MULTI-MEASUREMENT CORRELATIONS IN THE NEAR-FIELD OF A COMPLEX SUPERSONIC JET USING TIME-RESOLVED SCHLIEREN IMAGING

Abstract

In this study, we aim to characterize the dynamics and visualize the propagation of fluctuations in the near field of a complex multi-stream jet over an aft deck plate. The flow is visualized using time resolved schlieren photography (up to 400,000 frames per second) while pressure on the aft deck plate is simultaneously sampled using Kulite pressure sensors. Time series are constructed using the schlieren photographs and conditioned to reduce the effects of signal drift. The analysis is focused in several regions identified in previous studies as dynamically interesting through their high levels of pressure variance and rich spectral content. Space-Time cross correlations are utilized to visualize propagation of fluctuations from the near field to the far field, and several propagation pathways are identified. This process is repeated for band-pass filtered schlieren signals, and a case is made for the separability of near field dynamics into distinct frequency bands. Introduction Over the last several decades, performance requirements of both commercial and military aircraft have led to increasingly complex propulsion systems, often including rectangular multi-stream nozzle configurations. Only within the last few years has noise become a design parameter for propulsion and airframe engineers [1]. While a complete theoretical understanding of jet noise has remained elusive, there is a wealth of information on which to build our knowledge base. There is a large number of theoretical and experimental studies focused on the flow regimes with which this study deals. In his 1991 chapter in Aeroacoustics of Flight Vehicles [2], Tam gives a thorough description of possible acoustic sources in the near field of supersonic jets. He highlights the importance of large scale motions in noise production where M j (jet Mach number) is greater than 1. Of particular interest are the phenomena of Mach wave radiation and broadband shock associated noise, particularly because they have firm theoretical descriptions [3], [4], and will be prominent sources of noise in our experiment due to our chosen design conditions. In addition to the fundamental theoretical and experimental work outlined in [2], more recent applied work has been performed by Bridges cataloguing the acoustic character of pure retangular jets [5] and those exhausting over flat plates. In the aforementioned study, it was shown that noise increases with deck length and that the deck edge acts like a dipole in its velocity scaling and directivity [6]. The goal of the present study is to utilize time resolved schlieren photography to observe and characterize the nearfield phenomena contributing to far-field noise, as described in the existing literature. With this time resolved view of the near-field, we hope to make more reliable predictions about far-field noise, particularly in the complex flows seen in modern day and next-generation aircraft exhaust. To this end, a multi-stream supersonic jet flow is visualized with a time-resolved schlieren system. In the lab, this complex flow is simplified, and is represented by a main flow, a wall jet along the aft deck plate, and a low-speed coflow to mimic modest forward motion. The aft deck plate is configurable to several different lengths. The general con-