Interaction of Twin Jets from Nozzles of Spanwise Oblique Geometry

Abstract

An experimental study was conducted to investigate the coupling of twin jets of complex geometry. The study was motivated by the need for nozzles with spanwise oblique exits on modem stealth aircraft and the possibility of plume coupling that could result in structural damage. Although several studies have examined aspects of twin jet coupling, very little data is available on the coupling of jets from nozzles of complex geometry. Our study focuses on twin convergent nozzles with an aspect ratio of 7 with spanwise oblique exits operated over the fully expanded Mach number range from 1.3 to 1.6. The internozzle spacing (sh) was varied from 7.5 to 13.5. Several interesting results were produced by this study: (a) Coupling of twin single beveled nozzles was observed only when the beveled edges faced each other and the nozzles formed a 'V' shape in the internozzle region. (b) Despite the presence of spanwise antisymmetric, spanwise symmetric and spanwise oblique modes for the single nozzles, only the first two coupling modes were seen. (c) The symmetric coupling produced higher dynamic pressures in the internozzle region. (d) Dynamic tests conducted by moving the nozzles apart while they were operating or by rapidly changing the operating pressure at fixed internozzle spacing revealed that coupling modes could co-exist at non-harmonically related frequencies. These dynamic tests reproduced the static test data. (e) The frequency of both coupling modes agrees with the higher order waveguide modes based on Tam's theory. Our results provide an understanding of complex twin jet coupling and will serve as benchmark data for validating computational models.