Element Numerical Simulation Method Research Articles

Enhanced fluorescence emission of a single quantum dot in a porous silicon photonic crystal—plasmonic hybrid resonator

Abstract Currently, much interest is attracted to investigating the potential of hybrid systems that exhibit plasmon-induced photoluminescence (PL) enhancement of quantum emitters in terms of optoelectronics and biosensing applications. The implementation of these systems based on photonic microcavities offers benefits due to a stronger localization of the field within the resonant cavity. Porous silicon is one of interesting materials for engineering such microcavities thanks to the simplicity of its fabrication and the possibility to embed emitters from the solution into a ready-made resonator. In this theoretical study, the fluorescence enhancement of a quantum dot (QD) in a hybrid system based on a porous silicon microcavity (pSiMC) and silver nanoplatelets (AgNPs) was investigated using finite element method (FEM) numerical simulations. For this purpose, infinite arrays were simulated by using a periodic unit cell. The pSiMC was designed as two λ/4 distributed Bragg reflectors with alternating refractive indices and a cavity layer of a double thickness between them. For comparison, simulations were also performed for an AgNP and a QD in a reference monolayer with a constant refractive index without a microcavity structure. The results show QD fluorescence enhancement in the AgNP/pSiMC hybrid system, mainly due to the higher excitation rate.

Mechanism analysis of the abnormal head-cover vibration of an ultra-high-head pump-turbine operating in pump mode by CFD and FEM

ABSTRACT Hydraulic excitation causes abnormal head-cover vibration in some pump-turbines, particularly as design heads continue to rise. Pump-turbines with heads exceeding 600 m have reported such vibrations in pump mode, and it is urgent to reveal the reasons. A study was conducted on a head-cover of a pump-turbine with a maximum head of 660 m by 3D computational fluid dynamics (CFD) and finite element method (FEM) numerical simulations to investigate the hydraulic excitation source and vibration mechanism. Results show that the intensive vibration is related to the coincidence of frequency and mode between abnormal pressure pulsations with frequency 6–8f n and the low-order wet mode of the head-cover. A new flow pattern termed the rolling vortex was discovered in the vaneless space under high-head pumping conditions. The 6–8f n pressure pulsations result from the interaction between rolling vortices and guide-vanes, similar to the rotor-stator interaction. The flow mechanism of the rolling vortices is that the outflow velocity near the hub at the runner outlet is higher than that near the shroud, due to the positive blade lean angle and the X-shape distorted blades. Suppressing rolling vortices may help eliminate the abnormal head-cover vibration, which will be attempted in further study.

Particle dynamic behaviors in bubble curtain barriers blocking the diffusion of sediment

Bubble curtains are linear multiphase plumes that are used in industry and engineering to reduce diffusive flows between two zones. The circulating shear flow generated by the bubble plume can effectively create a particle barrier in the water; thus, it is also widely used to suppress the diffusion of suspended particles during dredging operations. To clarify the interaction mechanism between the bubble curtain and suspended particles under pressure-driven flow, experimental studies and numerical simulations were conducted to investigate the particle dynamics in the flow field around the bubble curtain. In this study, we established a formal analogy between a bubble curtain and an air curtain and qualitatively identified four typical distribution zones of the particles in the flow field. Based on the quantitative measurements and theoretical considerations, the optimal operating conditions and the upper limit of effectiveness of the bubble curtain were determined. In addition, the blocking behavior and efficiency of bubble plumes on particles with different ReU and FrUgnumbers and different particle properties were simulated via the computational fluid dynamicsdiscrete element method (CFDDEM) numerical simulation method.

Numerical study on nanofluid heat transfer and fluid flow within a micro-channel equipped with an elastic baffle

In this study, the incorporation of a flexible baffle structure at the base of a microelectronic channel is examined to determine its impact on thermal and fluid dynamics. The investigation covers two configurations: case 1, where the heat source is situated at the top section of the microchannel, and case 2, with the heat source placed at the bottom. To cool the electronic component, a nanofluid consisting of water with aluminum oxide (Al2O3) nanoparticles, in concentrations varying from 0% to 6%, is injected into the microchannel, exhibiting a periodic velocity profile at the inlet. The governing equations for the system are resolved using the finite element method for numerical simulation. The results show that the shape and curvature of the baffle significantly affect the heat dissipation efficacy. In particular, the setup with the heat source located at the bottom, adjacent to the flexible baffle, demonstrates the most advantageous cooling performance. The data also show a positive relationship between the nanoparticle volume fraction and enhanced heat transfer, particularly when the heat source is near the bottom baffle for shapes with S = −20 and −10, implying superior convective heat transfer. The findings indicate optimal outcomes are achieved when there is a reduction in pressure drop and fluid resistance.

