Granular Matter最新文献

A numerical simulation based on volume-of-fluid and discrete particle method has been accomplished to analyze the effect of particle holdup on bubble formation in suspension medium. A two-way coupled model involving some essential interphase forces (liquid-particle suspension inertial force, particle-bubble contact force, Basset force, bubble inertial force, virtual mass force, pressure gradient force, etc.) is firstly set up and validated with the experimental and simulation results. On the basis of the analysis on two-stage bubble formation, some potential influence factors (bubble neck length, bubble detachment size and period, bubble shape and wakes) are discussed. The results show that the neck length of bubble detachment increases with the increase of particle holdup due to the greater drag coefficient. For 0.6% < ({varepsilon }_{p}) < 0.8%, the neck length at (t/{t}_{det}) = 1 varies greatly due to a significant increase in apparent viscosity at ({varepsilon }_{p}) = 0.55%. Additionally, the trajectory instability of the bubble is attributed to the increase of bubble aspect ratio, leading to a strong entrainment ability and transport enhancement in gas–liquid–solid medium. The particles can weaken the entrainment of bubble-induced flow to the surrounding fluid, thus the influence of shortening the detachment period caused by the wake weakening is weakened.

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Carbonate sands (CS) have the potential to be utilized as construction materials, and their porous particles may bring unique thermal properties, which is critical for the thermal design of geosystems; however, this area of research is highly understudied. The present work provides a new contribution in this field by investigating the thermal conductivity (λ) of five uniform fractions of CS from the South China Sea, with emphasis on the influence of particle size and relative density of the sand. The impact of the size of particles on the index void ratios and thermal conductivity of the samples was profoundly different from that of silica sands. Contrary to silica sands, the extreme void ratio (e_max, e_min), which is critical for calculation of relative density, of the CS increased as the mean grain size increased. The maximum thermal conductivity of each fraction was negatively correlated with the particle size, and the thermal conductivity of the finer fractions exhibited higher sensitivity to the packing density. Literature models were found to be ineffective in predicting the thermal conductivity of the CS given the unique thermal energy transferring mechanisms of the porous particles. Two thermal conductivity models, stemming from semi-analytical and empirical approaches, were proposed in the light of providing useful guidance for the thermal designs that include CS.

For complex shaped materials, computational efficiency and accuracy of DEM models are usually opposing requirements. In the literature, DEM models of railway ballast often use very complex and computationally demanding particle shapes in combination with very simple contact laws. In contrast, this study suggests efficient DEM models for railway ballast using simple particle shapes together with a contact law including more physical effects. In previous works of the authors, shape descriptors, calculated in a shape analysis of two types of ballast, were used to construct simple particle shapes (clumps of three spheres). Using such a shape in DEM simulations of compression and direct shear tests, accurate results were achieved only when the contact law included additional physical effects e.g. edge breakage. A parametrisation strategy was developed for this contact law comparing DEM simulations with the measurements. Now, all the constructed simple particle shapes are parametrised allowing to study their suitability and relating their shape descriptors to those of railway ballast. The most suitable particle shapes consist of non-overlapping spheres, thus have a high interlocking potential, and have lowest sphericity and highest convexity values. In a micromechanical analysis of the four best performing shapes, three shapes show similar behaviour on the bulk and the micro-scale, while one shape differs clearly on the micro-scale. This analysis shows, which shapes can be expected to produce similar results in DEM simulations of other tests/load cases. The presented approach is a step towards both efficient and accurate DEM modelling of railway ballast.

