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Jet Noise and its Reduction

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This is the first lecture in a new series of lunchtime seminars on Thursdays. Fellows and other participants can take their lunches from Hall into the Drawing Room from 12.30 onwards. The talk will be from 12.45 until 1.15, with time for questions until 1

Jet noise is a dominant component of aircraft noise, particularly at takeoff. It is generated by turbulent mixing of the high-speed propulsive jet with ambient air. Lighthill’s celebrated 8th power law showed that the sound power is proportional to the 8th power of the jet velocity multiplied by the jet cross-sectional area. Historically jet noise has been reduced by increasing the engine bypass ratio, whereby the required thrust is obtained by increasing the exit area of the propulsive jet and decreasing its speed, resulting in a jet noise reduction. But this increases the fan noise. Fan and jet noise are now appropriately equal and more subtle ways of reducing jet noise are required. Chevrons or corrugations around the lip of the jet pipe have been shown experimentally to reduce the jet noise. Time consuming high-resolution computational fluid mechanics can predict jet noise reduction due to chevrons, but to date the development and optimisation of chevrons has required extensive large-scale empirical testing. A simple theory that captures the physics of jet noise generation and its reduction by chevrons will be described. The effects of the geometry of the chevrons can be calculated sufficiently quickly that the method can be used in preliminary nozzle design to optimise the noise reduction. The same approach can also be used to predict the effects of microjets installed around the rim of the jet pipe exit, another technique that leads to reduced noise.

This talk is part of the Homerton Seminars series.

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