INSTABILITY OF VORTICAL AND ACOUSTIC MODES IN SUPERSONIC ROUND JETS

The stability of ''top-hat'' and fully developed jet profiles is inves tigated by an inviscid linear stability theory for compressible flow. The study covers a wide range of the Mach number and the temperature r atio. Two types of instabilities are found: vortical and acoustic, eac h of which can be subdivided into non-radiating (subsonic) and radiati ng (supersonic) modes. The vortical mode is the continuation of the Ke lvin-Helmholtz instability from incompressible flow. The acoustic mode is a compressible flow phenomenon, which becomes important at large M ach numbers. Temperature-ratio effects can be destabilizing or stabili zing, depending on the Mach number and mode of instability. A spectrum of unstable acoustic modes, including axisymmetric ones, are found to exist in the fully developed jet. For this jet, acoustic axisymmetric waves become more unstable than both vortical and acoustic helical wa ves at Mach numbers over about 3. Strong evidence of a resonance mecha nism for acoustic modes is seen in the growth rate curves at high Mach numbers, where a spectrum of local peaks and valleys appears at regul arly distributed frequencies. (C) 1997 American Institute of Physics.