Aeroacoustic Measurements by Thomas J. Mueller, Christopher S. Allen, William K. Blake,

By Thomas J. Mueller, Christopher S. Allen, William K. Blake, Robert P. Dougherty, Denis Lynch, Paul T. Soderman, James R. Underbrink

The aim of aeroacoustic measurements is to supply a foundation for assessing mechanisms of noise iteration, and to boost tools of lowering noise to extra appropriate degrees. in spite of the fact that, the measurements themselves are complicated, and require a deep knowing of the experimental facility applied (such as a wind tunnel), dimension instrumentation, and information research options.
In this quantity fresh advances within the size and realizing of aerodynamically generated sound are provided by means of leaders within the improvement of recent options during this box. either uncomplicated and utilized difficulties are coated in detail.

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They found that for the frequencies below 30 kHz, the effect of turbulence absorption was not important. 5 The wave convection of intensity is not the same as acoustic pressure convective amplification discussed with Eq. 24). Intensity is the acoustic energy that flows through a unit area in unit time and, in still air, is equal to the time average product of acoustic pressure and particle velocity. However, in a moving stream, a fixed noise source will experience a convective amplification of the pressure and velocity terms.

100 1000 frequency, 1f3 octave bands, Hz ~~ 10000 Fig. 15. 40 x 80 background noise at 445 km/h before and after modification of the test section acoustic lining (Soderman et al. 2000) Open-Jet Background Noise In addition to drive-fan and wind noise, an in-airstream microphone in an open-jet wind tunnel is also exposed to sound from the shear layers - particularly as the shear layers induce unsteady pressures on the nozzle and collector. That is mitigated to some extent by the lack of test section walls, which eliminates wall boundary-layer noise and allows drive-fan sound waves to propagate into the open room.

2000) Background Noise from Wall Boundary Layer Depending on wind tunnel wall roughness and distance from strut-mounted microphones, wall boundary layers can be a source of background noise. Between roughly 5 kHz and 12 kHz, the 40 X 80-background noise was dominated by noise from the test section wall boundary layer before steps were taken to smooth the surface. 15 measured before and after installation of a new test section lining (Soderman et al. 2000). Prior to the modification, a 152-mm deep sound absorbent lining with a 40 %-open perforated metal face sheet covered the walls.

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