PREDICTION OF BOTH DANGER ZONE OF FLOOD AND ACCUMULATION DISTRIBUTION OF VOLCANIC ASH IN REVERE BY MEANS OF LAGRANGE METHODS

本文通过实验和数值模拟研究，预测了洪水危险区和火山灰堆积分布。实验装置是一个矩形的明渠（模拟一个在流向和法向都有二维流场的倒圆），侧墙是用丙烯酸玻璃制成的窗口，用于照明和观察。特别地，使用CCD相机和图像处理来观察板（模拟桥墩）周围的流动特性。相应的数值分析进行了使用的控制方程，这是离散的有限差分技术，以确定流向和横向速度分量。揭示了雷诺数和缝宽d与缝厚δ之比对速度场的影响。研究发现：（i）在Re和/或d/delta较小的情况下，缝隙上的流场不随时间变化;（ii）当Re和d/delta超过一定值时，从板的一侧到另一侧的流动的交替交叉发生在槽上，以及（iii）流体流动的交替变化扰乱了沿着板形成的速度边界层，并引起板的在槽下游的上部流和下部流的混合。在速度场的这些结果证实了使用离子交换树脂的流动可视化。

An experimental and numerical study has been performed to predict both the danger zone of flood and the accumulation distribution of volcanic ash in reveres. The experimental apparatus used here was consisted of a rectangular open channel (simulating a revere with a 2-D flow field in the streamwise and normal directions) with a side wall which Was made of acrylic windows for illumination and observation. In particular, the flow characteristics around the plate (simulating a bridge pier) is observed using a CCD camera and an image processing. The corresponding numerical analysis was carried out using the governing equations, which are discretized by means of a finite-difference technique to determine streamwise and transverse velocity components. The roles of both the Reynolds number and the ratio of the slot width, d, to its thickness, delta, on the velocity field are disclosed. It is found from the study that : (i) at a small value of Re and/or d/delta, the flow field over the slot exhibit no timewise variation, (ii) when Re and d/delta exceed certain values, an alternate crossing of flow from one side of the plate to the other occurs across the slot and (iii) the alternating change in the fluid flow disturbs the velocity boundary layer formed along the plate and induces mixing of the upper and lower streams of the plate downstream from the slot. These results in the velocity field are confirmed by the flow visualization using ion-exchange resins.