Abstract
Strip spoilers of varying size have been inserted into the 112-in. diam exit pipe of a model jet system, 6 in. upstream from the exit plane. Measurements of the noise radiation as a function of flow velocity and spoiler dimension have been made. It was expected that the noise radiation for these configurations would be dominated by dipole-like sources arising from turbulence-generated surface forces. However, the expected levels were greatly exceeded by a tone of narrow bandwidth occurring near the resonance frequency of the pipe for cross modes (5 kHz), with additional tones appearing at multiples of this frequency. Experiments have been conducted to determine whether the resonances are driven by acoustic feedback or by the turbulent wake of the spoiler. [This work is partially supported by the Air Force Office of Scientific Research.]
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