Generation and Growth of Coherent Structures and Turbulent Elementary Vortices

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The main objectives of the present studies are to clarify the generation and growth process of large-scale coherent vortical structures and their subsequent breakdown into small-scale elementary vortices and then to develop elemental manipulation methods for turbulence control. In the study on the wall turbulence mechanism the so-called near-wall streaks are experimentally examined to find that the algebraic growth mechanism is responsible for their occurrence in a laminar wall layer excited by high-intensity random turbulence, and that their instability gives rise to a train of quasi-streamwise vortices of alternating sign and further that the sinuous streak instability can maintain the same low speed streaks of 100 wall units as in fully developed wall turbulence. In the study on the vortical structures in a uniformly sheared turbulence in a rotating system, direct numerical simulations demonstrate that owing to the Coriolis force high-vorticity blobs easily develop to local vortex layers, which soon evolve into tubular vortices. The tubular vortices are intensified and maintained long under a combined effect of the Coriolis force and the vortex-induced pressure gradient. In the study on the turbulence manipulation our research efforts are focused onto the supersonic mixing enhancement using streamwise vortices. The instability and the mixing capability of the so-called hollow vortex are examined theoretically and numerically. Experimentally it is found that the supersonic streamwise vortices can be excited by high-frequency disturbances due to cavity-flow oscillations to have energetic fluctuations in the Kolmogorov's -5/3 power law frequency range for the vortex Reynolds numbers beyond about 10^4. In the study on the vorticity manipulation using pile fabrics our model describing their aerodynamic effect is verified through experimental and numerical efforts in particular for the case of TS waves over the pile fabric wall boundary layer.