CFD–DEM modelling of the infiltration of non-spherical slurry particles in granular soils

The slurry filtration process at an excavation surface plays an important role in supporting pressure transmission, which is crucial to the stability of a tunnel face during slurry shield tunnelling. In this paper, a series of coupled computational fluid dynamics (CFD)–discrete element method (DEM) numerical simulations were carried out to model the slurry filtration column test. Oblate spheroids of varying aspect ratios were adopted to represent slurry particles. A simplified pitching torque model was implemented to better simulate the motion of slurry particles. The pressure distribution, void fraction, permeability and orientations of the particles were analysed as the aspect ratio was systematically changed. The pressure distribution, void fraction and the permeability within a stratum are highly dependent on the aspect ratio of the particles. Particles with larger aspect ratios tend to effectively seal infiltration channels and transfer slurry pressure to effective pressure, thereby helping to maintain stability at the tunnel face. This research study explores how particle shape impacts the slurry infiltration procedure and the microstructure of the filter cake.

Dynamic Behavior Analysis of Wax Crystals during Crude Oil Gelation.

The motion behaviors, rheological properties, and mechanical properties of a wax crystal mesoscale force chain network of waxy crude oil during gelation were experimentally studied and simulated by rheo-optic in situ measurement and computational fluid dynamics-discrete element method (CFD-DEM) numerical simulation, respectively. The motion behavior characteristics of wax crystals and the changes in the average coordination number of wax crystal networks were obtained at different temperatures. The study investigates the rule of crude oil rheological deterioration, the increase of wax crystals, and the changes in wax crystal motion behaviors with decreasing temperature. The relationship between the structure of the mesoscale force chain of the wax crystal network and the motion behaviors of the wax crystal and its rheological properties was analyzed. The results show that the average motion velocity of wax crystals or aggregates decreases from 28.48 to 22.56 μm/s when the temperature decreases from wax appearance temperature (48 °C) to 25 °C. The rotation and rolling trend of wax crystals gradually flatten, and the average coordination number increases 4.39 times. The viscosity of waxy crude oil increases from 6.27 to 8369.7 mPa·s, and the average coordination number of wax crystals obtained by CFD-DEM also increases significantly, which confirms the experimental results. We also found that when the system tended to gelation, a complex and stable force chain network was formed between the wax crystals, with a force chain coverage of 87.93% and a significant increase in the pressure drop in the flow field, which is consistent with the variation pattern of the system viscosity. The micro-meso-dynamic behavior analysis of waxy crude oil combined with CFD-DEM coupling provides a new way to explore the rheological properties of waxy crude oil and the microscopic mechanism of its modification.

SHPB experiments and FDM-DEM coupled simulations on calcareous sand under uniaxial to triaxial deformation

The impact behavior of sand is paramount to geotechnical and protective engineering. Although the one-dimensional impact responses of sand are extensively studied, the behavior of shear under impact loading remains poorly understood as the shear stresses in uniaxially compressed sand cannot attain the failure state. This paper reports the macroscopic responses of crushable calcareous sand under uniaxial to triaxial deformation by split Hopkinson pressure bar (SHPB) experiments. In addition, the mesoscopic mechanism is analyzed by coupled finite difference method (FDM) and discrete element method (DEM) numerical simulations. With an increasing lateral confined stiffness, the p-q curves exhibit a clockwise rotation, signifying a deviation of the stress path from the critical state line (CSL) during the transition from triaxial to uniaxial deformation, meanwhile the Mohr circle experiences a shift from intersecting to tangential contact with the failure line, indicating that shear failure occurs under triaxial deformation. The stress-strain relationships for both compression and shear exhibit four distinct regimes, the initial elastic stage mainly overcomes intergranular friction through particle rotation; the yield stage mainly compresses the skeleton by contribution of displacement; the hardening stage experiences critical particle breakage and then promotes particle displacement and rotation; and the unloading stage with strain softening caused by the asynchrony between impact loading and deformation. Both compressed and shear yield stress increases with an increasing confined stiffness, but the shear friction angle decreases. This indicates that the governing factor in triaxial compression under impact loading is the stress state rather than kinetic friction. Particle breakage in triaxial compression state tends to approach the splitting mode, while in uniaxial squeezing state it tends to approach the Mixed mode.

