明渠紊流三维涡旋结构的特征及对泥沙输移的影响

Sediment transport is the key problem in many environmental and industrial processes. Transport formulas are based on sediment transport mechanics, while transport phenomena are always believe to be induced by vortex structures. With advances in experimental and computational techniques, the study of vortex structures has been promoted remarkably, while most of published papers are only about two dimensional vortex structures with less investigation on the mutual interaction between vortices and sediment particles included.. The study is to quantitatively investigate three dimensional vortex structures, on their spatial structure, transportation of turbulent kinetic energy and interaction with suspended sediment load. With the help of 3D Time-Resolved PIV (3D-TRPIV), time series on velocity fields for both water flow and sediment particles in open channel flow will be measured. With the comparison between properties of vortex structures (such as, strength, density, size and orientation) in closed and open channel flows, the general property of vortex structures in near wall turbulence will be established. By analyzing the difference and correlation between fields for eddy viscosity (or mixing length) and swirling strength, the transfer mode of turbulent kinetic energy by vortices will be revealed. By analyzing the relatively spatial distribution between sediment particles and vortices and the connection between dynamic parameters of sediment particles and vortices, the transportation mechanism of suspended sediment load by vortices will be investigated.. Focusing on the intersection between coherent structures and sediment transport, this study would help to understand the fundamental turbulence and sediment transport physics, and improve the accuracy of related semi-empirical formulas in simulation and engineering.

泥沙输移一直是理论研究和实际工程普遍关心的问题。早期的泥沙输移公式建立在力学和随机理论等基础上，虽能基本满足工程要求，但尚无法阐释输移的物理本质。随着实验和模拟技术的发展，人们发现涡旋结构是紊流的基本组成单元，并试图利用其定性解释泥沙输移现象，但已有成果主要集中于二维涡旋结构方面，关于涡旋结构与泥沙相互作用的定量研究甚少。本项目拟定量研究三维涡旋结构的形态、动力特征及其与悬沙的相互作用。通过开展明渠低含沙水流试验，采用高频高时空分辨率的三维粒子图像测速技术，获得三维涡旋结构的特征参数沿水深的变化规律，并结合涡粘性系数的三维分布，研究涡旋传递紊动动能的方式；分析泥沙与涡旋结构的空间相对位置及泥沙运动参数与涡旋结构特征参数的关系，揭示涡旋对悬沙运动形态的影响机理。研究内容将有助于探明悬沙输移的物理本质，提高现有涡粘模型及悬沙输移公式的预测精度，并为紊流和泥沙精细数学模型的开发提供参考。

泥沙输移一直是理论研究和实际工程普遍关心的问题。随着实验和模拟技术的发展，人们发现涡旋结构是紊流的基本组成单元，并试图利用其解释泥沙输移机理。. 本项目通过开展理论分析和明渠低含沙水流试验，研究涡旋提取方法的理论解、三维涡旋结构定量特征、涡旋与经典紊流理论的关系及水沙相互作用机理等内容。. 主要结论有以下几点：（1）二维PIV能否检测出三维涡旋，主要受涡旋环量和旋涡与测量平面之间的倾角两个因素所影响，且仅当涡旋环量大于某一特定值（是倾角的函数）时，三维涡旋才可以被测量出。（2）以DNS槽道紊流数据为基础，利用切片法提取涡旋三维结构等值面的特征，主要参数包括涡旋数量、等效半径、圆形度及狭长度四个指标。随着水深的增加，涡旋个数不断减小，圆形度不断增大，狭长度不断减小并趋于稳定；半径的变化规律与阈值大小相关，并不呈唯一性。（3）由POD各阶模态分析可知，一阶模态下的颗粒浓度分布主要由雷诺应力导致，而二阶模态下颗粒浓度分布主要由旋转强度导致。（4）悬沙分布主要取决于涡旋的数量和强度，离床面较近的区域，涡旋密度大、强度高，造成悬沙浓度高；远离床面，涡旋密度小、强度弱，造成悬沙浓度低。. 本项目研究内容将有助于推动紊流和泥沙两学科的交叉领域物理机理的认识，从而帮助理解紊流的物理本质，揭示涡旋结构对紊动动能和泥沙的输移机理。