Density Evolution Research Articles

Size-dependent errors in real-time electron density propagation.

Real-time (RT) electron density propagation with time-dependent density functional theory (TDDFT) or Hartree-Fock (TDHF) is one of the most popular methods to model the charge transfer in molecules and materials. However, both RT-TDHF and RT-TDDFT within the adiabatic approximation are known to produce inaccurate evolution of the electron density away from the ground state in model systems, leading to large errors in charge transfer and erroneous shifting of peaks in absorption spectra. Given the poor performance of these methods with small model systems and the widespread use of the methods with larger molecular and material systems, here we bridge the gap in our understanding of these methods and examine the size-dependence of errors in RT density propagation. We analyze the performance of RT density propagation for systems of increasing size during the application of a continuous resonant field to induce Rabi-like oscillations, during charge-transfer dynamics, and for peak shifting in simulated absorption spectra. We find that the errors in the electron dynamics are indeed size dependent for these phenomena, with the largest system producing the results most aligned with those expected from linear response theory. The results suggest that although the RT-TDHF and RT-TDDFT methods may produce severe errors for model systems, the errors in charge transfer and resonantly driven electron dynamics may be much less significant for more realistic, large-scale molecules and materials.

Temporal Evolution of Dune Number Density in a Barchan Dune Field

AbstractThe present work examines the temporal evolution of dune number density in desert barchan dune fields and the effects of wind velocity, particle diameter and sand supply thickness using the Comprehensive Scale‐Coupled Model of dune field considering turbulence. Results show that the dune number density exhibits a temporal evolution trend of “increase—decrease—tend to be stable,” which is similar to the “overshoot” phenomenon of the per unit width sand transport rate evolving with time. The larger the wind velocity and the smaller the particle diameter are, the enhanced surface sand transport by wind causes the dune number density peak to be larger and appear earlier, but the more active dune interactions result in a higher decay rate during the decline stage. In addition, a small sand supply thickness slows sand accumulation and thus delays the initial development of barchans. With an increase in the sand supply thickness, more small‐scale barchans are formed earlier in dune fields due to gradually sufficient sand amounts, which promotes dune interactions and thus accelerates the decay of dune number density. Nevertheless, as the sand supply thickness continuously increases, the barchans link together to form dune chains, resulting in a slowed migration velocity and a reduced spatial density of interactions. The present work makes up for the lack of understanding of the initial evolution stage of barchan dune fields and the dune interactions in existing studies, and thus may be helpful for gaining insight into the dynamic evolution process of aeolian dune systems.

Construction of protograph LDPC codes based on the convolution neural network

This paper presents an intelligent protograph construction algorithm. Protograph LDPC codes have shown excellent error correction performance and play an important role in wireless communications. Random search or manual construction are often used to obtain a good protograph, but the efficiency is not high enough and many experience and skills are needed. In this paper, a fast searching algorithm is proposed using the convolution neural network to predict the iterative decoding thresholds of protograph LDPC codes effectively. A special input data transformation rule is applied to provide stronger generalization ability. The proposed algorithm converges faster than other algorithms. The iterative decoding threshold of the constructed proto-graph surpasses greedy algorithm and random search by about 0.53 dB and 0.93 dB respectively under 100 times of density evolution. Simulation results show that quasi-cyclic LDPC (QC-LDPC) codes constructed from the proposed algorithm have competitive performance compared to other papers.

In-situ study on the tensile property and fracture behavior of Cr coating for nuclear fuel cladding

It has been suggested that Cr-coated zircaloy is an excellent material for accident-tolerant fuel (ATF) cladding. It is found that Cr coating shows multi-cracking behavior under external load due to its embrittlement. Different fabrication parameters will lead to different crack resistances of the coatings, but the brittle cracking characteristic is common. The estimation of the strength and structural integrity of the fuel cladding depend on the cracking characteristic of the coating. The Cr coating’s tensile characteristic for ATF cladding was examined utilizing the in-situ mechanical testing method. The surface crack initiation, propagation, and fracture process were captured in real time, and the crack density evolution of two different process parameters was compared. The results demonstrate that the first long channel crack occurs at a small strain, then the crack density increases rapidly and gradually reaches saturation with increasing stress. Different fabrication parameters will change the cracking resistance. Compared with the previous sample, the new sample started to crack at a larger strain, the crack density reached saturation faster with a larger saturated crack density, and the crack resistance is inferior. Many small cracks in different directions were observed near the fracture surface. Moreover, there was no interfacial spallation under stress, demonstrating the Cr coating’s excellent interfacial adhesive property.

