High-resolution numerical analysis of turbulent secondary motion in open duct flow

矩形截面直管道中的流体流动表现出小幅度的连续性诱导的二次运动，但对动量、热量和质量传输有很大的影响。相应的开放管道流（具有自由表面）的特征在于明显的二次流模式，导致了所谓的“下沉现象”（指的是在流体表面未发现平均流向速度的最大值）等实际重要效应。在本项目中，我们建议调查的机制，二次流形成的明渠流，刚性/刚性和混合（刚性/自由表面）的角落存在。首先，我们将通过伪谱直接数值模拟生成高保真数据。第二，我们将分析流场，特别强调相干结构的动力学和后果的雷诺数和纵横比的缩放。第三，我们将考虑二次流在开口管道中的泥沙输运过程中通过拉格朗日粒子跟踪（在一个点粒子的框架方法）。本项目将填补现有的知识空白有关的精确机制，二次流形成的混合角落的存在。由于相干结构的方法，它可以预期，我们的研究结果将铺平道路的流量控制策略的设计。这些流动过程直接相关的一个应用实例是人工运河和河流中由移动的沉积物颗粒形成的纵向脊的出现。

Fluid flow in a straight duct with rectangular cross-section exhibits turbulence-induced secondary motion of small amplitude, but with large consequences for momentum, heat and mass transport. The corresponding open duct flow (featuring a free surface) is characterized by a distinct secondary flow pattern, leading to such practically important effects as the so-called „dip phenomenon'' (referring to the fact that the maximum of average stream wise velocity is not found at the fluid surface). In the present project we are proposing to investigate the mechanism of secondary flow formation in open channel flow, where rigid/rigid and mixed (rigid/free-surface) corners exist. First, we will generate high-fidelity data by means of pseudo-spectral direct numerical simulation. Second, we will analyze the flow fields with particular emphasis on the dynamics of coherent structures and the consequences for Reynolds-number and aspect-ratio scaling.Third, we will consider the effect of secondary flow in open ducts upon the process of sediment transport by means of Lagrangian particle tracking (in the framework of a point-particle approach). The present project will fill existing knowledge gaps concerning the precise mechanism of secondary flow formation in the presence of mixed corners. Due to the coherent structure approach, it can be expected that our results will pave the way for the design of strategies for flow control. One example of an application where these flow processes are of direct relevance is the appearance of longitudinal ridges forming from mobile sediment particles in man-made canals and rivers.