Author: T. F. Balsa

Publisher:

Published: 1978

Total Pages: 719

ISBN-13:

This report presents the results of Task 2 conducted under the subject program over a period of 45 months. Task 2 was formulated as a fundamental theoretical and experimental study aimed at an understanding of the noise generation and suppression mechanisms of high velocity jets. The mechanisms investigated included changes in turbulence structure, fluid shielding, and alteration of convective amplification of jet noise sources. Several other areas such as physical shielding, shock associated noise, lip noise, effect of fluid/particle additives on jet noise, orderly structure in jets, ejector aeroacoustics, and flight effects on jet noise were also investigated. The most significant achievements of Task 2 were as follows. A unified aeroacoustic theory composed of three ingredients were developed: (1) the prediction of mean properties of the jet plume, (2) deduction of turbulence properties relevant to jet noise by similarity arguments, and (3) the prediction of far-field noise including the effects of fluid shielding. A semiempirical shock noise prediction procedure was also developed. Exhaustive theory-data comparisons for a wide range of nozzle configurations and velocity/temperature combinations were conducted and have confirmed the essential validity of this model as a prediction tool. A comprehensive series of experiments with simple suppressor elements (such as a single rectangular tube, twin jets, linear arrays of jets, circular arrays of jets) was carried out and revealed the importance of acoustic shielding by adjacent jets. A fundamental series of experiments, specifically tailored to reveal fluid shielding as a jet noise suppression mechanism, was successfully conducted.