. 10. On the other hand, the measurement result when sound-absorbing material is installed is shown in Fig. 13, relative to back-ground noise (B.G.N.) at circular cylinder diameters of 20mm-40mm. Two sound pressure peaks are seen in each figure. The first peak (1st peak) on the low frequency side is a Karman vortex shedding frequency, and the second peak (2nd peak) on the high frequency side is twice the Karman vortex shedding frequency. Moreover, the magnitude correlation of the two peaks is different in each circular cylinder. In the case of circular cylinders with diameters of 10mm, 15mm, and 30mm, the first peak (1st peak) on the low frequency side is larger. In the case circular cylinder diameters of 20mm, 25mm, and 40mm, the second peak (2nd peak) on the high frequency side is larger. It is shown that there is a change in the interference pattern of the sound wave in a vertical direction in the flow in the measurement section. The two peaks can also be observed in a blow-type wind tunnel with a half-open measurement section as shown in Fig. 11. In this case, however, the microphone is set up at and angle of 45 **¶** 

¶

¶ ¶

¶ **¶** 

**3.5 Comparison of measurement results with a blow-type wind tunnel** 

be a good measurement technique for examining sound effects.

70

40

**3.6 Effect of acoustic material and sound directivity** 

**3.6. Effect of acoustic material and sound directivity**

60

*SPL* dB 80

100

80

90

100

*SPL* dB

110

120

158 Wind Tunnel Designs and Their Diverse Engineering Applications

Figure 10 shows the variation of the peak frequency of the sound pressure level at the time of varying a circular cylinder diameter. The back ground noise is also shown for comparison. The abscissa is frequency *f* and the ordinate is a sound pressure level *SPL*. Increase of a cylinder diameter can see the tendency for a sound pressure level to increase and for peak frequency to decrease. The experimental result (Tomita et al., 1982) in the wind tunnel of a blow type with a half-opening type test section is shown in Fig. 11 for comparison with this experimental result. Although the variation of a sound pressure level or peak frequency to the variation of the diameter of the circular cylinder shows the same tendency, in each circular cylinder, one large peak and its harmonics component are seen, and spectrum distribution of the fluid-dynamic noise made when a circular cylinder is installed into an air current constitutes a larger sound pressure level than a back ground noise by the high frequency side which passed over the large peak. This suggests containing other sounds potential in not only the fluid-dynamic sound to be measured but also the flow noise. Therefore, it appears that the use of a blow-type wind tunnel with a half-open measurement section is rather inconvenient for measuring a sound effect. On the other hand, the results from a sealed-type measurement section of a suction-type wind tunnel becomes a sound pressure level that only the section of the frequency of the aimed fluid-dynamic sound is big as shown in Fig. 10, and the other frequency components are the same degree of the sound pressure level as the back ground noise. This is convenient for the examination of sound effects. The suction wind tunnel with a sealed-type measurement section can be expected to

<sup>10</sup><sup>1</sup> <sup>10</sup><sup>2</sup> <sup>10</sup><sup>3</sup> <sup>10</sup><sup>4</sup> <sup>60</sup>

d=15 d=20

<sup>10</sup><sup>1</sup> <sup>10</sup><sup>2</sup> <sup>10</sup><sup>3</sup> <sup>10</sup><sup>4</sup> <sup>20</sup>

Fig. 11. Characteristics of fluid-dynamic noise, in blow-type wind tunnel (Tomita et.al., 1982)

