鉛直上昇流による飽和粒子層の内部流動化に対する流体・固体連成計算

Abstract

底面から局所的に流入する鉛直上昇水流(平均流速約 0.77 m/s) により，粒子層で内部流動化が発生し，最終的に水流が粒子層を貫通して崩壊に至る過程を対象として，実験および粒子スケールの流体・固体連成計算を行った．実験では，2 種類の礫粒子（粒径約 7 mm と 4 mm）および粒径約 7 mm のガラスビーズを用いて，高速ビデオで粒子の動きを撮影し，超小型間隙水圧計による計測を行った．一方，計算では個々の粒子を四面体要素で表現する粒子モデルを利用し，最小粒径粒子の計算では，粒子モデル数を 21, 936，流体計算セル数を 117, 504, 000 として，粒子周辺の流体計算に十分な分解能を設定し，2, 176 プロセスの並列処理により演算を高速化した．実験および計算結果を用いて，粒子層内に生ずる過剰間隙水圧の時間的・空間的分布を比較するとともに，計算結果を利用した粒子移動パターンを示し，一連の過程が進行する力学的な要因や，粒子形状および粒径による現象の相違について考察を加えた．

The internal fluidization and the failure of a saturated granular bed due to the vertically-upward water flow entering from the bottom surface were investigated with the experiments and the computations taking account of the particle-scale fluid-solid interactions. Two types of gravel particles (average diameters are about 7 mm and 4 mm) and glass particles (diameter is about 7 mm) were used in the experiments, in which the particle movements were captured with a high-speed camera and the pore-water pressure was measured with four ultra-compact pore pressure gauges. In the computations, the fluid forces acting on up to about 21, 936 particle models, each of which is represented with multiple tetrahedron elements, were calculated from the pressure and viscous terms. The number of the fluid cells was up to 117, 504, 000 and the maximum number of the cores in parallel computations was 2, 176 to decrease the elapsed time. As a result, in the fluidized area of the granular layer, it was shown that the computation method enables us to obtain adequately the distributions of the excess pore-water pressure and the typical flow patterns of particles, which are important to understand the mechanisms on the behaviors of the particles driven by the water flows entering from the bottom.