Enhanced Stretching of Material Lines by Antiparallel Vortex Pairs in Turbulence

"Deformations of material lines in homogeneous isotropic turbulence of an incompressible viscous fluid are numerically investigated to understand physical mechanism of the enhanced stretching of material lines in turbulence. It is shown qualitatively by careful visualizations of numerical data and quantitatively by detailed numerical analysis that tubular vortical coherent structures in small scales of turbulence play crucial roles in such intensive stretching of material lines. In particular, the tubular vortices tend to align to each other in an antiparallel manner, and a pair of antiparallel vortices strongly stretch material lines in two regions between them.