Seismic response analysis of subway station structure under random excitation based on probability density evolution method

The subway station structure mainly faces the uncertainty of ground motion and surrounding soil mass. Comprehensive analysis of the seismic response of subway station structures under random excitation is the key to ensuring the safety of urban people's lives and property. In this paper, the random shear wave velocity profile model and the random ground motion model characterizing uncertainty are established. Using a high-dimensional space optimization point selection strategy significantly reduces the number of sample points and computational costs of nonlinear time history analysis (NLTHA). Combining the probability density evolution method (PDEM) with finite element analysis, the stochastic seismic response analysis of the subway station structure under dual random excitations is realized. In addition, statistical methods are used to analyze the distribution characteristics of the most unfavorable seismic responses of the subway station structure. The research results show that when random excitation is not considered, the adverse seismic damage risk of subway station structures will be ignored. The evolution of the seismic dynamic response of subway station structures is mainly controlled by the amplitude of random ground motion acceleration. Additional random shear wave velocities can increase the lateral deformation range of the subway station structure, leading to a decrease in the dynamic reliability of the structure. PDEM can conduct stochastic seismic response analysis of the subway station structure from both time and response dimensions. The proposed method provides strong support for a comprehensive analysis of the seismic response of underground structures.

The Evolution of Whistler Hiss‐Like Waves Across Plasmapause: Hiss to Exohiss

AbstractThe exohiss waves are hiss‐like whistler mode waves that occur outside plasmapause. Although these waves can cause the loss of radiation belt electrons ∼<MeV through resonant scattering, the formation and evolution processes of exohiss waves remain incompletely covered yet. In this letter, by virtue of measurement from Van Allen Probes and ray tracing simulation, we provide a direct observational evidence for the origin of exohiss waves. We analyze the evolution of power spectrum density of hiss‐like whistler waves during their propagation. The results suggest that exohiss waves originate from mixed result of hiss waves leaked out from different L shells inside the plasmapause. Additionally, we found that frequencies of exohiss waves are primarily below 700 Hz, it is probably because the hiss waves with higher frequencies experience more severe Landau damping. Our results significantly improve understanding of the evolution of whistler waves in radiation belts.

Turnaround density evolution encodes cosmology in simulations

Turnaround density evolution encodes cosmology in simulations

Statistical investigation of the widths of supra-arcade downflows observed during a solar flare

ABSTRACT Supra-arcade downflows (SADs) are dark voids descending towards the post-reconnection flare loops and exhibit obvious variation in properties like width. However, due to the lack of further statistical studies, the mechanism behind such variations hitherto remains elusive. Here, we statistically investigated widths of 81 SADs observed in one flare by the Solar Dynamics Observatory (SDO). For each of SADs, six moments were selected with equal time intervals to measure their widths at different stages of their evolution. It is found that most SADs show a roughly monotonous width decrease during their descents, while some SADs with small initial widths can have complex evolutions. 3D reconstruction results based on SDO and Solar Terrestrial Relations Observatory Ahead (STEREO-A) images and thermal properties analysis reveal that differences in magnetic and plasma environments may result in that SADs in the north are overall wider than those in the south. Additionally, correlation analysis between the width and other parameters of SADs was further conducted and revealed that (1) SADs with different initial widths show no significant differences in their temperature and density evolution characteristics; (2) SADs with small initial widths usually appear in lower heights, where more frequent collisions between SADs could lead to their intermittent acceleration, width increment, and curved trajectories. These results indicate that SADs with different initial widths are produced the same way, while different environments (magnetic field or plasma) could affect their subsequent width evolutions.

