Particle Simulation Method Research Articles

Electrochemical migration and dendrite growth between two electrodes: Experiments and Brownian dynamics simulations

Electrochemical migration (ECM) is a common cause of failure in printed circuit boards (PCB) due to dendrite growth between exposed metallic conductors under moisture especially in the presence of flux residues. Understanding this phenomenon, therefore, is of interest from technological and economical points of view. Here, we report experimental results of water drop tests on surfaces contaminated by various amount of flux residues, which are supplemented with Brownian dynamics (BD) results to gain molecular insight into ECM and dendrite growth. BD is a particle simulation method where the ions are modeled explicitly. We show dendrite growth curves (the length of the longest dendrite as a function of time) obtained from visual examination with digital video microscope and from BD simulations. Results for the time to failure are also shown for various values of the electric field strength and cation (Sn2+) concentration (dilution of contamination). We report a mapping of experimental and simulation results that is necessary due to the large concentrations and electric field strengths used in the BD simulations.

Simulation of a Pelton turbine using the moving particle simulation method: application to two challenging situations

ABSTRACT Simulation of the flow in a Pelton turbine is a challenging task for mesh based methods due to the air/water mixture and the rotation of the runner. Meshless based methods are better suited for such flows and take advantage of the graphical power unit to speed up the calculation. Simulations of a Pelton turbine are carried out using the software Particleworks based on moving particle simulation, which is a meshless method. The efficiency drop due to the ‘falaise’ effect and the erosion of the splitter are computed and compared with either experimental measurements or the literature. The results show that this method is able to capture the tendency of the efficiency drop in agreement with the available data and at a lower computational cost than the mesh based methods. These encouraging results should motivate the community to test and validate such an approach.

Design of a compact coaxial distributed intense relativistic electron beam focusing system

In order to achieve the objectives of compactness and miniaturization in high-power microwave systems, along with reducing system energy consumption, this paper presents a design of a compact coaxial distributed intense relativistic electron beam (IREB) focusing system based on the transit time oscillator. The design methodology integrates theoretical analysis with a 2.5-dimensional particle simulation method. The entire IREB focusing system is composed of three sets of distributed magnetic rings and front-end focusing structures. The layout of this magnetic system is optimized based on periodic permanent magnets, leading to the formation of a quasi-trapezoidal wave magnetic field configuration. This optimization reduces the required number of magnetic rings while ensuring the efficient transmission of the IREB. To prevent the breakdown of the loaded magnetic rings and to optimize the radial electric field in the diode region, an additional front-end focusing structure was added. Based on the aforementioned structural configuration, a 100% electron beam transmission rate was achieved within a 150 mm range.

Investigation of hydraulic characteristics of Pelton turbine with casing structure using a moving particle simulation method

Abstract This paper reports the initial results of three dimensional simulation of the jet/bucket interactions in the vertical Pelton turbine with different casing structures. The casing is an important part of the Pelton turbine, and a reasonable casing structure can reduce the interaction between the jet and bucket, so as to improve the hydraulic efficiency of the turbine. Previous research has little numerical simulation analysis on the casing. In this paper, a Moving Particle Simulation(MPS) method without grid generation is adopted to compare and analyze the hydraulic characteristics of the runner under the regular hexagonal and circular casing structures. The simulation results show that the hydraulic efficiency of the runner under the hexagonal casing structure is better, and the runner time average torque is improved by 3.09%, which compared with the circular casing. The distribution of free surface flows in the hexagonal casing is analysis, and the obvious interference phenomenon between jets can be observed, which has an important guiding role in the design of the casing structure of Pelton turbine in the future.

