宾汉姆流体的LBM-DEM方法及自密实混凝土复杂流动研究

发布时间：2018-02-16 22:57

本文关键词： 格子Boltzmann-离散元耦合方法 Bingham流体 Bingham两相流自密实混凝土多孔介质　出处：《清华大学》2014年博士论文　论文类型：学位论文

【摘要】：堆石混凝土的浇筑过程主要是自密实混凝土在堆石孔隙中的流动填充过程，本质上是宾汉姆（Bingham）两相流在多孔介质中运动的过程。本论文建立了耦合的格子Boltzmann-颗粒离散元方法（简称LBM-DEM），并将其应用于自密实混凝土在多孔介质中的流动研究。LBM作为一种新兴的计算流体力学方法，在处理复杂边界条件、计算简单稳定、易于并行等方面有着传统数值方法无法比拟的优势。而DEM在模拟固体颗粒的摩擦、碰撞、堵塞和迁移等方面物理概念清晰，在岩土力学和颗粒流等领域也得到了广泛的应用。因此，充分吸收了上述两种方法特点的LBM-DEM的耦合框架在处理多相流的复杂流动方面有着明显的优势。 Bingham流体是一种典型的非牛顿流体，因此需要对常用于牛顿流体的线性LBM方法进行修正。本文采用了Papanastasiou提出的改进指数模型，并使用了多松弛时间的LBM（简称MRT-LBM）对Bingham流体进行数值模拟和研究。在保持计算稳定性的同时，MRT-LBM很好地模拟了非屈服区，并捕捉到了死区的存在。另外，通过对Bingham流体中的圆形颗粒曳力系数进行研究发现，曳力系数CD不仅和雷诺数Re有关，还与宾汉姆数Bn有关，因此，本论文根据理论推导和数值计算结果，给出了Bingham流体中统一的曳力系数公式。本论文在上述工作的基础上建立了Bingham两相流的LBM-DEM耦合框架，并将其应用于Bingham两相流在二维单孔道和多孔道中的堵塞问题研究。在单孔道流动中，利用通过系数定量地研究了孔径比、颗粒固含率和压力梯度对Bingham两相流堵塞的影响，，发现二维条件下堵塞的临界孔径比为2，且孔径比是影响堵塞最关键的因素。随着颗粒固含率的增加，堵塞的风险会有所增加，而压力梯度对于两相流的堵塞没有影响。多孔道的研究发现，Bingham两相流的流动是自调整的，而且窄通道数（孔径比小于2）的增加会显著增加堵塞的风险。最后，本论文实现了三维条件下Bingham两相流的LBM-DEM方法的应用。利用该方法，不仅得到多孔介质内部丰富的流动信息，还有效地模拟出两相流中的颗粒运动、堵塞等现象。三维定性的研究结果表明：LBM-DEM耦合方法是一种研究自密实混凝土在堆石孔隙中的流动、堵塞等细观力学过程的有效数值方法。
[Abstract]:The pouring process of rockfill concrete is mainly the flow filling process of self-compacting concrete in the rockfill pore. In this paper, a coupled lattice Boltzmann- particle discrete element method (LBM-DEMN) is developed and applied to study the flow of self-compacting concrete in porous media. As a new method of computational fluid dynamics, In dealing with complex boundary conditions, simple and stable calculation, easy to parallel and so on, DEM has incomparable advantages over traditional numerical methods, while DEM has clear physical concepts in simulating the friction, collision, blockage and migration of solid particles. It has been widely used in the fields of geotechnical mechanics and particle flow, so the coupling frame of LBM-DEM, which fully absorbs the characteristics of the above two methods, has obvious advantages in dealing with the complex flow of multiphase flow. Bingham fluid is a typical non-Newtonian fluid, so it is necessary to modify the linear LBM method, which is often used in Newtonian fluid. In this paper, an improved exponential model proposed by Papanastasiou is used. The Bingham fluid is numerically simulated and studied by means of multiple relaxation time LBM (MRT-LBM). While maintaining the computational stability, MRT-LBM can well simulate the non-yielding zone and capture the existence of the dead zone. By studying the drag coefficient of circular particles in Bingham fluid, it is found that drag coefficient CD is not only related to Reynolds number re, but also to Bingham number B n. The uniform drag coefficient formula in Bingham fluid is given. Based on the above work, the LBM-DEM coupling framework of Bingham two-phase flow is established and applied to the study of the blockage of Bingham two-phase flow in two-dimensional single-channel and multi-channel. The effects of pore size ratio, particle holdup and pressure gradient on the blockage of Bingham two-phase flow are quantitatively studied by using the coefficient. It is found that the critical pore size ratio is 2, and the pore size ratio is the most important factor affecting the blockage. With the increase of particle solid holdup, the risk of blockage will increase. However, the pressure gradient has no effect on the blockage of two-phase flow. The multi-channel study shows that the flow of Bingham two-phase flow is self-adjusted, and the increase in the number of narrow channels (pore size ratio less than 2) will significantly increase the risk of blockage. In this paper, the application of LBM-DEM method for Bingham two-phase flow under three-dimensional condition is realized. By using this method, not only the abundant flow information in porous media is obtained, but also the movement of particles in two-phase flow is effectively simulated. The results of three-dimensional qualitative study show that the coupling method of the two dimensional LBM-Dem is an effective numerical method for studying the flow and blockage of self-compacting concrete in the pores of rockfill.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TV431;TV544

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