Wavepacket models for supersonic twin-jets

Twin-jet configurations, composed of two jets placed in close proximity to each other, are a common feature of many military aircraft. Multi-tube jets designed for noise suppression and distributed-propulsion systems also share some of the features of twin jets, such as the hydrodynamic interactions of the jet plumes and the modification of the radiated acoustic fields. The existence of large-scale coherent flow structures in the turbulent jet flow, usually referred to as wavepackets, has been demonstrated in the literature for both subsonic and supersonic round jets, along with their relation to the generation of highly directional noise emitted in the aft direction. An extension of the classical parabolized stability equations to flows strongly dependent on the two cross-stream spatial directions and weakly dependent on the streamwise one, known as plane-marching parabolized stability equations (or PSE-3D) is applied to model the large-scale structures present in twin-jet configurations. The present study considers twinjet configurations with di fferent separations at high Reynolds number and perfectly-expanded M-j = 1.5 exhaust conditions. The existing instability modes for the twin-jets mean flow, their dependence on the separation of the two jets and the interaction between the wavepackets originating from the two jets is investigated here. The near pressure field associated with the wavepackets is examined with the aim of documenting its azimuthal structure, and shedding new light on the mixing noise radiated by twin jets and associated phenomena, like noise shielding in the jet axes plane or the noise level increase in the perpendicular plane. Present results show that these e ffects are already present for linear models in which the jet-jet interaction occurs due to the coherence of the pressure fields. Significant variations of the near pressure field result from this interaction, that exceed by up to +/- 50% the pressure field associated with two ideal non-interacting jets at the same axes distances.