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arXiv 2023年

作者： Xu, Xiaojing Jiang, Song Sun, Wenjun School of Mathematics and Information Science Guangxi University Guangxi Nanning530004 China School of Mathematics and Physics Southwest University of Science and Technology Sichuan Mianyang621010 China Institute of Applied Physics and Computational Mathematics Fenghao East Road 2 Beijing100094 China Center for Applied Physics and Technology College of Engineering Peking University Beijing100871 China

A spatial second-order scheme for the nonlinear radiative transfer equations is introduced in this paper. The discretization scheme is based on the filtered spherical harmonics (FPN) method for the angular variable and the unified gas kinetic scheme (UGKS) framework for the spatial and temporal variables respectively. In order to keep the scheme positive and second-order accuracy, firstly, we use the implicit Monte Carlo (IMC) linearization method [6] in the construction of the UGKS numerical boundary fluxes. This is an essential point in the construction. Then, by carefully analyzing the constructed second-order fluxes involved in the macro-micro decomposition, which is induced by the FPN angular discretization, we establish the sufficient conditions that guarantee the positivity of the radiative energy density and material temperature. Finally, we employ linear scaling limiters for the angular variable in the PN reconstruction and for the spatial variable in the piecewise linear slopes reconstruction respectively, which are shown to be realizable and reasonable to enforce the sufficient conditions holding. Thus, the desired scheme, called the PPFPN-based UGKS, is obtained. Furthermore, we can show that in the regime ϵ 1 and the regime ϵ = O(1), the second-order fluxes can be simplified. And, a simplified spatial second-order scheme, called the PPFPN-based SUGKS, is thus presented, which possesses all the properties of the non-simplified one. Inheriting the merit of UGKS, the proposed schemes are asymptotic preserving. By employing the FPN method for the angular variable, the proposed schemes are almost free of ray effects. Moreover, the above-mentioned way of imposing the positivity would not destroy both AP and second-order accuracy properties. To our best knowledge, this is the first time that spatial second-order, positive, asymptotic preserving and almost free of ray effects schemes are constructed for the nonlinear radiative transfer equations without operato

关键词： Radiative transfer

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A Two-Phase Image Dejittering Approach on the Perspective of Mixed Noise Removal

SSRN

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SSRN 2023年

作者： Zhang, Yingxin Zhang, Wenxing Yin, Junping School of Mathematical Sciences University of Electronic Science and Technology of China Chengdu611731 China Institute of Applied Physics and Computational Mathematics Beijing100094 China Shanghai Zhangjiang Academy of Mathematics Shanghai201203 China

The jittery image, which is visually abnormal in jags of edge and loss of coherence, arises commonly from the area of wireless video transmission and multimedia data compression. The problem of image dejittering is challenging to resolve because of the ubiquitous noise and blur in jittery data. In this paper, we are devoted to the pixel-jitter (possibly blurry) image recovery on the perspective of mixed noise removal. By viewing a jittery image as the contamination of an ideal image with spatially-varying Gaussian+impulse noise, we proposed a two-phase (including filtering and diffusing phases) approach for recovering pixel-jitter images. At the filtering phase, outliers posed by jitters in the vicinity of edges are inhibited and inspected by median filters. At the diffusing phase, structure tensor based anisotropic diffusion is exploited to reduce the spatially-varying perturbations in piecewise smooth image regions. Upon the spectral decomposition of structure tensor and algorithmic advances in separable convex optimization, the anisotropic diffusion model can be efficiently solved with all subproblems admitting closed-form solutions. Numerical simulations on recovering jittery data demonstrate the compelling performance of the two-phase approach. © 2023, The Authors. All rights reserved.

