COUPLING OF TWIN RECTANGULAR SUPERSONIC JETS

Twin jet plumes on aircraft can couple, producing dynamic pressures si gnificant enough to cause structural fatigue. For closely spaced jets with a moderate aspect ratio (e.g. 5), previous work has established t hat two coupling modes (antisymmetric and symmetric) are kinematically permissible. However, the dynamics of twin-jet coupling have remained unexplored. In this paper a more fundamental assessment of the steady and unsteady aspects of twin-jet coupling is attempted. While we docu ment and discuss the nozzle spacings and Mach numbers over which phase -locked coupling occurs, our concentration is much more on answering t he following questions: (a) What mechanism causes the jets to couple i n one mode or the other? (b) Why do the jets switch from one mode to a nother? (c) Are the two modes mutually exclusive or do they overlap at the transition point? Our results reveal, among many things, the foll owing. (i) For very closely spaced twin jets in the side-by-side confi guration phased feedback based on source to nozzle exit distance of ad jacent jets does not fully explain the coupling modes. However, the 'n ull' phase regions surrounding the jets where the phase of an acoustic wavefront (arriving from downstream) does not vary appears to correla te well with the existence of the symmetric mode. When the 'null' regi ons of adjacent jets do not overlap antisymmetric coupling occurs and when they do overlap the jets couple symmetrically. We provide a simpl e correlation using a parameter (a) that can be used as a simple test to determine the mode of coupling. (ii) The switch from the antisymmet ric to the symmetric mode of coupling appears to occur because of an a brupt shift in the effective screech source from the third to the four th shock, which in turn causes the 'null' phase region surrounding the jets to grow abruptly and overlap. (iii) The two modes are mutually e xclusive. Our results provide considerable insight into the twin-jet c oupling problem and offer hope for designing twin-jet configurations t hat minimize damage to aircraft components.