bluuebunny/crossref_metadata_2025 huggingface dataset:
Consititutive modeling of time-dependent flows in frictional suspensions
Authors: Cates Michael
Published: June 2021

This paper summarizes recent joint work towards a constitutive modelling framework for dense granular suspensions. The aim is to create a time-dependent, tensorial theory that can implement the physics described in steady state by the Wyart-Cates model. This model of shear thickening suspensions supposes that lubrication films break above a characteristic normal force so that frictional contact forces come into play: the resulting non-sliding constraints can be enough to rigidify a system that would flow freely at lower stresses [1]. Implementing this idea for time-dependent flows requires the introduction of new concepts including a configuration-dependent ‘jamming coordinate’, alongside a decomposition of the velocity gradient tensor into compressive and extensional components which then enter the evolution equation for particle contacts in distinct ways. The resulting approach [2, 3] is qualitatively successful in addressing (i) the collapse of stress during flow reversal in shear flow, and (ii) the ability of transverse oscillatory flows to unjam the system. However there is much work required to refine this approach towards quantitative accuracy, by incorporating more of the physics of contact evolution under flow as determined by close interrogation of particle-based simulations.
Energy propagation in 1D granular soft-stiff chain
Authors: Taghizadeh K. , Steeb H. , Luding S.
Published: June 2021

When a mechanical wave travels through a medium, its intensity diminishes with distance. The research focuses on the energy transfer with distance as well as across different wavenumbers, as the mechanical wave propagates. The diffusive characteristic of energy propagation has been discussed for one-dimensional chains composed of random, pre-stressed soft and stiff particles interacting through Hertzian repulsive forces, which can be solved analytically after linearization. The effect of soft-stiff ratio (disorder in property) on energy transfer across wavenumbers is examined using a standing sinusoidal wave initial condition (with a specific wavenumber). From the total energy signals in wavenumber space, as function of time, it is observed that stronger disorder leads to more rapid loss of energy of the signal and faster transfer of energy to other wavenumbers.
Strain dependent vorticity in sheared granular media
Authors: Wang Dong , Dijksman Joshua A. , Barés Jonathan , Zheng Hu
Published: June 2021

Displacement fields in sheared particle packings often display vortex-like structures that reveal essential features about the mechanical state of the collection of particles. There are several metrics to quantify these flow field features, yet extracting such quantitative metrics from flow field or particle tracking data involves making numerous choices on the time and length scales over which to average. Here we employ a much used experimental data set on sheared disk packings to explore how such arbitrary data mining choices affect the obtained results. We focus on calculating the strain dependent vorticity, as this metric is a differential method hence potentially sensitive to the way it is computed. We find that the total surface area with an absolute vorticity above a certain threshold approaches a plateau value as shear progresses. This plateau value exhibits a non-monotonic dependence on packing fraction. We also show which range of choices yields results that can support an analysis method independent, physical interpretation of the flow field data.
Influence of the particle shape on the impact force of lahar on an obstacle
Authors: Chehade Rime , Chevalier Bastien , Dedecker Fabian , Breul Pierre
Published: June 2021

Lahars represent natural phenomena that can generate severe damage in densely populated urban areas. The evaluation of pressures generated by these mass flows on constructions (buildings, infrastructure…) is crucial for civil protection and assessment of physical vulnerability. The existing tools to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the estimation of mechanical efforts. A discrete numerical model is developed for evaluating debris flow (DF) impact pressures at the local scale of one structure. The large-sized solid particles are modelled explicitly using Distinct Element Method (DEM) and the fine-grained solid particles are integrated in a fluid phase which generates two effects on the movement of particles, i.e. buoyancy and drag. Fluid velocity field and the fluid free surface are obtained from Computational Fluid Dynamics (CFD) code then imported in the DEM simulation in a one way coupling scheme. In this paper, the influence of particle shape on the impact forces generated on the obstacle is investigated: spherical particles and polygonal rigid blocks (r-blocks) are considered. The shape of the particle influences the contact surface and therefore the impact pressure. With an angular shape and several facets like r-blocks, the impact pressure on an obstacle is more important for a flow with the same characteristics.
The effect of boundary roughness on dense granular flows
Authors: Fazelpour Farnaz , Daniels Karen E.
Published: June 2021

In the field of granular rheology, an important open question is to understand the influence of boundary conditions on granular flows. We perform experiments in a quasi-2D annular shear cell subject to 6 different boundaries with controlled roughness/compliance. We characterize the granular slip at the boundaries to investigate which aspects of a dense granular flow can be controlled by the choice of boundary condition. Photoelastic techniques are implemented to measure the stress fields P(r) and τ(r) throughout the material. A full inverse-analysis of the fringes within each disk provides the vector force at each contact. This allows us to measure the continuum stress field by coarse-graining internal forces. We have observed that boundary roughness and compliance strongly controls the flow profile v(r) and shear rate profile γ˙(r). We also observed that boundary roughness and compliance play a significant role in the pressure profile P(r) and shear stress profile τ(r).
Multiplicity of states in Taylor-Couette flow of a dense granular gas
Authors: Gopan Nandu , Alam Meheboob
Published: June 2021

