Influence of nozzle asymmetry on supersonic jets

Abstract

The flowfield and noise-generation charactaristics of jets from a series of nonaxisymmetric nozzles have been investigated at supersonic pressure ratios. Detailed acoustic measurements have revealed a strong azimuthal vari- ation in the far-field components of screech and mixing noise. A phase-conditioned schlieren technique has been used to determine the screech modes and the characteristics of the time-mean flowfield. A variety of flow prop- erties has been revealed, including mean jet vectoring, spreading enhancement, and screech suppression. A two- dimensional screech mode has been observed in contrast with the toroidal and helical modes of the reference jet. ODIFICATION of nozzle geometry by introduction of azimuthal asymmetry is a technique for controlling de- velopment of high-speed jets. Applications of this technique include thrust vectoring, mixing enhancement, and noise re- duction. A series of nonaxisymmetric nozzles with azimuthally varying lip position has been investigated for the control of subsonic jets.1'2 The supersonic performance of these nozzles is discussed in this paper. The dynamics of supersonic jets with imbedded shocks and high-amplitude pure-tone screech have been studied by a number of investigators since the pioneering work of Powell.3 Several noise-control techniques involving the use of tabs and baffles have been investigated. Norum4 tested a variety of asymmetric nozzles and had some success in alleviating the screech feedback loop. Krothapalli et al.5 clarified the relevant mechanisms in low-aspect-ratio rectangular jets. Phase-averag- ing techniques were used by Yu and Seiner6 to determine the instability modes of axisymmetric jets. The present level of understanding of supersonic axisymmetric jets is detailed in the review by Seiner. This study was directed at both the gross flowfield char- acteristics and the modes and magnitudes of screech instabili- ties in supersonic jets from asymmetric nozzles. A detailed mapping of the acoustic far field was obtained from anechoic chamber tests. Phase-conditioned flow visualization tech- niques were used to clarify jet instability modes and the mean flowfield. The noise measurements are explained in the context of the visualization results, and a phenomeno- logical grouping is used to categorize broadly the variety of observations.