<DOC>
<DOCNO>
721
</DOCNO>
<TITLE>
near noise field of a jet engine exhaust .
</TITLE>
<AUTHOR>
callaghan,e.e., howes,w.l. coles,w.d. and mull,r.h.
</AUTHOR>
<BIBLIO>
naca r.1338.
</BIBLIO>
<TEXT>
  aircraft structures located in the near noise field of a jet
engine are subjected to extremely high fluctuating pressures that
may cause structural fatigue .  studies of such structures have
been limited by lack of knowledge of the loadings involved .
  the acoustic near field produced by the exhaust of a stationary
turbojet engine having a high pressure ratio was measured for
a single operating condition without afterburning .  the maximum
over-all sound pressure without afterburning was found
to be about 42 pounds per square foot along the jet boundary in
the region immediately downstream of the jet-nozzle exit .
with afterburning the maximum sound pressure was increased
by 50 percent .  the largest sound pressures without afterburning
were obtained on a constant percentage band width basis in
the frequency range from 350 to 700 cps .
  additional tests were made at a few points to find the effect
of jet velocity on near-field sound pressures and to determine
the difference in value between sound-pressure levels at rigid
surfaces and corresponding free-field values .  near the jet
nozzle, over-all sound pressures were found to vary as a low
power (approx. unity) of the jet velocity .  over-all sound-pressure
levels considerably greater than the corresponding free-field
levels were recorded at the surface of a rigid plate placed along
the jet boundary .
  the downstream locations of the maximum sound pressure
at any given frequency along the jet-engine-exhaust boundary
and the longitudinal turbulent-velocity maximum of the same
frequency along a small cold-air jet at 1 nozzle-exit radius from
the jet axis were found to be nearly the same when compared on
a dimensionless basis .  also, the strouhal number of the corresponding
spectra maximums was found to be nearly equal at
similar distances downstream .
  in addition to the magnitude and frequency distribution of
the acoustic pressures, it is necessary to know the cross correlation
of the pressure over the surface area .  cross-correlation
measurements with microphones were made for a range of jet
velocities at locations along the jet and at a distance from the
jet .  free-field correlations of the over-all sound pressure and
of the sound pressure in frequency bands from 100 to 1000 cps
were obtained both longitudinally and laterally .  in addition,
correlations were obtained with microphones mounted at the
surface of a rigid plate that was large compared with the distance
over which a positive correlation existed .
  the region of positive correlation was generally found to
increase with distance downstream of the engine to 6.5 nozzleexit
diameters, but remained nearly constant thereafter .  in
general, little change in the correlation curves was found as a
function of jet velocity or frequency-band width .  the distance
from unity correlation to the first zero correlation was greater for
lateral than for longitudinal correlations for the same conditions
and locations .  the correlation curves obtained in free
space and on the surface of the plate were generally similar .
  the results are interpreted in terms of pressure loads on
surfaces .
</TEXT>
</DOC>
