The Dynamics of Bursting Process and Secondary Currents in River Turbulence'

理解复杂的物理现象是计算物理学的一个重要主题。特别是紊流是最有趣的课题之一，许多研究人员和工程师对它进行了大量的研究。在这种情况的背景下，发现湍流虽然具有随机性，但具有连贯的结构，并且可以通过Navier-Stokes方程进行确定性估计。基于计算流体动力学的方法已经增加了对湍流的理解。特别是一种称为DNS的方法，由于其准确性高，发挥了更大的作用。由于它仅基于物理定律而没有湍流模型，因此DNS有望普遍适用于任何流场。然而，谱法等DNS并不适用于复杂的边界流，而从工程角度要求其适用性。使用有限差分方法的DNS方法最近越来越多，因为它们可用于越来越复杂的系统。然而，分析仍然只对包括通道流在内的简化湍流场进行。关于曲线边界的一般流场的报道很少（Krettenauer等人，1992，De Angelis等人，1997，Mito等人，1998，Ohta等人，1998）。与此同时，流动可视化技术的发展揭示了在复杂时空结构的湍流中存在一种寿命相对较长的相干结构（Kline et al.,1967），并证明了相干结构与湍流的输运密切相关，包括湍流的产生、混合和扩散。这种相干结构形成被螺旋流包围的细长圆柱形高涡度区，称为圆柱螺旋涡（低压涡）。它在壁面附近产生条纹和雷诺剪切应力方面起着重要作用，因此被认为是表达湍流结构的基本结构。然而，还有很多东西需要澄清，包括与普朗特尔的第二类次级流的关系。因此，人们对粒子图像测速技术（PIV）的未来发展寄予了期望。了解湍流结构不仅需要时均场和瞬时场的数据，还需要流场随时间变化发展的信息。因此，目前包括DNS在内的数值分析被认为是非常有效的方法。少

Understanding complicated physical phenomena is a major theme of computational physics. Especially, turbulent flows are one of the most interesting subjects, and have been studied energetically by many researchers and engineers. In the background of the situation is the finding that turbulence, although of a random nature, has a coherent structure, and can be estimated deterministically by a Navier-Stokes equation. Approaches based on computational fluid dynamics have already increased the understanding of turbulent flows. In particular, a method called DNS plays a greater role owing to its high accuracy. Since it is based only on physical laws without a turbulence model, DNS is expected to be universally applicable to any flow field. DNS such as the spectral method, however, is not applicable to complex boundary flows while the applicability is required from an engineering viewpoint. DNS methods using a finite difierence method have recently been increasing due to their expasibility t … More o more comptex systems. Analyses have, however, still been made only of simplified turbulence fields including channel flows. Few reports are available on general flow fields with a curvilinear boundary(Krettenauer et al.,1992, De Angelis et al.,1997, Mito et al., 1998,Ohta et al.,1998).Technological development for flow visualization, in the meantime, has revealed the existence of a coherent structure of relatively long life in turbulent flows of a complex time-space structure(Kline et al.,1967), and has also been demonstrating that the coherent structure is closely related to the transport of turbulent flows involving the generation, mixing and diffusion of turbulence. The coherent structure forms a slender cylindrical high-vorticity area surrounded by spiral flows, and is called cylindrical spiral vortex(low-pressure vortex). It plays a important role in generating streaks and Reynolds shear stress near the wall, and is thus regarded as a basic structure that expresses the turbulence structure. Much, however, needs to be clarified including the relationship to the secondary currents of Prandtl's second kind. Expectations are, therefore, placed on future development in particle-image velocimetry(PIV). Understanding the turbulence structure requires not only the data on the time-average and instantaneous fields but also information on ever-changing time development of the flow field. At present, therefore, numerical analyses including DNS are considered highly effective methods. Less