Granular Systems Research Articles

Modeling and analysis for landing airbag–lunar soil interaction using a CPU–GPU-based FMBD-DEM computational framework

To investigate the landing performance of airbag cushion systems on the discrete lunar soil, this study proposes a novel modeling approach that couples multi-physics fields, including the flexible multibody systems (FMBS), the granular systems, and the gas effects. In this model, the GPU parallelism technique is utilized to address challenges arising from the large number of particles and complex contact detection involving large deformation and overall motions, and an efficient contact implementation between finite elements and particles is introduced. Additionally, contact modeling between airbags is realized using master–slave techniques. Then, a modified conventional serial staggered procedure is employed to couple flexible multibody dynamics (FMBD) and discrete element method (DEM) modules. Finally, a series of numerical examples is presented to validate the proposed dynamic model. The validated model successfully simulates the interaction between landing airbags and lunar soil. Subsequent parametric analysis contributes to the design optimization of the landing system. Moreover, discussions regarding computational efficiency illustrate that the proposed CPU–GPU-based FMBD-DEM is advantageous in engineering scenarios involving flexible components and large-scale granular systems.

Statistics of a granular cluster ensemble at a liquid-solid-like phase transition.

We report on the construction of a granular network of particles to study the formation, evolution, and statistical properties of clusters of particles developing at the vicinity of a liquid-solid-like phase transition within a vertically vibrated quasi-two-dimensional granular system. Using the data of particle positions and local order from Castillo etal. [G. Castillo, N. Mujica, and R. Soto, Phys. Rev. Lett. 109, 095701 (2012)0031-900710.1103/PhysRevLett.109.095701], we extract granular clusters taken as communities of the granular network via modularity optimization. Each one of these communities is a patch of particles with a very well defined local orientational order embedded within an array of other patches forming a complex cluster network. The distributions of cluster sizes and lifespans for the cluster network depend on the distance to the liquid-solid-like phase transition of the quasi-two-dimensional granular system. Specifically, the cluster size distribution displays a scale-invariant behavior for at least a decade in cluster sizes, while cluster lifespans grow monotonically with each cluster size. We believe this systematic community analysis for clustering in granular systems can help to study and understand the spatiotemporal evolution of mesoscale structures in systems displaying out-of-equilibrium phase transitions.

Granular vibration pumping system for handling and characterizing particulate materials.

A unique setup for a granular vibration pumping system was developed to ease the installation into a wide variety of granular handling processes and provide flexible options for granular physics research. Although the granular vibration pumping system can notably lift granular materials with an oscillating pipe, the climbing mechanism and related granular physics are yet to be clarified thoroughly. The new setup employs an eccentric cam mechanism as the excitation source, a linear system with dust tolerance, and a recording system, making it simple, compact, and adaptable for extending experiments. The excitation mechanism generated a clear sinusoidal vibration in the pipe, realizing better reproducibility of the climbing motions of glass beads. Moreover, the compact design facilitates the close placement of multiple pipes vibrating individually, which affects the transport performance. In addition, various types of sample cells that store particles and the imaging system allow for the detailed observation of particle motions in the pipes and even the sample cells. This developed system provides easy and accurate tuning of the existing parameters of the granular vibration pumping system, as well as new options for a further understanding of granular physics.

Exploring the combined effect of particle shape and friction on cell structures in granular packings via LS-DEM modeling

To comprehensively understand and predict the macroscopic behavior of a granular system, one must consider not only the properties of individual particles but also the local structures. In this study, we prepare mechanically stable granular packings of super disks using the two-dimensional level set discrete element method (LS-DEM). We identify the cells, the irreducible loops enclosed by contacting particles, in the prepared granular packings and then analyze the statistics of these structures. We find the following. (1) The packing fraction or mean coordination number of studied systems exhibits a non-trivial dependency on particle shapes and inter-particle frictions, making it challenging to establish a direct correlation between them. (2) Using the cell-based description as a bridge, we can measure the packing behavior without considering the influence of particle shape and inter-particle friction. Specifically, the mean coordination number can be explicitly represented as a function of the mean cell order according to Euler’s topological relation. Upon the assumption that all cells are regular polygons, we can also approximately estimate the rattlers-free packing fraction via the mean cell order. (3) We have observed a nearly linear relationship between the mean cell order and rattler fraction. This enables us to determine the packing fraction as well.

