Nonparallel and nonlinear stability of supersonic jet flow

Linear and nonlinear evolution of disturbances in an axisymmetric, superson ic, low Reynolds number jet is studied using parabolized stability equation s approach. Both axisymmetric and helical modes are considered and nonparal lel effect is found to increase the disturbance growth rate, although there is very little effect on the wavenumber. Nonlinear interaction of the heli cal modes, which are the dominant instability modes of the jet, results in disturbance saturation, spectrum filling and large mean flow distortions. S imilar to that for the supersonic boundary layer flow (Chang, C.-L., Malik, M.R., Oblique-mode breakdown and secondary instability in supersonic bound ary layers. J. Fluid Mech. 273 (1994) 323-359), interaction of the helical modes induces streamwise vortices which cause significant mean flow distort ion and growth of other harmonics. The computed evolution of disturbances i s in reasonably good agreement with the experimental data of Morrison and M cLaughlin (Morrison, G.L., McLaughlin, D.K., Instability process in low Rey nolds number supersonic jets. AIAA J. 18(7) (1980) 793-800). The present ma rching scheme is able to compute evolution of supersonic disturbances witho ut any numerical reflections and the induced far-field pressure disturbance is found to be almost in phase with the acoustic wave at ambient condition s. (C) 2000 Elsevier Science Ltd. All rights reserved.