Sound generation by flow over relatively deep cylindrical cavities

Abstract

An experimental investigation has been conducted into the production of high-intensity tones by deep cylindrical cavities in a flat surface at relatively low Mach numbers (i.e., 0.12–0.24). The application of such a mechanism is to the acoustic coding of moving objects containing drilled cylindrical cavities. The sound intensity and frequency have been determined as functions of flow velocity, diameter, and depth of the cavities resulting in correlations to be used for the design of a whistle in the range of f=5–17 kHz. As a practical matter, it is possible to produce a whistle producing 106 dB at a 30.5-cm distance from a cylindrical cavity of 0.508 cm in diameter and 1.32 cm in depth with an airflow of 57.7 m/s past the hole. An eddy-driven oscillator is represented by a one-dimensional oscillator with a forcing term dependent on a time-delayed displacement. This simple model shows how the natural frequencies of the shear layer are pulled to the acoustic frequencies at resonance, describes the shape of resonance, and shows how the natural frequencies of a pipe with no flow correspond to actual frequencies of the whistle which depend on the flow through the factor (1−M)−1.