Geometric cohesion in two-dimensional systems composed of star-shaped particles.

Using a discrete element method, we investigate the phenomenon of geometric cohesion in granular systems composed of star-shaped particles with 3 to 13 arms. This was done by analyzing the stability of columns built with these particles and by studying the microstructure of these columns in terms of density and connectivity. We find that systems composed of star-shaped particles can exhibit geometric cohesion (i.e., a solidlike behavior, in the absence of adhesive forces between the grains), depending on the shape of the particles and the friction between them. This phenomenon is observed up to a given critical size of the system, from which a transition to a metastable behavior takes place. We also have evidence that geometric cohesion is closely linked to the systems' connectivity and especially to the capability of forming interlocked interactions (i.e., multicontact interactions that hinder the relative rotation of the grains). Our results contribute to the understanding of the interesting and potentially useful phenomenon of geometric cohesion. In addition, our work supplements an important set of experimental observations and sheds light on the complex behavior of real, three-dimensional, granular systems.

Multiscale modeling of coupled thermo-hydro-mechanical behavior in ice-bonded granular media subject to freeze-thaw cycles

We present a novel multiscale framework that integrates the single-point multiphase material point method (MPM) and the discrete element method (DEM) to model the complex freeze-thaw behavior of ice-bonded granular media. The proposed numerical framework is featured by (a) employing the continuum-based MPM to solve the macroscopic governing equations for granular systems involving thermo-hydro-mechanical (THM) coupling and phase transitions, and (b) using the grain-scale discontinuum-based DEM to capture the thermodynamically sensitive mechanical behaviors of ice-bonded granular media. The multiscale framework is constructed by attaching a DEM-based representative volume element (RVE) at each material point in MPM. This RVE serves as a live sample of each material point to track the state-dependent effective stress with respect to the local deformation and thermodynamic conditions like ice saturation, bridging the macroscopic phenomena and the underlying microstructural evolution. In particular, we implement a semi-implicit staggered integration scheme for the macroscale THM-coupled MPM to boost computational efficiency and enhance numerical stability. We also propose an innovative ice saturation-dependent bond contact to effectively reproduce the thermodynamically sensitive mechanical behaviors. The new multiscale framework is first benchmarked against analytical solutions for 1D non-isothermal consolidation problems. We then demonstrate its exceptional capability in simulating intricate freeze-thaw behavior of granular media through a boundary value problem involving cyclic freeze-thaw actions. Further cross-scale analyses reveal its potential in capturing key loading- and state-dependent THM responses with explainable microstructural mechanisms during complex freezing and thawing loading conditions.

Deformation and Frictional Failure of Granular Media in 3D Analog and Numerical Experiments

Frictional sliding along grain boundaries in brittle shear zones can result in the fragmentation of individual grains, which ultimately can impact slip dynamics. During deformation at small scales, stick–slip motion can occur between grains when existing force chains break due to grain rearrangement or failure, resulting in frictional sliding of granular material. The rearrangement of the grains leads to dilation of the granular package, reducing the shear stress and subsequently leading to slip. Here, we conduct physical experiments employing HydroOrbs, an elasto-plastic material, to investigate grain comminution in granular media under simple shear conditions. Our findings demonstrate that the degree of grain comminution is dependent on both the normal force and the size of the grains. Using the experimental setup, we benchmark Discrete Element Method (DEM) numerical models, which are capable of simulating the movement, rotation, and fracturing of elasto-plastic grains subjected to simple shear. The DEM models successfully replicate both grain comminution patterns and horizontal force fluctuations observed in our physical experiments. They show that increasing normal forces correlate with higher horizontal forces and more fractured grains. The ability of our DEM models to accurately reproduce experimental results opens up new avenues for investigating various parameter spaces that may not be accessible through traditional laboratory experiments, for example, in assessing how internal friction or cohesion affect deformation in granular systems.

