Noise Generated by Boundary-Layer Interaction with Perforated Acoustic Liners

Abstract

A problem that occurs in the application of perforated plate acoustic duct liners is the noise generated by the. turbulent boundary-layer flow over the holes in the liner surface. This flow not only generates noise but also thickens the boundary layer. To observe the noise generation, a series of tests has been run at the McDonnell Douglas Aerophysics Laboratory. These tests demonstrated that the sound pressure level generated at a liner surface can be as large as 158 dB for a duct Mach number of 0.4. The liner self-noise as measured in one liner panel was found to be affected by changes in the impedance of other liner panels. The tests showed that liner self-noise can be an important consideration in liner design. ELF-NOISE caused by flow over solid and porous surfaces, and in particular noise generated by airflows over perforated plate acoustic liners has been known to exist for some time.1'2'3 Such flows not only generate noise but also increase boundary-layer thicknesses and reduce thrust from aircraft engine ducts. It is possible that self-noise in an engine duct with a large amount of acoustic lining could be high enough so that an increase in lining area would actually increase the duct noise level. Recently, tests were conducted at the Douglas Aircraft Company to measure the impedance of acoustic liners using the two-microphone method. A specially designed siren was used to generate the high amplitude sound waves needed for the tests. It was a surprise to find that the self-noise amplitude could be of the same order as the high amplitude sound introduced into the duct from the siren. The self-noise waveform was periodic, with a frequency much different from the liner resonant frequency. Because selfnoise appears to be a basic problem connected with the use of perforated plate liners, a separate study of liner self-noise was carried out. The phenomenon of self-noise reminds the authors of two related problems in unsteady aeroacoustics, that of the edge tone,4'5 and that of vortex shedding behind a twodimensional body.6 In both problems an unsteady and periodic motion is generated by airflow over a rigid surface. Furthermore, the acoustic wavelength is much larger than a characteristic dimension of the surface so that near the surface the flowfield is primarily a hydrodynamic or a pseudosound7 field. That is, pressure fluctuations in the near field are of the order pu2 rather than pew, where p is the fluid density, c is the speed of sound, and u is the magnitude of the fluid velocity at a typical location in the flowfield. In both the edge tone and the self-noise problems a of air impinges on a rigid surface; in the vortex shedding problem the jet is really the entire freestream, which comes to rest on the forward part of the bluff body. In each case hydrodynamic instability is responsible for the unsteady motion, and a portion of the stream energy is radiated as acoustic energy. This portion is quite small if the freestream Mach number Uao/c is small compared to one, but it grows rapidly with Mach number.