A series of convergent rectangular nozzles of aspect ratios 2:1,
4:1, and 8:1 were constructed with uniform exit velocity profiles.
Additional nozzles were constructed that extended the wide lip on
one side of these nozzles to form beveled nozzles. Far-field
acoustic measurements were made and analyzed, and the results
presented. The impact of aspect ratio on jet noise was similar to
that of enhanced mixing devices: reduction in aft, peak frequency
noise with an increase in broadside, high frequency noise.
Azimuthally, it was found that rectangular jets produced more noise
directed away from their wide sides than from their narrow sides.
The azimuthal dependence decreased at aft angles where noise
decreased. The effect of temperature, keeping acoustic Mach number
constant, was minimal. Since most installations would have the
observer on the wide size of the nozzle, the increased high
frequency noise has a deleterious impact on the observer. Extending
one wide side of the rectangular nozzle, evocative of an aft deck
in an installed propulsion system, increased the noise of the jet
with increasing length. The impact of both aspect ratio and bevel
length were relatively well behaved, allowing a simple bilinear
model to be constructed relative to a simple round jet.
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