Almost all of the noise from a jet is caused by the violent turbulent mixing at the boundary between the jet and the atmosphere. Initially the edge of the jet is clearly defined and there is thus a high-speed shearing action between static and rapidly moving molecules of air and gas. This quickly breaks down into small eddies which cause high-frequency noise of great intensity. The eddies progressively mix the jet with the surrounding atmosphere, blurring the jet boundary. Continued mixing makes the eddies larger and larger, progressively reducing the eddy frequency and thus the pitch of the noise, and also reducing the jet velocity. Eventually, at a distance of perhaps 200 ft, the jet is no longer clearly definable, but is replaced by a much larger mass of eddies characterised by the fact that, in general, the air in line with the jet axis is moving away from the aircraft. By this time the writhing air mass is also distorted by the natural wind, so that it can constitute a hazard to light aircraft beside an airport runway in a crosswind.
Once the gas has left the nozzle, nothing can be done to quieten its noise, but noise can be significantly reduced by the design of the aero engine exhaust system modules. Remembering that awesome '8th-power of Vi' effect, everything possible must be done to minimize jet velocity. By far the best answer, for subsonic aircraft, is to replace a turbojet by a high-BPR turbofan. In any turbofan, provided there are no severe penalties in engine thrust and weight, it pays to mix the fan airflow with the core jet inside a full-length jet pipe. This can be done within the available length either with a chute-type mixer or by making the inner wall of the bypass duct (ie, the core jet pipe) progressively change from a circular section to deep corrugations.
Jet noise became an important environmental factor with the introduction of commercial transports powered by turbojets in the mid 1950s. These engines were noisy by any standard, and whereas the pioneer Comet 1 and Tu104 had plain jet pipes, the Comet 4, 707 and DC-8 were all provided from the outset with exhaust nozzles which promoted more rapid mixing of the jet with the atmosphere. The objective was to maximize the length of the nozzle periphery.
In the Avon engines of the Comet and Caravelle the exhaust nozzle (named for F. B. Greatrex of RollsRoyce) had an expanding diameter so that, whilst keeping cross-section area constant, it could incorporate six large inward-pointing chutes bringing fresh air to mix with the jet. In the DC-8 and Conway-engined 707 the jet was diverted outwards by internal vanes to escape through eight radial lobes. The effect with this corrugated-perimeter nozzle was the same as with the Greatrex but even more pronounced. With the Pratt & Whitney-engined 707 the exhaust nozzle was terminated in 20 separate circular tubes.
In the Avon 531 of some Caravelle VIs the nozzle was fitted with a hinged flap between two of the lobes to trim area to different flight conditions. Concordes have unique exhaust nozzles, the inner primary petals having retractable 'spades' to break up the periphery and the outer (reverser) eyelids, in effect squashing the jet on take-off to reduce noise.
All of these exhaust nozzles had little effect on the high-frequency noise from immediately downstream (in fact, by lengthening the periphery they mostly increased it), but this noise is rapidly attenuated by the atmosphere, and some that may reach the listener is beyond the audible range of frequencies.
Their main achievement was to reduce the more enduring lower-frequency noise. The fact that they were heavy and also degraded engine performance was obvious, and by modem standards the aircraft were still unacceptably noisy.
The breakthrough came when BPR (turbofan bypass ratio) was raised from below unity up to 5 or more. This has resulted in aero engines which obtain their thrust by accelerating a far greater mass of air more gently, and their noise is of a much less-obtrusive character. The fan jet is slower and cooler than that from a turbojet, and the annular nozzle already has a large periphery. The hot core jet is relatively small, and has had most of its energy removed by the multiple turbine stages, and again presents no significant problem.
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By John Routledge
