Turbulence Structures and the Acoustic Far Field of a Mach 1.3 Jet

The temporal characteristics of the acoustic far field of a Mach 1.3, high-Reynolds-number, ideally expanded axisymmetric jet and their potential correlation with large-scale turbulence structures within the jet were explored. A dual microphone array, placed approximately 30 deg from the jet axis in the acoustic far field, was used to determine the temporal variations of the acoustic field and the approximate locations of intense noise sources within the jet, as well as the time of noise emission with respect to the acquired planar flow images. Simultaneous double-pulse flow visualizations were used to identify turbulence structures, as well as their development and interaction in the region of intense noise generation. The time history of the acoustic data showed individual large-amplitude noise events, periodic large-amplitude noise events, and long periods of relative quiet without any large-amplitude noise production. These three particular noise events were shown to constitute a significant portion of the sound pressure time traces. It is believed that a fundamental understanding of the mechanisms involved in the production of these events could lead to substantial reductions in the turbulent mixing noise caused by large-scale structures. Based on these preliminary results, interactions between and tearing of large-scale structures within the mixing layer were speculated as mechanisms of large-amplitude noise generation.