Parametric study of noise from dual-stream nozzles

Abstract

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 ($M_{p}^{2}$ – $M_{d}^{2})^{2}$ or ($M_{s}^{2}$ – $M_{d}^{2})^{2 }$ ($M_{p}$ and $M_{s}$ denote the Mach numbers of the primary and secondary streams and $M_{d}$ 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.