In order to verify the effect of the sound-absorbing material (fibered glass) in the measurement section, the acoustic frequency from the circular cylinder was measured. At this time, the microphone is set up from the bell mouse to 500mm upstream side by equal height to the circular cylinder installation position. The air flow velocity of the measurement section was *U*=28m/s. As a result, the measurement result differed according to the existence of the sound-absorbing material. Figure 12 shows the measurement results for circular cylinders 20mm, 25mm, 30mm, and 40mm in diameter when upper and lower sidewalls made of an acrylic board are used. Here, the back-ground noise (B.G.N.) is shown for comparison. The sound pressure levels for the peak frequency of each circular cylinder are small, and the peak frequency is twice the value of fluid oscillating frequency, based on the Karman vortex shedding shown in Fig. 10. On the other hand, the measurement result when sound-absorbing material is installed is shown in Fig. 13, relative to back-ground noise (B.G.N.) at circular cylinder diameters of 20mm-40mm. Two sound pressure peaks are seen in each figure. The first peak (1st peak) on the low frequency side is a Karman vortex shedding frequency, and the second peak (2nd peak) on the high frequency side is twice the Karman vortex shedding frequency. Moreover, the magnitude correlation of the two peaks is different in each circular cylinder. In the case of circular cylinders with diameters of 10mm, 15mm, and 30mm, the first peak (1st peak) on the low frequency side is larger. In the case circular cylinder diameters of 20mm, 25mm, and 40mm, the second peak (2nd peak) on the high frequency side is larger. It is shown that there is a change in the interference pattern of the sound wave in a vertical direction in the flow in the measurement section. The two peaks can also be observed in a blow-type wind tunnel with a half-open measurement section as shown in Fig. 11. In this case, however, the microphone is set up at and angle of 45

In order to verify the effect of the sound-absorbing material (fibered glass) in the measure‐ ment section, the acoustic frequency from the circular cylinder was measured. At this time, the microphone is set up from the bell mouse to 500mm upstream side by equal height to the circular cylinder installation position. The air flow velocity of the measurement section was *U*=28m/s. As a result, the measurement result differed according to the existence of the sound-absorbing material. Figure 12 shows the measurement results for circular cylinders 20mm, 25mm, 30mm, and 40mm in diameter when upper and lower sidewalls made of an

B.G.N.

d=40 d=30 d=25

B.G.N.

Fig. 10. The characteristics of fluid-dynamic noise, in the case of present test section

**Figure 10.** The characteristics of fluid-dynamic noise, in the case of present test section

*U*=35m/s

**Figure 11.** Characteristics of fluid-dynamic noise, in blow-type wind tunnel (Tomita et.al., 1982)

d=20 d=15 d=10

d=6

d=6 d=10

*f* Hz

*f* Hz

acrylic board are used. Here, the back-ground noise (B.G.N.) is shown for comparison. The sound pressure levels for the peak frequency of each circular cylinder are small, and the peak frequency is twice the value of fluid oscillating frequency, based on the Karman vortex shedding shown in Fig. 10. On the other hand, the measurement result when sound-absorb‐ ing material is installed is shown in Fig. 13, relative to back-ground noise (B.G.N.) at circular cylinder diameters of 20mm-40mm. Two sound pressure peaks are seen in each figure. The first peak (1st peak) on the low frequency side is a Karman vortex shedding frequency, and the second peak (2nd peak) on the high frequency side is twice the Karman vortex shedding frequency. Moreover, the magnitude correlation of the two peaks is different in each circular cylinder. In the case of circular cylinders with diameters of 10mm, 15mm, and 30mm, the first peak (1st peak) on the low frequency side is larger. In the case circular cylinder diame‐ ters of 20mm, 25mm, and 40mm, the second peak (2nd peak) on the high frequency side is larger. It is shown that there is a change in the interference pattern of the sound wave in a vertical direction in the flow in the measurement section. The two peaks can also be ob‐ served in a blow-type wind tunnel with a half-open measurement section as shown in Fig. 11. In this case, however, the microphone is set up at and angle of 45 degrees and positioned 500mm behind the circular cylinder, aiming at the sound around the circular cylinder. The first peak (1st peak) on the low frequency side is always larger than the second peak (2nd peak) on the high frequency side in the magnitude correlation of the peak because of the po‐ sition of the microphone and the directivity of the microphone. A comparison of the results between a suction-type and a blow-type wind tunnel with sound-absorbing material instal‐ led shows that the acoustical free space of an internal flow can become an acoustical free space similar to the case of an external flow. The sound is fluctuation of the pressure which transmits the inside of fluid, the size of the amplitude is the size of sound, and the height of oscillation frequency is the height of sound. The fluid force acts on the circular cylinder by the fluid fluctuation according to the vortex shedding from the circular cylinder. The oscilla‐ tion of the fluid force can be divided into a lift component and a drag component, at a ratio of 1:2. The circular cylinder placed on the air flow is a source of two kinds of sound waves as two peaks are apparent in the fluid sound. When acrylic upper and lower sidewalls are used, the acoustical free space becomes the only flow direction. The sound by the oscillation in the direction of the lift is canceled by acoustical interference. Therefore, the peak frequen‐ cies of each circular cylinder shown in Fig. 12 are considered to be the oscillation a sound from the drag direction. On the other hand, when the sound-absorbing material is installed on the upper and lower sidewalls, the acoustical free space is two directional (a parallel di‐ rection and a vertical direction) for the flow. It is an acoustical free space similar to the blowtype wind tunnel with a half-open measurement section. Therefore, the sounds of the lift and drag oscillations are measured as shown in Fig. 13.

