Kinetic Simulations Research Articles

Adsorption and evolution of hydrogen molecules on hexagonal boron nitride monolayer: a combined DFT and kinetic monte-carlo simulations study

Density functional theory (DFT) and kinetic Monte-Carlo (kMC) simulation code has been combinedly used to study the adsorption and evolution dynamics of hydrogen molecules over a hexagonal boron nitride (h-BN) monolayer. Maximum adsorption energy from van der Waals curve is predicted to be around 60 to 70 meV using two different DFT functionals. Repulsive lateral interaction between two hydrogen molecules plays a key role in determining the maximum number of adsorptions inside one unit cell of h-BN. Bader charge analysis, electron localization function (ELF), total and partial density of states (DOS) plots have been included to understand the weak interaction going on between the adsorbent and substrate. The input energy parameters from the DFT calculation has been used to perform the kMC simulation for describing the adsorption, desorption and the diffusion pattern of hydrogen molecules with a given time of exposure to an empty h-BN substrate along with the overall surface coverage.

Investigation of Flame Structures of Double-Base Propellant and Modified Double-Base Propellant Containing Nitramine Using OH-PLIF and Kinetic Simulation.

The combustion behavior of various propellant samples, including double-base propellants, pressed nitramine powders, and modified double-base propellants containing nitramine, was examined using OH-PLIF technology. The combustion process took place within a combustion chamber, and images capturing the flame at the moment of stable combustion were selected for further analysis. The distribution and production rate of OH radicals in both the double-base propellant and the nitramine-modified double-base propellant were simulated using Chemkin-17.0 software. The outcomes from both the experimental and simulation studies revealed that the concentration of OH radicals increased with a higher content of NG in the double-base propellant. In the modified double-base propellant containing RDX, the OH radical concentration decreased as the RDX content increased, with these tendencies of change aligning closely with the simulation results.

Thermodynamic and Kinetic Simulations Used for the Study of the Influence of Precipitates on Thermophysical Properties in NiTiCu Alloys Obtained by Spark Plasma Sintering.

The thermodynamic and kinetic simulations based on the re-assessment of the thermodynamic and kinetic database of the Ni-Ti-Cu system were employed to predict the phenomena of mechanical alloying, spark plasma sintering and thermal properties of the intriguing Ni-Ti-Cu system. Thermodynamic calculations are presented for the stable and unstable phases of NiTiCu materials and support a correlation with the evolving microstructure during the technological process. Also, the thermal conductivity, the thermal diffusivity and the specific heat of spark plasma sintered and aged Cu-alloyed NiTi-based shape memory alloys (NiTiCu) with two compositions, Ni45Ti50Cu5 and Ni40Ti50Cu10, are evaluated and the influence of mechanical alloying and precipitates on thermal properties is discussed. Measurements of these thermal properties were carried out from 25 °C up to 175 °C using the laser flash method, as well as differential scanning calorimetry. The thermal hysteresis of the 20 mm diameter samples was between 8.8 and 24.5 °C. The observed T0 temperatures from DSC experimental transformation features are in reasonable accordance with the thermodynamic predictions. The determined k values are between 20.04 and 26.87 W/m K and in agreement with the literature results. Moreover, this paper can provide some suggestions for the preparation of NiTiCu shape memory alloys and their applications.

A method for determining the locations and configurations of magnetic reconnection within three-dimensional turbulent plasmas

Context. Three-dimensional (3D) reconnection is an important mechanism for efficiently releasing energy during astrophysical eruptive events, which is difficult to be quantitatively analyzed especially within turbulent plasmas. Aims. In this paper, an efficient method for identifying locations and configurations of 3D reconnection from magnetohydrodynamical (MHD) data is developed. Methods. This method analyzes the local nonideal electric field and magnetic structure at an arbitrary position. As only performing algebraical manipulations on the discrete field data and avoiding computationally expensive operations such as field-line tracing and root-finding, this method naturally possesses high efficiency. To validate this method, we apply it to the 3D data from a high-resolution simulation of a Harris-sheet reconnection and a data-driven simulation of a coronal flux rope eruption. Results. It is shown that this method can precisely identify the local structures of discrete magnetic field. Through the information of nonideal electric field and the geometric attributes of magnetic field, the local structures of reconnection sites can be effectively and comprehensively determined. For fine turbulent processes, both qualitative pictures and quantitative statistical properties of small-scale reconnection structures can be obtained. For large-scale solar simulations, macro-scale magnetic structures such as flux ropes and eruption current sheets can also be recognized. Conclusions. We develop a powerful method to analyze multi-scale structures of 3D reconnection. It can be applied not only in MHD simulations but also in kinetic simulations, plasma experiments, and in situ observations.

