Vorticity and large-scale structures in the transition region of a turbulent jet

Though the existence of large-scale structures in the near and far field of jets has been demonstrated, their exact nature in the self-similar region is still open to debate. Furthermore, the evolution of these structures in the transition region is not well understood. In this thesis, a hybrid experimental-numerical approach is developed to investigate the flow structure of the jet transition region. The spatial-temporal characteristics of these large eddies and their effect on local entrainment are examined.; The velocity field in planes normal and parallel to the jet axis are measured using the digital particle image velocimetry (DPIV) technique with a computationally efficient algorithm designed to minimize sub-pixel bias. The numerics employ a second order projection method with adaptive mesh refinement to simulate a natural unforced jet. The link between the experiments and numerics is provided by a temporal sequence of DPIV nozzle velocity data which serves as the inflow boundary condition to the computation. Successful comparisons of the development of near-field primary and secondary structures validate the simulation. The mean jet growth rate is identical to the experiments.; Analysis of the experimental data with a proper orthogonal decomposition identifies and reveals the evolution of large-scale structures. After the first pairing the rings develop a tilting instability which is amplified by the interaction of adjacent rings. After the potential core, the inclined rings have a unique time scale and are responsible for a significant fraction of the local turbulent transport and rapid growth of the jet. Further downstream, beginning at nine diameters, the rings evolve into a flow that alternates between ejection and entrainment of fluid. This motion persists through the remainder of the transition region and increases its contribution to lateral mixing with downstream distance. The time scale governing the inward and outward flow oscillations increases throughout transition.; Azimuthal vorticity dominates the near-field but the streamwise and radial components increase rapidly and by eight diameters all three components reach a maximum of comparable magnitude. The computations confirm the existence of a preferred azimuthal wavenumber in the development of the secondary streamwise structures. In the transition region the streamwise vorticity remains aligned with the flow while the azimuthal filaments are arranged in a helical distribution. Instantaneous entrainment is more sensitive to fluctuations in streamwise vorticity than azimuthal even though the strength of the two components is comparable. With an increasing ability to mix ambient and jet fluid, streamwise vorticity becomes the primary mechanism for entrainment in the transition region.