关键词： Pixels

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Modeling and Flux Reconstruction of Unstructured Geometric Reactor

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SSRN 2024年

作者： Xiao, Bowen Zheng, Youqi Wei, Linfang Institute of Applied Physics and Computational Mathematics Beijing100094 China School of Nuclear Science and Technology Xi’an Jiaotong University Shaanxi Xi’an710094 China School of Optoelectronic Engineering Xidian University Shaanxi Xi’an710071 China

Innovative reactors are typically designed with complex geometric structures, which put forward higher requirements for deterministic reactor core calculation. In the complex geometric reactor core calculation, the LAVENDER code in the SARAX code system was used, which is based on the three-dimensional neutron transport nodal method. A modeling method based on constructive solid geometry was applied to generate arbitrary triangular-z meshes. To meet the requirements of coupling core physics calculation with thermal and other physics fields, a flux reconstruction method based on the statistics of biharmonic spline interpolation of Green’s function has been proposed, which was used to reconstruct the flux of specified region. Three reactors with different types were calculated to verify the accuracy of the flux reconstruction method. Numerical results show that the fluxes of specified region calculated by the proposed flux reconstruction method in the SARAX code were in good agreement with the direct full-core Monte-Carlo results. © 2024, The Authors. All rights reserved.

关键词： Interpolation

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Imperfectly coordinated water molecules pave the way for homogeneous ice nucleation

arXiv

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arXiv 2023年

作者： Chen, Mingyi Tan, Lin Wang, Han Zhang, Linfeng Niu, Haiyang State Key Laboratory of Solidification Processing International Center for Materials Discovery School of Materials Science and Engineering Northwestern Polytechnical University Xi’an710072 China Laboratory of Computational Physics Institute of Applied Physics and Computational Mathematics Beijing100094 China HEDPS CAPT College of Engineering Peking University Beijing100871 China DP Technology Beijing100080 China AI for Science Institute Beijing100080 China

Water freezing is ubiquitous on Earth, affecting many areas from biology to climate science and aviation technology[1–3]. Probing the atomic structure in the homogeneous ice nucleation process from scratch is of great value but still experimentally unachievable[4, 5]. Theoretical simulations have found that ice originates from the low-mobile region with increasing abundance and persistence of tetrahedrally coordinated water molecules[6–12]. However, a detailed microscopic picture of how the disordered hydrogen-bond network rearranges itself into an ordered network is still unclear. In this work, we use a deep neural network (DNN) model to "learn" the interatomic potential energy from quantum mechanical data, thereby allowing for large-scale and long molecular dynamics (MD) simulations with ab initio accuracy. The nucleation mechanism and dynamics at atomic resolution, represented by a total of 36 µs-long MD trajectories, are deeply affected by the structural and dynamical heterogeneity in supercooled water. We find that imperfectly coordinated (IC) water molecules with high mobility pave the way for hydrogen-bond network rearrangement, leading to the growth or shrinkage of the ice nucleus. The hydrogen-bond network formed by perfectly coordinated (PC) molecules stabilizes the nucleus, thus preventing it from vanishing and growing. Consequently, ice is born through competition and cooperation between IC and PC molecules. We anticipate that our picture of the microscopic mechanism of ice nucleation will provide new insights into many properties of water and other relevant materials. Copyright © 2023, The Authors. All rights reserved.

关键词： Ice

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Machine learning the Kondo entanglement cloud from local measurements

arXiv

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arXiv 2023年

作者： Aikebaier, Faluke Ojanen, Teemu Lado, Jose L. Department of Applied Physics Aalto University Espoo00076 Finland Computational Physics Laboratory Physics Unit Faculty of Engineering and Natural Sciences Tampere University TampereFI-33014 Finland Helsinki Institute of Physics P.O. Box 64 FI-00014 Finland

A quantum coherent screening cloud around a magnetic impurity in metallic systems is the hallmark of the antiferromagnetic Kondo effect. Despite the central role of the Kondo effect in quantum materials, the structure of quantum correlations of the screening cloud has defied direct observations. In this work, we introduce a machine-learning algorithm that allows to spatially map the entangled electronic modes in the vicinity of the impurity site from experimentally accessible data. We demonstrate that local correlators allow reconstructing the local many-body correlation entropy in real-space in a double Kondo system with overlapping entanglement clouds. Our machine learning methodology allows bypassing the typical requirement of measuring long-range non-local correlators with conventional methods. We show that our machine learning algorithm is transferable between different Kondo system sizes, and we show its robustness in the presence of noisy correlators. Our work establishes the potential of machine learning methods to map many-body entanglement from real-space measurements. Copyright © 2023, The Authors. All rights reserved.