Relevance of the P+O2 Reaction for PO Formation in Astrochemical Environments: Electronic Structure Calculations and Kinetic Simulations

Phosphorus monoxide (PO) is a key brick of prebiotic chemistry since it is a potential precursor of phosphates, which are present in all living systems. Prompted by the lack of information on the different processes involved in the formation of PO, we have revisited and analyzed in detail the P(4 S) + O2(3Σ−) and P(4 S) + O2(1Δ) reactions leading to PO. The former process has been widely studied from both experimental and theoretical points of view, however, with contradictory results. We have used high-level quantum-chemical calculations to accurately describe the reaction mechanisms. Next, rate constants have been computed using a master equation approach based on ab initio transition state theory. By incorporating the P(4 S) + O2(3Σ−) reaction in an astrochemical model, we have found that this reaction cannot be overlooked when aiming at a complete understanding of the PO abundance in regions dominated by shocks with speeds below 40 km s−1.

Kinetic instability of whistlers in electron beam-plasma systems

The whistlers in space plasmas and in magnetic fusion experiments are destabilized by beams of fast electrons. While the linear regime of instability is analytically tractable, in most practical cases, the instability operates at the saturated level during the stages of observation and measurement. The saturated states, however, involve nonlinear whistlers, which remain best accessible for analysis by kinetic simulations. Results of electromagnetic Vlasov simulations are presented, analyzing an anisotropic electron beam driven whistler instability. The simulations cover the initially unstable regime followed by a saturated or marginally stable regime. Both regimes are separated by an intermediate nonlinear regime during which the electron distribution undergoes a kinetically self-consistent modification. A linearly obtained generalized marginal stability condition is applied to the stabilized state. The condition obtained in its dispersive version shows the β|| at threshold and, in turn, the residual anisotropy, to be a function of the whistler mode number k.

On efficient modelling of radical production in cavitation assisted reactors

Process intensification by cavitation is gaining widespread attention due to the benefits that the intense bubble collapse conditions can provide, yet, several knowledge gaps exist in the modelling of such systems. This work studies the numerical prediction of single bubble dynamics and the various approaches that can be employed to estimate the changes in the chemical composition of cavitating bubbles. Specific emphasis is placed on the prediction of the radical production rates during bubble collapse and the computational performance, with the aim of coupling the single bubble dynamics to flow models for reactor hydrodynamics. The results reveal that the choice of chemical reaction approach has virtually no effect on the bubble dynamics, whereas the predicted radical production rates can differ substantially. It is found that evaluating the radical production only on temperature peaks, an approach commonly followed in literature, may result in the most erroneous estimations (on average 12.8 times larger than those of the full kinetic model), while a simplified kinetic model yields more accurate predictions (2.3 times larger) at the expense of increased computational times. Continuous evaluation of the bubble content by assuming equilibrium when the bubble temperature is above a certain threshold (≈1500K) is shown to be capable of predicting total radical production values close to those estimated by solving the kinetics of a detailed reaction model (19.8% difference), as well as requiring only 22.2% more computational costs compared to simulations without chemical reaction modelling. Such an equilibrium approach is therefore recommended for future studies aiming to couple flow simulations with single bubble dynamics to accurately predict radical production rates in cavitation devices, involving numerous bubbles following different flow trajectories. Furthermore, an algebraic expression that successfully approximates the full kinetic simulation results is proposed as a function of the initial nucleus size and the time integral of the liquid pressure when it is under vapor pressure. Such a model can be applied in modelling efforts that do not require local instantaneous radical concentrations, and paves the way for efficient closure modelling of radical production in CFD simulations of hydrodynamic reactors.

