arXiv subjects

Dong Wu

Publications and source records attributed to Dong Wu.

At least 55 records · Page 3Linked to original sources

arXiv2025

Heat transfer simulation of window frames with SPHinXsys

Maintaining a comfortable temperature inside a building requires appropriate thermal insulation of windows, which can be optimised iteratively with numerical simulation. Smoothed particle hydrodynamics(SPH) is a fully Lagrangian method widely used for simulating multi-physics applications with high computational efficiency and accuracy. It is advantageous in physically coupled problems such as heat-fluid-solid or any other type of physically coupled simulations. The focus of this study is to simulate the heat transfer process in various window frames under convective boundary conditions according to ISO10077-2:2012. This paper demonstrates the accuracy and compatibility of SPH when dealing with heat transfer problems, which ensures further development of thermal coupling with other physical fields. The results and methods used in this paper provide some guidance on how to properly handle heat transfer simulations using SPH, which can be extended to multi-physics coupled simulations in the future.

cs.CE↗

arXiv2024

Selective excitation of collective modes in multiband superconductor MgB2

Recent developments in nonequilibrium and nonlinear terahertz (THz) spectroscopies have significantly advanced our understanding of collective excitations in superconductors. However, there is still debate surrounding the identification of Higgs or Leggett modes, as well as BCS charge fluctuations, in the well-known two-band superconductor MgB$_2$. Here, we utilized both multi-cycle and single-cycle THz pump-broadband THz probe techniques to investigate the THz nonlinear response of MgB$_2$. Through multicycle THz pump-THz probe experiments on MgB$_2$, we observed distinct nonlinear signals at both the fundamental frequency ($ω$) and the second harmonic frequency (2$ω$) of the pump pulses, which exhibited resonant enhancement at temperatures where their frequencies respectively match 2$Δ_π(T)$. They are mainly attributed to the $π$-band Higgs mode. By adjusting the THz pump pulse to a single-cycle waveform that satisfies non-adiabatic excitation criteria, we observed an over-damped oscillation corresponding to the Leggett mode. Our findings contribute to solving the ongoing debates and demonstrate the selective excitation of collective modes in multiband superconductors, offering new insights into the interaction between Higgs and Leggett modes.

cond-mat.supr-con↗

arXiv2024

Structure of weakly collisional shock waves of multicomponent plasmas inside hohlraums of indirect inertial confinement fusions

In laser-driven indirect inertial confinement fusion (ICF), a hohlraum--a cavity constructed from high-Z materials--serves the purpose of converting laser energy into thermal x-ray energy. This process involves the interaction of low-density ablated plasmas, which can give rise to weakly collisional shock waves characterized by a Knudsen number $K_n$ on the order of 1. The Knudsen number serves as a metric for assessing the relative importance of collisional interactions. Preliminary experimental investigations and computational simulations have demonstrated that the kinetic effects associated with weakly collisional shock waves significantly impact the efficiency of the implosion process. Therefore, a comprehensive understanding of the physics underlying weakly collisional shock waves is essential. This research aims to explore the formation and fundamental structural properties of weakly collisional shock waves within a hohlraum, as well as the phenomena of ion mixing and ion separation in multicomponent plasmas. Weakly collisional shocks occupy a transition regime between collisional shock waves ($K_n \ll 1$) and collisionless shock waves ($K_n \gg 1$), thereby exhibiting both kinetic effects and hydrodynamic behavior. These shock waves are primarily governed by an electrostatic field, which facilitates significant electrostatic sheath acceleration and ion reflection acceleration. The differentiation of ions occurs due to the varying charge-to-mass ratios of different ion species in the presence of electrostatic field, resulting in the separation of ion densities, velocities, temperatures and concentrations. The presence of weakly collisional shock waves within the hohlraum is expected to affect the transition of laser energy and the overall efficiency of the implosion process.

physics.plasm-ph↗

arXiv2024

Room-temperature non-volatile optical manipulation of polar order in a charge density wave

