Study of the Structure of Wall-Turbulence and Turbulence Noise

这个研究项目涉及几个课题。即壁面单元中多达几十层的近壁层与外层的相互作用，外层的调制可能有助于减少阻力和壁面湍流发出的湍流噪声。对于相互作用问题，在一定人工条件下的模型模拟表明，光滑壁面上的近壁层具有显著的自维持能力和普遍性，与外层的性质无关。壁面的粗糙度破坏了这一特性，使得几乎整个层都被外层的特性所占据。具有波浪状壁面的河道水流证实了这一结论。此外，还研究了固体粒子调制的外层特性。新发明的跟踪运动中的粒子的技术工作令人满意，并揭示了湍流动力学的重要方面，可能适用于湍流控制。在没有使用任何模型的情况下，模拟了墙上方湍流发出的噪声。以时间推进的方式连续计算了源场和伴随声场。结果表明，并不是所有的涡旋都能产生声音，而涡旋的某些特定事件只会产生噪声。作为提高噪声计算可靠性的基础，本文对自由圆射流进行了高分辨率模拟。本文还利用整体稳定性分析的方法，对噪声产生最主要的射流近场进行了研究，探讨了旋涡结构产生的机理。

Several subjects are in concern in this research program. Namely, interaction between near-wall layer up to several tens in wall unit and outer layer, modulation of outer layer which may contribute to drag reduction and turbulence noise emitted from wall turbulence.As for the interaction problem, model simulations under certain artificial conditions revealed that near-wall layer over smooth wall has remarkable self-sustenance ability and universality, independent on the property of outer layer. Roughness on the wall has been found to destruct this property and makes almost whole layer occupied by the property of outer layer. Channel flow having wavy wall has confirmed the conclusion.Outer layer property modulated by solid particles has been investigated as well. New technique invented to trace particles in motion works satisfactorily and revealed important aspects of turbulence dynamics that may be applicable for turbulence control.Noise emitted from turbulence over a wall has been simulated without using any models. Source field and accompanying acoustic field have been consecutively calculated in time-marching manner. It turned out that not all the vortices generate sound but certain particular events of vortex only contribute for noise generation. As the basis of enhancing reliability of noise calculation, free round jet has been simulated with high resolution. Near field of a jet which is most responsible for noise generation has been also investigated with respect to the mechanism of birth of vortical structure, by global stability analysis.