Numerical Simulation of Torque Variation During Paste Mixing and Kneading

In the production process of aluminum electrolytic carbon anode, the kneading process is to fully mix asphalt and powder to obtain the best paste, because the kneading process is completed in a high-temperature kneading machine, there is no suitable automatic real-time judgment method for the uniformity of paste. Real-time online monitoring based on the viscoelastic change of paste during kneading is a potential method to judge the uniformity of paste. In this paper, the Discrete Element Method numerical simulation is used to study the variation law of stirring torque in the mixing and kneading wet mixing process, mainly examining the effect of viscosity on stirring torque. The results showed that the mixing and kneading torque varied periodically and the total torque of the stirrer was positively correlated with the viscosity.

Hybrid approach for graphene modeling in numerical simulations of graphene based plasmonic devices with COMSOL

We present a hybrid approach for graphene modeling in the finite element method numerical simulations of graphene based plasmonic devices with COMSOL. The surface conductivity approach for modeling graphene or other two-dimensional materials needs less number of meshing elements if compared to the volumetric permittivity approach; however, it is less intuitive and less compatible with the boundary mode analysis, which provides a very efficient way for the excitation of boundary ports. We adopt the surface conductivity approach in the scattering domain and the volumetric permittivity approach in the boundary ports. A triangular connector with a spatially varying permittivity is introduced to achieve reflection-less coupling between the surface conductivity and the volumetric permittivity. The hybrid model enables the numerical simulation of graphene based devices with COMSOL to be benefited from less storage and calculation time, keeping compatibility with the boundary mode analysis.

Material's Strength Analysis of the Coupling Node of Axle of the Truck Trailer.

Road transport plays an important role in the transport of goods and people and is important for the national economy. Damage usually excludes the means of transport from operation, which causes disruption of supply chains. One such damage is the failure of the suspension system of the vehicle or trailer, which usually occurs when the vehicle is heavily loaded. Such a defective system has been analyzed in this publication. Mathematical apparatus and finite element method (FEM) numerical simulations were used. A dangerous axle cross-section in terms of load was indicated and the maximum stresses in this area were calculated for two types of roads. On highways, the stress at the critical point was 199 MPa, and on uneven roads it increased to 304 MPa, which is comparable to the yield point. It was found that the second form of vibration may cause stresses in the damage area, but the excitation frequency would have to be quite high. The probability of such a load and failure event occurring is low under operating conditions.

Tensile behavior of Brazilian disks containing non-persistent joint sets subjected to diametral loading: experimental and numerical investigations

Structural defects are part of the inherent characteristics of rock masses. They can be found in fishers, joints, and beddings and can be divided into persistent or non-persistent ones. The coalescence of non-persistent cracks may lead to the formation of persistent joints under the tensile stress field, leading to rock mass instability. The mechanical behavior of non-persistent jointed disks under tensile stress has important implications for rock structures. In this paper, experimental and Discrete Element Method (DEM) numerical simulation approaches were adopted to investigate the effect of joint continuity factor (the relationship between joint length and rock bridge length), bridge angle, joint spacing, and loading direction in relation to joint angle on the tensile strength and stiffness as well as failure pattern. Cement-mortar-based Brazilian disks containing open non-persistent joints were constructed and subjected to diametral loading and simulated using a two-dimensional Particle Flow Code (PFC2D). The investigations revealed that the tensile strength, stiffness, and failure pattern of Brazilian disks are highly affected by non-persistent pre-existing cracks. The increase of joint continuity factor and loading direction leads to the reduction of both tensile strength and stiffness. However, when spacing increases, tensile strength and stiffens have a minimum at the average spacing, but it was concluded that bridge angle does not affect both parameters. Finally, the results showed that the experimental and numerical approaches are in good agreement, and almost all the parameters affect the failure pattern, and some failure patterns such as step-path failure, splitting, or sliding may occur as a function of non-persistent joint sets.

Study on the Influence of Water Content on Mechanical Properties and Acoustic Emission Characteristics of Sandstone: Case Study from China Based on a Sandstone from the Nanyang Area

In order to study the influence of water content on the mechanical properties of sandstone and evolution of crack propagation, laboratory compression tests and Engineering Discrete Element Method (EDEM) numerical simulation of sandstone under different conditions were carried out by the RMT-150B rock mechanics test system. The sandstone samples were from Nanyang, Henan Province, containing a total of 12 rock samples. Under the confining pressure of 0, 5, 10, and 20 MPa, the rock samples with 0%, 1.81%, and 3.24% water content were tested. The findings demonstrated that as the sample’s water content grew, the peak strain increased but the peak strength, elastic modulus, maximum energy rate of individual acoustic emission events, and cumulative acoustic emission energy rate all reduced. While the ratio of tensile cracks to shear cracks inside the rock samples rose with increasing water content, the failure mode of sandstone changes from shear failure to tensile failure with the increase of water content, but the sandstone specimens in the three conditions exhibited shear macroscopic fracture surfaces. Research results will provide reference for the safe construction of underground projects in water rich areas.