Molecular dynamics simulations with a purely repulsive Lennard-Jones potential and a normal damping force is used to simulate the granular flow in the annular region between two differentially-rotating cylinders, called the Taylor-Couette flow. The flow transition from the azimuthally-invariant Circular Couette flow (CCF) to the Taylor-vortex flow (TVF) is studied by increasing the rotation rate (ωi) of the inner cylinder, with the outer cylinder being kept stationary. Multiplicity of states, highlighting the hysteretic nature of the “CCF ↔ TVF” transition, is observed over a wide range of rotation rates. The onset of Taylor vortices is quantified in terms of the maximum radial velocity and the net circulation per vortex.
Mechanically driven active and passive grains as models for egress dynamics
Authors: Mohammadi Mahdieh , Harth Kirsten , Puzyrev Dmitry , Trittel Torsten , Hanselka Tina , Stannarius Ralf
Published: June 2021

Passages of people or cattle through narrow entrances or exits occur in manifold situations. They are difficult to study experimentally, because one has to carefully separate objective, physical parameters from subjective, individual motivations, manners and temperament. Mechanically excited physical model systems can help to discriminate some of these classes of parameters. We characterize active and passive particles of equal shape and mass on a vibrating plate and study their bottleneck passage dynamics. They show fundamentally different scaling behavior.
Robust experimental study of avalanche precursory events based on reproducible cycles of grain packing destabilizations
Authors: Oger Luc , Delannay Renaud , Le Gonidec Yves
Published: June 2021

Quasi-periodic collective displacements of grains at the free surface of a tilted grain packing constitute precursors of granular avalanches. Laboratory experiments are commonly performed by slowly tilting the packing from 0° to the maximal stability angle θA. In these conditions, the number of precursors is too small to assess reproducible and robust statistical analyses of the precursor activity. To go beyond this limitation, we have developed a specific experimental protocol consisting of tilting the packing with successive oscillation cycles. We use a high-resolution optical camera and process the images of the packing free surface to identify precursory events during many consecutive cycles of a single packing. We observe the same behavior for all half-cycles, forth and back: appearance of the first precursors after the same variation of inclination, exponential evolution of the weak surface activity for the first precursors and linear growth of stronger surface activity for the following ones. The experimental protocol provides both reproducible precursor measurements based on large sample statistical inferences and a quasi-stationary state after one full-cycle. This approach is very promising for highlighting the effects of external parameters, including humidity and packing geometry.
Coupling non-local rheology and volume of fluid (VOF) method: a finite volume method (FVM) implementation
Authors: Faroux Dorian , Washino Kimiaki , Tsuji Takuya , Tanaka Toshitsugu
Published: June 2021

Additional to a behavior switching between solid-like and liquid-like, dense granular flows also present propagating grain size-dependent effects also called non-local effects. Such behaviors cannot be efficiently modeled by standard rheologies such as µ(I)-rheology but have to be dealt with advanced non-local models. Unfortunately, these models are still new and cannot be used easily nor be used for various configurations. We propose in this work a FVM implementation of the recently popular NGF model coupled with the VOF method in order to both make non-local modeling more accessible to everyone and suitable not only for single-phase flows but also for two-phase flows. The proposed implementation has the advantage to be extremely straightforward and to only require a supplementary stabilization loop compared to the theoretical equations. We then applied our new framework to both single and two-phase flows for validation.
Experimental investigation of segregation of granular mixtures during heap formation
Authors: Gharat Sandip H.
Published: June 2021

The present study is on segregation of granular mixtures during heap formation in a quasi two-dimensional rectangular bin where binary mixture of a specified composition is poured intermittently into the auxiliary hopper and then allowed to pass through the gap (k = 10 mm) between the divider and the plate and finally settle on to the heap. The profiles of number fraction of big particles are plotted along the flow directions to study the segregation phenomena for surface profile. It shows that larger particles travel more distance and smaller particles settle near the pouring point for all cases studied.
The effect of rotation speed and mixer size on granular flow and mixing in bladed mixers
Authors: Herman Angga , Gan Jieqing , Yu Aibing
Published: June 2021

Bladed mixers are widely used in the industry for granular mixing. In the past decades, the mixing of particles in bladed mixers has been extensively investigated experimentally and numerically. Recently, GPU-based DEM has been employed to simulate various industrial-scale applications. This work aims to apply the GPU-based DEM to investigate the effect of rotation speed and mixer size on granular mixing in bladed mixers of different sizes. The simulation in a larger mixer revealed distinct particle flow patterns that well-describe the mixing mechanism which is difficult to observe in a smaller mixer. The Lacey’s mixing index curves revealed a delay in mixing as the mixer size increases. The mixing rate decreases as the mixer size increases and it can be improved with increasing rotation speed. The average particle velocity increase significantly with increasing rotation speed and mixer size.
Time scales and rheology of visco-cohesive granular flows
Authors: Radjai Farhang
Published: June 2021

