This paper provides an assessment from an experimental point of view of the present understanding of high speed jet noise primarily as it pertains to shock containing supersonic jet plumes. The nature of this assessment involves an examination of the complex flow and related acoustic field associated with this problem. A certain emphasis is placed on prediction of the near acoustic field to satisfy a motivation driven by a new set of guiding principles, namely the high performance tactical fighter and second generation space transportation vehicles. The review concludes that after weighing all the experimental evidence, only after consideration of the role of large scale coherent structure is adopted can a consistent unifying theme be achieved to physically interpret and properly predict noise generation by the fundamental mechanisms.

Advances in high speed jet aeroacoustics

Prediction of jet noise in flight from static tests

Jet noise is a byproduct of turbulence. Until recently turbulence was assumed to be known statistically, and jet noise was computed therefrom. As a result of new findings though on the behavior of vortices and instability waves, a more integrated view of the problem has been accepted lately. After presenting a simple view of jet noise, the paper attempts to resolve the apparent differences between Lighthill's and Lilley's interpretations of mean-flow shear, and examines a number of ad hoc approaches to jet noise suppression.

Perspectives on Jet Noise

A detailed parametric study of the noise from dual-stream jets has been carried out to assess the importance of the noise generated by the primary and secondary shear layers and their principal radiation directions under different operating conditions. Realistic geometries and engine conditions are chosen to enhance the relevance and the usefulness of the experimental results reported here. The effects of different operating conditions in the two streams on both the turbulent mixing noise and shock-associated noise are evaluated under static conditions as well as in the presence of an external co-flowing stream. The results indicate that the secondary-to-primary jet velocity ratio is an important parameter for mixing noise while its effect is negligible on shock-associated noise. The shock-associated noise, not surprisingly, is dependent on the geometric details of the nozzle. The characteristics of this noise component are very different depending on whether shocks are present in the primary or secondary stream. There is strong radiation of shock noise to the aft angles when the secondary stream is supersonic. This trend is troublesome since this component is transmitted into the aft cabin of certain aircraft with engines mounted close to the fuselage. The intensity of the shock-associated noise at the lower angles, in general, is proportional to the strength of the shocks and scales with ( is the design Mach number). Just as for a single jet, the effect of forward flight on mixing noise and shock-associated noise is very different. While there is a progressive reduction in levels of mixing noise with increasing free-stream velocity, there is amplification of the shock-associated noise. This is particularly so for the aft-radiated component of shock noise from the secondary stream. This effect could further exacerbate the interior noise in the aft cabin.

P. Drevet

Papers

The effect of flight on jet noise as observed on the Bertin Aérotrain

Effect of flight on the noise from a convergent nozzle as observed on the Bertin Aerotrain