Page 211 - The Jet Engine

Noise suppression

5. Jet exhaust noise is caused by the violent and 6. Compressor and turbine noise results from the
hence extremely turbulent mixing of the exhaust interaction of pressure fields and turbulent wakes
gases with the atmosphere and is influenced by the from rotating blades and stationary vanes, and can
shearing action caused by the relative speed be defined as two distinct types of noise; discrete
between the exhaust jet and the atmosphere. The tone (single frequency) and broadband (a wide range
small eddies created near the exhaust duct cause of frequencies). Discrete tones are produced by the
high frequency noise but downstream of the exhaust regular passage of blade wakes over the stages
jet the larger eddies create low frequency noise. downstream causing a series of tones and
Additionally, when the exhaust jet velocity exceeds harmonics from each stage. The wake intensity is
the local speed of sound, a regular shock pattern is largely dependent upon the distance between the
formed within the exhaust jet core. This produces a rows of blades and vanes. If the distance is short
discrete (single frequency) tone and selective ampli- then there is an intense pressure field interaction
fication of the mixing noise, as shown in fig. 19-2. A which results in a strong tone being generated. With
reduction in noise level occurs if the mixing rate is the high bypass engine, the low pressure
accelerated or if the velocity of the exhaust jet compressor (fan) blade wakes passing over
relative to the atmosphere is reduced. This can be downstream vanes produce such tones, but of a
achieved by changing the pattern of the exhaust jet lower intensity due to lower velocities and larger
as shown in fig. 19-3. blade/vane separations. Broadband noise is

Fig. 19-3 Change of exhaust jet pattern to reduce noise level.

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