Hall physics during magnetic reconnection with collision effect

The Hall effect, decoupling between the ion and electron motions, is the core mechanism triggering fast reconnection. In plasmas with collision effects such as laboratory facilities, collision can suppress the Hall effect and influence the triggering of fast reconnection. Here, by conducting a series of kinetic simulations with varying collision parameters, we show that collisions can suppress the electron outflow, impairing the quadrupole Hall magnetic field. Besides, collision weakens the inflow of magnetic flux by reducing the charge separation and increasing the thermal pressure at the reconnection site, leading to a reduction of the Hall electric field. As the collisionality becomes larger, the Hall electric field diminishes more easily than the Hall magnetic field. We propose that the quadrupolar Hall magnetic field can be a significant indicator in reflecting Hall reconnection.

Intermittency and Dissipative Structures Arising from Relativistic Magnetized Turbulence

Kinetic simulations of relativistic turbulence have significantly advanced our understanding of turbulent particle acceleration. Recent progress has highlighted the need for an updated acceleration theory that can account for particle acceleration within the plasma’s coherent structures. Here, we investigate how intermittency modeling connects statistical fluctuations in turbulence to regions of high-energy dissipation. This connection is established by employing a generalized She–Leveque model to characterize the exponents ζ p for the structure functions Sp∝lζp . The fitting of the scaling exponents provides us with a measure of the codimension of the dissipative structures, for which we subsequently determine the filling fraction. We perform our analysis for a range of magnetizations σ and relative fluctuation amplitudes δ B 0/B 0. We find that increasing values of σ and δ B 0/B 0 allow the turbulent cascade to break sheetlike structures into smaller regions of dissipation that resemble chains of flux ropes. However, as their dissipation measure increases, the dissipative regions become less volume filling. With this work, we aim to inform future turbulent acceleration theories that incorporate particle energization from interactions with coherent structures within relativistic turbulence.

Kinetic plasma-sheath self-organization

The interaction between a plasma and a solid surface is studied in a (1D–1V) kinetic framework using a localized particle and convective energy source. Matching the quasineutral plasma region and sheath horizon is addressed in the fluid framework with a zero heat flux closure. It highlights non-polytropic nature of the physics of parallel transport. Shortfalls of this approach compared to a reference kinetic simulation highlight the importance of the heat flux as the measure of kinetic effects. Non-collisional closure and higher moment closure are used to determine the sound velocity. Within these frameworks, no gain in the fluid predictive capability is obtained. The kinetic constraint at the sheath horizon is discussed and modified to account for conditions that are actually met in simulations, namely quasineutrality with a small but finite charge density. Analyzing the distribution functions shows that collisional transfer is mandatory to achieve steady-state self-organization on the open field lines.

Revisit of bake hardening mechanism: Influence of baking on tensile properties of press hardening steels

Paint baking is a process in automotive industries to accelerate the curing of paint and increase yield strength of steel panel for improving the dent resistance due to the formation of well-known Cottrell's atmosphere. However, our careful study on bake hardening mechanism of three press hardening steels (PHS) indicates a different explanation. We developed two novel Nb/Si alloyed steels with superior tensile properties when compared with the commercial grade of 22MnB5. It was found that the paint baking at 170 °C led to the significant increase of yield strength but the decreases of both uniform and total elongation for all the three PHSs. According to the neutron diffraction results, both the baking and the tensile straining until the elastic-plastic transition strain (εep) led to the decrease of full width at half maximum (FWHM), indicating that both should release the residual stress (RS) that was generated during martensite transformation; whilst the internal friction examinations reveal no presence of Snoke peak and no influence of the baking on Snoek-Kê-Köster peak. Based on these results, the kinetics simulation on forming Cottrell's atmosphere and the relevant microstructural characterization results, we can safely conclude that the strengthening caused by the baking is not ascribed to the change of Cottrell atmosphere, carbide precipitates or retained austenite (RA) grains during the baking although commonly believed, but just due to the release of RS. Moreover, the relaxation of compressive RS in martensite and the transformation of RA grains to martensite are the main contributor to the strain hardening of invented PHSs before εep. Therefore, the baking treatment decreases both strain hardening capability and ductility of studied PHSs due to the release of the RS.

