The use of counterflow for noise suppression of a supersonic rectangular jet

Abstract

The effect of counterflow on the mixing and acoustic characteristics of a Mach 1.44 rectangular jet operated at on and off design conditions is 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. Changes in both shock cell spacing and strength occur when counterflow is applied to nonideally expanded jets. Consequently, screech tones are either reduced or totally eliminated, and breadband shock associated noise is observed to shift to higher frequencies. In the under-expanded mode, a Mach disk is formed at certain levels of counterflow, which significantly weakens the subsequent periodic shock cell structure and reduces the breadband shock associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Significantly, it was also discovered that a jet operating at over-expanded conditions can be decelerated nearly isentropically by applying the proper amount of counterflow. This modification leads to a 4-dB reduction in OASPL. It is suggested that counterflow should be further investigated as a potential noise reduction technique in the future. (Author)