Study on the 3-D Spatial Structure of Particle Cluster

研究了气固流动中颗粒团簇的时空结构。对垂直管道内重力沉降流进行了实验研究。利用高速相机测量的管道截面颗粒分布时间序列，重构了管道内颗粒团簇的时空三维结构。此外，通过数值模拟研究了颗粒团簇的三维空间结构。在周期边界包围的矩形-平行六面体计算域中对气固流动进行了数值模拟。流体运动采用DNS计算，固体流动采用拉格朗日模型计算，并考虑粒子间碰撞。在最大的情况下，超过1600万个粒子被跟踪。所得结果总结如下：(1)粒子分布的三维时空结构。发现粒子分布的三维表示使我们能够发现粒子簇的三维性质。(2)通过分析模拟得到的颗粒浓度空间分布的功率谱，定量研究了颗粒团簇的空间尺度特征。结果表明，该星团由多个空间尺度组成，但以低波数为主，具有大尺度结构；(3)提出了一种基于波数空间低通滤波技术的颗粒团簇三维空间结构重构方法，并将其应用于本次数值模拟得到的流场数据。成功地提取了粒子簇的优势三维结构。对其三维结构和性能的详细研究成为可能。我们发现在二维观测中可以识别的逆u型结构是弯曲管状簇元的一部分。在三维空间中，这些集群元素以一种非常复杂的方式相互作用。少

Spatial and temporal structure of particle clusters formed in gas-solid flows is studied. Experiments are made for gravitational settling flow in a vertical duct. Spatial and temporal three-dimensional structure of particle clusters formed in the duct are reconstructed from the measured time series of high-speed camera images of particle distributions in a duct cross-section.In addition, numerical simulations are conducted to investigate the three-dimensional spatial structure of particle clusters. The numerical simulations are made for the gas-solid flows in a rectangular-parallelepiped calculation domain surrounded by periodic boundaries. The fluid motion is calculated by using DNS, while the solid flow is calculated by using a Lagrangian model with considering the particle-particle collision. More than sixteen million particles are tracked in the maximum case.The obtained results are summarized as follows.(1) Three-dimensional spatial and temporal structure of particle distributions … More are obtained from the experiments. It is found that three-dimensional presentation of particle distributions enables us to find the three-dimensional nature of particle clusters.(2) Spatial-scale characteristics of particle clusters were investigated quantitatively by analyzing the power spectrums of spatial particle concentration distributions obtained by the present simulation. The results showed that the cluster consists of multiple-spatial scales while the low wave-number, hence large-scale structure, is dominant.(3) A reconstruction method of three-dimensional spatial structure of particle clusters based on the low-pass filtering technique in the wave-number space are proposed and applied to the flow field data obtained from the present numerical simulations. Dominant three-dimensional structures of particle cluster are successfully extracted. It becomes possible to have the detailed investigations on its three-dimensional structures and behaviors. We found that inverse U-structure which is recognizable in two-dimensional observations is a part of bent tube-like cluster elements. In three-dimensions, these cluster elements are interacting in a very complex way. Less