Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

Abstract

The influence of counterflow on the mixing and acoustic characteristics of a Mach 1.4 rectangular jet operated at on- and off-design conditions were studied experimentally for different levels of counterflow. The results show that counterflow significantly enhances shear-layer mixing and reduces the jet potential core length under all operating conditions. Significant changes in both shock-cell spacing and strength were ohserved when counterflow was applied to nonideally expanded jets. Consequently, screech tones were either reduced or totally eliminated, and broadband shock-associated noise was shifted to higher frequencies. In the underexpanded mode, a Mach disk was formed at certain levels of counterflow, which substantially weakened the subsequent periodic shock-cell structure and reduced the broadband shock-associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Interestingly, it was also discovered that a jet operating at overexpanded conditions could be decelerated nearly isentropically by applying the proper amount of counterflow. This modification led to a 4 dB reduction in OASPL. Based on the present study, it is suggested that counterflow warrants further investigation as a potential noise reduction technique.