In general, because the oscillation amplitude of the lift is far larger than that of the drag, it is expected that the sound pressure level in the direction of the lift is far larger than the sound pressure of the drag direction. However, the sound from the oscillation of the drag direction is easily detected because the directivity microphone is located on the upstream side of the circular cylinder in this measurement, and the fluid-dynamic sound by the oscillation in the direction of the lift is not detected easily. In addition, because the interference pattern of the ¶

¶

shown in Fig. 13.

sound wave in the direction of the lift is different in each circular cylinder, the sound pres‐ sure level in the direction of the lift is small. The sound pressure level of the drag direction has a large value, as shown in Figs. 12(c) (d) and (f). Moreover, when the microphone is set up in the measurement section, only the fluid-dynamic sound in the direction of the lift is measured as shown in Fig. 8. Such a phenomenon suggests that the directivity of the sound source and the directivity of the microphone are at issue, and this is the subject of future in‐ vestigation.

comparison of the results between a suction-type and a blow-type wind tunnel with soundabsorbing material installed shows that the acoustical free space of an internal flow can become an acoustical free space similar to the case of an external flow. The sound is fluctuation of the pressure which transmits the inside of fluid, the size of the amplitude is **Figure 12.** Characteristics of fluid-dynamic noise at the measurement point from the up-stream side of the test sec‐ tion, in the case of solid wall

the size of sound, and the height of oscillation frequency is the height of sound. The fluid force acts on the circular cylinder by the fluid fluctuation according to the vortex shedding from the circular cylinder. The oscillation of the fluid force can be divided into a lift component and a drag component, at a ratio of 1:2. The circular cylinder placed on the air flow is a source of two kinds of sound waves as two peaks are apparent in the fluid sound. When acrylic upper and lower sidewalls are used, the acoustical free space becomes the only flow direction. The sound by the oscillation in the direction of the lift is canceled by acoustical interference. Therefore, the peak frequencies of each circular cylinder shown in Fig. 12 are considered to be the oscillation a sound from the drag direction. On the other hand, when the sound-absorbing material is installed on the upper and lower sidewalls, the acoustical free space is two directional (a parallel direction and a vertical direction) for the flow. It is an acoustical free space similar to the blow-type wind tunnel with a half-open measurement section. Therefore, the sounds of the lift and drag oscillations are measured as

In general, because the oscillation amplitude of the lift is far larger than that of the drag, it is expected that the sound pressure level in the direction of the lift is far larger than the

degrees and positioned 500mm behind the circular cylinder, aiming at the sound around the circular cylinder. The first peak (1st peak) on the low frequency side is always larger than the second peak (2nd peak) on the high frequency side in the magnitude correlation of the peak because of the position of the microphone and the directivity of the microphone. A

Fig. 12. Characteristics of fluid-dynamic noise at the measurement point from the up-

stream side of the test section, in the case of solid wall

Experimental Study of Internal Flow Noise Measurement by Use of a Suction Type Low Noise Wind Tunnel http://dx.doi.org/10.5772/53828 161

