We study the effect of size polydispersity on the stress distributions and structural properties of static frictionless packings under isotropic compressions.More than 50 isostatic packings with constant mean stress of 1 kPa are generated for each size polydispersity s with a uniform distribution of diameter between(d_(0-s)/2)and(d_(0+s)/2).In order to vary the degree of positional order,the size polydispersity s ranges from 0 to 0.5.Several typical structural characterizations,(i.e.,the height of the first pair correlation peak,the global and the local order parameters),the probability distribution of the normalized mean stress and the stress-stress correlation are calculated.The result shows that(i)the stress distribution scales as a power law in the limit of small stresses,and the distribution displays a Gaussian tail in the limit of large stresses;(ii)s has no evident influence on the structural and mechanical properties when s>0.2.
Fresh cement mortar is a type of workable paste, which can be well approximated as a Bingham plastic and whose flow behavior is of major concern in engineering. In this paper, Papanastasiou＇s model for Bingham fluids is solved by using the multiple- relaxation-time lattice Boltzmann model （MRT-LB）. Analysis of the stress growth exponent m in Bingham fluid flow simulations shows that Papanastasiou＇s model provides a good approximation of realistic Bingham plastics for values of m 〉 108. For lower values of m, Papanastasiou＇s model is valid for fluids between Bingham and Newtonian fluids. The MRT-LB model is validated by two benchmark problems： 2D steady Poiseuille flows and lid-driven cavity flows. Comparing the numerical results of the velocity distributions with corresponding analytical solutions shows that the MRT-LB model is appropriate for studying Bingham fluids while also providing better numerical stability. We further apply the MRT-LB model to simulate flow through a sudden expansion channel and the flow surrounding a round particle. Besides the rich flow structures obtained in this work, the dynamics fhi d force on the round particle is calculated. Results show that both the Reynolds number Re and the Bingham number Bn affect the drag coefficients Co, and a drag coefficient with Re and Bn being taken into account is proposed. The relationship of Bn and the ratio of unyielded zone thickness to particle diameter is also analyzed. Finally, the Bingham fluid flowing around a set of randomly dispersed particles is simulated to obtain the apparent viscosity and velocity fields. These results help simulation of fresh concrete flowing in porous media.
A granular material is a conglomeration of discrete solid particles.It is intrinsically athermal because its dynamics always occur far from equilibrium.In highly excited gaseous states,it can safely be assumed that only binary interactions occur and a number of kinetic theories have been successfully applied.However,for granular flows and solidlike states,the theory is still poorly understood because of the internally correlated structures,such as particle clusters and force networks.The current theory is that the mesoscale characteristics define the key differences between granular materials and homogeneous solid materials.Widespread interest in granular materials has arisen among physicists,and significant progress has been made,especially in understanding the jamming phase diagram and the characteristics of the jammed phase.In this paper,the underlying physics of the mesoscale structure is discussed in detail.A multiscale framework is then proposed for dense granular materials.
颗粒介质是由大量离散颗粒构成的无序材料,在工业生产与自然界中广泛存在.在外界作用下,颗粒材料可以类似固体保持稳定,也可类似流体发生流动,且类固体和类流体之间可以自然转化,目前人们难以采用统一的连续本构进行描述.本文提出了物质点法(Material Point Method,MPM)与离散元法(Discrete Element Method,DEM)的多尺度建模框架,亦即宏观尺度采用适用于大变形问题的MPM,颗粒尺度则采用DEM描述每个颗粒运动,每个物质点的力学性质由该点处若干颗粒构成的代表性体积元的力学性质计算得到,宏观尺度MPM所得变形梯度作为边界条件施加到体积元中,基于DEM计算得到域内的平均柯西应力,反馈到MPM计算,以此实现跨尺度研究.以沙堆倒塌作为算例,验证了MPM/DEM多尺度建模在描述颗粒复杂力学行为的有效性,并对倒塌过程中,宏观应变等信息与内在接触力链网络的关联进行了讨论.MPM/DEM多尺度建模没有引入唯象本构,且适用于大变形问题,为颗粒介质的多力学状态研究提供了新的思路.
