Large- and small-scale characteristics in a temporally developing shearless turbulent mixing layer

Abstract

Direct numerical simulation of a temporally developing shearless turbulent mixing layer is performed. Two quasi-homogeneous isotropic turbulent (HIT) regions with different turbulent kinetic energies (TKEs) and a mixing-layer region temporally develop. The small-scale properties are analyzed with the velocity gradient tensor. The statistics on the velocity variances show that the development of the mixing layer is divided into two stages. In the first stage, grid turbulence in the large-TKE region has not fully developed, and the center of the mixing layer hardly moves. Large-scale intermittency grows in the mixing-layer region at this stage. In the second stage, grid turbulence in the large-TKE region has fully developed, and the center of the mixing layer moves toward the small-TKE region. The small-scale intermittency is most significant in the mixing-layer region in both stages. The statistics on the velocity gradient tensor show that stronger vortex compression occurs more frequently in the mixing-layer region than in the quasi-HIT regions at late times. In addition, the extensive and compressive eigenvalues of the rate-of-strain tensor exhibit the strongest intermittency in the mixing-layer region at late times.