Study of coupled supersonic twin jets of complex geometry using higher-order spectral analysis

The purpose of this paper is to examine nonlinear processes occurring during the coupling of complex twin jet plumes using higher-order spectral methods. Inadequacies of linear spectral analysis when closely spaced multiple screech sources exist in complex configurations are demonstrated. Cross-bicoherence is used for identifying the nonlinear interactions between jets emanating from rectangular nozzles with spanwise oblique exit geometries. Two basic topologies in twin jets are compared in the paper; one that promotes coupling (referred to as ‘co-directed’) and another that inhibits coupling (referred to as ‘contra-directed’). The primary findings are: (i) some configurations that appeared to be uncoupled by linear spectral analysis metrics were actually found to be nonlinearly coupled, and (ii) two types of nonlinear coupling were observed—one dominated by the fundamental and its interaction with higher modes, and another that displayed clusters of interactions between a frequency component and its sidebands. Further, three new metrics have been proposed: (i) the first metric, ‘quadratic phase coupling index’, was developed and used to indicate the propensity of jets to couple, (ii) the second metric, namely, ‘interaction density’ was developed to quantify nonlinear coupling, and (iii) the third metric known as the ‘average interaction density’ was shown to increase sharply during coupling mode transition.