AN INVESTIGATION OF SONIC & SUPERSONIC AXISYMMETRIC JETS: CORRELATIONS BETWEEN FLOW PHYSICS AND FAR-FIELD NOISE

Abstract

An axisymmetric convergent jet is studied at ideal and underexpanded conditions using velocity and acoustic data. Time-resolved and large-window PIV capture near-field velocities and are simultaneously sampled with far-field microphones. POD is used to extract modes representative of physical processes in the flow. Specifically, screechcontaining and turbulent mixing modes are isolated in the supersonic case. The decoupled velocity fields are then correlated with acoustic data to identify modes related to specific noise spectra. Finally, selective flow reconstruction is carried out to reduce flow features associated with an imperfectly expanded jet. INTRODUCTION Understanding and reducing jet noise are difficult problems due to the inevitable turbulence encountered. The aerospace industry continues to invest considerable effort into mitigating jet noise as it creates unwanted acoustic pollution near airports, generates negative health consequences to flight deck crews, and compromises the stealth of military aircraft. Today, supersonic flight is becoming a standard for military aircraft and is being revisited for commercial applications. Furthering the understanding of turbulence in supersonic flow is a critical step towards noise source identification and suppression for future aircraft. While research in the area of aeroacoustics has progressed considerably since its introduction, Tam (1998), engine technologies are advancing at increasing rates, which push aircraft to greater speeds. Advanced designs utilize exotic nozzle and flow configurations to increase performance and abate noise generation in supersonic jets, Henderson et al. (2012). However, many of the heuristic solutions employed to date have been guided by partially anecdotal evidence, leaving incomplete understanding of the fluid mechanics involved. Prior to studying such configurations, the axisymmetric nozzle is revisited to interpret shock-turbulence interactions as anticipated in future designs. This prompted an investigation of supersonic noise generation in Syracuse University’s anechoic chamber. Current research is focused on identifying important flow features in a cold, axisymmetric convergent jet; in particular, sonic and supersonic flows are investigated to further recognize differences in noise generation associated with shocks. Particle imaging velocimetry (PIV) is simultaneously sampled with far-field pressures to allow for rigorous analyses. Reduced-order modeling (e.g. POD) of the jet plume in the streamwise plane (complementing work by Caraballo et al. (2003)), directivity and magnitude calculations of acoustic radiation, and correlations between the two measurements are carried out. Flow physics are then related to far-field noise signatures. Previously, subsonic test campaigns at Syracuse University’s Skytop Turbulence Laboratory by Low et al. (2013) and Berger et al. (2014) have presented evidence correlating deterministic spatial structures in the near-field flow with far-field noise. At present, data sets containing ideally and underexpanded jets (M j = 1.0 & M j = 1.1) are presented to similarly locate noise generation mechanisms.