Granular Matter最新文献

In this experimental study, granular bed response to horizontal vibrations of various frequencies and amplitudes are examined with high-speed imaging. Ideal granular beds consisting of spherical glass beads are horizontally vibrated in a quasi-two-dimensional arrangement, firstly with homogeneous granular media and then with a ternary mixture to explore how bed response deviates with changes to material composition. Phenomena of note are the tendency for the homogeneous material to subdivide into discrete areas of crystalline lattice structures, bounded by non-crystalline lines of bead contacts, labelled in this paper as ‘shear lines’. Shear line failure arises as neighbouring crystalline areas slide relative to one another along their shared non-crystalline border, combining to form one larger crystalline area. Under vibration conditions where particle agitation and relative movement is high, sloshing occurs in the upper bed and triangular granular-gas regions form in the top corners. The ternary mixture also exhibits sloshing at low frequency and large amplitude, but the inhomogeneity of its composition prevents formation of ordered crystalline regions and shear lines, instead promoting low percolation and a jamming effect underneath the sloshing region. Surprisingly strong convective responses are induced in the inhomogeneous bed with more energetic vibrations. From the analysis of shear lines in the homogeneous beds, and of convection in the inhomogeneous beds, comparisons between homogeneous and inhomogeneous bed behaviour are drawn. Results are used to discuss how behavioural response of non-cohesive granular material to horizontal vibrations is ultimately tied to, and changes with, the geometric complexity of the internal packing structure. The concept of ‘geometric compatibility’ between constituent particle species in an inhomogeneous granular medium is proposed as an explanation for the low percolation and strong convective response to vibration.

Graphical abstract

The study describes a comprehensive methodology to evaluate X-Ray micro-computed tomography data from sand samples and to characterize their 3D microstructural properties. Fine and medium-grained sands are analyzed in their natural and bio-cemented states. While the two materials exhibit similar peak and residual strengths in their untreated state, they yield distinctly different strength improvements in their bio-cemented state, despite similar cementation contents. To understand the underlying mechanisms that govern this behavior, a recently developed approach is presented to gain new insights into the specimen’s micro-architecture. Results capture a series of properties such as the volume distribution of pore bodies, pore throats, particles, interparticle contacts, precipitation bonds, and distribution of tortuous paths. It is found that the intrinsic, i.e., pre-cementation microstructural properties, are crucial in determining the spatial distribution of post-cementation bonds. Furthermore, the volume of bonds at interparticle contacts and in throats governs the overall contact area, directly reflecting interparticle stress transmission. Contact area increases by 180% for the medium-grained sand compared to 120% for the fine-grained. Overall, the methodology introduced in this study forms a new basis for understanding biocementation and can contribute to a more robust formulation of simulation models incorporating pore and contact mechanics in porous media.

Graphic abstract

For a wide range of applications, we need DEM simulations of granular matter in contact with elastic flexible boundaries. We present a novel method to describe the interaction between granular particles and a flexible elastic membrane. Here, the standard mass-spring model approach is supplemented by surface patches given by triangulation of the membrane. In contrast to standard mass-spring models, our simulation method allows for an efficient simulation even for large particle size dispersion. The novel method allows coarsening of the mass-spring system leading to a substantial increase in computation efficiency. The simulation method is demonstrated and benchmarked for a triaxial test.

The discrete element method is used to study the dissipation behavior of the granular balls in a rotating closed cylinder under the Earth gravity environment, and five kinds of particle motion forms with different dissipation characteristics are obtained. These phases are slumping, rolling, cascading, cataracting and centrifuging. Combined with the simulation of the dissipation behavior of the granular system under the Mars and Moon environment, the universality and the mechanism of the high dissipation effect of the cataracting phase is studied. In addition, based on the dissipation behavior of the granular balls in the rotating closed cylinder, the optimization design of the autogenous mill lifter is studied, and finally a design method of the structure parameters of the autogenous mill based on the material dissipation behavior is proposed.