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Understanding and modeling the contact behavior of particles provides enhanced constitutive laws in discrete-based simulations as well as a strong linkage between micro- and macroscopic behavior of granular materials. In the present study, we studied the influence of a polymer-based additive composed of Methocel cellulose ether on the tribological response of rough particles with grain-scale experiments. We performed both monotonic and cyclic tests on pairs of particles subjected to repeated shearing so that to explore the additional influence of abrasion in relation to previous loading history. Each sample was tested at first in a dry state and consecutively was immersed into the polymer-based fluid. The test results indicated increased friction at the contacts of the particles when immersed in the polymer-based additive, suggesting promising applications of the Methocel cellulose ether in improving the stability of geo-systems. For the shearing tests in a dry state, abrasion influences were amplified at higher magnitudes of normal load, whereas the polymer additive acted as a fluid film (or thin coating) mitigating in this way the continuous abrasion due to repeated shearing. We incorporated the Mindlin and Deresiewicz (M–D) contact model in a modified form to analyze the tangential response of the particles. It was found that the power of the modified M–D model was directly correlated with the natural logarithm of the microslip displacement threshold, when this threshold was normalized with respect to the normal load. Based on the analysis of the closed loops from the cyclic tests, the energy dissipation was found to be reduced when the samples were immersed in the polymer fluid compared with the tests in a dry state. The test results from the present work offer some insights into the micromechanical-based analysis of particulate materials and the interactions of rough particles in the presence of polymeric fluids with potential applications in contact mechanics modeling using discrete-based tools. These results also contribute to unveiling the prevailing micromechanisms in order to understand (or interpret) bulk behavior, thus the database from the present work may provide helpful guidance in understanding granular material behavior and the interactions between particles with polymer-based additives.

The maximum shear modulus (G_max) is a key material characteristic that is incorporated in advanced soil constitutive models. Numerous experimental studies have been conducted to describe the effects of particle sizes and packing characteristics on G_max. However, most of these studies were conducted on quartz-based sands. A review of the literature revealed that few studies have described the effects of grain size distribution (GSD) on G_max in calcareous sands. Therefore, bender element (BE) tests were performed on calcareous sands with different mean grain sizes (d₅₀), uniformity coefficients (C_u), and void ratios to obtain G_max. The BE results revealed that the G_max of calcareous sand increases slightly with increasing d₅₀ but decreases significantly with increasing C_u. A modified model of G_max incorporating the effects C_u and d₅₀ was therefore developed for calcareous sand. Moreover, microscopic observations of pore size distributions (PSD) obtained from nuclear magnetic resonance (NMR) tests were presented to demonstrate the effect of GSD on PSD and its correlation with G_max. The NMR results revealed that the interaggregate pore structure proportion and uniformity of the PSD decreased significantly with increasing C_u but increased slightly with increasing d₅₀. The underlying mechanism for the effect of GSD on G_max was related to its substantial impact on microstructure. The significant decrease in G_max with increasing C_u can be attributed to the remarkable reduction in the ratio of the interaggregate void ratio to the intraaggregate void ratio. Additionally, G_max was enhanced as the heterogeneity of the microporosity structure distribution decreased.

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We compute the mean square displacement (MSD) of intruders immersed in a freely cooling granular gas made up of smooth inelastic hard spheres. In general, intruders and particles of the granular gas are assumed to have different mechanical properties, implying that non-equipartition of energy must be accounted for in the computation of the diffusion coefficient D. In the hydrodynamic regime, the time decay of the granular temperature T of the cooling granular gas is known to be dictated by Haff’s law; the corresponding decay of the intruder’s collision frequency entails a time decrease of the diffusion coefficient D. Explicit knowledge of this time dependence allows us to determine the MSD by integrating the corresponding diffusion equation. As in previous studies of self-diffusion (intruders mechanically equivalent to gas particles) and the Brownian limit (intruder’s mass much larger than the grain’s mass), we find a logarithmic time dependence of the MSD as a consequence of Haff’s law. This dependence extends well beyond the two aforementioned cases, as it holds in all spatial dimensions for arbitrary values of the mechanical parameters of the system (masses and diameters of intruders and grains, as well as their coefficients of normal restitution). Our result for self-diffusion in a three-dimensional granular gas agrees qualitatively, but not quantitatively, with that recently obtained by Blumenfeld [arXiv: 2111.06260] in the framework of a random walk model. Beyond the logarithmic time growth, we find that the MSD depends on the mechanical system parameters in a highly complex way. We carry out a comprehensive analysis from which interesting features emerge, such a non-monotonic dependence of the MSD on the coefficients of normal restitution and on the intruder-grain mass ratio. To explain the observed behaviour, we analyze in detail the intruder’s random walk, consisting of ballistic displacements interrupted by anisotropic deflections caused by the collisions with the hard spheres. We also show that the MSD can be thought of as arising from an equivalent random walk with isotropic, uncorrelated steps. Finally, we derive some results for the MSD of an intruder inmersed in a driven granular gas and compare them with those obtained for the freely cooling case. In general, we find significant quantitative differences in the dependence of the scaled diffusion coefficient on the coefficient of normal restitution for the grain-grain collisions.

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