Modelling dynamic breakage in rock blocks with different elongation and flatness ratios upon impact

The two crucial shape factors (elongation ratio and flatness ratio) of brittle particles may influence the dynamic breakage of brittle particles upon impact. Hence, three-dimensional discrete element method simulations of brittle rock blocks with different shapes upon normal impact were performed. The simulated results indicate that the elongation ratio, that is, ratio of width to length and flatness ratio, that is, ratio of thickness to width can significantly affect the breakage of brittle rock blocks. Three fracture mechanisms, that is, fragmentation, horizontal tensile fracture and vertical tensile fracture, were revealed, which determine the dynamic breakage of rock blocks. The fragmentation results in numerous single-sphered fragments with velocities even larger than 2 times of the initial velocities. Fragmentation can provide a buffering effect at high impact velocities of larger than 4 m/s. With an increasing elongation ratio or flatness ratio, the phenomenon of fragmentation gradually disappears. The reflection of a compression stress wave results in horizontal tensile fracture. The expansion in the plane perpendicular to the impact velocity results in vertical tensile fracture.     

Discrete simulations of a triaxial compression test for sand by DEM

A quasi‐static homogeneous drained triaxial compression test on cohesionless sand under constant lateral pressure was simulated using a three‐dimensional discrete element method. Grains were modelled by means of particle clusters composed of rigid spheres or spheres with contact moments imitating irregular particle shapes. Attention was paid to the effect of initial void ratio and grain shape mixture on the shear strength, volume changes, force chains, kinetic, elastic and dissipated energies. In addition, the effect of the mean grain size, grain size distribution, grain size range, specimen size and roughness and stiffness of boundaries was numerically analysed in initially dense sand. Some numerical results were compared with available experimental results. Copyright © 2014 John Wiley & Sons, Ltd.     

颗粒粒径对滑坡碎屑流动力特征及能量转化的影响——以四川省三溪村滑坡为例

滑坡碎屑流的能量演化机制涉及复杂的碰撞、摩擦和能量转化,对滑坡灾害的防治具有重要意义。以四川省三溪村滑坡为例,采用离散元法建模,研究了不同土颗粒粒径下滑坡的运动堆积特征、能量演化过程及其对建筑物的影响。结果表明,在不同粒径条件下,滑坡的堆积特征变化不大,但它们对能量转换和建筑物的冲击力有显著的影响。粒径越大,滑坡启动速度越快,峰值动能速度越高,碰撞耗散能量越大,摩擦耗散能量越少。粗粒土中颗粒间距越大,颗粒间的碰撞效应越明显,有利于能量传递,因此,对房屋的冲击力越大。因此,在模拟过程中不能忽视颗粒粒径对滑坡碎屑流动力学特征的影响。这些研究结果揭示了不同粒径土粒在滑坡运动过程中的能量演化机制,为能量演化对建筑冲击的影响提供了初步的认识,可为滑坡碎屑流的防治提供指导依据。     

破碎岩体压实特性的三维离散元数值计算方法研究

研究破碎岩体的压实特性是矿井地下工程的基础工作之一,由于破碎岩体所处环境的隐蔽性与危险性,常采用实验室测试和数值计算的研究方法。提出了一种三维破碎岩体模型构建方法,即在3D Voronoi建立完整岩体数值模型的基础上,通过预定孔隙率,随机删除完整岩体中的块体反演破碎岩体结构,测定破碎岩体的压实特性。该方法可较真实地反映破碎岩体的块度特征、碎胀特性与压实特性,与现有研究方法有较高的吻合度,为矿山地下工程的安全控制提供了新的有效的研究方法。     

Extended CFD–DEM for free‐surface flow with multi‐size granules

Computational fluid dynamics and discrete element method (CFD–DEM) is extended with the volume of fluid (VOF) method to model free‐surface flows. The fluid is described on coarse CFD grids by solving locally averaged Navier–Stokes equations, and particles are modelled individually in DEM. Fluid–particle interactions are achieved by exchanging information between DEM and CFD. An advection equation is applied to solve the phase fraction of liquid, in the spirit of VOF, to capture the dynamics of free fluid surface. It also allows inter‐phase volume replacements between the fluid and solid particles. Further, as the size ratio (SR) of fluid cell to particle diameter is limited (i.e. no less than 4) in coarse‐grid CFD–DEM, a porous sphere method is adopted to permit a wider range of particle size without sacrificing the resolution of fluid grids. It makes use of more fluid cells to calculate local porosities. The developed solver (cfdemSolverVOF) is validated in different cases. A dam break case validates the CFD‐component and VOF‐component. Particle sedimentation tests validate the CFD–DEM interaction at various Reynolds numbers. Water‐level rising tests validate the volume exchange among phases. The porous sphere model is validated in both static and dynamic situations. Sensitivity analyses show that the SR can be reduced to 1 using the porous sphere approach, with the accuracy of analyses maintained. This allows more details of the fluid phase to be revealed in the analyses and enhances the applicability of the proposed model to geotechnical problems, where a highly dynamic fluid velocity and a wide range of particle sizes are encountered. Copyright © 2015 John Wiley & Sons, Ltd.