Abstract
Mixing of primary and secondary flows in a conventional turbofan engine provides a means of reducing jet noise. By shaping the nozzle exit velocity profile, noise reduction greater than that resulting from fully mixed flow has been achieved. In a static jet noise experiment, five primary flow nozzles were used with a common secondary nozzle to simulate exhaust flows of turbofan engines with bypass ratios from 1 to 5. Data are shown which relate jet noise to the location, extent, and magnitude of the peak velocity region. In general, minimum noise is obtained for inverted profiles where the outer area peak velocity is 5. to 15% greater than the reference uniformly mixed velocity, and the area of the peak velocity region is 40 to 50% of the total flow area. The inverted flow profiles produce noise characteristics similar to multielement jet suppressor nozzles, i.e., low frequencies are reduced and high frequencies are increased. It is shown that these spectral effects can be used to obtain a balanced noise signature.
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