Small‐scale features of vorticity and passive scalar fields in homogeneous isotropic turbulence

Small‐scale structures of the vorticity and passive scalar fields have been examined by means of direct numerical simulations of homogeneous isotropic turbulence with 96³ grid points and R_λ≊60. Both statistical and visual techniques have been used to examine the structure of certain quantities from the evolution equations for enstrophy and the scalar gradient. Tubelike regions of intense enstrophy contain large positive and sometimes large negative enstrophy production, and mostly moderate‐valued energy dissipation regions surround these tubes. The most intense regions of the scalar gradient are dissociated from the vortex tubes, and occur as large flat sheets. Within these sheets the scalar gradient production is large, the energy dissipation is small, and in their vicinity only moderate‐valued sheetlike enstrophy regions exist. The statistical techniques show that although activity in these intense regions is strong, on a volume normalized basis, by far the largest contributions to the terms in the evolution equations, along with the energy dissipation, are from low‐level ‘‘background’’ activity.