Numerical simulation study of hydraulic performance of the ball valve implied in ultra-high head Pelton turbine

Abstract This paper is deal with the hydraulic performance of the ball valve served in ultra-high head Pelton turbine systems, the Moving Particle Simulation method is used for numerical simulation uncommonly instead of the Finite Volume method. A valve rotating range of 0 degree to 90 degree is simulated by the software Particleworks, according to the pressure change through the valve, we get the flow resistance coefficient which is consistent with those of other similar literature. The change of the internal flow distribution can be visually observed. At the small rotation degree, the flow velocity entering the ball valve is very high and the disturbance in flow field is large. With the rotation degree increases, the flow in ball valve and the outlet pipe is gradually stabilized, the pressure difference before and after the ball valve is gradually reduced. It brings reference value for the structure and size design of the ball valve in the future.

An Efficient Explicit Moving Particle Simulation Solver for Simulating Free Surface Flow on Multicore CPU/GPUs

The moving particle simulation (MPS) method is a simulation technique capable of calculating free surface and incompressible flows. As a particle-based method, MPS requires significant computational resources when simulating flow in a large-scale domain with a huge number of particles. Therefore, improving computational speed is a crucial aspect of current research in particle methods. In recent decades, many-core CPUs and GPUs have been widely utilized in scientific simulations to significantly enhance computational efficiency. However, the implementation of MPS on different types of hardware is not a trivial task. In this study, we present an implementation method for the explicit MPS that utilizes the Taichi parallel programming language. When it comes to CPU computing, Taichi’s computational efficiency is comparable to that of OpenMP. Nevertheless, when GPU computing is utilized, the acceleration of Taichi in parallel computing is not as fast as the CUDA implementation. Our developed explicit MPS solver demonstrates significant performance improvements in simulating dam-break flow dynamics.

Hundreds-petawatt laser pulses shaping and heavy ion acceleration based on conical plasma channels

In this paper, the effects of conical plasma channels on the laser pulses shaping and the heavy ion acceleration under the extreme light field conditions of hundreds-petawatt are investigated using a particle simulation method. The law of influence of the conical plasma channel on the spatio-temporal waveform and intensity of the incident laser is analyzed, when the QED effect is taken into account. The reason for the shaping laser-enhanced heavy ion acceleration is given, and the role of the QED effect in the acceleration process is explained.<br>It is found that, due to the non-linear interference and focusing effects, the conical plasma channel can shape the spatio-temporal waveform of the laser pulse and enhance the laser intensity. A tightly focused (beam waist radius < 1 μm) and ultra-high intensity (enhanced 6 times) shaping laser is obtained for a linearly polarized laser with an intensity of 5.46 × 10<sup>22</sup> W/cm<sup>2</sup> and a waist radius of 10 μm at an incident angle of <i>θ</i> = 10°. In the simulation, the conical plasma channel is fully ionized high-Z gold plasma with the electron densities up to <i>n<sub>e</sub></i>=2626.5<i>n<sub>c</sub></i>. Therefore most of the laser energy in the channel is reflected by the channel wall, and the QED effect has less impact on laser focusing and shaping. Using this laser to accelerate an ultra-thin flat target placed at the end of the channel. It is found that, the radiation reaction force can effectively suppress the transverse expansion of the ultra-thin flat target caused by the electron heating and the transverse non-uniform of the laser intensity. The transparency time of the ultra-thin flat target is prolonged, which will allow the gold ions to be fully accelerated. Ultimately, gold ions with a cutoff energy of up to ~ 240 GeV can be obtained. The results are expected to provide theoretical references and technical support for the design of future hundreds-petawatt laser heavy ion acceleration experiments and their application research of high-quality ion source, such as nucleus-nucleus collisions.

Collision integrals of electronically excited atoms in air plasmas. I. N–N and O–O interactions