Graphical Abstract

Jammed granular matter can be considered a meta-material that behaves viscoelastic for small deformations. We characterize the elastic properties of the meta-material through the response of a simply supported bending beam consisting of jammed granular matter under weak load and quasistatic deformation.

Soils can have direction-dependent characteristics reflected in the anisotropy of their responses. Studies have demonstrated the impact of the stress state and history (i.e., stress-induced anisotropy) and the depositional processes and particle arrangements (i.e., fabric-induced anisotropy) on the anisotropy of macroscopic behaviors. However, quantifying the stress- and fabric-induced anisotropies remains a challenge. This study presents two investigations on the effects of stress- and fabric-induced anisotropy on the anisotropy of shear wave velocity (V_S). A framework based on the V_S measurements along various orientations and polarization planes obtained from discrete element method (DEM) simulations and experimental bender element (BE) tests is presented; this framework is tested using the results from specimens of spherical and non-spherical particles under isotropic and 1D compression. The observed trends indicate that the angular distributions of V_S are related to the angular distributions of particle alignment and interparticle contact forces. This framework, when presented in terms of the ratio of V_S measurements along different orientations and polarization planes and of the newly introduced Anisotropy parameter (A_e), can assist in evaluating the stress- and fabric-induced anisotropy of soil specimens. The results also highlight the challenges in discerning the effects of stress and fabric anisotropy when both influence the soil response.

Graphical abstract

Dynamic sublevel caving technology (DSCT) proposed by the researchers is one of effective methods to solve the problems of low top coal recovery, poor drawing balance and support stability in longwall top coal caving (LTCC) with large dip angle. To investigate the reasonable number of supports in a sublevel (N) and the top coal drawing mechanisms under DSCT, this research takes Panel 7401 in Zouzhuang Coal Mine as the geological background. Firstly, the optimal threshold value of N is theoretically analyzed, and the numerical simulations of drawing experiments under different Ns are calculated. The results show that when N = 3, the top coal recovery is the highest, the number of excessive drawing top coal at the upper end is relatively small, and the drawing balance is great, which is conducive to improving the resource recovery and safety management. With increasing N, the over-development of right top coal boundary towards the upper end increases, the range of coal ridge in the lower sublevel also gradually increases, while the strong force chain area at the upper end gradually decreases, resulting in the support stability becoming worse. In addition, the displacement of top coal at the upper end gradually increases with increasing N, and the permanent loss feature of residual top coal exists in the upper sublevel. The field top coal recovery under DSCT was measured finally, obtaining that DSCT can improve the top coal recovery by about 5% and promote the stability and working efficiency of the support. The research results have great theoretical and guiding significance for the high yield and high efficiency LTCC technology for thick coal seam with large dip angle.

Graphical Abstract

Fabric evolution monitoring of sandy specimens during shearing up to critical state is characterized by continuous, bidirectional shear wave velocity measurements along the vertical and horizontal directions (V&H). The specimens are prepared by water sedimentation methods and then subjected to drained compression and extension loading paths. The results exhibit a significant differences between shear wave velocities in two orthogonal directions, and subsequently shear moduli, as shear develops. Not only do the differences between shear wave velocities in V and H directions illuminate a severe and increasing soil anisotropy during the shearing, but the results also signify promising information related to the current fabric and stress state. Comparison between compression and extension results highlight different fabric evolution trends and consequently dissimilar fabric states at the critical state. Considering the conforming results with recent findings on the basis of the discrete element method (DEM), the proposed method can be used as an experimental method facilitating the macroscopic investigation of the effects of fabric anisotropy on the soil elastic response. The fabric anisotropy and its evolution are assessed consecutively using three methods, including quantitative evaluation of shear moduli, proposing a fabric function to account for the soil fabric, and 3D microscopic inspection of Micro-CT slices. The findings of the mentioned methods agree on the importance of fabric anisotropy in shear wave propagation and microscopic variations towards the critical state evolving from the initial state to dissimilar anisotropic states at the critical state under different shear modes.