Utilizing ultrafast light-matter interaction to manipulate electronic states of quantum materials is an emerging area of research in condensed matter physics. It has significant implications for the development of future ultrafast electronic devices. However, the ability to induce long-lasting metastable electronic states in a fully reversible manner is a long-standing challenge.Here, by using ultrafast laser excitations, we demonstrate the capability to manipulate the electronic polar states in the charge-density-wavematerial EuTe4 in a non-volatile manner. The process is completely reversible and is achieved at room temperature with an all-optical approach. Each induced non-volatile state brings about modifications to the electrical resistance and second harmonic generation intensity. The results point to layer-specific phase inversion dynamics by which photoexcitation mediates the stacking polar order of the system. Our findings extend the scope of non-volatile all-optical control of electronic states to ambient conditions, and highlight a distinct role of layerdependent phase manipulation in quasi-two-dimensional systems with inherent sublayer stacking orders.

cond-mat.str-el↗

arXiv2024

Manipulation of annular electron beams in plasmas

The annular electron beam has significant practical potential in high-energy physics and condensed matter physics, which can be used to edge-enhancement electron imaging, collimation of antiprotons in conventional linear accelerators, acceleration of positively particles like positrons, structured X-ray generation and manipulation of nanomaterials. The quality of an annular electron beam depends on its energy, flux and topology. In this article, we study the transport characteristics of annular electron beam in a plasma medium and propose a scheme to modify it. According to our theory and full three-dimensional LAPINS simulations, we have found that the self-generated magnetic field focuses the incident annular electron beam, enabling the adjustment of its annular width (AW). Besides, the annular electron beam, endowed with orbital angular momentum (OAM), exhibits contrasting transport characteristics compared to an electron beam devoid of OAM. The former requires an external magnetic field to ensure stable transportation in the plasma. Under the influence of this magnetic field, the radius of the annular electron beam oscillates periodically, with the direction of change whether increasing or decreasing dependent on the field's strength. In this case, the radius of annular electron beam will be affected by the external magnetic field and allows for the simultaneous adjustment of its radius and AW, significantly broadening its application range.

physics.plasm-ph↗

arXiv2024

A generalized non-hourglass updated Lagrangian formulation for SPH solid dynamics

Hourglass modes, characterized by zigzag particle and stress distributions, are a common numerical instability encountered when simulating solid materials with updated Lagrangian smoother particle hydrodynamics (ULSPH). While recent solutions have effectively addressed this issue in elastic materials using an essentially non-hourglass formulation, extending these solutions to plastic materials with more complex constitutive equations has proven challenging due to the need to express shear forces in the form of a velocity Laplacian. To address this, a generalized non-hourglass formulation is proposed within the ULSPH framework, suitable for both elastic and plastic materials. Specifically, a penalty force is introduced into the momentum equation to resolve the disparity between the linearly predicted and actual velocities of neighboring particle pairs, thereby mitigating the hourglass issue. The stability, convergence, and accuracy of the proposed method are validated through a series of classical elastic and plastic cases, with a dual-criterion time-stepping scheme to improve computational efficiency. The results show that the present method not only matches or even surpasses the performance of the recent essentially non-hourglass formulation in elastic cases but also performs well in plastic scenarios.

cs.CE↗

arXiv2024

Learn to Memorize and to Forget: A Continual Learning Perspective of Dynamic SLAM

Simultaneous localization and mapping (SLAM) with implicit neural representations has received extensive attention due to the expressive representation power and the innovative paradigm of continual learning. However, deploying such a system within a dynamic environment has not been well-studied. Such challenges are intractable even for conventional algorithms since observations from different views with dynamic objects involved break the geometric and photometric consistency, whereas the consistency lays the foundation for joint optimizing the camera pose and the map parameters. In this paper, we best exploit the characteristics of continual learning and propose a novel SLAM framework for dynamic environments. While past efforts have been made to avoid catastrophic forgetting by exploiting an experience replay strategy, we view forgetting as a desirable characteristic. By adaptively controlling the replayed buffer, the ambiguity caused by moving objects can be easily alleviated through forgetting. We restrain the replay of the dynamic objects by introducing a continually-learned classifier for dynamic object identification. The iterative optimization of the neural map and the classifier notably improves the robustness of the SLAM system under a dynamic environment. Experiments on challenging datasets verify the effectiveness of the proposed framework.

