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National Advisory Committee for Aeronautics, Technical Notes - Effect of Jet Temperature on Jet-Noise Generation

An experimental investigation was conducted in order to determine
the effect of jet temperature on jet-noise generation. Jet pressure
ratios from 1.5 to 1.9 and temperatures from 80° to 10000 F were used.
Results showed that sound power can be adequately predicted by the Light-
hill parameter based on ambient temperature over the range of tempera-
tures investigated. The dimensionless frequency spectra of the jet was
shown to be affected by temperature; increasing jet temperature resulted
in a shift of acoustic energy from high to low Strouhal numbers. Shifts
in the jet spectra were explained on the basis of the effect of tempera-
ture on the spreading characteristics of the jet, and a method of cor-
recting the spectra for jet temperature was presented.

The far-field noise of jets and jet engines has received consider-
able attention in recent years (refs. 1 to 9). A survey of the litera-
ture indicates that the effect of temperature is not as immediately evi-
dent as the effect of jet velocity. The temperature effect may be sig-
nificant because the temperature range of interest is quite large. It
would be desirable to know, for instance, whether cold-model—jet tests
will correctly simulate turbojet and rocket noise.

Reference 4 indicates that jet temperature has a negligible effect
on sound pressure at a single point in the_sound field. Early experi—
ments with various gases (ref. 5) shOWed that sound pressure varies lin-
early with jet density. Since jet density varies inversely with tempera—
ture, sound power would be expected to vary inversely with the square of
the temperature. For a first approximation one might expect that the
variation of jet density either by the use of temperature variation or
by the use of gases of various molecular weights should give similar
results. However, the experiments of reference 3 indicate that data
from both full—scale tests with jet engines and small cold-air jets can
be correlated on a total-sound—power basis and that no significant effect
of jet temperature was observed.