The current work presents the collision integral data for N(4 S)–N(4 S, 2 D, 2 P) and O(3 P, 1 D, 1 S)–O(3 P, 1 D, 1 S) interactions in the temperature range of 500–50 000 K. The collision integrals are calculated based on high-quality potential energy curves (PECs) obtained from fitting the high-level calculation data in a wide energy range to the neural network (NN) functions. In the construction of PECs, the diabatic PECs are adopted when avoided crossings exist because the diabatic paths are much more likely to be followed for such situations. Moreover, the nonadiabatic transition effects are estimated to be negligible for PECs crossings. The accuracy of traditional analytical formulas to fit PECs are also examined. It is found that the collision integral calculations are sensitive to the accuracy of PECs and the NN based PECs overwhelm the others. The contribution of inelastic excitation exchange processes to the diffusion collision integrals are also computed by using an accurate evaluation of the differences of PECs for gerade and ungerade pairs of excited atoms. Finally, based on the new collision integral data, we calibrate the collision model parameters suitable for the widely used particle simulation methods. The collision integrals and collision models developed in this work can be used to support high-confidence simulations of weakly ionized air plasma problems.

An autonomous coverage path planning algorithm for maritime search and rescue of persons-in-water based on deep reinforcement learning

The prevalence of maritime transportation and operations is increasing, leading to a gradual increase in drowning accidents at sea. In the context of maritime search and rescue (SAR), it is essential to develop effective search plans to improve the survival probability of persons-in-water (PIWs). However, conventional SAR search plans typically use predetermined patterns to ensure complete coverage of the search area, disregarding the varying probabilities associated with the PIW distribution. To address this issue, this study has proposed a maritime SAR vessel coverage path planning framework (SARCPPF) suitable for multiple PIWs. This framework comprises three modules, namely, drift trajectory prediction, the establishment of a multilevel search area environment model, and coverage search. First, sea area-scale drift trajectory prediction models were employed using the random particle simulation method to forecast drift trajectories. A hierarchical probability environment map model was established to guide the SAR of multiple SAR units. Subsequently, we integrated deep reinforcement learning with a reward function that encompasses multiple variables to guide the navigation behavior of ship agents. We developed a coverage path planning algorithm aimed at maximizing the success rates within a limited timeframe. The experimental results have demonstrated that our model enables vessel agents to prioritize high-probability regions while avoiding repeated coverage.

Particle method simulation of direct-chill casting process including breakout

Particle method simulation of direct-chill casting process including breakout

Extension of moving particle simulation by introducing rotational degrees of freedom for dilute fiber suspensions

AbstractWe develop a novel Moving Particle Simulation (MPS) method to reproduce the motion of fibers floating in sheared liquids accurately. In conventional MPS schemes, if a fiber suspended in a liquid is represented by a one‐dimensional array of MPS particles, it is entirely aligned to the flow direction due to the lack of shear stress difference between fiber–liquid interfaces. To address this problem, we employ the micropolar fluid model to introduce rotational degrees of freedom into the MPS particles. The translational motion of liquid and solid particles and the rotation of solid particles are calculated with the explicit MPS algorithm. The fiber is modeled as an array of micropolar fluid particles bonded with stretching, bending and torsional potentials. The motion of a single rigid fiber is simulated in a three‐dimensional shear flow generated between two moving solid walls. We show that the proposed method is capable of reproducing the fiber motion predicted by Jeffery's theory which is different from the conventional MPS simulations.

Application of Moving Particle Simulation Method on Water-skipping

Inspired by the phenomenon of stone-skipping in life, the problem of continuous water-skipping in trans-media vehicles is widely concerned, which is characterized by multiphase flow-solid coupling, dynamic boundary, liquid surface fragmentation and high impact loads. Based on the Moving Particle Semi-implicit numerical method, this paper investigates the disc’s motion characteristics and liquid surface variation, focusing on the influence of the water entry parameters on the sloshing phenomenon of the water-skipping. The simulation results indicate that the disc water skipping process can be divided into three stages: water entry, sliding and re-flying, during which the disc is with large impact loads and the water surface is with large scale breaking and droplet splashing. With certain rotation velocity, the disc could maintain a smooth water skipping, while there will be a lateral displacement due to the Magnus effect. The larger the water-entry velocity and angel is, the larger impact load is, the faster velocity decay rate is, the larger vacuole area is, and larger the water surface broking area and the droplet splashing is. Predicted water-skipping phenomenon agrees well with the experimental result obtained from high-speed photography.

