Page 213 - The Jet Engine

Noise suppression

11. As previously described, the major source of
noise on the pure jet engine and low by-pass engine
is the exhaust jet, and this can be reduced by
inducing a rapid or shorter mixing region. This
reduces the low frequency noise but may increase
the high frequency level. Fortunately, high
frequencies are quickly absorbed in the atmosphere
and some of the noise which does propagate to the
listener is beyond the audible range, thus giving the
perception of a quieter engine. This is achieved by
increasing the contact area of the atmosphere with
the exhaust gas stream by using a propelling nozzle
incorporating a corrugated or lobe-type noise
suppressor (fig. 19-5).

12. In the corrugated nozzle, freestream
atmospheric air flows down the outside corrugations
and into the exhaust jet to promote rapid mixing. In
the lobe-type nozzle, the exhaust gases are divided
to flow through the lobes and a small central nozzle.
This forms a number of separate exhaust jets that
rapidly mix with the air entrained by the suppressor
lobes. This principle can be extended by the use of a
series of tubes to give the same overall area as the
basic circular nozzle.

13. Deep corrugations, lobes, or multi-tubes, give
the largest noise reductions, but the performance
penalties incurred limit the depth of the corrugations
or lobes and the number of tubes. For instance, to
achieve the required nozzle area, the overall
diameter of the suppressor may have to be
increased by so much that excessive drag and
weight results. A compromise which gives a
noticeable reduction in noise level with the least
sacrifice of engine thrust, fuel consumption or
addition of weight is therefore the designer's aim.

14. The high by-pass engine has two exhaust
streams to eject to atmosphere. However, the
principle of jet exhaust noise reduction is the same
as for the pure or low by-pass engine, i.e. minimize
the exhaust jet velocity within overall performance
objectives. High by-pass engines inherently have a
lower exhaust jet velocity than any other type of gas
turbine, thus leading to a quieter engine, but further
noise reduction is often desirable. The most Fig. 19-5 Types of noise suppressor.
successful method used on by-pass engines is to
mix the hot and cold exhaust streams within the
confines of the engine (fig. 19-5) and expel the lower a good understanding of the mechanisms of noise
velocity exhaust gas flow through a single nozzle generation and comprehensive noise design rules
(Part 6). exist. As previously indicated, these are founded on
the need to minimize turbulence levels in the airflow,
15. In the high by-pass ratio engine the reduce the strength of interactions between rotating
predominant sources governing the overall noise blades and stationary vanes, and the optimum use of
level are the fan and turbine. Research has produced acoustically absorbent linings.

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