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A detailed study on similarities and differences of monodisperse dry and wet dense granular flow down on rough and smooth inclined planes was carried out by discrete element method simulations. Despite implementing a minimal model for capillary bridge cohesive force, all leading regimes of a granular flow, i.e. low-dissipation, high-dissipation, and oscillatory flow, can be developed in wet granular flow, similar to what we knew in the dry one. A smooth and rough based inclined planes as well as different inclination angels were used as parameters to create various flow regimes in dry and wet granular flow. In the oscillatory flow regime, the frequency of velocity profile variation is lower than that of the dry one. The velocity profile of the wet system in the low-dissipation flow regime exhibits an abrupt slope change at shear band bottom. As a measure of particle velocity fluctuations we have studied granular temperature in layers parallel to the inclined base. We found the temperature profile is increasing from the top to bottom, which means the shear band can be considered as a frozen region. By calculation of Radial Distribution Function (RDF) and using the adaptive Common Neighbor Analysis (a-CNA), the evolution of ordered/disordered structures in both dry and wet models is studied. In a wet system in the low-dissipation regime, the shear band exhibits frozen polycrystalline structure and in the bottom slice, in spite of having layered flow in the scale of one granule, we have a low fraction of crystallization. This study gives insightful key differences between wet and dry monodisperese granular flows, specifically the appearance of ordering and presence of crystalizations in different parts of high and low dissipation flow.Graphic abstract

The two-fluid model (TFM) coupled with the kinetic theory of granular flow (KTGF) has gradually been used for modeling dense granular flows and mixing in rotating drums in recent years. In the present paper, a review is made from the perspective of model development and model application. It is found that several frictional viscosity models were proposed to consider the enduring contact of dense particles for the specific rotating studied, but there is still a lack of a universal model. The model is validated by various experiment results and the applicability is indicated. The model is used for investigating dynamic particle flow, and the effects of the parameters on granular flow behavior and flight design. Although the model theoretically has the advantage of saving computing resources, and is suitable for industrial-scale modeling, it is found that the model is used for the research of laboratory-scale rotating drums (diameter less than 0.5 m) and has not been used for industrial rotating drum analysis. Moreover, recommendations for future work are provided.

In this paper, we give an overview of the results established in Alonso (http://arxiv.org/org/abs/2008.05173, 2020) which provides the first rigorous derivation of hydrodynamic equations from the Boltzmann equation for inelastic hard spheres in 3D. In particular, we obtain a new system of hydrodynamic equations describing granular flows and prove existence of classical solutions to the aforementioned system. One of the main issue is to identify the correct relation between the restitution coefficient (which quantifies the rate of energy loss at the microscopic level) and the Knudsen number which allows us to obtain non trivial hydrodynamic behavior. In such a regime, we construct strong solutions to the inelastic Boltzmann equation, near thermal equilibrium whose role is played by the so-called homogeneous cooling state. We prove then the uniform exponential stability with respect to the Knudsen number of such solutions, using a spectral analysis of the linearized problem combined with technical a priori nonlinear estimates. Finally, we prove that such solutions converge, in a specific weak sense, towards some hydrodynamic limit that depends on time and space variables only through macroscopic quantities that satisfy a suitable modification of the incompressible Navier–Stokes–Fourier system.

In this paper, a model is presented for modeling saturated granular‐liquid free‐surface flows, in which the volume‐averaged mixture bulk velocity is employed to derive the balance equations for the mass and momentum of mixture flow. Additionally, an evolution equation of the slip velocity between granular‐and liquid constituents is derived to describe the separation between these constituents. The frictional‐collisional constitutive relation for granular‐constituent is employed to determine the stress due to particles interaction. The governing equations for mixture flows are numerically solved by a finite difference two‐step projection method. The volume of fluid (VOF) method is employed to track the free surface of the mixture flow in the present numerical model. Good agreements between numerical results and experimental data are observed by modeling the dam‐break process of granular‐liquid mixture flow, dam‐break waves over the saturated erodible beds and surge waves induced by submarine landslides along an inclined plane. Furthermore, the difference between the volume‐averaged mixture bulk velocity and mass‐averaged mixture bulk velocity is found to vary as the instinct density ratio of granular‐constituent and liquid‐constituent and the volumetric concentration ns of the granular‐constituent, and the evolution in the slip velocity during the process of the settlement of sediments is numerically analyzed. Plain Language Summary In this study, we developed a mathematical model for saturated granular‐liquid free‐surface flows, in which the volume‐averaged mixture bulk velocity is employed to describe the balance equations for the mass and momentum of the mixture flow of granular‐liquid flows and the evolution equation of the slip velocity between granular‐constituent and liquid‐constituent is derived to describe the separation between the granular‐constituent and liquid‐constituent. The stress due to the interaction of particles is determined based on the frictional‐collisional constitutive relations. The finite difference two‐step projection method is employed to numerically solve the governing equations and the volume of fluid (VOF) method is employed to track the free surface of the mixture flow in the present numerical model. Good agreements between numerical results and experimental data are observed. Finally, the role of slip velocity on the dynamics of granular‐liquid flows is analyzed. Key Points A novel mathematical model for saturated granular‐liquid free‐surface flows is presented Good agreements between the numerical results and experimental data are observed The evolution of slip velocity plays a pivotal role in the dynamics of granular‐liquid flows