Applying surface tension as pressure boundary condition in free surface flow analysis by moving particle simulation method

A model that introduces surface tension as a pressure boundary condition, named the surface tension as pressure (STP) model, was developed for free surface flow analyses by the moving particle simulation (MPS) method. The STP model assigns to surface particles the liquid pressure of Laplace’s formula. The model is an alternative to previous models that apply surface tension as volume force such as the continuum surface force model. Problems that appeared when using the volume force models, such as the dependencies of calculation results on particle resolution and pressure gradient accuracy, were solved by using the STP model. Calculations predicted the theoretical values of the internal pressure of a 3D spherical droplet and the oscillation period of a 2D elliptic droplet over a wide range of surface tension coefficients and droplet sizes with errors less than 10%. Since the STP model is easy to implement, does not increase computation cost from previous models, and does not require surface reconstruction or additional marker particles, the model is suitable for practical and large-scale free surface flow problems that involve violent deformation of the liquid surface such as liquid atomization.

A δf PIC method with forward–backward Lagrangian reconstructions

In this work, we describe a δf particle simulation method where the bulk density is periodically remapped on a coarse spline grid using a forward–backward Lagrangian approach. This method is designed to handle plasma regimes where the densities strongly deviate from their initial state and may evolve into general profiles. We describe the method in the case of an electrostatic particle-in-cell scheme and validate its qualitative properties using a classical two-stream instability subject to a uniform oscillating drive.

Concurrent multi-scale modeling of granular materials: Role of coarse-graining in FEM-DEM coupling

The finite element method (FEM) is commonly used for modeling continuum media, while particle simulation methods like the so-called discrete element method (DEM) are used for discrete systems. Coupling the discrete (DEM) and continuum (FEM) methods is conventionally achieved through a direct mapping between discrete particles and finite elements. Coarse-graining (CG) is a micro–macro transition (discrete-to-continuum) method that maps discrete particle data onto smooth, differentiable fields that satisfy the continuum equations. By choosing an appropriate length scale (the coarse-graining width c), the coarse-grained fields are then homogenized and projected onto a FEM spatial discretization.This concept is utilized here to reformulate FEM-DEM coupling methods, both surface and volume, where in the limiting case of c→0, the classical coupling is recovered. For surface coupling, the discrete particle–surface contact forces are first mapped onto a continuous surface traction field (using CG) which is then coupled to the continuum FEM model. For volume coupling (also known as the Arlequin framework), the homogenization operators are enriched with CG functions, offering a non-local coupling approach between discrete particles, their continuum fields, and the finite elements.The CG enrichment represents a new strategy that consists of (1) a discrete-to-continuum mapping and (2) a continuum-to-continuum coupling based on “CG-enriched homogenization” (CGH). It is shown for surface coupling that the CG-enriched formulation not only leads to more accurate results, conserving symmetry, but also reduces energies generated by the coupling. For volume coupling, there is consistently less numerical dissipation with than without CG-enrichment, especially when the dynamic load contains high-frequency content. Finally, the optimal CG widths are identified for very simple test cases, with which the surface or volume coupling performs best.CGH can be potentially extended beyond the present examples, by considering other continuum fields (e.g., higher-order) and equations (e.g., multi-physics), and used to formulate other multi-scale modeling methods.

Fast asymptotic-numerical method for coarse mesh particle simulation in channels of arbitrary cross section

Particles traveling through inertial microfluidic devices migrate to focusing streamlines. We present a numerical method that calculates migration velocities of particles in inertial microfluidic channels of arbitrary cross section by representing particles by singularities. Refinements to asymptotic analysis are given that improve the regularity of the singularity approximation, making finite element approximations of flow and pressure fields more effective. Sample results demonstrate that the method is computationally efficient and able to capture bifurcations in particle focusing positions due to changes in channel shape and Reynolds number.