In the presence of viscous and cohesive interactions between particles, a granular flow is governed by several characteristic time and stress scales that determine its rheological properties (shear stress, packing fraction, effective viscosities). In this paper, we revisit and extend the scaling arguments used previously for dry cohesionless granular flows and suspensions. We show that the rheology can be in principle described by a single dimensionless control parameter that includes all characteristic times. We also briefly present simulation results for 2D sheared suspensions and 3D wet granular flows where the effective friction coefficient and packing fraction are consistently described as functions of this unique control parameter.
A modified calculation of particle buoyant forces in vibro-fluidized beds
Authors: Zhang Zhixiong , Chu Xihua , Wang Yanran
Published: June 2021

Segregation of granular materials under vibration or flow conditions such as the Brazil nut effect has been well known, however, there is yet no consensus mechanisms to explain this phenomenon. This study attempts to investigate particle buoyant forces in the segregation process. To explain the difference of the segregation behavior for the large particle with different size, a modified calculation method of particle buoyant force is suggested for considering the effect of particle size ratio. A simple verification illustrates its validity.
DEM modeling of segregation and stratification in pouring heaps of bidispersed mixtures of rounded particles
Authors: Vuilloz Tristan , Cantor David , Ovalle Carlos
Published: June 2021

This study aims to assess and analyse the patterns of segregation and stratification in pouring heaps of granular mixtures composed by binary sized and uniformly shaped particles. We present 2D and 3D simulations which respectively build deposits of poured disks and spheres by means of a discrete-element approach known as contact dynamics (CD). In order to identify preferable conditions for segregation and stratification, we try several deposition scenarios varying the pouring flow rate, injection height, heap’s width and mass ratio between large and small grains in our binary samples. Although some authors assert that shape dispersity might not be necessary to obtain stratification, the phenomenon seems hard to seize with mono-shaped granular media as it appears to require a close control on pouring conditions. The introduction of our DEM models and statistical analysis intend to provide examples of what could constitute efficient numerical tools to study the remaining open problems related to heap segregation patterns prediction.
Computationally efficient approach to quantify 2D particle morphological descriptors
Authors: Kalyan N.S.S.P. , Kandasami Ramesh Kannan
Published: June 2021

The macro-scale behavior of granular materials is strongly influenced by grain kinematics. The mobility of the grains in turn is affected by grain morphology which needs to be comprehensively characterized. Initially, morphological descriptors were determined by manual processes which were tedious and cumbersome. But with the help of image processing techniques and computational geometry, this problem can be handled with ease. The long-established descriptors such as sphericity and roundness can be estimated by operating over the grain boundary obtained from 2D images of the particle. The main objective of this study is to quantify these descriptors in a computationally effective way. The roundness quantification in previous studies involves the sequence of processes such as removing the noise, corner identification and fitting circles. This paper details the necessary modifications to the quantification process required to reduce the cost of time for a single particle. Further, the influence of different smoothing techniques and a new corner identification method will be detailed.
Rheology of soft granular materials: uniaxial compression
Authors: Nezamabadi Saeid , Radjai Farhang , Mora Serge , Delenne Jean-Yves , Ghadiri Mojtaba
Published: June 2021

Soft granular materials are assemblies of highly deformable grains interacting via surface forces. The large grain deformations of these materials differ them from hard granular systems, in which, their behaviors are essentially governed by grain rearrangements. In this paper, we study the uniaxial compression of soft granular materials using a numerical approach based on the Material Point Method allowing for large grain deformations, coupled with the Contact Dynamics method for the treatment of unilateral frictional contacts between grains. Considering the neo-Hookean and elasto-plastic grains, the compaction of 2D soft granular packings is analyzed. We focus essentially on the evolution of the packing vertical stress as a function of the packing fraction and the predictive models are proposed.
Structural characteristics of ordered clusters in packs of ellipses
Authors: Jiang Xiaoyu , Matsushima Takashi , Blumenfeld Raphael
Published: June 2021

Structural characteristics of two-dimensional elliptic granular packs with various aspect ratios and intergranular friction coefficients were studied using the Discrete Element Method (DEM). Isotropic compaction from random unjammed state leads to a jammed state with polycrystals of orientationally ordered clusters (OOC). The OOCs were identified using a cluster labelling algorithm, based on the relative angle Δθ between the major axes of two contacting particles. The threshold value of Δθ was optimised to give the strongest correlation between OOCs and the force chain network. We found that the resulting OOC size distribution decays algebraically with an exponent of −2, independently of grain aspect ratio and material properties.
Self-assembly of self-propelled magnetic grains
Authors: Madrid Marcos A. , Irastorza Ramiro M. , Meyra Ariel G. , Carlevaro C. Manuel
Published: June 2021

