Linear instability of compound jets with nonaxisymmetric disturbances

A three-dimensional linear instability analysis is carried out for a double infinitely, incompressible, and viscous compound jet moving in an inviscid surrounding gas. An analytical form of dispersion relation is derived and then solved by a direct numerical procedure. A detailed parametric study is performed to explore the instability mechanisms that cause the growth of nonaxisymmetric disturbances in compound jets. The results show that the Weber number defined on the outer interface between the ambient gas and the shell-liquid layer and the gas-to-shell density ratio both have significant influences on the growth of nonaxisymmetric instability modes, whereas the other parameters including the radius ratio, the core-to-shell density ratio, the Reynolds numbers, and the Weber number defined on the inner interface are relatively less important to the onset of the nonaxisymmetric modes. Particularly, it is found that the nonaxisymmetric sinuous mode may prevail over the axisymmetric mode for a compound jet operating at high Weber number but with low Reynolds number. The instability characteristics exhibit that the aerodynamic drag is the dominant mechanism resulting in the onset of nonaxisymmetric instability in compound jets. The present results complement the existing axisymmetric investigations and give a more complete theoretical understanding for the instability behaviors of compound jets. (C) 2009 American Institute of Physics. [DOI:10.1063/1.3059633]