Evolution of the Large-Scale Vortices in Turbulent Mixing-Layers

The evolution of the large-scale vortices in turbulent mixing layers was investigated both theoretically and experimentally. In the theoretical study, a novel approach was used to model the dynamics of the large-scale vortices. The model accounts for the amalgamation processes as well as the advection of the vortices between the amalgamations. The vortices are characterized by the non-dimensional parameter xi equiv lambda/x at formation, where lambda is the wavelength of the vortex and x is its downstream location at formation. The evolution of xi from one vortex generation to the next was investigated and modeled. The resulting equation could be viewed as the first-return map of a nonlinear dynamical system. Such a deterministic model exhibits a wide range of dynamical behavior and the dynamical states were explored as a function of a defined control parameter a. The value of the parameter a for a turbulent mixing layer was obtained using the statistical theory of the large-scale vortices discussed by Bernal (1988). The picture of the randomization of the large-scale vortices that emerged from the theoretical study is a stable, persistent oscillation of the strengths of the vortices around an equilibrium value. This randomization process was found to increase whenever neighboring vortices of different strengths exist in the mixing layer. Other factors that change the randomization process are also discussed. In order to obtain a better understanding of the evolution of the large-scale vortices during their lifespan an experimental study was carried out. This study was based on flow visualization and Particle Image Velocimetry (PIV) measurements. The PIV results showed that small vortices were highly stretched during their lifespan and that their peak core vorticity dropped as they convected. Flow reversal and positive vorticity (with negative vorticity being in the vortex cores) were measured in the neighborhood of the large-scale vortices near the jet exit. In addition, a considerable amount of three-dimensionality in the braid regions was observed at the early stages of a vortex lifespan. (Abstract shortened by UMI.).