Jet Noise : 3rd Report, Effect of the Shape of Nozzle-Exit

Abstract

The effects of the shape of nozzle exit, the size of nozzle exit, the multiple nozzle (jetinterference) etc. on the generation of noise and the mechanism of noise reduction of the widely used noise suppressors are investigated. With the change of the shape or the size of nozzle exit, the noise intensity per unit frequency increases proportionally to the second power of the characteristic length of nozzle exit at the lower frequencies and to the fourth power of that length at the higher frequencies. The interference between jets causes a change in the turbulent velocity field and reduces the strength of acoustic sources in the mixing region to the degree of the strength in the turbulent region. This accounts for the mechanism of noise reduction of tubular or lobed nozzles. The rate of associated noise-attenuation is evaluated on the basis of experimental results of the turbulent velocity field. The closer the ratio of the width of nozzle to the space between nozzles to unity on average, the more improved are the characteristics of noise reduction.