Turbulent Structures and the Related Sediment Transport in Unsteady Open-Channel Flows

在河流、河口等明渠水流中，经常会出现各种相干涡旋，并对泥沙输移产生很大影响。在离床层很近的内区，爆发现象呈准周期性发生，并对湍流起支撑作用。另一方面，在靠近树木表面的外部区域，出现了Kolk-Boil涡旋，在床层附近存在一个强烈的向上倾斜的流向运动，称为Kolk，类似于大气中的西红柿运动，Kolk涡旋发展到自由面，然后变成“Kolil”。典型的沸腾开始时泥沙浓度很高，就像水面上隆起的圆形或椭圆形斑块；随着时间的推移，它会变宽，并逐渐向河流水位下降，直到完全消退，并与周围较少搅动的部分融合。因此，研究非恒定明渠水流中非恒定流对湍流结构和拟序结构的影响，对于动态地解释沙丘的生长过程和悬沙的行为具有重要的意义。作者研究了明渠非恒定流中的湍流结构。结果表明，非定常对自由面附近的外部区域有显著影响。考察了沙丘外部区域湍流结构的时间变化，并与无自由面的非定常管道流动进行了比较；利用两套光纤激光多普勒风速仪(FLDA)和自动穿越系统，精确测量了实际形状沙丘上的详细湍流结构。精确测量了沙丘后和再附着点后产生的相干涡的三维结构。

In open-channel flows such as rivers and estuaries, various coherent vortices are often observed and greatly affect sediment transport. In the inner region which is very near the bed, bursting phenomena occur quasi-periodically and sustain the turbulence. On the other hand, in the outer region which is near the tree surface, a kolk-boil vortex occurs.There exists a strong upward-tilting streamwise vortex motion, called the "kolk" , near the bed, in a analogy to a tomado motion in the atmosphere, and the kolk vortex develops up to the free surface and then becomes a "boil". A typical boil starts life with high sediment concentration, as a raised circular or oval patch on the water surface ; in the course of time, it widens and gradually settles back down toward the river level until it subsides completely and merges with the surrounding, less-agitated parts. Such coherent vortices are generated especially in flood rivers.Therefore, it is very important to investigate unsteadiness effects on the turbulent structures and the coherent structures in unsteady open-channel flows so that the growing process of the dunes and the behavior of the suspended sediment can be explained dynamically. The authors have investigated turbulent structures in unsteady open-channel flows. It has been clarified that the unsteadiness has significant effects on the outer region near the free surface. The time variations of turbulent structure in the outer region and comparison with unsteady duct flow which has no free surface have examined.Furthermore, the detailed turbulent flow structures over the actual form of sand dunes were measured accurately by making use of two sets of fiber-optic laser Doppler anemometers (FLDA) and automatic traversing system. The three-dimensional structures of coherent vortices generated behind the dune crest and the reattachment point were measured accurately.