Effect of nozzle on the global eigenfunctions of a Ma=1 jet

The effect of the nozzle on instability mechanisms in perfectly expanded Ma=1 jets is assessed by means of global stability analysis. The first considered configuration represents the geometry of the nozzle within the computational domain. A second setup replaces the flow through the nozzle with a parallel jet. In both cases, a globally unstable eigenfunction is computed, indicating of absolute instability. The origin of the instability is identified as the sharp streamwise gradient in the base flow immediately downstream of the nozzle exit location. The destabilizing role of the nozzle is attributed to the generation of the steep streamwise gradient while the constraining effect of the nozzle walls leads to a reduction of the instability growth rate. Finally, the vortical, acoustic and thermal components of the eigenfunctions are assessed using the momentum potential theory by Doak (1989).