Sound radiation from surface cutouts in high speed flow
In an experimental investigation of subsonic and supersonic flows of air past rectangular cavities cut into a flat surface it was discovered that the cavities emit a strong acoustic radiation. The frequency of the sound-producing oscillations measured by a hot wire in the cavity was found to be inv...
| Summary: | In an experimental investigation of subsonic and supersonic flows of air past rectangular cavities cut into a flat surface it was discovered that the cavities emit a strong acoustic radiation.
The frequency of the sound-producing oscillations measured by a hot wire in the cavity was found to be inversely proportional to the breadth for fixed depth. For fixed breadth the frequency was found to increase, though not systematically, with a decrease in depth.
A non-dimensional frequency S is defined in terms of the frequency of emission, the gap breadth, and the free stream velocity. The dependence of S on the various parameters in the problem, such as Mach number, Reynolds number and ratio of the boundary layer thickness to a dimension of the cavity, is discussed in light of appropriate experiments.
An estimate of the intensity of the radiation was obtained by means of an optical interferometer of the Mach-Zehnder type. For points located at 3 to 4 cavity breadths from the cavity, intensities of the order of 100 - 150 decibels were found for sound fields from cavities 0.1" deep and 0.1 to 0.2 inch broad at Mach numbers 0.7 to 0.85.
Possible mechanisms for the sound production by the cavities are discussed. |
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