In this work, we study bidisperse mixtures of self-propelled magnetic particles of different shapes via discrete element method simulations. We show how these particles self-assemble into clusters and how these clusters depend on the ratio of the mixture, the magnetic interaction, and the shape of the grains. It is found that the mix ratio of the system controls the cluster size. Besides, the intensity of the magnetic dipoles and the shape of the grains in the mixture rule the average number of neighbors in contact and the shape of the clusters. By varying the intensity of the interactions, globular, linear and branched clusters were obtained.
Particle dynamics in two-dimensional point-loaded granular media composed of circular or pentagonal grains
Authors: Kozlowski Ryan , Zheng Hu , Daniels Karen E. , Socolar Joshua E. S.
Published: June 2021

Granular packings exhibit significant changes in rheological and structural properties when the rotational symmetry of spherical or circular particles is broken. Here, we report on experiments exploring the differences in dynamics of a grain-scale intruder driven through a packing of either disks or pentagons, where the presence of edges and vertices on grains introduces the possibility of rotational constraints at edge-edge contacts. We observe that the intruder’s stick-slip dynamics are comparable between the disk packing near the frictional jamming fraction and the pentagonal packing at significantly lower packing fractions. We connect this stark contrast in packing fraction with the average speed and rotation fields of grains during slip events, finding that rotation of pentagons is limited and the flow of pentagonal grains is largely confined in front of the intruder, whereas disks rotate more on average and circulate around the intruder to fill the open channel behind it. Our results indicate that grain-scale rotation constraints significantly modify collective motion of grains on mesoscopic scales and correspondingly enhance resistance to penetration of a local intruder.
Effect of particle shape on particle breakage inside rotating cylinders
Authors: Orozco Luisa Fernanda , Delenne Jean-Yves , Sornay Philippe , Radjai Farhang
Published: June 2021

We study the influence of particle shape on the evolution of particle breakage process taking place inside rotating cylinders. Extensive particle dynamics simulations taking into account the dynamics of the granular flow, particle breakage, and polygonal particle shapes were carried out. We find that the rate of particle breakage is faster in samples composed of initially rounder particles. The analysis of the active flowing layer thickness suggests that for samples composed of rounder particles a relatively lower dilatancy and higher connectivity lead to a less curved free surface profile. As a result, rounder particles rolling down the free surface have a higher mobility and thus higher velocities. In consequence, the faster breakage observed for rounder initial particles is due to the larger particles kinetic energy at the toe of the flow.
Strength and energy consumption of inherently anisotropic rocks at failure
Authors: Cantor David , Ovalle Carlos , Azéma Emilien
Published: June 2021

Using a discrete-element approach and a bonding interaction law, we model and test crushable inherently anisotropic structures reminiscent of the layering found in sedimentary and metamorphic rocks. By systematically modifying the level of inherent anisotropy, we characterize the evolution of the failure strength of circular rock samples discretized using a modified Voronoi tesselation under diametral point loading at different orientations relative to the sample’s layers. We characterize the failure strength, which can dramatically increase as the loading becomes orthogonal to the rock layers. We also describe the evolution of the macroscopic failure modes as a function of the loading orientation and the energy consumption at fissuring. Our simulation strategy let us conclude that the length of bonds between Voronoi cells controls the energy being consumed in fissuring the rock sample, although failure modes and strength are considerably changing. We end up this work showing that the microstructure is largely affected by the level of inherent anisotropy and loading orientation.
Energy dissipation due to breakage during confined compaction of granular materials
Authors: Kuwik Brett , Hurley Ryan C.
Published: June 2021

The dissipation of energy during the compaction of granular materials was studied by performing confined drop tower experiments on Ottawa sand. Energy dissipated due to breakage was quantified by evaluating the creation of new surfaces at varying drop heights. Post-compaction particle size distributions (PSD) were measured and the amount of breakage was quantified by the position of the current PSD relative to the pre-compaction and ultimate PSD. Our observations revealed that the percentage of input energy dissipated due to breakage accounted for less than 0.5% of the total energy budget and was a constant proportion regardless of the total energy applied to the system. We also evaluated the effects of die wall friction by measuring post-compaction PSD in various positions within the sample.
Oedometric compression of a granular material: computation of energies involved during breakage with a discrete element modelling
Authors: Nader François , Silvani Claire , Djeran-Maigre Irini
Published: June 2021

A numerical model able to simulate the grain breakage with the discrete element method, using the “Non-Smooth Contact Dynamics” is presented. The model reproduces 3D grains having complex shapes and is tested in single grain and in oedometric compressions. Numerical simulations are then carried out to evaluate the different energies active during breakage (surface creation and redistribution energies). The surface creation energy is estimated. Results are closed to the ones found in the literature.
The effect of liquid viscosity on sliding friction coefficient of wet granular materials
Authors: Aliasgari Mojgan , Maleki-Jirsaraei Nahid , Rouhani Shahin
Published: June 2021