Fig. 13. Characteristics of fluid-dynamic noise at the measurement point from the up-stream side of the test section, in the case of fibered glass wall; (a) cylinder diameter *d* is 10mm, (b) *d*=15mm, (c) *d*=20mm, (d) *d*=25mm, (e) *d*=30mm, (f) *d*=40mm **Figure 13.** Characteristics of fluid-dynamic noise at the measurement point from the up-stream side of the test sec‐ tion, in the case of fibered glass wall; (a) cylinder diameter *d* is 10mm, (b) *d*=15mm, (c) *d*=20mm, (d) *d*=25mm, (e) *d*=30mm, (f) *d*=40mm

sound pressure of the drag direction. However, the sound from the oscillation of the drag

#### direction is easily detected because the directivity microphone is located on the upstream side of the circular cylinder in this measurement, and the fluid-dynamic sound by the **4. Conclusion**

¶

sound wave in the direction of the lift is different in each circular cylinder, the sound pres‐ sure level in the direction of the lift is small. The sound pressure level of the drag direction has a large value, as shown in Figs. 12(c) (d) and (f). Moreover, when the microphone is set up in the measurement section, only the fluid-dynamic sound in the direction of the lift is measured as shown in Fig. 8. Such a phenomenon suggests that the directivity of the sound source and the directivity of the microphone are at issue, and this is the subject of future in‐

<sup>10</sup><sup>1</sup> <sup>10</sup><sup>2</sup> <sup>10</sup><sup>3</sup> <sup>10</sup><sup>4</sup> <sup>20</sup>

B.G.N.

Fig. 12. Characteristics of fluid-dynamic noise at the measurement point from the up-

degrees and positioned 500mm behind the circular cylinder, aiming at the sound around the circular cylinder. The first peak (1st peak) on the low frequency side is always larger than the second peak (2nd peak) on the high frequency side in the magnitude correlation of the peak because of the position of the microphone and the directivity of the microphone. A comparison of the results between a suction-type and a blow-type wind tunnel with soundabsorbing material installed shows that the acoustical free space of an internal flow can become an acoustical free space similar to the case of an external flow. The sound is fluctuation of the pressure which transmits the inside of fluid, the size of the amplitude is the size of sound, and the height of oscillation frequency is the height of sound. The fluid force acts on the circular cylinder by the fluid fluctuation according to the vortex shedding from the circular cylinder. The oscillation of the fluid force can be divided into a lift component and a drag component, at a ratio of 1:2. The circular cylinder placed on the air flow is a source of two kinds of sound waves as two peaks are apparent in the fluid sound. When acrylic upper and lower sidewalls are used, the acoustical free space becomes the only flow direction. The sound by the oscillation in the direction of the lift is canceled by acoustical interference. Therefore, the peak frequencies of each circular cylinder shown in Fig. 12 are considered to be the oscillation a sound from the drag direction. On the other hand, when the sound-absorbing material is installed on the upper and lower sidewalls, the acoustical free space is two directional (a parallel direction and a vertical direction) for the flow. It is an acoustical free space similar to the blow-type wind tunnel with a half-open measurement section. Therefore, the sounds of the lift and drag oscillations are measured as

**Figure 12.** Characteristics of fluid-dynamic noise at the measurement point from the up-stream side of the test sec‐

In general, because the oscillation amplitude of the lift is far larger than that of the drag, it is expected that the sound pressure level in the direction of the lift is far larger than the

d=40 d=30

d=25

d=20

*f* Hz

vestigation.

¶

¶

shown in Fig. 13.

tion, in the case of solid wall

40

*U*=28m/s

stream side of the test section, in the case of solid wall

60

*SPL* dB 80

100

160 Wind Tunnel Designs and Their Diverse Engineering Applications

oscillation in the direction of the lift is not detected easily. In addition, because the This study proposed a technique to measure the fluid-dynamic noise of an internal flow in a wind tunnel, and the fluid-dynamic noise from a circular cylinder placed on the air flow of a suction-type wind tunnel with a sealed-type measurement section with sound-absorbing material (fibered grass) was measured. The following conclusions were obtained.