Using discrete element numerical simulation to determine effect of persistent fracture morphology on permeability stress sensitivity

AbstractThe stress sensitivity of coal seams with different fracture structures affects gas migration characteristics in the fracture zone. In this study, permeability stress experiments for intact and persistent fracture coal samples are carried out. A discrete element method numerical simulation that adopts the changing joint stiffness method is proposed. Based on the experimental data, the corresponding parameters of persistent fractures and elastic joints for fluid–solid coupling simulations are determined. Subsequently, the internal influence mechanisms of fracture morphology on axial and confining stress sensitivities are determined. The simulation results show that sectional fractures with different dip angles in the persistent fracture control the sensitivity to axial and confining stresses. The horizontal segment fracture aperture decreases with an increase in axial stress, which reduces its flow capacity. This results in a decrease in the flow rates in the persistent fracture. When the confining stress increases, the vertical fracture aperture is significantly reduced, considerably reducing the flow rate in the persistent fracture.

Study of In-Plane Mechanical Properties of Novel Ellipse-Based Chiral Honeycomb Structure

In this paper, we propose an elliptical anti-tetrachiral honeycombs structure (E-antitet) with in-plane negative Poisson’s ratio (NPR) and orthogonal anisotropy. The analytical and numerical solutions of the in-plane Poisson’s ratio and Young’s modulus are given by theoretical derivations and finite element method (FEM) numerical simulations and are verified experimentally by a 3D printed sample. Finally, we analyzed the influences of different parameters on the in-plane Poisson’s ratio and Young’s modulus of E-antitet. The results show that the proposed E-antitet can achieve a smaller Poisson’s ratio and larger Young’s modulus in the desired direction compared with the anti-tetrachiral honeycombs structure (antitet), and moreover, the E-antitet has a more flexible means of regulation than the antitet. The analytical results of this paper provide meaningful guidance for the design of chiral honeycomb structures.

The uplift resistance of submarine pipelines buried in medium dense sand

Buried submarine pipelines need to be operated under high temperature and high pressure, and ensuring sufficient buried depth is a necessary factor for safe operation. Based on the plane strain assumption, the purpose of this study is to study the evolutionary mechanism of pipe-soil interaction under the monotonic uplift of buried submarine pipelines through model tests and the Discrete Element Method (DEM) numerical simulation. The results show that the uplift resistance is affected by the embedment ratio and the pipe diameter; during the pipeline uplift process, the development of the uplift resistance is divided into three stages: the development stage, the softening stage, and the residual stabilization stage. The displacement at the peak uplift resistance increases with the increase of the embedment ratio, and the value is about 0.5–1.5%H, and the sand particle size has a great influence on the displacement of the peak uplift resistance and the post-peak softening behavior. According to the comparison of the test results, the prediction model of the load-displacement response of pipe-soil interaction is proposed.

Influenced Zone of Deep Excavation on Adjacent Tunnel Displacement and Control Effect of Ground Improvement in Soft Soil

The aim of this study is to predict the influenced zone of deep excavation on adjacent tunnel displacement, evaluate the control effect of ground improvement, and give the optimal parameters for ground improvement. Based on the current research, a series of finite element method (FEM) numerical simulations were conducted to study the deep excavation-induced tunnel displacement behaviors, considering different tunnel positions outside the pit. On this basis, the influenced zone of deep excavation on an adjacent tunnel was divided corresponding to 3-level tunnel displacement control standards. Then, the commonly used control measure of ground improvement was chosen to study the effects of strength, depth, and width of the improved soil outside the pit on the displacement behaviors of the tunnel. An index of tunnel displacement control effectiveness (η) was proposed to quantitively characterize the control effect on tunnel displacement. Considering the control effect and engineering economy, the suggested values of strength, depth, and width of the improved soil were provided. Finally, the control effect of ground improvement outside the pit on the influenced zone of deep excavation was studied using the suggested parameters. The research indicates that the range outside the pit can be divided into: I—primary influenced zone, II—secondary influenced zone, III—general influenced zone, and IV—weak influenced zone. Considering the control effect and engineering economy, it is suggested that the ground improvement strength should be kept within 1.5~2 MPa, the ground improvement depth should be 2 times the excavation depth, and the ground improvement width should be increased as much as possible if the site condition allows. After the ground improvement using the suggested parameters, the scope of the influenced zone of deep excavation is reduced and the I—primary influenced zone no longer exists.