A numerical study on the effect of dust particles on tritium deposition on plasma-facing materials

Tritium deposition is a key issue for fusion devices. The existence of dust leads to increase of tritium deposition rates due to the large specific surface area (SSA) of dust grains. Experiments in JET-ILW have indicated that the deposition and retention behavior of hydrogen on dust grains is complicated. In present work, the deposition behavior of tritium on the dust is studied by a hybrid particle simulation method. Our results indicate that the large SSA of dust is not the only mechanism that increases tritium deposition. Dust size and the electrostatic sheath induced by the dust are also important factors that influence the tritium deposition. The retention rate enhanced by dust grains is also predicted based on our study.

Three‐dimensional weakly compressible moving particle simulation coupled with geometrically nonlinear shell for hydro‐elastic free‐surface flows

AbstractA three‐dimensional fluid–structure interaction solver based on an improved weakly compressible moving particle simulation (WC‐MPS) method and a geometrically nonlinear shell structural model is developed and applied to hydro‐elastic free‐surface flows. The fluid–structure coupling is performed by a polygon wall boundary model that can handle particles and finite elements of distinct sizes. In WC‐MPS, a tuning‐free diffusive term is introduced to the continuity equation to mitigate nonphysical pressure oscillations. Discrete divergence operators are derived and applied to the polygon wall boundary, of which the numerical stability is enhanced by a repulsive Lennard–Jones force. Additionally, an efficient technique to deal with the interaction between fluid particles placed at opposite sides of zero‐thickness walls is proposed. The geometrically nonlinear shell is modeled by an unstructured mesh of six‐node triangular elements. Finite rotations are considered with Rodrigues parameters and a hyperelastic constitutive model is adopted. Benchmark examples involving free‐surface flows and thin‐walled structures demonstrate that the proposed model is robust, numerically stable and offers more efficient computation by allowing mesh size larger than that of fluid particles.

A novel all-metal metamaterial for constructing relativistic slow wave structure

In recent vacuum electronic devices, metamaterials have shown an obvious advantage of miniaturization. Due to increasing miniaturization demand, research of metamaterials in high-power microwave (HPM) field is now also a hotspot. For better applications of metamaterials, there are still two issues to be solved, the low space limit current of the structure and the low uniformity of the working electric field. To solve these problems, we construct a novel all-metal metamaterial structure by applying a 45° rotational arrangement in space and a four-support rod structure. This new metamaterial structure has a great uniformity of the axis electric field, and its electric field fluctuation is reduced from 8.6% to 2.8%. Based on this metamaterial structure, we proposed a novel relativistic slow wave structure, and its working mode has a negative dispersion characteristic. Using the method of expanding the electron beam channel, the space limit current of the relativistic slow wave structure is greatly improved to 12.3 kA, and the particle simulation method is used to prove the increase in the space limit current. In addition, it is found that this slow wave structure has higher coupling impedance, which provides a better working environment for an annular relativistic electron beam. Therefore, this metamaterial structure has great potential in the HPM field.

Gyro-average method for global gyrokinetic particle simulation in realistic tokamak geometry

Gyro-average is a crucial operation to capturing the essential finite Larmor radius (FLR) effect in gyrokinetic simulation. In order to simulate strongly shaped plasmas, an innovative multi-point average method based on non-orthogonal coordinates has been developed to improve the accuracy of the original multi-point average method in gyrokinetic particle simulation. This new gyro-average method has been implemented in the gyrokinetic toroidal code (GTC). Benchmarks have been carried out to prove the accuracy of this new method. In the limit of concircular tokamak, ion temperature gradient (ITG) instability is accurately recovered for this new method and consistency is achieved. The new gyro-average method is also used to solve the gyrokinetic Poisson equation, and its correctness is confirmed in the long-wavelength limit for realistically shaped plasmas. The improved GTC code with the new gyro-average method is used to investigate the ITG instability with EAST magnetic geometry. The simulation results show that the correction induced by this new method in the linear growth rate is more significant for short-wavelength modes where the FLR effect becomes important. Due to its simplicity and accuracy, this new gyro-average method can find broader applications in simulating shaped plasmas in realistic tokamaks.