Irregular Bloch-Zener oscillations in two-dimensional flat-band Dirac materials

When a static electrical field is applied to a two-dimensional (2D) Dirac material, Landau-Zener transition (LZT) and Bloch-Zener oscillations can occur. Employing $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathcal{T}}_{3}$ lattices as a paradigm for a broad class of 2D Dirac materials, we uncover two phenomena. First, due to the arbitrarily small energy gaps near a Dirac point that make it more likely for LZTs to occur than in other regions of the Brillouin zone, the distribution of differential LZT probability in the momentum space can form a complicated morphological pattern. Second, a change in the LZT morphology as induced by a mutual switching of the two distinct Dirac points can lead to irregular Bloch-Zener oscillations characterized by a nonsmooth behavior in the time evolution of the electrical current density associated with the oscillation. These phenomena are due to mixed interference of quantum states in multiple bands modulated by the geometric and dynamic phases. We demonstrate that the adiabatic-impulse model describing Landau-Zener-St\"uckelberg interferometry can be exploited to calculate the phases, due to the equivalence between the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathcal{T}}_{3}$ lattice subject to a constant electrical field and strongly periodically driven two- or three-level systems. The degree of irregularity of Bloch-Zener oscillations can be harnessed by selecting the morphology pattern, which is potentially experimentally realizable.

Hypoelliptic and spectral estimates for some linear operators of Landau type

We are interested in the inhomogeneous Landau equation which describes the evolution of a particle density f = f (t, x, v) representing at time t $\ge$ 0, the density of particles at position x $\in$ R 3 and velocity v $\in$ R 3. The study is motivated by the linearization of the Landau equation near Maxwellian distribution. In this article, we establish hypoelliptic estimates, a localization of the spectrum and estimates of the resolvent of the linear Landau operator with hard potentials and Maxwellian molecules. The proof is based on a multiplier method and requires fine pseudo-differential calculus tools.

FIREbox: simulating galaxies at high dynamic range in a cosmological volume

ABSTRACT We introduce a suite of cosmological volume simulations to study the evolution of galaxies as part of the Feedback in Realistic Environments project. FIREbox, the principal simulation of the present suite, provides a representative sample of galaxies (∼1000 galaxies with $M_{\rm star}\gt 10^8\, M_\odot$ at z = 0) at a resolution ($\Delta {}x\sim {}20\, {\rm pc}$ , $m_{\rm b}\sim {}6\times {}10^4\, M_\odot$ ) comparable to state-of-the-art galaxy zoom-in simulations. FIREbox captures the multiphase nature of the interstellar medium in a fully cosmological setting (L = 22.1 Mpc) thanks to its exceptionally high dynamic range (≳106) and the inclusion of multichannel stellar feedback. Here, we focus on validating the simulation predictions by comparing to observational data. We find that star formation rates, gas masses, and metallicities of simulated galaxies with $M_{\rm star}\lt 10^{10.5-11}\, M_\odot$ broadly agree with observations. These galaxy scaling relations extend to low masses ($M_{\rm star}\sim {}10^7\, M_\odot$ ) and follow a (broken) power-law relationship. Also reproduced are the evolution of the cosmic HI density and the HI column density distribution at z ∼ 0–5. At low z , FIREbox predicts a peak in the stellar-mass–halo-mass relation but also a higher abundance of massive galaxies and a higher cosmic star formation rate density than observed, showing that stellar feedback alone is insufficient to reproduce the properties of massive galaxies at late times. Given its high resolution and sample size, FIREbox offers a baseline prediction of galaxy formation theory in a ΛCDM Universe while also highlighting modelling challenges to be addressed in next-generation galaxy simulations.

Numerical Prediction of Microstructure Evolution of Small-Diameter Stainless Steel Balls during Cold Skew Rolling.

The wear resistance and hardness of stainless steel (SS) balls formed by cold skew rolling are effectively improved due to the change in internal microstructure. In this study, based on the deformation mechanism of 316L stainless steel, a physical mechanism-based constitutive model was established and implemented in a subroutine of Simufact to investigate the microstructure evolution of 316L SS balls during the cold skew rolling process. The evolution of equivalent strain, stress, dislocation density, grain size, and martensite content was studied via simulation during the steel balls' cold skew rolling process. The corresponding skew rolling experiments of steel balls were carried out to verify the accuracy of the finite element (FE) model results. The results showed that the macro dimensional deviation of steel balls fluctuates less, and the microstructure evolution agrees well with the simulation results, which proves that the established FE model has high credibility. It shows that the FE model, coupled with multiple deformation mechanisms, provides a good prediction of the macro dimensions and internal microstructure evolution of small-diameter steel balls during cold skew rolling.

