KINEMATICS OF THE QUASI-COHERENT VORTICAL STRUCTURE IN NEAR-WALL TURBULENCE

Near-wall turbulence structures in a direct numerical simulation database of turbulent channel flow were investigated by using a three-dimensional (3-D) computer graphics software tool. Several candidate methods to detect near-wall vortical structures were tested. They were based on instantaneous flow properties such as pressure, vorticity, enstrophy, dissipation rate, and the second invariant of deformation tenser. Among them, the low-pressure regions and those of negative second invariant of the deformation tenser corresponded well to the cores of vortical fluid motion. The spatial distribution of each term in the Reynolds stress transport equations was also examined in the instantaneous field to explore the role of vortical structures in production and destruction of the Reynolds stresses. It is found that these terms are distributed highly intermittently in space; intense production occurs mostly on both sides of near-wall streamwise vortices, and high redistribution, diffusion, and destruction regions also exist near around the vortices. High helicity regions are found to be associated with the elongated near-wall streaky structures.