Vortex-ring dynamics in a transitional subsonic free jet. A numerical study

The spatial evolution of a high Reynolds number, axisymmetric, laminar free jet is computed numerically with a compressible, fourth-order phase-accurate, finite-difference Flux-Corrected Transport algorithm and appropriate inflow and open-boundary conditions. The effects of initial conditions, Mach number and controlled axial excitation on the physics of large-scale, two-dimensional, organized structures in the transitional region of the jet are investigated. The results show the detailed connection between the formation and interaction of coherent structures and streamwise momentum flux increases associated with pressure drops and transverse velocity fluctuations in the shear layer. More pronounced momentum flux increases are found for lower Mach numbers, for larger ratios of jet-diameter to initial-shear-layer-thickness, and when the shear-layer organization is enhanced by controlled excitation of the shear-layer mode.