鉛直噴流による礫粒子群輸送とsaltation-collapse平衡の数値解析

Abstract

本研究では，静止状態から約 3.2 秒間，鉛直下方に向かう平均流速約 1.2 m/s の円管噴流を平均粒径約 7 mm の礫群に衝突させる現象を対象として，実験と数値計算を行った．計算では，流体・固体連成並列計算手法を用いて，約 16, 700 個の礫モデルと流体間の相互作用と，実際の礫粒子の概略形状を表す礫モデル間の衝突を扱う 1, 088 プロセスの並列計算を行った．礫面洗掘現象の非定常過程を，(A) 非平衡洗掘状態，(B) saltation-collapse 平衡状態，そして，(C) 安息角静止状態，という 3 段階に分け，各段階の礫面形状を再現できることを確認した．saltation-collapse 平衡状態では跳躍・崩落礫モデル数ががほぼ等しい状態で大局的な礫面形状はほぼ一定であることを明らかにした．礫モデルの移動特性と，洗掘孔近傍で礫間流体圧が増加する可能性があることを示した．

Hydraulic experiments and numerical predictions were conducted for the unsteady process of local scour on the gravel bed, in which the average diameter of gravel particles is about 7 mm, caused by an impinging vertical water jet. The average velocity of the vertical jet was about 1.2 m/s and the flow was stopped at t = 3.18 [s]. In the computations, the fluid-solid interactions are taken into account in a similar way to the so-called an immersed domain method. This method was introduced into our parallel computational method in which governing equations are discretized with a finite volume method in the three-dimensional collocated grid system. The parallel computations were conducted for the local scour with 16, 700 gravel particle models with 1, 088 processes. The unsteady process of the local scour is categorized into three stages, (A) unsteady-scouring stage, (B) saltation-collapse equilibrium, and (C) stationary state with angle of repose. It was confirmed that the calculated gravel-bed shapes in all three stages are in good agreement with the experimental results. In particular, it was shown that the numbers of rising saltation particles and falling collapsed ones are approximately equivalent and that as a result almost uniform scoured surfaces are maintained during the saltation-collapse equilibrium. In addition, the total traveling lengths of the gravel particles were tracked in a Lagrangian way with the computational results. The distributions of fluid pressure were obtained in the computations and the pore water pressure near the scoured area is estimated higher than the initial hydrostatic pressure.