Perturbation-induced granular fluidization as a model for remote earthquake triggering.

Studying the effect of mechanical perturbations on granular systems is crucial for understanding soil stability, avalanches, and earthquakes. We investigate a granular system as a laboratory proxy for fault gouge. When subjected to a slow shear, granular materials typically exhibit a stress overshoot before reaching a steady state. We find that short seismic pulses can reset a granular system flowing in steady state so that the stress overshoot is regenerated. This feature is shown to determine the stability of the granular system under different applied stresses in the wake of a perturbation pulse and the resulting dynamics when it fails. Using an analytical aging-rejuvenation model for describing the overshoot response, we show that our laboratory-derived theoretical framework can quantitatively explain data from two fault slip events triggered by seismic waves.

Effect of gravity on granular material flows

Most geohazards and industrial processes are essentially gravity-driven dense granular flows. Deep understanding of the effect of gravity on granular flow is of crucial importance in analyzing the evolution of planetary surfaces. By using a discrete element numerical simulation method, we obtain the evolutionary characteristics of velocity, volume fraction, shear rate and granular temperature during granular flow, and analyze the effect of gravity on the rheological behavior of granular materials. The results indicate that gravity significantly affects the flow characteristics of granular materials, especially in the case of large inclination angle. With the increase of gravity, the flow velocity, shear rate and granular temperature of the granular material tend to increase. When the inclination angle exceeds the critical value, the volume fraction of the granular system in the high gravity environment decreases significantly, while the inertia number increases.

An electro-mechanical contact model for particulate systems

This study presents an electro-mechanical contact model to investigate the electrical response within a particulate system under mechanical loading. The model considers the electrical resistance across particle-to-particle and particle-to-wall contacts by formulating bulk and contact resistances, separately. Hertz and Holm's models are utilised for the contact resistance in the overlapping region. The bulk resistance for a particle is estimated considering particle geometry transformations during the compression. The proposed electro-mechanical contact model is verified and validated against published analytical solutions and experimental data. Finally, the contact model is applied to simulate a high-pressure torsion test to investigate the influence of three different materials on the electrical conduction properties of the system after particle fragmentation. The intensity distributions of electric potential, current and resistance of the granular system are analysed at both particle and system scales. An electroactivity index is proposed for evaluating the contribution of each fragment sieve cut on the bulk electrical conduction behaviour.

Discrete element method study of the mixing and heat transfer behaviour of a binary‐size granular system in a rotating drum

AbstractUnderstanding the internal solid motion and heat transfer behaviour within rotating drums is paramount for their design and operation across various industries. The discrete element method (DEM) is utilized to elucidate the general flow, mixing, and heat transfer characteristics of particles within rotating drums. Following model validation, this study delves into the mixing behaviour and heat transfer patterns of binary‐size particles in the rotating drum, while also assessing the impact of size ratio and rotating speed. The findings reveal that variations in particle size result in noticeable radial segregation, consequently affecting the heat transfer dynamics of solid phase within the system. Higher rotating speeds enhance mixing and dispersion of solid phase but lead to a decrease in the averaged particle temperature. Furthermore, the heat flux exhibits a negative correlation with particle size. Distinct heat transfer behaviours are observed among particles of different sizes in both active and passive areas, with larger particle size ratios exacerbating segregation, potentially impacting final product quality. In summary, these findings offer crucial insights into heat transfer phenomena in rotating drums, aiding in the design and operation of apparatus.

Computational analysis of a dense granular system driven by a propagating shock wave in an Eulerian–Eulerian framework

Computational analysis of a dense granular system driven by a propagating shock wave in an Eulerian–Eulerian framework

Adsorption of Pb (II) ions on variable charge oxidic calcined substrates with chemically modified surface.