cs.CV↗

arXiv2024

Coexistence of interacting charge density waves in a layered semiconductor

Coexisting orders are key features of strongly correlated materials and underlie many intriguing phenomena from unconventional superconductivity to topological orders. Here, we report the coexistence of two interacting charge-density-wave (CDW) orders in EuTe4, a layered crystal that has drawn considerable attention owing to its anomalous thermal hysteresis and a semiconducting CDW state despite the absence of perfect FS nesting. By accessing unoccupied conduction bands with time- and angle-resolved photoemission measurements, we find that mono- and bi-layers of Te in the unit cell host different CDWs that are associated with distinct energy gaps. The two gaps display dichotomous evolutions following photoexcitation, where the larger bilayer CDW gap exhibits less renormalization and faster recovery. Surprisingly, the CDW in the Te monolayer displays an additional momentum-dependent gap renormalization that cannot be captured by density-functional theory calculations. This phenomenon is attributed to interlayer interactions between the two CDW orders, which account for the semiconducting nature of the equilibrium state. Our findings not only offer microscopic insights into the correlated ground state of EuTe4 but also provide a general non-equilibrium approach to understand coexisting, layer-dependent orders in a complex system.

cond-mat.str-el↗

arXiv2024

Dynamical pressure boundary condition for weakly-compressible smoothed particle hydrodynamics

This paper introduces a novel dynamical pressure boundary condition for weakly-compressible smoothed particle hydrodynamics (WCSPH). Unlike previous methods that rely on indirect approaches or ghost particles, our method integrates the dynamical boundary pressure directly into the SPH approximation of the pressure gradient on near-boundary particles. Additionally, we develop a meshfree bidirectional in-/outflow buffer by periodically relabelling buffer particles at each time step, a concept that has not been explored before. This simple yet effective buffer facilitates the simulation of both uni- and bidirectional flows, especially those with mixed in-/outflow boundary conditions. We validate the accuracy and convergence of our method through benchmark cases with available analytical solutions. Furthermore, we demonstrate its versatility in hemodynamic simulations by investigating generic carotid and aorta flows with the Windkessel model, paving the way for studying the cardiovascular system within a unified meshfree computational framework.

physics.flu-dyn↗

arXiv2024

A generalized essentially non-hourglass total Lagrangian SPH solid dynamics

In this paper, we tackle a persistent numerical instability within the total Lagrangian smoothed particle hydrodynamics (TLSPH) solid dynamics. Specifically, we address the hourglass modes that may grow and eventually deteriorate the reliability of simulation, particularly in the scenarios characterized by large deformations. We propose a generalized essentially non-hourglass formulation based on volumetric-deviatoric stress decomposition, offering a general solution for elasticity, plasticity, anisotropy, and other material models. Comparing the standard SPH formulation with the original non-nested Laplacian operator applied in our previous work \cite{wu2023essentially} to handle the hourglass issues in standard elasticity, we introduce a correction for the discretization of shear stress that relies on the discrepancy produced by a tracing-back prediction of the initial inter-particle direction from the current deformation gradient. The present formulation, when applied to standard elastic materials, is able to recover the original Laplacian operator. Due to the dimensionless nature of the correction, this formulation handles complex material models in a very straightforward way. Furthermore, a magnitude limiter is introduced to minimize the correction in domains where the discrepancy is less pronounced. The present formulation is validated, with a single set of modeling parameters, through a series of benchmark cases, confirming good stability and accuracy across elastic, plastic, and anisotropic materials. To showcase its potential, the formulation is employed to simulate a complex problem involving viscous plastic Oobleck material, contacts, and very large deformation.

cs.CE↗

arXiv2024

Validation of Classical Transport Cross Section for Ion-Ion Interactions Under Repulsive Yukawa Potential

Value of cross section is a fundamental parameter to depict the transport of charged particles in matters. Due to masses of orders of magnitude higher than electrons and convenience of realistic calculation, the cross section of elastic nuclei-nuclei collision is usually treated via classical mechanics. The famous Bohr criterion was firstly proposed to judge whether the treatment via classical mechanics is reliable or not. Later, Lindhard generalized the results of Coulomb to screening potentials. Considering the increasing importance of detailed ion-ion interactions under modern simulation codes in inertial confinement fusion (ICF) researches, the validation of classical transport cross section for ion-ion interactions in a big range of parameter space is certainly required. In this work, the transport cross sections via classical mechanics under repulsive Yukawa potential are compared with those via quantum mechanics. Differences of differential cross sections are found with respect to scattering angles and velocities. Our results generally indicate that the classical picture fails at the cases of both low and high velocities, which represent a significant extension of the famous Bohr criterion and its generalized variations. Furthermore, the precise validation zones of classical picture is also analysed in this work. This work is of significant importance for benchmarking the modern ion-kinetic simulation codes in ICF researches, concerning the stopping power of $α$ particles in DT fuels, ion-ion friction and viscous effects in the formation of kinetic shocks.

