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 multiplerelaxation-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 fluid force on the round particle is calculated.Results show that both the Reynolds number Re and the Bingham number Bn afect the drag coefcients CD,and a drag coefcient 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.
