**Nomenclature**


*I*0: intensity of sound of standard, W/m2

*L*: level of sound intensity, dB

*L*IN: level of sound intensity inside fan room, dB

*L*OUT: level of sound intensity outside fan room, dB

*PL*/*PL*max: dimensionless sound pressure level, non-dimensional

*r*c: critical distance, m

suction-type wind tunnel with a sealed-type measurement section with sound-absorbing

**1.** The acoustic performance and fluid-dynamic performance of a test wind tunnel were good. The following results were obtained for the performance of the test wind tunnel. The noise in the blower room is effectively intercepted. The position of the sound source and the microphone are not influenced by directivity. The uniformity of the flow of the measurement section narrows when sound-absorbing material is used for the

**2.** The following results were obtained from installing sound-absorbing material in the measurement section. The acoustical free space can be made from the closed space. When the surface of the microphone was arranged and set up on the surface of the sound-absorbing material, the measurement of the fluid sound of an internal flow be‐

**3.** The acoustic frequency measured by the microphone was confirmed to have a frequen‐ cy based on the fluid oscillation caused by the Karman vortex shedding measured with

**4.** The following results were obtained when a comparison was made with the results from a blow-type wind tunnel. The aimed acoustic frequency was measured by the large sound pressure level. Other frequency elements were the same degrees of the sound pressure level as the back ground noise. It has been understood that such a result

**5.** When an acoustical effect was examined, it was understood that the following consider‐ ation is necessary. The distance between the sound source and the microphone must be set in consideration of the influence of the pressure fluctuation of the near-field. The lower bound frequency must be understood. The microphone must be arranged in con‐

**6.** From the results outlined in (2)-(4), this present measurement technique is considered to be a technique useful for the measurement of the fluid sound of an internal flow.

material (fibered grass) was measured. The following conclusions were obtained.

measurement section of the test wind tunnel.

162 Wind Tunnel Designs and Their Diverse Engineering Applications

the hot-wire anemometer.

**Nomenclature**

*f*: frequency, Hz

*a*: acoustic velocity, m/s

*B*: height of test section, m

*I*: intensity of sound, W/m2

B.G.N.: back ground noise, dB

*d*: diameter of circular cylinder, m

came possible without any disarrangement of the flow-field.

was convenient when a sound effect was examined.

sideration of the sound directivity with the sound source.

*S*: Strouhal number, non-dimensional

*SPL*: sound pressure level, dB

*U*: main flow velocity, m/s

*U*max: maximum flow velocity, m/s

*u*: component of fluctuating flow velocity, m/s

*x*: measurement position in test section, m

x: coordinate component (flow direction), m

*Y*: measurement position in test section, m

y: coordinate component (vertical direction), m

z: coordinate component (horizontal direction), m

δ: thickness of boundary layer, m

ν: kinematic viscosity of air, m<sup>2</sup> /s
