Vortex dynamics, transition to turbulence, and aeroacoustics in rectangular free jets

Abstract

An overview of recent studies of compressible, rectangular, free jets is presented, with special focus on understanding the dynamics and topology of coherent vortical structures controlling the jet development, and identifying major near-jet noise generation mechanisms. Relevant issues of subgrid and supergrid modeling in free-jet numerical simulations are addressed in this context. The vortex dynamics underlying axis-switching and vortex bifurcation phenomena are examined, including: the roles of self-induced vortex-ring deformation, reconnection, braid vortices, aspect ratio (AR), and the transition to turbulence from laminar initial jet conditions. Qualitatively different vorticity geometries characterizing the near field of low-AR, M<2, ideally expanded, rectangular jets are demonstrated, involving: (i) self-deforming and (ii) bifurcating vortex rings; interacting ring and rib (braid) vortices—including, (iii) single ribs aligned with corner regions (AR>1), and (iv) rib pairs aligned with the corners (AR=1); (v) smaller-scale, elongated, ‘‘worm’’ vortices in the turbulent jet regime. The near-field entrainment of low-AR rectangular jets is shown to be largely determined by the characteristic geometry of the ring–rib interactions; progress in the study of noise-generation mechanisms in low-AR jets is discussed. [Work sponsored by AFOSR, ONR, and the DoD HPC-MP.]