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 decreasing noise to extra appropriate degrees. notwithstanding, the measurements themselves are advanced, and require a deep knowing of the experimental facility applied (such as a wind tunnel), dimension instrumentation, and knowledge research thoughts.
In this quantity fresh advances within the dimension and figuring out of aerodynamically generated sound are offered through leaders within the improvement of latest thoughts during this box. either uncomplicated and utilized difficulties are lined in detail.
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M . . H ........ N . . N ••••••••••••••••••••••••••••• -"'--. ------- -----HFF --HFR ...................................... --+-HpR 0 r:Q "0 ::rf -5 -10 -15 0 20 40 60 80 100 frequency, kHz (oM = 187 Hz) Fig. 28. Typical B&K 4135 pressure response curve (lower), which when combined with a free-field effect curve (top) result in a free-field response curve (middle). Sound waves are normally incident to the diaphragm. 35 mm (Allen et al. 1995) Microphone Corrections at High Frequency 47 field response curve (middle).
A is the wavelength of the propagating wave in still air. e' > 90° front sweeps through the shear layer at some angle, the wave-shear layer intersection point moves along the shear layer at the trace velocity. The velocities are shown as proportional distances (the distance covered in one period, T). \ in the wave normal direction outside the jet and a trace distance L along the shear layer. For propagation from a stream into still air, the trace velocity must be greater than the speed of sound outside the flow, Co' In the open jet, the trace velocity includes contributions from the flow velocity, U=, and the sound speed in the jet, Ct.
O N ~ 100 a:l -c c: ~ 90+······································ ,............................................................................ +. . . . . . . . . . . . . . . . . 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.
Aeroacoustic Measurements by Thomas J. Mueller, Christopher S. Allen, William K. Blake, Robert P. Dougherty, Denis Lynch, Paul T. Soderman, James R. Underbrink