Computational Particle Mechanics最新文献

Abstract

Fine particles of ash and sand can deposit on the surfaces of cooling ducts, diminishing heat transfer efficiency and threatening the operation of turbine engines. The surface roughness of deposits can alter the nearby flow dynamics, and result in changes of subsequent particle collision and deposition. In this work, the effects of rib turbulence on particle deposition in cooling duct are numerically studied based on the wall modeled shear stress transport k–ω model with a UDF code correction for particle–wall impacts and the discrete particle model. A Gaussian probability density function is adopted to give the topology of deposited particles on the surface impacted by micron particles. We investigate how variables such as particle diameter and temperature impact collision and deposition processes. Additionally, the impact of ribbed turbulence on particle deposition is also discussed. The findings indicate that the impact ratio increases with particle diameter while exhibiting less sensitivity to temperature. Deposition ratios experience a significant decrease when particle size exceeds 1 μm. The temperature of the particles has a noteworthy influence on surface profile of deposits. Specifically, deposits on the wall surface, where particles are introduced by fluid injection, tend to assume a crane-like shape as the temperature rises. Notably, a more uniform deposition pattern is achieved when the particle temperature is low. In terms of particle distribution, low-velocity particles are more likely to accumulate in the windward region of the rib, especially at the junction of the rib wall, where the maximum deposition height is observed. Furthermore, deposits on the rib surface tend to grow, and the gap between the peak and valley widens as the particle temperature increases, as evident from the roughened rib surface features.

Viscoelastic fluids are central in numerous applications from polymer manufacturing to the pharmaceutical industry and biological research. However, since analytical solutions are generally not available or too complex, it is common practice to study free-surface viscoelastic flows through numerical simulation techniques. This work proposes the use of the so-called particle finite element method (PFEM), a Lagrangian approach combining standard FEM techniques with a remeshing strategy. The PFEM is able to efficiently handle mesh distortion and to accurately track the free-surface evolution. Therefore, it is exploited in this work to deal with large displacements problems in the context of nonlinear viscoelasticity. An implementation of the Oldroyd-B constitutive model in the PFEM framework is here presented including details regarding how to deal with the transfer of the internal variables during remeshing events. Additionally, an innovative approach to impose unilateral Dirichlet boundary conditions ensuring optimal mass conservation is presented. The implementation is verified with two free-surface highly viscous benchmark flows: the impacting drop and the jet buckling problems. The results show perfect agreement with those obtained with other numerical techniques. The proposed framework opens the way for using PFEM in various applications, ranging from polymer extrusion to more sophisticated scenarios involving viscoelastic and viscoelasto-plastic constitutive laws.

Abstract

The microstructures and local characteristics of ordinary refractory ceramics are heterogeneous. The discrete element (DE) method was used to consider the variation in particle spatial distributions and statistically distributed interface properties (uniform, Weibull) between elements. In addition, three Weibull distributions with different shape parameters were evaluated. A uniaxial tensile test was used to study the effects of particle spatial distributions and interface property distributions on the stress–strain curve, tensile strength, and crack propagation. The results of the test show that the particle spatial distribution significantly influences crack propagation and fracture patterns, and the interface condition plays an important role in mechanical responses, crack propagation, and fracture mechanisms and patterns. The discrete element modelling of uniaxial tensile and compressive tests shows that brittle materials exhibit asymmetric mechanical responses to compression and tension loading including static Young’s modulus.

Tunnel excavation in weak surrounding rock areas is prone to landslide accidents, and the use of high-pressure rotary piles to pre-strengthen the soil in the local area can enhance the strength and bearing capacity of the surrounding rock. Discrete lattice spring model is established with the three-dimensional morphology modeling system of the rotary pile reinforcement. It is used to quantitatively characterize the reinforcement effects of high-pressure rotary piles, to analyze the influence of the reinforcement ratio and reinforcement function. The results show that compared with the deformation of unreinforced stratum, the high-pressure rotary pile can better control the ground surface settlement. The larger the reinforcement ratio is, the better the reinforcement effect of the rotary spray pile is, especially with the increase in reinforcement ratio, the contact between individual piles bites to form a row of piles, which can significantly improve the ability of the formation to resist deformation. Under the same reinforcement situation, the square root type reinforcement function has the best reinforcement effect, the line function has the middle reinforcement effect, and the quadratic type reinforcement function has the worst effect.

This research investigates how inserting notched gypsum filling between granite specimens affects their breakage under uniaxial compressive testing. Various thicknesses of gypsum filling slabs were placed between granite specimens, incorporating different dimensions and notch configurations. The investigated parameters include elastic modulus, Poisson’s ratio, uniaxial compressive strength, and Brazilian tensile strength of 5 GPa, 0.18, 7.4, and 1 MPa, respectively. Compression testing, at an axial load rate of 0.05 mm/min, was conducted on a total of 9 different models. Numerical simulations were performed on models with notched gypsum filling, varying thicknesses, and notch angles using Particle Flow Code in 2D. The results demonstrated that breakage behavior was primarily influenced by filling thickness and notch angle. The uniaxial compressive strengths in samples were found to be affected by fracture patterns and the breakage mechanism of the filling. The study revealed that the behavior of discontinuities is influenced by the number of induced tensile cracks, which increase with thicker filling. Acoustic emission (AE) hits during loading’s initial phase, a rapid increase in AE hits before the applied stress reached its peak, and significant AE hits accompanying each stress drop were observed. The breakage patterns and strengths were found to be similar in both experimental and numerical approaches.

