Aeroacoustic Test Research Articles

Jet Noise Characteristics of Unsuppressed Duct Burning Turbofan Exhaust System

Recent aeroacoustic tests of model coannular nozzles have shown that less noise is generated if the higher velocity jet is exhausted from the outer annular passage rather than from the primary nozzle. These findings are of particular significance to the duct burning turbofan engine being studied for application to an advanced supersonic transport airplane. Unlike conventional turbofan engines that have peak velocities from the primary nozzle, it is possible to design a DBTF engine to have a fan velocity higher than that of the primary flow. In this paper are presented the results of a model test program that covers a range of fan to primary area ratios from 0.75 to 1.2, and a range of fan-to-primar y velocity ratios from 0.4 to 2.8. Correlations are presented that relate radiated sound power to fan velocity, fan-to-primary velocity ratio, and fan-to-primary area ratio. Corresponding exhaust plume velocity traverse data are presented which suggest that the observed noise benefits may be because of the more rapid decay of the annular flow caused by shear stresses on the inner surface which result from the lower velocity primary flow.

A State-of-the-Art Report on Aeroacoustic Testing in Conventional Wind Tunnels

The increasing attention given to aerodynamically generated noise brings into focus the need for quality experimental research in this area. To meet this need, several specialized anechoic wind tunnels have been constructed. In many cases, however, it is desirable to use conventional wind tunnels. Two basic problems are inherent in conventional facilities; high background noise and strong frequency-dependent reverberation effects. This paper reviews the known acoustic characteristics of several existing tunnels and presents new data obtained in a conventional 7 × 10 wind tunnel and a smaller 4 × 5 wind tunnel, which is convertible from a closed-jet to an open-jet mode with an anechoic test section. The data from the smaller tunnel serve as a guideline for evaluating design techniques involving a closed-jet and sandwich-wall construction. Model scaling parameters and data reduction techniques are also considered. It is concluded that existing conventional wind tunnels can be utilized for aeroacoustic measurements. The overall technique, however, will require a careful balance of acoustic treatment, selection of proper model scaling, experimental technique, and data reduction schemes. Several suggestions are offered that are pertinent to a relatively large, low-speed wind tunnel facility.