<DOC>
<DOCNO>
1244
</DOCNO>
<TITLE>
on the aerodynamic noise of a turbulent jet .
</TITLE>
<AUTHOR>
cheng,sin-i.
</AUTHOR>
<BIBLIO>
j. ae. scs. 1961, 321.
</BIBLIO>
<TEXT>
  a new model is advanced for analyzing the broad-spectrum
noise of a turbulent jet .  the shear layer bounding the turbulent
jet is assumed to play an important role in modifying the /quadrupole
sound radiation/ from the interior .  to the sound-emitting
small-scale turbulent eddies (with frequencies much higher
than those of large-scale eddies), the laminar shear layer has an
irregular contour, as if the large-scale turbulent motions were
frozen .  the linearized analysis is then applied to the laminar
shear layer to relate the acoustic oscillations across it .
  the concept of geometrical acoustics is generalized to represent
the passage of an acoustic ray through a laminar shear layer .
acoustic rays may be traced across the shear layer as transmission
and refraction, but they may also be apparently /absorbed/
or /generated/ by the laminar layer .  this /generation/ is visualized
as the schematic representation, within the framework of
geometrical acoustics, of the action of the reynolds stress in
transferring energy from the shearing mean flow to the acoustic
waves .  such action of the reynolds stress can be neglected in
ordinary acoustics when the acoustic medium is not moving at
speeds comparable to the speed of sound in the medium .  however,
this action is of crucial importance in the aerodynamic noise
of high-speed turbulent jets where the reynolds stress is the fundamental
element of the radiating quadrupoles, according to
lighthill .
  those acoustic waves that become /stationary/ with respect
to the local mean flow somewhere in the interior of the shear layer
are significantly modified by the viscous action through the
critical layer .  the shear layer therefore serves as a selective
amplifier of the acoustic waves passing through it .  kinematically,
the shear layer brings about the preferred downstream
emission ..  dynamically, the shear-layer augmentation significantly
increases the polar peak noise level .  the acoustic power
output per unit solid angle for such downstream emissions augmented
by the shear layer (including the polar peak) varies as,
predicted by lighthill, but without lighthill's convective
corrections .  on the other hand, the acoustic power output per
unit solid angle nearly normal to the jet, due to the transmitted
downstream-propagating waves, varies roughly as .  heating
the jet gas increases the shear-layer augmentation and may increase
the polar peak noise level by several db .  the silencing
action of the edge notches and edge teeth may also be interpreted
as due apparently to the result of possible distortion of
the shear-layer profiles .
</TEXT>
</DOC>