Research progress and development trends of hydrogen explosion suppression materials and mechanisms

In order to effectively control and reduce the destructive effects caused by hydrogen leakage and explosion accidents, continuously improving the theoretical system of hydrogen suppression. Based on a survey of literature, this article reviews and discusses the current research status of hydrogen explosion characteristics, explosion suppression materials, and explosion suppression mechanisms. In terms of research on hydrogen explosion characteristics, the impact of initial conditions on macroscopic parameters such as explosion limit, explosion pressure and flame propagation speed were mainly studied. In terms of research on hydrogen suppression, summarizes the research progress of hydrogen explosion suppression materials in recent years from five aspects: inert gas, water mist, solid powder, halogenated hydrocarbons and composite suppression. The suppression effects and suppression mechanisms of different materials were comparatively analyzed. The analysis found that the explosion suppression mechanism of inert gas is mainly physical inerting, and a higher concentration of inert gas is required to completely suppress hydrogen explosion. The explosion suppression mechanism of fine water mist is mainly evaporation and heat absorption, but it cannot completely suppress lean fuel hydrogen explosion. Solid powder materials have almost no inhibitory effect on hydrogen explosion. The explosion suppression mechanism of halogenated hydrocarbons is mainly to eliminate H radicals, but halogenated hydrocarbons have an promoting effect on lean fuel hydrogen. The suppression effect of composite explosion suppressants is better than that of a single explosion suppressor. However, there are currently few studies on the suppression of hydrogen explosions by composite suppressants, and the mechanism of explosion needs to be improved. The article proposes that hydrogen absorption materials, eliminate H radical targeted suppression materials and composite explosion suppression materials are the focus of research on hydrogen suppression materials. Using advanced optical diagnostic technology to study the concentration of radicals and explosion flow field characteristics, and using kinetic simulation software to reveal the complex chain reaction process are important means to study the mechanism of hydrogen explosion suppression. Large-scale explosion suppression experiments are more instructive for engineering applications, but there is a lack of large-scale equipment and experimental research.

Learning fast, accurate, and stable closures of a kinetic theory of an active fluid

Important classes of active matter systems can be modeled using kinetic theories. However, kinetic theories can be high dimensional and challenging to simulate. Reduced-order representations based on tracking only low-order moments of the kinetic model serve as an efficient alternative, but typically require closure assumptions to model unrepresented higher-order moments. In this study, we present a learning framework based on neural networks that exploits rotational symmetries in the closure terms to learn accurate closure models directly from kinetic simulations. The data-driven closures demonstrate excellent a-priori predictions comparable to the state-of-the-art Bingham closure. We provide a systematic comparison between different neural network architectures and demonstrate that nonlocal effects can be safely ignored to model the closure terms. We develop an active learning strategy that enables accurate prediction of the closure terms across the entire parameter space using a single neural network without the need for retraining. We also propose a data-efficient training procedure based on time-stepping constraints and a differentiable pseudo-spectral solver, which enables the learning of stable closures suitable for a-posteriori inference. The coarse-grained simulations equipped with data-driven closure models faithfully reproduce the mean velocity statistics, scalar order parameters, and velocity power spectra observed in simulations of the kinetic theory. Our differentiable framework also facilitates the estimation of parameters in coarse-grained descriptions conditioned on data.

Particle-in-Cell simulation of radio-frequency ion thruster RIT-1.0

Ion propulsion systems with a small thrust can be used in space missions for highly accurate controllability while possessing low power and propellant consumption. At the University of Giessen, radio-frequency ion thrusters are miniaturized with thrusts in the order of micro-Newtons (μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}N-RIT). As the size of the thruster becomes smaller the plasma properties can not be measured inside a closed plasma chamber. The simulations are needed to investigate plasma properties like temperature, density profile which would aid further opimization. In this work, we discuss kinetic simulations of plasma inside the RIT-1.0 thruster using in-house developed PlasmaPIC code that incorporates Monte-Carlo collisions (MCC) and Direct Simulation Monte Carlo (DSMC) for neutral gas distribution. We briefly present the features of PlasmaPIC and simulation of stable plasma generation. We also show the influence of neutral gas and extraction properties for the RIT-1.0 thruster under consideration.