Displacement discontinuity method for interfacial cracks in one-dimensional hexagonal quasi-crystal coating under thermal-mechanical loading

This study derived Green’s functions of the concentrated interfacial displacement and temperature discontinuities. Green’s functions and the displacement and temperature discontinuity method were used to analyze interfacial cracks in a one-dimensional (1D) hexagonal quasi-crystal (QC) coating under thermal-mechanical loading. The boundary integral-differential equations were established for an interfacial crack, the fundamental solutions were obtained for uniformly distributed temperature, and the displacement discontinuities were applied over a line element. The near-crack-tip oscillatory singularity of stresses was eliminated by introducing the Gaussian distribution function to approximate the delta function. After this treatment, the expressions of the stress and heat flux intensity factors without oscillatory singularity, and the energy release rate (ERR) at the crack tips were obtained. The displacement and temperature discontinuity boundary element method for numerical simulation is proposed, and the influence of relevant factors is briefly discussed.

Experimental and numerical studies of the impact breakage of granite with high ejection velocities.

The impact-induced fragmentation of rock is widely and frequently encountered when natural hazards occur in mountainous areas. This type of fragmentation is an important and complex natural process that should be described. In this study, laboratory impact tests under different impact velocities were first conducted using a novel gas-driven rock impact apparatus. The three-dimensional digital image correlation (3D DIC) technique was used to monitor the dynamic fragmentation process upon impact. Then, coupled 3D finite-discrete element method (FDEM) numerical simulations were performed to numerically investigate the energy and damage evolutions and fragmentation characteristics of the sample under different impact velocities. The laboratory test results show that as the impact velocity increases, the failure pattern of the rock sample gradually changes from shear failure to splitting failure, and the fragmentation intensity increases obviously. The strain localization area gradually increases as the impact velocity increases and as the location gradually deviates away from the impacting face. In the numerical simulation, the proposed model is validated by quasi-static uniaxial compression tests and impact tests. The numerical simulations clearly show the progressive fracture process of the samples, which agrees well with the experimental observations. The evolutions of energy and damage variables were also derived based on the simulation results, which are markedly affected by the impact velocity. The fragment size distributions based on mass and number can be well fitted using a generalized extreme value law. Finally, the distribution of the fragment flying velocity and angle are analyzed.

Experimental Study on the Influence of Confining Pressure and Bedding Angles on Mechanical Properties in Coal

Extensive bedding planes have a great influence on the mechanical properties of coal. In order to study the mechanism of the effects of bedding angles on the mechanical properties and failure characteristics of coal in the Shanxi Baode coal mine, uniaxial and triaxial compression tests and numerical simulations were conducted. The strength deterioration and microstructural changes in the samples were then analyzed with discrete element method (DEM) numerical simulation. The experimental results reveal that the power function strength criterion has good applicability to the strength characteristics of this coal. It was also found that the bedding angles have a great influence on the mechanical properties of coal. The properties of peak strength at different bedding angles roughly showed a U-shaped changing trend. The maximum strength occurred at a bedding angle of 0°, whereas the minimum strength occurred at a bedding angle of 60°. The numerical simulation and test results prove that the forms of failure of different bedding coal samples are complicated and are mainly represented by tensile and shear failures.

Analysis of mechanical behaviour and fracture interaction of multi-hole rock mass with DIC measurement

To study the mechanical properties and fracture behaviour of rock containing multiple cavities, uniaxial compression experiments were carried out on marble specimens with three rectangular cavities of different layouts. The fracturing behaviour was revealed by digital image correlation (DIC) measurement. Then, discrete element method (DEM) numerical simulations were performed and the stress evolution was analysed to reveal the influence of cavity layout on fracture behaviour and coalescence patterns between them. Results showed that stress evolution and failure coalescence pattern between two cavities can be directly or indirectly influenced by the third cavity. The compressive stress concentrated around the sidewalls of a cavity may suppress the propagation of tensile cracks from other cavities. Combining laboratory and numerical results shows that during the failure evolution, firstly comes the internal coalescence of cavities before instability point, then follows failure from the three-cavity system boundary to the specimen borders resulting in the whole instability.