Evolution of the largescale vortices in turbulent mixinglayers
Abstract
The evolution of the largescale 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 largescale 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 nondimensional parameter xi is equivalent to 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 firstreturn 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 largescale vortices discussed by Bernal. The picture of the randomization of the largescale 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 largescale vortices during their lifespan an experimental study was performed. 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 largescale vortices near the jet exit. In addition, a considerable amount of threedimensionality in the braid regions was observed at the early stages of a vortex lifespan.
 Publication:

Ph.D. Thesis
 Pub Date:
 1992
 Bibcode:
 1992PhDT........34A
 Keywords:

 Advection;
 Flow Visualization;
 Mixing Layers (Fluids);
 Nonlinear Systems;
 Particle Image Velocimetry;
 Statistical Analysis;
 Turbulence;
 Turbulent Mixing;
 Vortex Generators;
 Vortices;
 Vorticity;
 Cores;
 Mathematical Models;
 Stable Oscillations;
 Fluid Mechanics and Heat Transfer