Shock-Tracking Procedure for Studying Screech-Induced Oscillations

Abstract

A SHOCK-CONTAINING supersonic jet radiates two so-called shock-associated noise components in addition to the classical mixing noise. These two components are the broadband shockassociated noise and the screech. The latter was studied in the early 1950s by Powell [1]. Screech is a tonal noise component. Its generation mechanism has been explained with some success as a feedback loop involving sound production through shock– turbulence interaction. The broadband shock-associated noise component has been investigated at least since Martlew [2]. HarperBourne and Fisher [3] have adapted Powell’s stationary source array model to derive some observed properties of this noise component. In his review about high-speed jet aeroacoustics, Seiner [4] points out the necessity of studying shockmotion to confirmPowell’s [1]model of stationary sources. Furthermore, fluid disturbances having a relative motion to shocks may be responsible for broadband shock noise [4]. A certain insight into shock behavior within a jet plume is thus essential to accurately understand shock-associated noise generation. The screech phenomenon in a round jet involves different modes. They have been studied extensively in the past, for example, by Davies and Oldfield [5], Merle [6], or Powell et al. [7]. Mode switching appears as a sudden change of jet plume structure and screech frequency with operating conditions. Five modes are traditionally quoted: A1 and A2 are axisymmetric, B is sinuous or flapping, C is helical, andD is again sinuous. The reader is referred to Raman [8,9] for a review of screech properties. The coincidence between shock oscillation frequency and screech frequency has been first observed by Lassiter and Hubbard [10] from a shadowgraph technique. Sherman et al. [11] havemade use of highspeed schlieren recording to investigate the shock distortion during screech. They have also found the coincidence between oscillation and screech frequencies and estimated oscillation amplitudes for the third shock. Using an optical shock detection technique based on laser light scattering by a shock, Panda [12,13] has significantly contributed to the characterization of shock motion during screech. He has reported that every shock oscillates at the screech frequency andmotion amplitudes have been assessed. In the latter reference, an analyticalmodel for shock oscillation is also proposed and is found to be in good agreement with measurements. To the authors’ knowledge, Sherman et al.’s [11] and Panda’s [12,13] contributions are the only occurrences of shock oscillation measurements in the screech literature. In the present Note, oscillation frequency and amplitude are examined by means of high-speed schlieren images and simultaneous near-field acoustic recordings. A procedure for shock tracking is developed and applied on two different screech modes. The experimental setup is presented in Sec. II. The procedure is explained in Sec. III, and some results are discussed. Conclusions are finally given in Sec. IV.