Acoustic Coupling of Twin Jets from Single-Beveled Nozzles

Abstract

A shock containing jet from even the simplest geometric configuration such as the circular nozzle, is rich in its flow features and mechanisms, and poses considerable challenge to its physical understanding and prediction. The problem is further complicated when two shock-containing jets are located in close vicinity. It has been shown in the past that such jets can interact, leading to acoustic characteristics of the resultant composite jet being significantly different from either individual jet and such interactions have become known as “jet coupling”. Although there is significant information available in the literature on coupling of axisymmetric jets as well as twodimensional (rectangular) jets of uniform geometry, in single and twin configurations, there is very little information on the coupling of jets from nozzles of complex geometry. Jets emanating from such nonuniform nozzles can also potentially couple, and, similar to the coupling phenomena observed in uniform exit rectangular jets, lead to very high fluctuating pressures in the internozzle regions that could lead to fatigue damage of the aircraft structure. In addition, jets with non-uniform exits exhibit spanwise modes that are substantially more complex than jets with uniform exits, due to the additional presence of spanwise oblique modes in non-uniform jets, that are absent in uniform geometries. The work presented in this paper was motivated by the need to understand the characteristics of the coupling between two such complex jets, with respect to how it affects the nearfield physics. The nozzles used in this experimental study were two dimensional with a single beveled exit geometry and an aspect ratio of 6.6. The twin jets were set up in a way that the beveled surfaces faced each other, forming a ‘V’-shape in the internozzle region. Experimental data was collected to obtain the directivity of the coupling modes in the azimuthal and horizontal planes and the broadband noise directivity was computed in the vertical plane. The variation of the instability modes along the downstream axial direction was also studied using both the linear and