DYNAMICS OF SMALL-SCALE VORTICITY AND STRAIN-RATE STRUCTURES IN THE TRANSITION FROM ISOTROPIC TO SHEAR TURBULENCE

摘要

Local topological and statistical measures of enstrophy and strain-rate structures are compared with global statistics to determine the effects of mean shear on the interactions between fluctuating vorticity and strain rate in DNS of transition-ing isotropic to shear turbulence. "Structures" are extracted as concentrations of turbulence fluctuations, allowing quantitative with visual analysis. We find that mean shear adjusts the alignment of fluctuating vorticity and strain rate so as to (1) enhance global and local alignments between vorticity and the second eigenvector of fluctuating strain rate, (2) two-dimensionalize fluctuating strain rate, and (3) align the compressional components of fluctuating and mean strain rate. Shear causes amalgamation of structures and suppresses strain-rate structures between enstrophy structures. Shear enhances "passive" strain-rate fluctuations-strain rate kinemalically induced by local vorticity concentrations with negligible enstrophy production-relative to "active," or vorticity-generating, strain-rate fluctuations. Enstrophy structures separate into "active" and "passive" based on the second eigenvalue of fluctuating strain rate. The time evolution of a shear-induced hairpin enstrophy structure was analyzed. The structure originated in the initial isotropic state as a vortex sheet, evolved into a vortex tube during a transitional period, and developed into a well-defined horseshoe vortex in the shear-dominated state.