Characterization and prediction of the “flowability” of powders are of paramount importance in many industries. However, our understanding of the flow of powders like cement or flour is sparse compared to the flow of coarse, granular media like sand. The main difficulty arises because of the presence of adhesive forces between the grains, preventing smooth and continuous flows. Several tests are used in industrial contexts to probe and quantify the “flowability” of powders. However, they remain empirical and would benefit from a detailed study of the physics controlling flow dynamics. Here, we attempt to fill the gap by performing intensive discrete numerical simulations of cohesive grains flowing down an inclined plane. We show that, contrary to what is commonly perceived, the cohesive nature of the flow is not entirely controlled by the interparticle adhesion, but that stiffness and inelasticity of the grains also play a significant role. For the same adhesion, stiffer and less dissipative grains yield a less cohesive flow. This observation is rationalized by introducing the concept of a dynamic, “effective” adhesive force, a single parameter, which combines the effects of adhesion, elasticity, and dissipation. Based on this concept, a rheological description of the flow is proposed for the cohesive grains. Our results elucidate the physics controlling the flow of cohesive granular materials, which may help in designing new approaches to characterize the “flowability” of powders.

Granular flow out of a silo is studied experimentally and numerically. The time evolution of the discharge rate as well as the normal force (apparent weight) at the bottom of the container is monitored. We show that particle stiffness has a strong effect on the qualitative features of silo discharge. For deformable grains with a Young modulus of about Y m ≈ 40 kPa in a silo with basal pressure of the order of 4 kPa, lowering the friction coefficient leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, it decreases during the discharge process. For hard grains with a Young modulus of about Y m ≈ 500 MPa the flow rate is much less sensitive to the value of the friction coefficient. Using DEM data combined with a coarse-graining methodology allows us to compute all the relevant macroscopic fields, namely, linear momentum, density and stress tensors. The observed difference in the discharge in the low friction limit is connected to a strong difference in the pressure field: while for hard grains Janssen-screening is effective, leading to high vertical stress near the silo wall and small pressure above the orifice region, for deformable grains the pressure above the orifice is larger and gradually decreases during the discharge process. We have analyzed the momentum balance in the region of the orifice (near the location of the outlet) for the case of soft particles with low friction coefficient, and proposed a phenomenological formulation that predicts the linear decrease of the flow rate with decreasing filling height.

The axial segregation of granular and particulate media is a well-known but little-understood phenomenon with direct relevance to various natural and industrial processes. Over the past decades, many attempts have been made to understand this phenomenon, resulting in a significant number of proposed mechanisms, none of which can provide a full and universally applicable explanation. In this paper, we show that several mechanisms can be simultaneously active within a single system, and that by considering all relevant mechanisms, it is possible to understand and explain a system's segregative behaviours over a wider range of parameter space than is possible by considering any one, single process. We explore the interrelation and competition between the individual mechanisms present within a given system and demonstrate that by understanding these interactions, we can predict and even, through carefully designed systems, control their behaviour. In particular, we demonstrate that it is possible to deliberately direct segregation, allowing an arbitrary number of pre-determined segregation patterns to be induced in a system. We also illustrate a manner in which the competition between two opposing segregation mechanisms may be exploited in order to enhance the mixing of two dissimilar species of particle-a much sought after ability.

Granular flows of 200 μm particles and the pile formation in a flat-bottomed hopper and bin in the presence of air and in a vacuum were predicted based on three-dimensional numerically empirical constitutive relations using Smoothed Particle Hydrodynamics and Computational Fluid Dynamics methods. The constitutive relations for the strain rate independent stress have been obtained as the functions of the Almansi strain including the large deformation by the same method as Yuu et al. [1]. The constitutive relations cover the elastic and the plastic regions including the flow state and represent the friction mechanism of granular material. We considered the effect of air on the granular flow and pile by the two-way coupling method. The granular flow patterns, the shapes of piles and the granular flow rates in the evolution are compared with experimental data measured under the same conditions. There was good agreement between these results, which suggests that the constitutive relations and the simulation method would be applicable for predicting granular flows and pile formation with complex geometry including free surface geometry. We describe the mechanisms by which the air decreases the granular flow rate and forms the convergence granular flow below the hopper outlet.

The smoothed particle hydrodynamics (SPH) method was recently extended to simulate granular materials by the authors and demonstrated to be a powerful continuum numerical method to deal with the post-flow behaviour of granular materials. However, most existing SPH simulations of granular flows suffer from significant stress oscillation during the post-failure process, despite the use of an artificial viscosity to damp out stress fluctuation. In this paper, a new SPH approach combining viscous damping with stress/strain regularisation is proposed for simulations of granular flows. It is shown that the proposed SPH algorithm can improve the overall accuracy of the SPH performance by accurately predicting the smooth stress distribution during the post-failure process. It can also effectively remove the stress oscillation issue in the standard SPH model without having to use the standard SPH artificial viscosity that requires unphysical parameters. The predictions by the proposed SPH approach show very good agreement with experimental and numerical results reported in the literature. This suggests that the proposed method could be considered as a promising continuum alternative for simulations of granular flows.

The time evolution of silo discharge is investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirm the widely known typical behavior with constant discharge rate (except for initial and final transients). For elongated particles with aspect ratios between 2 ⩽ L / d ⩽ 6.1, we find a peculiar flow rate increase for larger orifices before the end of the discharge process. While the flow field is practically homogeneous for spherical grains, it has strong gradients for elongated particles with a fast-flowing region in the middle of the silo surrounded by a stagnant zone. For large enough orifice sizes, the flow rate increase is connected with a suppression of the stagnant zone, resulting in an increase in both the packing fraction and flow velocity near the silo outlet within a certain parameter range.