Survey of the acoustic near field of three nozzles at a pressure ratio of 30 Page: 2 of 33
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TECH LIBRARY KAFB, NM
NACA TN 3978 0067051
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
TECHNICAL NOTE 3978
SURVEY OF T ACOUSTIC NEAR FIELD OF TEREE
NOZZLES AT A PRESSURE RATIO OF 30
By Harold R. Mull and John C. Erickson, Jr.
Detailed measurements were made in the acoustic near field of three
cold-air exhaust nozzles with pressure ratios of 30 exhausting into
quiescent air. Two of the nozzles were convergent-divergent, one having
a 150 conical expansion and the other an isentropic expansion. Throat
diameters were 5/8 inch and exit diameters, 1.2 inches. The third nozzle
was convergent with a 5/8-inch exit diameter.
The total acoustic power radiated by the jet stream was about the
same for the convergent-divergent nozzles and appeared to be in agree-
ment with predictions based upon tests at subsonic velocities and the
Lighthill parameter. The convergent nozzle created more than the pre-
dicted noise power based on the Lighthill parameter. Contour maps and
plots of the position of the maximum sound-pressure level for audible
frequencies revealed that there were no intense sources of noise upstream
of 10 nozzle-exit diameters from the jet exit. The 1/3-octave-band
acoustic spectra showed a broad nonresonant shape. There were no dis-
crete frequencies, and no high-level ultrasonic sources were detected.
The use of rocket power for missiles and aircraft has focused atten-
tion on the noise fields created by the exhaust stream from high-pressure-
ratio nozzles. The noise associated with high-velocity jets can be
destructive to adjacent structures and cause failure of electronic equip-
ment. Adequate knowledge of the intensity, spectrum, and directionality
of the near-field noise is necessary to provide for the protection of
delicate mechanisms and eliminate fatigue failure.
The near field, as the term implies, is that region which surrounds
a distributed source out to a distance that is not long compared with an
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Mull, Harold R. & Erickson, John C., Jr. Survey of the acoustic near field of three nozzles at a pressure ratio of 30, report, April 1957; (https://digital.library.unt.edu/ark:/67531/metadc56392/m1/2/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.