The mechanical properties of wet granular materials are different to those of dry granular matter. If the wet granular medium is not completely saturated, the capillary bridges form and the surface tension changes the elastic properties of the medium. We studied the sliding friction of a sledge over wet granular media and found that surface tension and viscosity have crucial effect on the dynamic friction coefficient of the wet granular media. Higher the viscosity of the interstitial liquid results in higher dynamic friction coefficient. Furthermore, viscous interstitial liquid shows time dependency behaviour. Viscosity makes the friction coefficient to increase since causes more the energy loss.
Erosion of a cohesive granular material by an impinging turbulent jet
Authors: Gong Mingze , Azadi Sivar , Gans Adrien , Gondret Philippe , Sauret Alban
Published: June 2021

The erosion of a cohesive soil by an impinging turbulent jet is observed, for instance, during the landing of a spacecraft or involved in the so-called jet erosion test. To provide a quantitative understanding of this situation for cohesive soils, we perform experiments using a model cohesion controlled granular material that allows us to finely tune the cohesion between particles while keeping the other properties constant. We investigate the response of this cohesive granular bed when subjected to an impinging normal turbulent jet. We characterize experimentally the effects of the cohesion on the erosion threshold and the development of the crater. We demonstrate that the results can be rationalized by introducing a cohesive Shields number that accounts for the inter-particles cohesion force. The results of our experiments highlight the crucial role of cohesion in erosion processes.
X-ray tomographies of a water-sensitive granular material (couscous) exposed to high relative humidity: an experimental study
Authors: Vego Ilija , Tengattini Alessandro , Andò Edward , Lenoir Nicolas , Viggiani Gioacchino
Published: June 2021

Several granular materials are hydro-sensitive, i.e., contact with water severely affects their morphology and mechanical behaviour. To this broad class belong a series of materials of great relevance for the pharmaceutical and food industry. Former studies have been conducted on the effect that moisture or humidity have on granular flow or on the individual grain mechanical response, but the processes occurring at the microlevel and their influence on the overall granular packing behaviour is yet to be fully understood. This study presents an experimental investigation of the response of a water-sensitive material (couscous) exposed to high relative humidity (97%). 4D (3D plus time) x-ray tomographies were acquired in operando. A data treatment approach based on Dicrete Digital Image Correlation (dDIC) is developed and detailed here. This allows for following each individual grain throughout the 4+ days of the humidification test. This, in turn, allows the study of the interand intra-granular strain, which is particularly pronounced in water sensitive materials. The expansion of the individual grains also severely affects the contact network, whose evolving properties are here analysed and correlated to the macroscopic (sample-scale) deformation.
Solute transport by suspended buoyant particles
Authors: Colecchio Iván , Arze Natalia , Flores Georgina , Quijandria Ana , Boschan Alejandro
Published: June 2021

The transport of a colouring solute, driven by the buoyant displacement of microscopic suspended particles, and in the absence of net flow, is studied experimentally in a Hele Shaw cell. Initially, a sharp interface between a transparent fluid without particles and an underlying coloured suspension is obtained. From this situation, the suspended particles rise, carrying the solute in the form of a fingering instability across the interface, where a light transmission technique is used to measure the local solute concentration. This one attains an asymptotic value that increases with the solid fraction ϕ of suspended particles, and decreases with the distance to the interface. The solute mass discharge also increases with ϕ, always being relatively small (< 3%). The onset and development of the instability as the mechanism driving the transport of the solute is discussed.
Dynamics of undeforming regions in the lead up to failure: jumping scales from lab to field
Authors: Tordesillas Antoinette , Zhou Shuo , Campbell Lachlan , Bellett Pat
Published: June 2021

Knowledge transfer from micromechanics of granular media failure to geohazard forecasting and mitigation has been slow. But in the face of a rapidly expanding data infrastructure on the motion of individual grains for laboratory samples – and ground motion data at the field scale – opportunities to accelerate this knowledge transfer are emerging. In particular, such data assets coupled with data-driven approaches enable ‘new eyes’ to re-examine granular failure. To this end, effective strategies that can jump scales from bench to field are urgently needed. Here we demonstrate one strategy that focusses on the study of deformation patterns in the precursory failure regime using kinematic data. Unlike previous studies which focus on regions of high strains, here we probe the development and evolution of near-undeforming regions through the lens of explosive percolation. We find a common dynamical signature in which undeforming regions, which are initially transient in the precursory failure regime, become persistent from the time of imminent failure. We demonstrate the robustness of these findings for data on individual grain motions in a classical laboratory test and ground motion in two real landslides at vastly different scales.
Mesoscale inertial transition in granular materials
Authors: Clerc Adriane , Wautier Antoine , Bonelli Stéphane , Nicot François
Published: June 2021

