| Research Abstract |
In this study, turbulent transport phenomenon is investigated based on the coherent fine scale structure in turbulence using direct numerical simulation and laser diagnostics. The universality of fine scale motions in turbulenceis clarified by analyzing DNS data of homogeneous isotropic turbulence, temporally and spatially developing turbulent mixing layer, turbulent channel flows, rotating homogeneous turbulence and MHD homogeneous turbulence. The mean azimuthal velocity profile of the fine scale eddies in all investigated flows can be approximated by the Burgers' vortex. Mean diameter and maximum azimuthal velocity is always about 10 times Kolmogorov microscale and about 0.6 times of r.m.s. velocity fluctuation (UィイD2rmsィエD2). The spatial distribution of the coherent fine scale eddies is closely related with the anisotropy of turbulence. The strain rate at the center of the coherent fine scale eddies is also independent on the Reynolds number and flow fields and can be scaled by UィイD2rmsィエD2 and the Taylor microscale. The maximum and minimum eigen vectors of the strain rate tensor are perpendicular to the rotating axis of the coherent fine scale eddy and intermediate eigen vector is parallel to the rotating axis. The balance of the principal strain rates leads to the elliptic feature on the rotating plane of the eddy, which results in the large energy dissipation around the coherent fine scale eddies. The evidence of the elliptic coherent fine scale eddies was shown by the experiments in turbulent mixing layer using high-resolution digital particle image velocimetry. To investigate turbulent transport mechanism, DNS of particle laden homogeneous isotropic turbulence, passive scalar transport in homogeneous isotropic turbulence and turbulent premixed flames were conducted. It was shown that the coherent fine scale eddy dominates spatial distribution of particles, the generation and dissipation of passive scalar fluctuation and local flame structure in turbulence.
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