The paper describes lead ion adsorption on variable charge oxidic calcined substrates with chemically modified surfaces. Amphoteric oxides of iron, aluminum, titanium, and manganese, change their surface electric charge after acid or alkaline treatment, letting cationic or anionic adsorption reactions from aqueous solutions. This property allows using them as adsorbing substrate for heavy metals retention in water treatment systems. Substrate was prepared by extruding cylindrical strips from a saturate paste of the oxidic lithological material-OLM; dries it up and thermally treated by calcination. The study was performed by triplicated trial, on batch mode, using 2 grams samples of treated with NaOH 0.1N and non-treated substrate. Lead analysis was performed by AAS-GF. Freundlich and Langmuir models were used to fit results. Comparing differential behavior between treated and non-treated substrates showed the variable charge nature of the OLM. Results show L-type isotherms for the adsorption of Pb(II) ions on the activated substrate, suggesting good affinity between Pb(II) ions and OLM's surface. Average value of adsorption capacity ( K) for activated substrate (1791.73±13.06), is around four times greater than the non-activated substrate (491.54±31.97), during the adsorption reaction, 0.35 and 0.26 mmolH + of proton are produced on the activated and non-activated substrate respectively using a 1 mM Pb(II) solution and 72.2 and 15.6 mmolH + using a 10 mM Pb(II) solution. This acidification agrees with the theoretic model of transitional metals chemisorption on amphoteric oxides, present in lithological material used for the preparation of adsorbent substrates, confirming the information given by the L-type isotherms. Results suggest that these variable charge oxidic adsorbent substrate show great potential as an alternative technique for water treatment at small and medium scale using granular filtration system. The easiness and low price make them suitable to apply in rural media where no treating water systems is available.

Unraveling the effect mechanism of sulfamethoxazole on nitrogen biotransformation in a PHBV supported denitrifying granular sludge

Sulfamethoxazole (SMX) is prevalent as one of the most frequently detected antibiotics in rivers worldwide, and is listed as one of the priority antibiotics in China based on the persistence, bioaccumulation, toxicity and resistance. Recently, the effect of SMX on aerobic granular sludge has attracted increasing interest. However, how SMX influences the biodegradable polymer supported denitrifying granular sludge is still obscure. Herein, two groups with or without exposure to 1 mg/L SMX with initial nitrate concentrations of 15, 30, and 50 mg/L were constructed. Results showed that there was a negligible difference on nitrate removal efficiencies between two groups under stable period (all exceeded 95 %) in batch bioreactors with an HRT of 24 h, supported by an insignificance difference (p > 0.05). However, the decline in the first-order decay kinetic constants of nitrate removal rate, NAR and NIR activities verified a slight inhibition of SMX on denitrification. Meanwhile, SMX also had an impact on the chemical and microbial properties on granular sludge. SMX boosted the EPS secretion with more humic acid, a delicate rise in PN/PS ratios and a decrease in zeta potential of EPS all favored the microbes' survival under SMX stress. SMX shifted the microbial compositions, namely, the microbes directly involving in PHBV degradation (Caldimonas, denitrificans Comamonas/Diaphorobacter) were enriched instead of some denitrifying microorganism that could not directly utilize PHBV (Denitratisoma, Azospira). Additionally, even though sul1 and sul2 were enriched, which was not readily accessible to receiving waters due to a good settling properties of granular sludge. This work contributes to a deeper understanding of the impact of SMX on this system, and is of great significance for the operation of granular sludge system for nitrate removal under antibiotic's stress.

Investigation on the rheological properties of dense granular inertial flow based on the contact features of four dual-partitioned subnetworks

The rheological properties of shear granular system under inertial flow regimes are investigated by fully considering both strong/weak and sliding/non-sliding contact features of four dual-partitioned subnetworks. Discrete element simulation results reveal that strong non-sliding subnetwork plays a dominant role in bearing external load. For strong sliding and weak non-sliding subnetworks, their anisotropy principal directions of normal contact forces undergo dynamic deflections during evolution, resulting in the lateral support of microstructures being primarily borne by unstable sliding contacts. Although the subnetworks display substantial variations in stability, their strong spatial correlation makes these variations complement each other, forming an intrinsic coupling relationship.