Granular assemblies can experience complex failure patterns along a given loading path, with a distribution of ephemeral inertial events marked by local outbursts in kinetic energy. However, investigating such mechanisms appears to be necessary to understand how a certain failure mode develops in a granular material. Using a discrete element method, this study highlights several microstructure reorganizations before the specimen reaches a proper failure state. Meso structures have proven to be efficient to understand the elementary mechanisms responsible for these outbursts in kinetic energy. Strain–like and stress-like quantities are thus defined at a mesoscale and they are used to characterize the nucleation and propagation of these local microstructural events.
Failure in granular materials based on acoustic tensor: a numerical analysis
Authors: Recchia Giuseppina , Cheng Hongyang , Magnanimo Vanessa , La Ragione Luigi
Published: June 2021

We investigate localization in granular material with the support of numerical simulations based upon DEM (Distinct Element Method). Localization is associated with a discontinuity in a component of the incremental strain over a plane surface through the condition of the determinant of the acoustic tensor to be zero. DEM simulations are carried out on an aggregate of elastic frictional spheres, initially isotropically compressed and then sheared at constant pressure p0. The components of the stiffness tensor are evaluated numerically in stressed states along the triaxial test and employed to evaluate the acoustic tensor in order to predict localization. This occurs in the pre-peak region, where the aggregate hardens under the circumstance to be incrementally frictionless: it is a regime in which the tangential force does not change as the deformation proceedes and, consequently, the deviatoric stress varies only with the normal component of the contact force.
Deformations of a 2D Elastica under a random distribution of normal loads
Authors: Trejo Miguel , Lazarus Arnaud , Vandembroucq Damien , Kolb Evelyne
Published: June 2021

We describe the deformations of a 2D elastic structure (beam, rod or filament) subjected to randomly distributed local orthogonal forces. The fiber is in quasistatic equilibrium condition when a given force distribution is acting on it. To analyze the effects of force fluctuations on the observed configurations, we study the behavior of the bending moment at the origin and the filament curvature, as a function of nominal values of the local mean forces and small, moderate and large fluctuations around them.
Toward a large-scale particle-based parallel simulator of Aeolian sand transport, including a model for mobile sand availability
Authors: Kamath Sandesh , Parteli Eric
Published: June 2021

We develop a numerical tool for particle-based simulations of Aeolian sand transport. Our model combines a Discrete-Element-Method for the sand particles with an efficient hydrodynamic description of the average turbulent horizontal wind velocity field over the granular bed, which has been developed in previous work and accounts for the two-way coupling of the granular and fluid phases. However, here we implement our model within the open source library LAMMPS for granular massively parallel simulations and incorporate a new grid coarsening scheme for the wind model. We show that our model quantitatively reproduces observed values of the steady-state (saturated) sand flux under various flow conditions. Furthermore, we model different conditions of mobile sand availability and find a strong dependence of the sand flux on this availability.
DEM modelling of 3D polyhedra with applications to gabion rockfall barriers
Authors: Knowles Jeffrey , Ma Yifei , Evans T. Matthew
Published: June 2021

Rock particle shape plays a crucial role in shear resistance and energy consumption during transient loads. The dynamics of such granular materials are complex and cannot be properly described using closed-form solutions when the problem involves more than a few particles. Thus, for the sake of computational efficiency, it is common practice to implement simplified numerical models that involve a limited number of particle interactions. In this study, a novel approach is used to capture realistic particle shapes while maintaining a relatively high simulation efficiency. The geometry of the particles is determined by a Delaunay triangulation which operates on a set of vertices and returns the corresponding network of facets and grid connections. Inertial and material properties are assigned to the rock prototype which are representative of realistic gravel particles. The algorithm is validated by performing a series of numerical simulations for various particle configurations, demonstrating that mass and momentum are conserved. A potential application of this work is related to rockfall barriers and their response to rigid boulder impacts. This innovative model, based on the discrete element method, is shown to be capable of simulating rock particles with realistic shapes and complex physical interactions.
DEM modelling of swelling of grains
Authors: Braile Domenica , Hare Colin , Wu Chuan-Yu
Published: June 2021

Swelling of grains due to water absorption is ubiquitous in many natural materials and industrial products. Hence, a thorough understanding of grain swelling is of great scientific importance. An experimental investigation can only provide limited information, whereas great insight could be gained from numerical modelling, rigorous numerical models for describing particle swelling are essential. Thus, the objective of this study is to develop and validate a discrete element method (DEM) model for swelling of grains. A first order kinetic model was introduced to describe the swelling of a single grain, and subsequently implemented into the DEM code LIGGGHTS. Model validation was performed by comparing the time evolution of the expansion of a packed bed made of super absorbent polymer (SAP) particles obtained numerically and experimentally. It was demonstrated that the developed model can accurately predict the bed expansion. The validated model was then used to investigate the effect of material properties on the swelling behaviour using rice and SAP as the model materials. It is shown that the swelling depends significantly on material properties, as expected; the expansion of the powder bed made of rice is much lower than that of SAP. The developed model could be further advanced to study consequences of swelling phenomena in granular materials, such as segregation and heat generation.
DEM based investigation of powder packing in 3D printing of pharmaceutical tablets
Authors: Sen Koyel , Mehta Tanu , W.K.Ma Anson , Chaudhuri Bodhisattwa
Published: June 2021