The GARD Prebiotic Reproduction Model Described in Order and Complexity.

Early steps in the origin of life were necessarily connected to the unlikely formation of self-reproducing structures from chaotic chemistry. Simulations of chemical kinetics based on the graded autocatalysis replication domain (GARD) model demonstrate the ability of a micellar system to become self-reproducing units away from equilibrium. Even though they may be very rare in the initial state of the system, the property of their endogenous mutually catalytic networks being dynamic attractors greatly enhanced reproduction propensity, revealing their potential for selection and Darwinian evolution processes. In parallel, order and complexity have been shown to be crucial parameters in successful evolution. Here, we probe these parameters in the dynamics of GARD-governed entities in an attempt to identify characteristic mechanisms of their development in non-covalent molecular assemblies. Using a virtual random walk perspective, a value for consecutive order is defined based on statistical thermodynamics. The complexity, on the other hand, is determined by the size of a minimal algorithm fully describing the statistical properties of the random walk. By referring to a previously published diagonal line in an order/complexity diagram that represents the progression of evolution, it is shown that the GARD model has the potential to advance in this direction. These results can serve as a solid foundation for identifying general criteria for future analyses of evolving systems.

Intermittent Electron-only Reconnection at Lunar Mini-magnetospheres

Lunar crustal magnetic anomalies (LCMA) are sub-ion-gyroradius structures that have been shown to stand off the solar wind (SW) plasma from the Moon’s surface, forming shock-like discontinuities and reflecting incident SW protons. In this Letter, the results of high-resolution, 2D fully kinetic simulations show a bursty electron-only magnetic reconnection in the SW-LCMA interaction region, characterized by the quasiperiodic formation and ejection of magnetic islands and strong parallel electron flows along the X-point separator lines. The islands are observed to modify the magnetic pressure pileup and Hall electric field above the LCMA, leading to sharp increases in reflected protons that drive electromagnetic fluctuations downstream and short distances upstream in the SW.

Role of Cr in Mn-rich precipitates for Al–Mn–Cr–Zr-based alloys tailored for additive manufacturing

Novel alloy concepts enabled via additive manufacturing processes have opened up the possibility of tailoring properties beyond the scope of conventional casting and powder metallurgy processes. The authors have previously presented a novel Al–Mn–Cr–Zr-based alloy system containing three times the equilibrium amounts of Mn and Zr. The alloys were produced via a powder bed fusion-laser beam (PBF-LB) process taking advantage of rapid cooling and solidification characteristics of the process. This supersaturation can then be leveraged to provide high precipitation hardening via direct ageing heat treatments. The hardening is enabled with Zr-rich and Mn-rich precipitates. Literature study confirms that Mn-rich precipitates have a notable solubility of Cr, for example, the Al12Mn precipitate. This study aims to clarify the effect of Cr solubility in the thermodynamics and kinetics simulation and compare the precipitation simulations with samples subject to >1000 h isothermal heat treatment, thus creating an equilibrium-like state. The results show that Cr addition to the precipitates stabilizes the Al12Mn precipitate while slowing the precipitation kinetics thus producing a favourable hardening response. Such observations could be insightful while designing such alloys and optimising heat treatments of the current or even a future alloy system.