Numerical investigation on the densification of granulated porous indium tin oxide powders before compaction

Dense and uniform initial packing structures of powders are of key significance in powder metallurgy process, which can provide the precondition for the fabrication of high-performance components with low cost and high efficiency. In this paper, the packing densification of indium tin oxide (ITO) granulated micro powders with continuous size distributions and internal porosity under three-dimensional vibrations was numerically investigated by discrete element method (DEM). Firstly, the DEM model was validated by physical experiments, and the contact parameters between particles and the porosity therein were determined. Then the effects of operating parameters on the macro/micro properties (e.g., packing density, coordination number (CN), pores, and forces) of the packing structures were analyzed. Results show that the mean CN cannot intuitively reflect the packing structure due to the complex and varied inter-particle contacts caused by the inhomogeneity of the particle size in a multi-sized granular system. Additionally, the excellent packing structure of porous ITO particles is always obtained from low amplitude when the vibration intensity is the same. Corresponding mechanism analyses reveal that higher amplitude normally causes strong convective diffusion, leading to more pores in the packing. This increases the probability of wedging effect occurring during settling, thereby reducing the packing density. However, optimal packing quality with low amplitude can only be achieved when coupled with high frequency. Therefore, applying vibration is an effective way to achieve dense packing of porous spherical particles, especially at low amplitude and high-frequency operating conditions.

Pulmonary function as a continuum of risk: critical care utilization and survival after allogeneic hematopoietic stem cell transplantation - a multicenter cohort study.

Abnormal pre-transplant pulmonary function tests (PFTs) are associated with reduced survival after allogeneic HCT. Existing scoring systems consider risk dichotomously, attributing risk only to those with abnormal lung function. In a multicenter cohort of 1717 allo-HCT recipients, we examined the association between pre-transplant PFT measures and need for ICU admission (120d), frequency of mechanical ventilation (120d) and overall survival (5 y). Predictive models were developed and validated using Cox proportional hazards, incorporating age, FEV1 (forced expiratory volume in 1-second) and diffusing capacity (DLCO). In univariate analysis, hazard ratios for each outcome (95% CI) were: mechanical ventilation (FEV1: 0.60 [0.52-0.69], DLCO: 0.69 [0.61-0.77], p < 0.001), ICU admission (FEV1: 0.74 [0.67-0.82], DLCO: 0.79 [0.72-0.86], p < 0.001) and overall survival (FEV1: HR 0.87 [0.81-0.94], DLCO: 0.83 [0.77-0.89], p < 0.001). A multivariable Cox model was developed and compared to the HCT-CI Pulmonary score in a validation cohort. This model was better at predicting need for ICU admission and mechanical ventilation, while both models predicted overall survival (p < 0.001). In conclusion, the risk conferred by pre-transplant pulmonary function should be considered in a continuous rather than dichotomous manner. A more granular prognostication system can better inform risk of critical care utilization in the early post-HCT period.

Impact response of physical analog of Martian regolith

A careful examination of collisional processes occurring on surfaces of Solar System objects requires data on the dynamic properties of porous geological materials. Shock waves traveling in sand with physical properties analogous to those of some types of Martian regolith are studied in a series of uniaxial plate impact experiments. The initial density of the samples and average grain size are 1.30 g cm−3 and 95 μm, respectively. The obtained Hugoniot of this simulant is in general agreement with those of previously studied sands. The effect of porosity and grain morphology on Hugoniots of these geological materials is also highlighted. A laser velocimetry technique is used to observe the structure of propagated waves. Wave fronts with rise times on the order of several hundred or tens of microseconds were recorded in the pressure range of 0.5 to 1.7 GPa. For the simulant, a value of 1.91 was found for an exponent in the relationship between rise time and pressure τ∼p−m. Additionally, exponents for different granular and silica-rich systems studied earlier were also provided. An analysis of wave thickness revealed that in the pressure range of 0.5–1.1 GPa, this parameter can be associated with the grain average size. At the highest pressure (1.7 GPa), the minimum particle size (∼20 μm) turned out to be a more suitable quantity to describe the shock wave structure.