3D printing is emerging as one of the most promising methods to manufacture Pharmaceutical dosage forms as it offers multiple advantages such as personalization of dosage forms, polypill, fabrication of complex dosage forms etc. 3D printing came into existence in 1980s but its use was extended recently to pharmaceutical industry along with the approval of first 3D printed tablet Spritam by FDA in 2015. Spritam was manufactured by Aprecia pharmaceuticals using binder jetting technology. Binder jet 3D printing involves a hopper for powder discharge and printheads for ink jetting. The properties of tablets are highly dependent upon the discharge quality of powder mixture from the hopper and jetting of the ink/binder solution from the printhead nozzle. In this study, numerical models were developed using Discrete element method (DEM) to gain better understanding of the binder jet 3D printing process. The DEM modeling of hopper discharge was performed using in-house DEM code to study the effect of raw material attributes such as powder bed packing density (i.e. particle size, particle density etc) on the printing process, especially during powder bed preparation. This DEM model was further validated experimentally, and the model demonstrated good agreement with experimental results.
Acoustic signals of rotating drums generated based on DEM simulations
Authors: Li Yaoyu , Bao Jie , Yang Runyu
Published: June 2021

Acoustic emission (AE) or vibration signal has been applied in detecting operations of grinding mills in many industries. This paper proposes an approach to generate AE signals based on the particle-wall impacts. Through a combination of multi-mode vibrations and the calibration of the key parameters, the model was able to reproduce experimental data. The AE model was then implemented into a discrete element method (DEM) modelling of particle flow in a rotating mill. The AE signals of the mill under different filling levels and rotation speeds were generated and analysed, mainly focusing on the frequency and magnitude of each vibration mode. The link between the AE signals and the particle-wall impact energy was explored.
Dilation as a precursor in a continuous granular fault
Authors: Levy dit Vehel Victor , Hatano Takahiro , Vanel Loïc , Måløy Knut Jørgen , Ramos Osvanny
Published: June 2021

We analyze the dilation of the system in a cylindrical granular fault consisting of one single layer of disks submitted to both normal pressure and continuous and slow shear, which results in intermittent and sudden energy release events that reproduce the main laws of seismicity. The dilation of the system can be separated into two parts: a smooth increase of dilation, plus sudden changes both contracting and dilating the medium, which are correlated to abrupt jumps -both positive and negative- in the measured resisting torque. We explain the four possible (and existing) general scenarios combining those two variables: dilation jumps and torque jumps, thanks to the assumption of an optimal local angle in the direction of force chains, and each reorganization of the structure as a replacement of the force chain holding most of the applied stress. The average rate of increase of global dilation varies monotonically with the size of the energy release event, making dilation a plausible candidate to predict catastrophic events in such earthquake-like systems.
Visualizing how zirconia ceramics toughen under shock
Authors: Bandari Anashe
Published: June 2021

A direct observation of the relationship between zirconia ceramics’ toughening behavior and their structure transformations helps better understand their failure modes for crack resistant applications.
Cydalima perspectalis (box tree moth).
Authors: Kenis Marc , Leuthardt Florine , Li Hong-mei
Published: June 2021

The box tree moth, Cydalima perspectalis, is native to East Asia (Inoue et al., 1982). It was first recorded in Europe in 2007, in southwest Germany and the Netherlands (Krüger, 2008; Straten and Muus, 2010). Since then it has been recorded in many other European countries, and climate models predict further spread of the species in Europe, invading most areas except for Northern Fenno-Scandinavia, Northern Scotland and high mountain regions (Nacambo et al., 2014). In the newly invaded regions, C. perspectalis larvae feed on the leaves of box trees, Buxus spp., resulting in defoliation, which can kill the trees. The most significant damage, however, can be from the larvae attacking the bark of box trees causing the trees to dry out and die. Besides cultural and economic effects, the most serious threat from C. perspectalis is on the natural Buxus populations (Kenis et al., 2013). The species is easily introduced accidentally with its host plant, which is extensively traded over Europe and therefore presents a serious threat (Leuthardt et al., 2010; Straten and Muus, 2010).
Uraba lugens (eucalypt leaf skeletonizer).
Authors: Berndt Lisa
Published: June 2021