关键词： Correlators

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Measurement of the Pu239(n,f) cross section from 4 keV to 100 MeV using the white neutron source at the CSNS Back-n facility

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Physical Review C 2023年第2期107卷 024606-024606页

作者： Yijia Qiu Changlin Lan Yonghao Chen Liyang Jiang Jie Bao Yiwei Yang Zhongwei Wen Rong Liu Xichao Ruan Jingyu Tang Jie Ren Hantao Jing Guangyuan Luan Ruirui Fan Yangbo Nie Xianlin Yang Xiaojun Li Han Yi Wei Jiang Tao Ye Yi Yang Shilong Liu Jincheng Wang School of Nuclear Science and Technology Lanzhou University Lanzhou 730000 China Institute of High Energy Physics Chinese Academy of Sciences (CAS) Beijing 100049 China Spallation Neutron Source Science Center Dongguan 523803 China Key Laboratory of Nuclear Data China Institute of Atomic Energy Beijing 102413 China Institute of Nuclear Physics and Chemistry China Academy of Engineering Physics Mianyang 621900 China Institute of Applied Physics and Computational Mathematics Beijing 100088 China

The neutron-induced fission cross section of Pu239 was measured relative to U235(n,f) at the back-streaming white neutron beam line (Back-n) of the China Spallation Neutron Source (CSNS). A multicell fast fission ionization chamber was used to perform the measurement. The reliability of the measurement was verified by the high consistency of U235's resonances between the measurement and the evaluation data. The Pu239 fission cross sections from 4keV to 100MeV are obtained with 1.7–5.8 % uncertainty when unfolding uncertainties are excluded. The total uncertainties including the unfolding errors, which reflect the effect of the double-bunch unfolding method, are 2.6–15 % from 10keV to 100MeV.

关键词： Fission A ≥ 220 Spectrometers & spectroscopic techniques

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First Principles Insights into Triboelectrification During Solid-Solid Contact: The Curious Case of 2d Mxenes and Aluminum

SSRN

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SSRN 2024年

作者： Abbas, Ghulam Alay-E-Abbas, Syed Muhammad Andreas, J. Larsson Shi, Yijun Division of Machine Elements Luleå University of Technology Luleå97187 Sweden Applied Physics Division of Materials Science Department of Engineering Sciences and Mathematics Luleå University of Technology Luleå97187 Sweden Computational Materials Modeling Laboratory Department of Physics Government College University Faisalabad38040 Pakistan

Recently, triboelectric nanogenerators (TENGs) have been widely used for energy harvesting and self-powered sensing due to their significant and unique advantages. However, the intrinsic mechanisms that contribute to tribo-electricification (TE) between two materials remain as a subject of rigorous debate. In addition to predicting the qualitative charge transfer in solid-solid contacts based on the difference in the work functions of the two moieties constituting the interface, we argue that it is essential to obtain atomic-level, first principles, insights into the bonding properties, quantitative charge transfer, and the possible presence of a electrostatic potential barrier at the interface to fully understand the TE mechanism of a system. We have utilized dispersion-corrected density functional theory (DFT) calculations in this study to systematically investigate the TE potential of the contact between Ti3X2 (X = C or N) MXene monolayers and Al(111). The effect of surface functionalization of MXene (Ti3X2R2 where X = C or N and R = -O, -OH, or -F) was also investigated. For these heterostructures, we have analyzed the adhesive energy of the interfaces, the nature of interaction through the electron localization function (ELF), and the charge distribution, which have revealed distinct characteristics of MXene/Al contacts for these monolayer/metal interfaces at their equilibrium distance and the changes in their properties under uniaxial pressure. Among all the metallic 2D MXene variants investigated in this study, we have determined that Ti3C2F2/Al and Ti3N2F2/Al interfaces show exceptional potential for TE. © 2024, The Authors. All rights reserved.