Weighted BATS Codes with LDPC Precoding

Batched Sparse (BATS) codes are a type of network coding scheme that use a combination of random linear network coding (RLNC) and fountain coding to enhance the reliability and efficiency of data transmission. In order to achieve unequal error protection for different data, researchers have proposed unequal error protection BATS (UEP-BATS) codes. However, current UEP-BATS codes suffer from high error floors in their decoding performance, which restricts their practical applications. To address this issue, we propose a novel UEP-BATS code scheme that employs a precoding stage prior to the weighted BATS code. The proposed precoding stage utilizes a partially regular low-density parity-check (PR-LDPC) code, which helps to mitigate the high error floors in the weighted BATS code We derive the asymptotic performance of the proposed scheme based on density evolution (DE). Additionally, we propose a searching algorithm to optimize precoding degree distribution within the complexity range of the precoding stage. Simulation results show that compared to the conventional weighted BATS codes, our proposed scheme offers superior UEP performance and lower error floor, which verifies the effectiveness of our scheme.

Impacts of climate change on wind energy potential in Australasia and South-East Asia following the Shared Socioeconomic Pathways

Wind energy is poised to play a major role in the energy transition. Fluctuations in global atmospheric circulation are expected as a result of climate change, and wind projections based on the most up-to-date scenarios of climate change, the Shared Socioeconomic Pathways (SSPs), anticipate significant changes in wind energy potential in many regions; so far, these changes have not been studied in Southeastern Asia and Australasia, a region with notable wind energy potential. This work investigates the evolution of wind power density and its temporal variability considering the latest scenarios of climate change, the SSPs. More specifically, two scenarios are considered, SSP2-4.5 and SSP5-8.5, corresponding to moderate and high emissions, respectively. As many as 18 global climate models are considered and compared against past-present data, and those that perform best are retained to build a large multi-model ensemble. The results show that projected changes in mean wind power density at the end of the 21st century are of little significance (typically below 5 %); nevertheless, this value can be far surpassed locally. In certain areas (e.g., Vietnam, Borneo) and seasons, remarkable changes in wind power density (exceeding 150 %) are anticipated. Typically, mean values and temporal variability changes are greater in the high-emissions scenario, however, seasonal variability is projected to be more pronounced in the moderate-emissions scenario. These effects of climate change on wind energy potential must be taken into account in the development of wind power in the region, for they will affect the energy production and, therefore, the economic viability of wind farms – not least in those areas where drastic changes are projected.

Impact of self-sputtering in high power impulse magnetron sputtering (HiPIMS) with helium

Conventional magnetron discharge is a widely used technology for many applications. In the last decade, the high current density sputtering regime has been an interesting alternative for tailoring thin film properties. In this paper, we focused on the electrical characterization of the helium magnetron plasma operated at average gas pressure (5 Pa) with a molybdenum target. Optical emission spectroscopy diagnostics also supports this study by providing information on electron density evolution. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (6 A cm−2), corresponding to different discharge regimes. The self-sputtering yield plays a key role in high-power impulse magnetron sputtering discharge operated with helium. The electron density is highly dependent on the presence of a metal.

Evaluating the influence of deformation variables on dynamic recrystallization behavior using a crystal plasticity model

The present study is an attempt to model dynamic recrystallization (DRX) in a single phase metal using a mean field crystal plasticity (MFCP) based approach. A new empirical equation is proposed to model nucleation, in which the nucleation rate is a function of microstructure and plasticity descriptors that are known to affect DRX behavior, such as the temperature, strain rate, grain fineness and stored energy. Grains undergo nucleation when their dislocation density exceeds a threshold value. Subsequently, new grains grow based on the difference in stored deformation energy with respect to the average value over all grains. The MFCP-DRX model is able to correctly predict trends for the flow stress, dislocation density evolution, grain size evolution and kinetics across a range of temperatures and strain rates for uniaxial compression. Transition of the flow stress from single to multiple peaks is observed with increasing temperature and decreasing strain rate, thus comparing well against known DRX trends. The evolutions of crystallographic texture during DRX in uniaxial compression and plane strain compression are compared against experimental observations. A sensitivity analysis is conducted to understand the effect of variables on nucleation and growth. The competition between nucleation and growth dominated deformation in different strain regimes is analyzed in detail across various temperatures and strain rates.

