Page 749 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 2. 1 Exterior noise: Assessment and control

ratio across the valve that is favourable to inflow is Both engine-breathing noise and flow noise are classes
maintained and maximised when the open period of of aerodynamic noise. Before concentrating on the dif-
the valveissuchtoallowatleast,butnomore than,one ferences between the two, it is useful to briefly consider
complete oscillation of the pressure at its resonant how they are similar. Howe (1975) provides the most
frequency to occur whilst the valve is open. convenient framework for this. Howe offered a contri-
bution to the theory of sound generation by flow turbu-
Harrison and Dunkley (2004) also identified the role for lence and vorticity by using the specific stagnation
intake flow momentum in the breathing performance of enthalpy as the fluctuating acoustic quantity rather than
higher-speed engines. vorticity (Powell, 1964) or fluctuating momentum
Secondary noise sources being noise created by the (Lighthill, 1952; 1954) as had been used before. This is
motionoftheflowthroughtheintakeandexhaustsystems. particularly convenient for the case of internal combus-
This self-induced noise is commonly called flow noise. tion (IC) engine intake and exhaust noise as the engine-
Shell noise being the structure-radiated sound from breathing process is well modelled by the component of
the intake or exhaust tailpipes as excited by either pri- enthalpy that is associated with inviscid irrotational flow
mary or secondary noise sources. (see Section 22.1.3.10.3). By contrast, the flow noise can
The first two classifications are the subject of this be considered as having its origin in the component of
section and are illustrated in Fig. 22.1-15. enthalpy associated with:
When analysing the sound recorded at the intake
orifice (or exhaust tailpipe), it is notoriously difficult to rotational flow;
distinguish between sound that is due to primary noise irreversibilities in the flow;
sources and that due to secondary noise sources. There is viscous forces;
a common misconception that whilst primary (engine local heat input or loss.
breathing) noise is tonal and dominated by low- Intuitively the formation of vortices in the flow are the
frequency components of the fundamental cycle fre- most likely manifestation of this component of enthalpy.
quency (which is true), flow noise is chaotic, broadband These might be formed:
and mostly of high frequency (which is not true). In re-
at sudden expansions of the flow such as at the
ality, flow noise in the intake or exhaust system is likely to
be tonal, with both low- and high-frequency components. entrance to reactive silencer chambers;
The fact that engine breathing noise and flow noise can at points of flow separation such as at tight radius
be confused and misdiagnosed is hardly surprising as they bends;
both result in similar effects: a level of sound power in at free jets such as at the exhaust tailpipe.
the duct that propagates away from some source towards Although in one way Howe’s model unifies both engine-
the open end of the pipe and along the way that sound breathing noise and flow noise as being related to
power flux can be filtered, attenuated or amplified. enthalpy, it also leads to a useful point of distinction:

Primary noise sources

Secondary
noise source
Secondary
noise source

Radiated sound resulting Radiated sound resulting
from both primary and from both primary and
secondary noise sources secondary noise sources
Fig. 22.1-15 Primary and secondary sources of intake and exhaust noise.

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