Transient growth mechanisms in a high-speed rapidly-swirling jet with vortex breakdown

We consider an experimentally-measured, high Reynolds and swirl number jet that has undergone an axisymmetric vortex breakdown, known to be dominated by a self-excited, lowest-order, absolutely unstable helical mode. We have shown this jet to also possess significant short-time transient growths in the wake region at even helical orders and moderate streamwise wavenumbers, easily exceeding the corresponding exponential growths at these times. We investigate transient growth mechanisms via analyzing the input and output optimal perturbation structures which resemble patterns similar to Kelvin waves of Lamb-Oseen and Batchelor vortices. The input perturbations appear as azimuthal velocity streaks in the streamwise planes, which once located exclusively inside the core region, uncoil via the classical Orr mechanism. If the output state resembles a compact spiral structure at the core, we show these situations to yield the strongest transient growths in the swirling jet. In contrast, initial perturbations that are spread out near the quasi-potential region rolls up to yield thin spirals outside the core region while a second compact region appears at the center via core contamination. Unlike in the Lamb-Oseen vortex, such waves yield only moderate transient amplifications.