Generation and Growth of Coherent Structures and Turbulent Elementary Vortices

本研究的主要目的是阐明大尺度拟序涡旋结构的产生和发展过程，以及随后分解为小尺度基本涡旋的过程，进而发展用于湍流控制的基本操纵方法。在对壁面湍流机制的研究中，对所谓的近壁条纹进行了实验研究，发现在高强度随机湍流激励的层流壁层中，近壁条纹是由代数增长机制引起的，它们的不稳定性产生了一串符号交替的准流向涡系，并且弯曲条纹不稳定性可以保持与充分发展的壁面湍流相同的100个壁面单元的低速条纹。在研究旋转系统中均匀剪切湍流的旋涡结构时，直接数值模拟表明，由于科里奥利力的作用，高涡度气泡很容易发展成局部涡层，而局部涡层又很快演变成管状涡。在科里奥利力和涡致压力梯度的共同作用下，管状涡被强化并维持了较长时间。在湍流操纵的研究中，我们的研究重点是流动涡对超音速混合的强化。对空心涡的不稳定性和混合能力进行了理论和数值分析。实验发现，在涡雷诺数大于10^4的情况下，空腔流振荡产生的高频扰动可以激发超音速流向涡，使其在Kolmogorov-5/3幂定律频率范围内产生能量波动。在毛绒织物涡度操纵的研究中，我们描述其气动效应的模型通过实验和数值计算得到了验证，特别是对于TS波在毛绒织物壁面边界层的情况。

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.