Experimental investigation of noise reduction from two parallel-flow jets

Abstract

Multitube nozzles have been developed to suppress turbojet engines in a number of cases. The concept of two parallel-flow jets forms a basic element of the multitube nozzle. Hence, the acoustic characteristics of two parallel-flow jets have been investigated as a fundamental study aimed at understanding jet noise suppression mechanisms which then could lead to improved jet noise suppressors. Model-scale tests were conducted in an anechoic environment. Acoustic measurements were made in the plane containing the axis of the two jets using a far-field microphone array. The effect of tube geometry (lateral tube spacing, longitudinal tube staggering, and tube size) was studied with the same flow through both tubes. The effect of flow parameters was investigated using twin coplanar jets. Detailed acoustic test results were evaluated in terms of engineering as well as subjective units. The following general conclusions were drawn from the study: 1) two coplanar parallel-flow jets with dissimilar unmixed flow can be quieter than an equivalent fully mixed single-flow jet; 2) two parallel-flow jets with the same flow can be up to 3 dB quieter than the equivalent single-flow jet; 3) two parallel-flow jets become quieter when lateral spacing is reduced, the nozzle near the observer is staggered upstream, or a smaller nozzle is placed near the observer; and 4) for two parallel-flow jets with dissimilar flows, the noise at 90 to 110 deg from the inlet axis depends mainly on the peak jet velocity, and noise at 120 to 160 deg depends very strongly on only that part of velocity profile which the observer can see directly.