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THE LARGE-SCALE MOTIONS IN NEUTRAL AND STABLY STRATIFIED WALL-TURBULENCE

Makoto Tsubokura
Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology; Department of Mechanical Engineering and Intelligent Systems, The The University of Electro-Communications

Tetsuro Tamura
Department of Environmental Science and Technology, Tokyo Institute of Technology 4259 Nagatsuta, Yokohama 226-8502, Japan

Résumé

The large-scale structures observed at or above the logarithmic-layer of wall-turbulence were investigated using Large Eddy Simulation. We have mainly focused on the Reynolds-number scaling of the large-scales and the influence of thermal stratification on these motions. The grid resolution required for the reproduction of the large-scale structures in plane channels was intensively studied and it was found that rather higher grid resolution that was sufficient to reproduce the near-wall structures was required to properly reproduce the large-scale motions. This fact indicates that there would be a strong relation between the small-scales near the wall and the large-scales in the outer-layer. The large-scales in the neutral state basically obey the outer-scaling and their spanwise size was about twice larger than the boundary-layer thickness independent of the Reynolds number tested here. The large-scales were also found to be strongly affected by the thermal stratification and were suppressed at a certain weakly stable stratification tested here, contrarily to the fine-scale structures being rather insensitive to the thermal stratification.