Coherent Structures and associated sediment transport in unsteady open-channel flows

有一些重要的课题需要考虑非定常对湍流结构的影响。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。这些相干结构经常出现，特别是在洪水泛滥的河流中。在汛期的涨落阶段，洪水建筑物的水动力特性有很大的不同。在上升期，由于Kolk-Boil涡与沙丘发育期相对应，以及从沙丘上分离出来的旋涡在下一个沙丘上间歇性地碰撞并冲刷周围的床沙，导致悬沙浓度增加。本文研究了非定常和压力梯度对正弦波和沙丘床面上非恒定明渠流和恒定流的相干结构的影响，以动态地解释沙丘的生长过程和悬浮泥沙的行为。特别是通过两套激光雷达的同时使用，精确测量了沙丘上相干涡的特殊三维行为，并通过评价条件时空相关性，讨论了相干涡对流特性的差异。

There are some important topics in which the unsteadiness effects on turbulent structures should be considered. In open-channel flows such as rivers and estuaries, various coherent vortices are often observed and greatly affect sediment transport. These coherent structures often occur especially in flooded fluvial rivers. The hydrodynamic behavior of flood structures is quite different between the rising and falling stages of flood period. In the rising stage, the concentration of suspended sediment increases due to the kolk-boil vortex because the rising stage corresponds to thegrowing period of dunes and because separated vortex from the dune collides intermittently at the next dune and washes out bed materials around there.In this study, unsteadiness and pressure gradient effects on the coherent structures in unsteady open-channel flows and steady flows over sine-wave and dune beds, were investigated so that the growing process of the dunes and the behavior of the suspended sediment can be explained dynamically. Especially, the specific 3-D behaviors of coherent vortices over dunes were accurately measured by means of simultaneous use of two sets of LDA.The differences of convection properties on coherent vortices were discussed by evaluating the conditional space-time correlation.