With the increasing number of projects in cold regions and the widespread use of artificial freezing methods, conducting research on the dynamic properties of frozen soil has become a considerable issue that cannot be avoided in permafrost engineering. Currently, the numerical simulation research on the dynamic mechanical behavior of frozen soil is less concerned with the changes in stress, strain, and particle damage inside the material. The necessary conditions for conducting this study are compatible with the core idea of smooth particle hydrodynamics (SPH). In this study, the Eulerian SPH method was modified to address numerical oscillations and errors in solid mechanics, particularly impact dynamics problems. A numerical scheme for simulating the split Hopkinson pressure bar test was developed within the modified Eulerian SPH framework and implemented using self-programming. The frozen soil dynamic mechanical behavior was simulated under three strain rates. The accuracy and superiority of the SPH method were verified through calculations and experiments. The simulation captures the stress and strain responses within the sample at different moments during the impact process, indicating that the frozen soil strain rate-strengthening effect resulted from microcrack expansion and inertial effects.

Rock bridges are important structures for maintaining rock mass stability, but their shapes are not well known. The researchers propose a method for determining the shape of rock bridges based on experiments, discrete element methods and machine learning, which is applicable to complex joints with arbitrary spatial distribution. Numerical models are constructed using the discrete element method, and parameter matching is performed based on experimental results. The particles were clustered using the k-means algorithm with the maximum principal stress (σ₁) as an indicator and the selection of initial values was optimized. The density-based spatial clustering of applications with noise (DBSCAN) algorithm was used to delete the noise from the particles. Finally, the boundary lines of the particles were extracted by self-programming, and the shape of the rock bridges was determined. Twenty-four sets of simulations were used to analyze the effect of rock bridges on the specimens. The results show that the failure mode of the specimen changes from shear to tensile damage as the cohesive force of the rock bridges increases. The peak strength and peak strain of the specimens increased with the increase of cohesion in the rock bridge. Rock bridges are the fastest growing areas of stress in the specimen.

Abstract

Railway ballast modeling can be performed by different approaches, through continuous or discrete models, which have their comparative advantages and disadvantages, such as excessive volumes of material for testing and calibration steps. This paper aims to adapt and propose the use of the Hybrid Lattice-Discrete Element Method for modeling railway ballast aggregates. The advantages of using this technique for this purpose are: (i) one-step calibration of the rock material from laboratory test results; (ii) simulation of fractures in rock materials; (iii) visualization of micromechanical phenomena, such as particle slippage and fracture modes; (iv) realistic representation of various geometries compared to the conventional use of the Discrete Element Method. First, parameter calibration was performed from laboratory test results on granite rock obtained from the literature. Then, particle generation, Voronoi discretization and packing algorithms were used to build models of railway ballast samples. These models were used to simulate mechanical tests, namely single particle compression, confined uniaxial compression, monotonic triaxial compression and cyclic triaxial compression. There was consistency between the results and the empirical observations reported in the literature. In addition, variations in particle size distribution were observed during the simulations, as well as the causes of failure in each specimen, either by shear or particle breakage, in addition to the fracture modes of the ballast aggregates. By analyzing these elements together, knowledge is obtained about the phenomena occurring inside the railway ballast under different loading conditions, in addition to the results of strength, failure and deformation. Finally, it is concluded that the proposed method is effective for modeling railway ballast, besides being versatile, allowing to simulate the material for different loading configurations and boundary conditions.

Since most of the current researches on the crushed-rock interlayer for highway embankment in permafrost region are based on thermal properties, there are few studies on their mechanical deformation characteristics. In order to study the deformation and failure process of crushed-rock interlayer under the long-term settlement deformation of permafrost foundation and to fully consider the discrete characteristics of the crushed-rock interlayer, the finite element model and discrete element model were coupled in this study to accomplish the numerical calculation of long-term settlement deformation of crushed-rock interlayer highway embankment as well as permafrost foundation. The results show that as for the granite blocks adopted in the Gonghe–Yushu expressway, the blocks in the interlayer would be rarely broken, and the deformation of crushed-rock interlayer is mainly caused by the relative movement and rearrangement of the blocks. Based on the calculation results, it is suggested to adopt the uncompacted randomly piled crushed-rock interlayer, which is composed of crushed blocks with more sharp corners. When the size of block varies from 20 to 40 cm, the block size has no obvious effect on the deformation of crushed-rock interlayer, and therefore, the block size could be determined only by the cooling effect of crushed-rock interlayer. At the meantime, the structure layer above the crushed-rock interlayer should also be rigid enough to ensure a smaller uneven settlement value for the superstructure.

A new kinetic particle modeling framework was developed to investigate electrostatic transport of lunar regolith dust particles with applications to the concept of electrostatic sieving. The new approach is based on kinetic particle dynamics and includes major modules of sampling the particle size distribution, solving electric fields, and tracking motion of charged dust grains. A case study for a concept of electrostatic sieving was chosen to validate the new model. The simulation achieved similar performance of particle size classification as reported in the literature. The new model is computationally efficient (takes a few minutes on a PC-type laptop computer) so that researchers can use it as a design and analysis tool to explore large parameter space for system optimization.