physics.plasm-ph↗

arXiv2024

Exact Normal Modes of Quantum Plasmas

The normal modes, i.e., the eigen solutions to the dispersion relation equation, are the most fundamental properties of a plasma, which also of key importance to many nonlinear effects such as parametric and two-plasmon decay, and Raman scattering. The real part indicates the intrinsic oscillation frequency while the imaginary part the Landau damping rate. In most of the literatures, the normal modes of quantum plasmas are obtained by means of small damping approximation (SDA), which is invalid for high-$k$ modes. In this paper, we solve the exact dispersion relations via the analytical continuation (AC) scheme, and, due to the multi-value nature of the Fermi-Dirac distribution, reformation of the complex Riemann surface is required. It is found that the change of the topological shape of the root locus in quantum plasmas is quite different from classical plasmas, in which both real and imaginary frequencies of high-$k$ modes increase with $k$ in a steeper way than the typical linear behaviour as appears in classical plasmas. As a result, the temporal evolution of a high-$k$ perturbation in quantum plasmas is dominated by the ballistic modes.

physics.plasm-ph↗

arXiv2023

Pump-induced terahertz conductivity response and peculiar bound state in Mn3Si2Te6

We report the significant enhancement on ultrafast terahertz optical conductivity and the unexpected formation of a polaronic-like state in semiconductor Mn3Si2Te6 at room temperature. With the absorption of pump photons, the low-frequency terahertz photoconductivity spectrum exhibits a significant rise, quickly forming a broad peak and subsequently shifting to higher energy. The short-lived nature of the broad peak, as well as the distribution of optical constants, strongly points towards a transient polaron mechanism. Our study not only provides profound insights into the remarkable photoelectric response of Mn3Si2Te6 but also highlights its significant potential for future photoelectric applications.

cond-mat.mtrl-sci↗

arXiv2023

Essentially non-hourglass and non-tensile-instability SPH elastic dynamics

Since the tension instability was discovered in updated Lagrangian smoothed particle hydrodynamics (ULSPH) at the end of the 20th century, researchers have made considerable efforts to suppress its occurrence. However, up to the present day, this problem has not been fundamentally resolved. In this paper, the concept of hourglass modes is firstly introduced into ULSPH, and the inherent causes of tension instability in elastic dynamics are clarified based on this brand-new perspective. Specifically, we present an essentially non-hourglass formulation by decomposing the shear acceleration with the Laplacian operator, and a comprehensive set of challenging benchmark cases for elastic dynamics is used to showcase that our method can completely eliminate tensile instability by resolving hourglass modes. The present results reveal the true origin of tension instability and challenge the traditional understanding of its sources, i.e., hourglass modes are the real culprit behind inducing this instability in tension zones rather that the tension itself. Furthermore, a time integration scheme known as dual-criteria time stepping is adopted into the simulation of solids for the first time, to significantly enhance computational efficiency.

cs.CE↗

arXiv2023

Giant nonlinear optical wave mixing in van der Waals compound MnPSe3

Optical nonlinearities, one of the most fascinating properties of two-dimensional (2D) materials, are essential for exploring novel physics in 2D systems and developing next-generation nonlinear optical applications. While tremendous efforts have been made to discover and optimize second-order nonlinear optical responses in various 2D materials, higher odd-order nonlinear processes, which are in general much less efficient than second order ones, have been paid less attention despite their scientific and applicational significance. Here we report giant odd-order nonlinear optical wave mixing in a correlated van der Waals insulator MnPSe3 at room temperature. Illuminated by two near-infrared femtosecond lasers simultaneously, it generates a series of degenerate and non-degenerate four- and six-wave mixing outputs, with conversion efficiencies up to the order of $10^{-4}$ and $10^{-6}$ for the four- and six-wave mixing processes, respectively, far exceeding the efficiencies of several prototypical nonlinear optical materials (GaSe, LiNbO3). This work highlights the intriguing prospect of transition metal phosphorous trichalcogenides for future research of the nonlinear light matter interactions in 2D systems and for potential nonlinear photonic applications.