U. lugens was first considered a serious pest of natural eucalypt forests in Western Australia in 1983 when the first severe outbreak occurred there (Strelein, 1988). Prior to that it was widely known as a pest of eucalypt forests in eastern Australia (Campbell, 1962; Harris, 1974). As these natural forests are or were managed for timber production, it is considered an economically important pest in its native range. Damage to amenity trees is also a common problem, although few trees are killed by this defoliation (Anonymous, 1979).U. lugens was recorded as invasive in New Zealand by Crabtree (1997). In its exotic range it has become a significant pest of amenity trees, particularly Lophestemon confertus (Australian brush box), which is commonly planted in some parts of Auckland City (Kriticos et al., 2007).
Beta‐alanine did not improve high‐intensity performance throughout simulated road cycling
Authors: Perim Pedro , Gobbi Nathan , Duarte Breno , de Oliveira Luana Farias , Costa Luiz Augusto Riani , Sale Craig , Gualano Bruno , Dolan Eimear , Saunders Bryan
Published: June 2021

This study investigated the effect of beta‐alanine supplementation on short‐duration sprints and final 4‐km simulated uphill cycling time‐trial performance during a comprehensive and novel exercise protocol representative of the demands of road‐race cycling, and determined if changes were related to increases in muscle carnosine content. Seventeen cyclists (age 38 ± 9 y, height 1.76 ± 0.07 m, body mass 71.4 ± 8.8 kg, V̇O2max 52.4 ± 8.3 ml·kg−1·min−1) participated in this placebo‐controlled, double‐blind study. Cyclists undertook a prolonged intermittent cycling protocol lasting 125 min, with a 10‐s sprint every 20 min, finishing with a 4‐km time‐trial at 5% simulated incline. Participants completed two familiarization sessions, and two main sessions, one pre‐supplementation and one post‐supplementation following 28 days of 6.4 g·day−1 of beta‐alanine (N=11) or placebo (N=6; maltodextrin). Muscle biopsies obtained pre‐ and post‐supplementation were analysed for muscle carnosine content. There were no main effects on sprint performance throughout the intermittent cycling test (all P>0.05). There was no group (P=0.69), time (P=0.50) or group x time interaction (P=0.26) on time‐to‐complete the 4‐km time‐trial. Time‐to‐completion did not change from pre‐ to post‐supplementation for BA (−19.2 ± 45.6 s, P=0.43) or PL (+2.8 ± 31.6 s, P=0.99). Beta‐alanine supplementation increased muscle carnosine content from pre‐ to post‐supplementation (+9.4 ± 4.0 mmol·kg−1dm; P<0.0001) but was not related to performance changes (r=0.320, P=0.37). Chronic beta‐alanine supplementation increased muscle carnosine content but did not improve short‐duration sprint performance throughout simulated road race cycling, nor 4‐km uphill time‐trial performance conducted at the end of this cycling test.

Highlights

Performance during prolonged cycling events often depends on the ability to maintain an increased power output during higher intensity periods. Thus, cyclists are likely heavily dependent on their ability to resist fatigue during these periods of high‐intensity activity.

Meta‐analytical data show beta‐alanine to be an effective supplement to improve exercise outcomes, but little work exists on its efficacy during dynamic actions that are common during prolonged cycling.

Beta‐alanine supplementation increased muscle carnosine content but did not generate improvements in the performance of high‐intensity cycling (10‐s sprints or 4‐km uphill time‐trial) during a simulated road race cycling protocol.

These data suggest that short duration sprints (≤10 s) and longer duration (>10 min) high‐intensity activity throughout endurance cycling may not be improved with beta‐alanine supplementation despite increases in muscle carnosine content.
New insights into correlated materials in the time domain—combining far-infrared excitation with x-ray probes at cryogenic temperatures
Authors: Mankowsky Roman , Sander Mathias , Zerdane Serhane , Vonka Jakub , Bartkowiak Marek , Deng Yunpei , Winkler Rafael , Giorgianni Flavio , Matmon Guy , Gerber Simon , Beaud Paul , Lemke Henrik Till
Published: June 2021

Modern techniques for the investigation of correlated materials in the time domain combine selective excitation in the THz frequency range with selective probing of coupled structural, electronic and magnetic degrees of freedom using x-ray scattering techniques. Cryogenic sample temperatures are commonly required to prevent thermal occupation of the low energy modes and to access relevant material ground states. Here, we present a chamber optimized for high-field THz excitation and (resonant) x-ray diffraction at sample temperatures between 5 and 500 K. Directly connected to the beamline vacuum and featuring both a Beryllium window and an in-vacuum detector, the chamber covers the full (2–12.7) keV energy range of the femtosecond x-ray pulses available at the Bernina endstation of the SwissFEL free electron laser. Successful commissioning experiments made use of the energy tunability to selectively track the dynamics of the structural, magnetic and orbital order of Ca2RuO4 and Tb2Ti2O7 at the Ru (2.96 keV) and Tb (7.55 keV) L-edges, respectively. THz field amplitudes up to 1.12 MV cm−1 peak field were demonstrated and used to excite the samples at temperatures as low as 5 K.
Three easy ways to the Hawking temperature
Authors: Pinochet Jorge
Published: June 2021

In this work, three heuristic derivations of the Hawking temperature are presented. The main characteristic of these derivations is their extreme simplicity, which makes them easily accessible to a wide and diverse audience.