Comparative evaluation of activated sludge and aerobic granular sludge for biological treatment of real domestic wastewater with oxytetracycline dosing

Aerobic granular sludge technology is an emerging choice for effective and sustainable wastewater treatment. The aim of this study was to determine the effect of antibiotics such as oxytetracycline on biological nutrient removal pathways in aerobic granular sludge and activated sludge sequencing batch reactors treating real domestic wastewater. The aerobic granular sludge and activated sludge reactors were operated in parallel for evaluating treatment of domestic wastewater and its response to 50–300 µg/L oxytetracycline dosing. The aerobic granular sludge and activated sludge sequencing batch reactors were dominated by compact granules (sludge volume index (SVI): 60 mL/g) and flocs (SVI: >100 mL/g), respectively, during treatment of real wastewater. Aerobic granular sludge has significantly outperformed activated sludge in achieving high phosphate removal of ∼99% against just 57%. This was corroborated with higher abundance of polyphosphate accumulating organisms and enhanced bio-P removal in the aerobic granular sludge system. In contrast, phosphate removal was achieved through normal bio-P removal in the activated sludge reactor. Oxytetracycline removals were distinct at 90% and 23%, respectively, in the aerobic granular sludge and activated sludge systems. High total nitrogen and phosphate removals of 97% and 96%, respectively, were maintained in the aerobic granular sludge system even during OTC dosing. In contrast, the total nitrogen and phosphate removals have dropped to 78% and 16%, respectively, in the activated sludge system due to oxytetracycline stress. Efficient biological nutrient removal and process stability along with environmental sustainability are critical for choosing aerobic granular sludge technology in wastewater treatment plants.

Achieving stable advanced nitrogen removal from mainstream municipal wastewater using integrated strategies in partial nitrification anammox (PNA) granular hybrid system

Partial nitrification anammox (PNA) is a promising technology for energy saving and nitrogen removal. The bottlenecks limiting the widespread application of PNA in mainstream municipal wastewater treatment include the difficulty in stably suppressing nitrite-oxidizing bacteria (NOB) and failure to meet discharge standards for total nitrogen (TN) due to high nitrate concentration in the effluent. To this end, this study aims to stably suppress NOB using integrated strategies (including the inoculated activated sludge pretreatment with FNA, low DO, residual ammonia nitrogen, and floc sludge discharge) while also investigating the feasibility and performance of coupling PNA with endogenous partial denitrification/anammox (EPDA) in a PNA granular hybrid system. During the 300 days of operation, the integrated strategies effectively stabilized the suppression of NOB, decreasing its abundance from 1.22 % to 0.55 %, and the final activity was reduced to 0.36 ± 0.12 mg-N/L/h. Meanwhile, the relative abundance of AnAOB as the dominant microbial community increased from 9.16 % to 24 %, with its activity reaching 2.84 ± 0.16 mg-N/L/h, limiting the supply of nitrite and ensuring the stable suppression of NOB. Typical cycle tests showed that EPDA and PNA were successfully coupled during the aeration stage with a nitrogen removal contribution rate of 80.55 %, ultimately achieving the average effluent TN and PO43--P were 4.22 mg/L and 0.09 mg/L, respectively. The metagenomic and metatranscriptomic sequencing results revealed that anammox bacteria (AnAOB) coexisted with ammonia-oxidizing bacteria (AOB), NOB, and endogenous denitrifying bacteria (EDB), and AnAOB played a primary role in the nitrogen removal process of mainstream PNA. This study brings mainstream PNA closer to widespread application and provides significant technical support for its engineering implementation.