Dynamic Reliability Analysis of Layered Slope Considering Soil Spatial Variability Subjected to Mainshock–Aftershock Sequence

The slope instability brought on by earthquakes frequently results in significant property damage and casualties. At present, the research on displacement response of a slope under earthquake has mainly emphasized the action of the mainshock, without accounting for the impact of an aftershock, and the spatial variability of material parameters is often neglected. The spatial variability of parameters is fully accounted for in this paper, and dynamic reliability of permanent displacement (DP) of a slope produced by the mainshock–aftershock sequence (MAS) is studied. A slope reliability analysis method is proposed based on the Newmark displacement method and the generalized probability density evolution method (GPDEM) to quantify the effect of the spatial variability of materials parameters on dynamic reliability. Firstly, the parameter random field is generated based on the spectral representation method, and the randomly generated parameters are assigned to the finite element model (FEM). In addition, the random simulation method of MAS considering the correlation between aftershock and mainshock is adopted based on the Copula function to generate the MAS. Then, the DP of slopes caused by the MAS considering the spatial variability is calculated based on the Newmark method. The impacts of the coefficient of variation (COV) and aftershock on the DP of slope is analyzed by means of mean values. Finally, the effect of COV and aftershock on the reliability of DP is explained from a probabilistic point of view based on the GPDEM. The results revealed that with the increase in the COV, the mean of the DP of the slope shows a trend of increasing gradually. The DP of slope is more sensitive to the coefficient of variation of friction angle (COVF). The mean DP of the slope induced by the MAS is larger compared to the single mainshock, and the PGA has a significant impact on the DP.

A physically-based constitutive model for a novel heat resistant martensitic steel under different cyclic loading modes: Microstructural strengthening mechanisms

Cyclic responses of a novel heat resistant martensitic steel, 9Cr3Co3W1CuVNbB steel, under different loading modes were studied to reveal its complex strengthening mechanisms at high temperature. Based on the experimental observations, dislocation strengthening, precipitation strengthening by M23C6 phase, MX phase, and Cu-rich phase, and subgrain boundary strengthening were the main mechanisms for its excellent fatigue and creep-fatigue properties. In particular, the dynamic process of interaction between phase and dislocation were studied with the help of molecular dynamics method, and the different contributions of hard and soft phases in the studied steel were determined in fatigue and creep-fatigue loading. Based on these phenomena, a physically-based constitutive model was proposed for both fatigue and creep-fatigue (dwell fatigue at elevated temperature) tests considering various micromechanical mechanisms. Three ways for dislocation annihilation were proposed to simulate the dislocation evolution under different loadings. In addition, the effect of Cu-rich phase was modeled by critical breaking angle and dislocation line tension. The capability of the proposed model under different loading modes was verified by comparing cyclic responses, hysteresis loops, stress relaxation, and dislocation density evolution. The proposed model provides an alternative perspective on understanding fatigue and creep-fatigue behaviors of heat resistant martensitic steels owning the similar strengthening mechanisms.

A nonparametric seismic reliability analysis method based on Bayesian compressive sensing – Stochastic harmonic function method and probability density evolution method

In engineering practice, the evaluation of seismic reliability of high-rise reinforced concrete structure relies on the probabilistic modelling of material properties. However, the measured data of material parameters in engineering practice is usually inadequate to determine the probability model accurately, especially for the problems involving spatial variability. Traditional parametric reliability analysis methods can be biased for this kind of problem. Therefore, a nonparametric seismic reliability analysis method is proposed based on the Bayesian compressive sensing – stochastic harmonic function method and the probability density evolution method. In this method, the conditional random fields are generated and applied to represent material properties of concrete. As a result, the seismic reliability of a high-rise reinforced concrete shear wall model structure is analyzed using the probability density evolution method.

Static and transient optical properties of thin film indium tin oxide during laser excitation

The mechanisms of ultrafast laser-induced free electron generation and the effect of the free electrons on the transient optical properties in indium tin oxide is investigated by means of numerically solving rate equations and ultrafast transient experiments. A dynamical model for the transient optical properties of indium tin oxide thin films during excitation with an ultrashort laser pulse with fluences between 0.4 J cm−2 to 2.3 J cm−2 is developed and compared to pump probe microscopy during and after excitation with laser pulses. In the non-excited case, thin film interferences dominate the shape of the reflection spectrum. During laser excitation, (quasi-)free electron dynamics drastically alter the optical properties. The temporal evolution of the (quasi-)free electron density is numerically computed using rate equations for irradiation with laser pulses with various peak fluences for a pulse duration of 700 fs and a central wavelength of 1056 nm. We found that a thermal ionization term that takes electronic states within the band gap into account has to be considered to reproduce the reflectivities obtained by pump probe measurements. Additionally, we found that cascade ionization dominates the multiphoton ionization for indium tin oxide.