Methane pyrolysis in packed bed reactors: Kinetic modeling, numerical simulations, and experimental insights

Pyrolysis of hydrocarbon feeds such as methane (CH4) and natural gas emerges as a pivotal carbon dioxide-free large-scale hydrogen (H2) production process combined with capturing the carbon as solid material. For fundamental understanding and upscaling, the complex kinetics and dynamics of this process in technically relevant reactors such as packed and moving beds still need to be explored, particularly concerning carbon formation and its impact on reactor performance. This study integrates kinetic modeling, numerical simulations, and experimental findings to comprehensively understand CH4 pyrolysis under industrially relevant conditions and its implications for efficient H2 production and carbon capture. The investigation covers temperatures from 1273 K to 1873 K, H2 addition with H2:CH4 ratios of 0 to 4, and hot zone residence time of 1 to 7 s. Two distinct pathways lead to carbon formation: soot formation and carbon deposition. Each pathway originates from different gas-phase precursors. An elementary-step-based gas-phase reaction mechanism is coupled with a soot formation model from polycyclic aromatic hydrocarbon and a newly developed deposition model from light hydrocarbons. Numerical simulations are performed in a packed bed reactor model, incorporating a method of moments for soot formation and a model for carbon deposition. The model is evaluated against experiments and predicts the effects of operating conditions on gas-phase product distribution and carbon formation. It also estimates the change in bed-voidage over operational time. The study reveals that at the temperature 1673 K, CH4 conversion exceeds 94 %, while both H2 and solid carbon yields surpass 96 %. The sophisticated modeling and simulation framework presented herein thus provides an enhanced understanding of the CH4 pyrolysis process and presents a valuable tool for optimizing this process.

Plasma kinetic simulation in Eulerian approach fully respecting f≥0 requirement and total particle number conservation

Abstract Plasma kinetic simulation in Eulerian approach (PKSiEA) can yield a dynamic description on plasmas and hence is a popular method in theoretical plasmas physics. Its mathematical model consists of 8 members: 5 partial differential equations (PDEs), a phase space boundary condition and an initial condition of the distribution function (PDF) f, and a mandatory non-negativity requirement on f. Because Vlasov equation (VE) is a PDE with variable coefficients and the non-negativity requirement is mandatory, when 5 PDEs (4 Maxwell equations and the VE) are treated as an initial-value problem, the temporal evolution of f is difficult to be attacked by existing popular computational mathematics techniques, such as Fourier analysis technique and finite-difference technique, and hence these techniques do not yield efficient and reliable schemes of the PKSiEA. In a universal scheme of the PKSiEA presented here, because the phase space boundary condition f∣ r=∞,∣υ∣=c = 0 causes the VE to yield a conservation of total particle number ∫fd 3 rd 3 υ, we propose an efficient and strict expression of the f-profile in terms of two classes of functions of phase space coordinates r , υ , and the VE displays a universal relation between two classes. By this new expression, we express the initial-value problem of 5 PDEs in terms of allowed deformation modes from an initial f-profile, and ensure the PKSiEA fully respecting the non-negativity requirement on f.

Critical assessment of hydrogen pipe diffusion at dislocations in metals

Hydrogen-dislocation interactions have always been a crucial problem in understanding and predicting the long-standing hydrogen embrittlement (HE) phenomenon in structural metals. Particularly with respect to hydrogen diffusion, dislocations have often been assumed with the ability to facilitate hydrogen transport via dislocation pipe diffusion (DPD). Yet the experimental and theoretical studies supporting DPD remain elusive and even controversial. In this work, hydrogen kinetics at dislocations in two representative metal systems, fcc Ni and bcc Fe, were systematically investigated combining comprehensive atomistic and kinetic activation relaxation technique simulations to reveal atomic details while enabling long-time (i.e., microsecond to second) hydrogen diffusion analysis. The dislocations were found to favor hydrogen trapping and provide regions of low migration barriers for hydrogen. However, these regions of low barriers only lead to localized fast short-circuit H diffusion, but do not translate into DPD over long time, which is attributed to fast hydrogen diffusion pathways along the dislocation line direction being periodically bound by high hydrogen migration barriers. Using the polyhedral structural units (PSUs) as an effective tool, we further quantitatively analyzed the correlation between hydrogen diffusion behaviors and local dislocation structures, illustrating the structural origin leading to local short-circuit diffusion and inhibition of DPD at dislocations. The new mechanistic insights gained not only are critical for understanding hydrogen-dislocation interaction, but also advances the general knowledge on hydrogen-microstructure interaction in structural metals.