关键词： Nanogenerators

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SHARP ILL-POSEDNESS FOR THE NON-RESISTIVE MHD EQUATIONS IN SOBOLEV SPACES

arXiv

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arXiv 2024年

作者： Chen, Qionglei Nie, Yao Ye, Weikui Institute of Applied Physics and Computational Mathematics Beijing100191 China School of Mathematical Sciences LPMC Nankai University Tianjin300071 China School of Mathematical Sciences South China Normal University Guangdong Guangzhou510631 China

In this paper, we prove a sharp ill-posedness result for the incompressible non-resistive MHD equations. In any dimension d ≥ 2, we show the ill-posedness of the non-resistive MHD equations in H d/2 −1(d)×H d/2 (d), which is sharp in view of the results of the local well-posedness in Hs−1(d) × Hs(d)(s > d/2) established by Fefferman et al.(Arch. Ration. Mech. Anal., 223 (2), 677-691, 2017). Furthermore, we generalize the ill-posedness results from H d/2 −1(d)×H d/2 (d) to Besov spaces (Equation presented) and (Equation presented) for 1 ≤ p ≤ ∞, q > 1. Different from the ill-posedness mechanism of the incompressible Navier-Stokes equations in B ∞,q−1 [3, 22], we construct an initial data such that the paraproduct terms (low-high frequency interaction) of the nonlinear term make the main contribution to the norm inflation of the magnetic field. Copyright © 2024, The Authors. All rights reserved.

关键词： Sobolev spaces

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Theory of dipole moment reconstruction by attosecond transient absorption spectroscopy

引用

Physical Review A 2021年第6期104卷 063121-063121页

作者： Xiaoxia Wu Linxuan Zhang Shaofeng Zhang Difa Ye Institute of Modern Physics Chinese Academy of Sciences Lanzhou 730000 China University of Chinese Academy of Sciences Beijing 100049 China Laboratory of Computational Physics Institute of Applied Physics and Computational Mathematics Beijing 100088 China Graduate School China Academy of Engineering Physics Beijing 100193 China

We propose a general procedure for the reconstruction of time-dependent dipole moment via transient absorption spectrum, within the framework of linear response theory. The reconstruction method is demonstrated with two examples: the self-reconstruction of extreme-ultraviolet-light-atom interaction (analytically) and the infrared-dressed helium system (numerically). A correlation coefficient is introduced to quantify the fidelity of the reconstruction, which reaches almost 100% over a wide range of laser parameters and thus indicates the robustness of our reconstruction method. Our theory provides a solid basis for understanding the recent experiment [Phys. Rev. Lett. 121, 173005 (2018)], explaining when and why the dipole moment can be well reconstructed solely from the absorption spectrum even without exact knowledge of the attosecond probe pulse.

关键词： Optical transient phenomena Ultrafast phenomena Electric moment Polarization Perturbative methods Schroedinger equation

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arXiv

引用

arXiv 2021年

作者： Yao, Jiansheng Zhao, Yingkui Zhang, Hantian Ye, Difa Ouyang, Biyao Institute of Applied Physics and Computational Mathematics Beijing100088 China

As proved by previous study, the similarity of electromagnetic processes in plasmas will be violated by Coulomb collisions between electron and ions. Therefore, there is no similarity in highly ionized collisional plasma. But the situation will be completely different for collisional plasmas with low ionization degree. The main collision type will change from electron-ion Coulomb collision to electron-molecule collision, and a new variable (the number density of neutral molecules) will be introduced into the similarity constraint, which can increase the degree of freedom. Thus, in this condition, the similarity restriction caused by the collision process doesn’t conflict with the other restrictions. Therefore, the similarity for electromagnetic process in collisional plasmas can be valid for partially ionized plasmas. In this paper, we propose the similarity in partially ionized plasmas, and prove it via particle in cell/Monte Carlo (PIC/MCC) simulation. Our research has a wide range of engineering applications. © 2021, CC BY.

关键词： Collisional plasmas

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