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National Advisory Committee for Aeronautics, Report - Far Noise Field of Air Jets and Jet Engines

An experimental investigation was conducted to study and
compare the acoustic radiation of air jets and jet engines. A
number of diferent nozzle-exit shapes were studied with air
jets to determine the efiect of exit shape on noise generation.
Circular, square, rectangular, and elliptical convergent nozzles
and convergent-divergent and plug nozzles were investigated.

At low jet pressure ratios (less than 93) the nozzle shape
had a negligible effect on the sound field; at higher pressure
ratios the convergent and plug nozzles exhibited discrete fre-
quencies associated with shock waves in the jet. The convergent-
divergent nozzle showed a substantial reduction in sound power
at its design pressure ratio. This reduction resulted from the
elimination of discrete frequencies caused by shock formations.

The acoustic power radiated by jets issuing from conical
convergent nozzles was correlated by the I/ighthill parameter
for both air jets and nonafierburning jet engines. The ratio
of sound power to Lighthill parameter was .29.?)(10‘5 for both
air jets and jet engines. This result shows that the principal
contribution to jet engine noise is the turbulent mixing of the
jet with the surrounding medium. The sound power radiated
by an afterburning jet engine was lower than indicated by the
I/ighthill relation.

Correction of sound-pressure-level directional data by the
nozzle area ratio and the eighth power of the velocity ratio gave
good agreement between engine and air-jet data.

The spectral distributions of the sound power for the engine
and the air jet were in good agreement for the case where the
engine data were not greatly afiected by reflection or jet inter—
ference ejects. Such power spectra for a subsonic or slightly
choked engine or air jet show that the peaks of the spectra occur
at a Strouhal number of 0.3.