physics.optics↗

arXiv2023

Large-scale Kinetic Simulations of Colliding Plasmas within a Hohlraum of Indirect Drive Inertial Confinement Fusions

The National Ignition Facility has recently achieved successful burning plasma and ignition using the inertial confinement fusion (ICF) approach. However, there are still many fundamental physics phenomena that are not well understood, including the kinetic processes in the hohlraum. Shan et al. [Phys. Rev. Lett, 120, 195001, 2018] utilized the energy spectra of neutrons to investigate the kinetic colliding plasma in a hohlraum of indirect drive ICF. However, due to the typical large spatial-temporal scales, this experiment could not be well simulated by using available codes at that time. Utilizing our advanced high-order implicit PIC code, LAPINS, we were able to successfully reproduce the experiment on a large scale of both spatial and temporal dimensions, in which the original computational scale was increased by approximately 7 to 8 orders of magnitude. When gold plasmas expand into deuterium plasmas, a kinetic shock is generated and propagates within deuterium plasmas. Simulations allow us to observe the entire progression of a strong shock wave, including its initial formation and steady propagation. Although both electrons and gold ions are collisional (on a small scale compared to the shock wave), deuterium ions seem to be collisionless. This is because a quasi-monoenergetic spectrum of deuterium ions can be generated by reflecting ions from the shock front, which then leads to the production of neutrons with unusual broadening due to beam-target nuclear reactions. This work displays an unprecedented kinetic analysis of an existing experiment, shedding light on the mechanisms behind shock wave formation. It also serves as a reference for benchmark simulations of upcoming new simulation codes and may be relevant for future research on mixtures and entropy increments at plasma interfaces.

physics.plasm-ph↗

arXiv2023

An explicit multi-time stepping algorithm for multi-time scale coupling problems in SPH

Simulating physical problems involving multi-time scale coupling is challenging due to the need of solving these multi-time scale processes simultaneously. In response to this challenge, this paper proposed an explicit multi-time step algorithm coupled with a solid dynamic relaxation scheme. The explicit scheme simplifies the equation system in contrast to the implicit scheme, while the multi-time step algorithm allows the equations of different physical processes to be solved under different time step sizes. Furthermore, an implicit viscous damping relaxation technique is applied to significantly reduce computational iterations required to achieve equilibrium in the comparatively fast solid response process. To validate the accuracy and efficiency of the proposed algorithm, two distinct scenarios, i.e., a nonlinear hardening bar stretching and a fluid diffusion coupled with Nafion membrane flexure, are simulated. The results show good agreement with experimental data and results from other numerical methods, and the simulation time is reduced firstly by independently addressing different processes with the multi-time step algorithm and secondly decreasing solid dynamic relaxation time through the incorporation of damping techniques.

cs.CE↗

arXiv2023

An SPH formulation for general plate and shell structures with finite deformation and large rotation

In this paper, we propose a reduced-dimensional smoothed particle hydrodynamics (SPH) formulation for quasi-static and dynamic analyses of plate and shell structures undergoing finite deformation and large rotation. By exploiting Uflyand-Mindlin plate theory, the present surface-particle formulation is able to resolve the thin structures by using only one layer of particles at the mid-surface. To resolve the geometric non-linearity and capture finite deformation and large rotation, two reduced-dimensional linear-reproducing correction matrices are introduced, and weighted non-singularity conversions between the rotation angle and pseudo normal are formulated. A new non-isotropic Kelvin-Voigt damping is proposed especially for the both thin and moderately thick plate and shell structures to increase the numerical stability. In addition, a shear-scaled momentum-conserving hourglass control algorithm with an adaptive limiter is introduced to suppress the mismatches between the particle position and pseudo normal and those estimated with the deformation gradient. A comprehensive set of test problems, for which the analytical or numerical results from literature or those of the volume-particle SPH model are available for quantitative and qualitative comparison, are examined to demonstrate the accuracy and stability of the present method.

math.NA↗