Abstract Excluding enclosed voids during design remains a critical challenge in additive manufacturing, impacting performance and manufacturability. This study presents a topology optimization method featuring a novel explicit void exclusion constraint. Unlike classical methods requiring additional physical fields, our innovative virtual displacement method addresses common mechanical problems through just one type of physical field, leveraging virtual compliance with polarized distribution for void or solid detection. The void exclusion constraint incorporates a penalized auxiliary volume term, which simplifies parameter tuning, eases computation, and facilitates integration. Reducing degrees of freedom, avoiding global stiffness matrix assembly, and simplifying sensitivity analysis sharply improve the efficiency of finite element analysis for the virtual displacement field. Besides, this approach combines density-based topology evolution for stable iteration and level-set-based result smoothing. The systematic method can eliminate predefined internal or boundary-adjacent voids, regulate local topology, simplify structures, and transform slender or thin-walled designs into thicker ones with minimal performance deterioration. It is implementable, cost-effective, performance-enhanced, efficient, and versatile. It has been validated via 2D/3D examples, 3D-printed prototypes, and comparisons with existing methodologies.

Soil arching is a common load transfer mechanism in geotechnical engineering, which is significantly influenced by soil particle size distribution (PSD). Existing studies have not fully understood the PSD effect, specifically the mean particle size (d50) and coefficient of uniformity (Cu), on the arching evolution and critical height. To this end, this study tries to investigate the PSD effect on the evolution of soil arching using the discrete element method. A series of two-dimensional trapdoor tests were simulated on eight specimens with varying d50 and Cu. The macroscopic responses and microscopic mechanisms were systematically analyzed. Simulations reveal that an increase in d50 or Cu leads to a reduction in the critical arching height. This indicates that coarser and better-graded granular soils promote a more rapid development of soil arching effect, thereby enhancing the initial load-transfer efficiency. At the microscopic level, specimens with larger d50 develop stronger yet sparser force chains and exhibit greater normal contact force anisotropy, while specimens with higher Cu form denser contact networks with larger coordination numbers, resulting in more stable force transmission. The findings of this study strongly suggest that PSD significantly controls the soil arching development process through its governing role in fabric formation and force chain structure. Besides, the implications of this study offer direct relevance for optimizing backfill material design in geotechnical practices, notably in pile-supported embankments and underground excavation projects.

Owing to the bio-geological origin, calcareous sands are characterized by irregular particle shapes and abundant internal pores. These weak soil structures undergo extensive particle breakage under specific stress conditions. In recent years, microbial-induced calcite precipitation (MICP), a green soil treatment technique, has been widely utilized to improve the mechanical properties of calcareous sands. The treatment effect of MICP on calcareous sands and the underlying mechanism are still relatively unclear owing to the complex microstructures of the sands. This study investigated the micromechanism of MICP treatment on calcareous sands via the discrete element method (DEM). First, a group of calcareous sand particles were individually treated via a proposed MICP process. Single-particle crushing tests were conducted on the MICP-treated calcareous particles and a control group of untreated calcareous particles. The experimental results were further used to verify and calibrate the DEM models and parameters. The DEM model of MICP-treated calcareous particles was established via a novel modeling technique using the commercial PFC3D platform, which considered the inter-particle bridging and intra-particle pore filling effects by MICP treatment. Through this method, a series of MICP-treated calcareous sand samples subjected to uniaxial compression were simulated. The effects of calcite content and its spatial non-uniformity on the compression behaviors of the bio-cemented calcareous sand samples were studied. The micromechanics in terms of crack propagation and distribution as obtained from DEM were investigated to elucidate the mechanism of the MICP treatment of calcareous sands.

To investigate the natural convective process in a hydrodynamically and thermally anisotropic porous medium at the representative elementary volume(REV) scale, the present work presented a multiplerelaxation-time lattice Boltzmann method(MRT-LBM) based on the assumption of local thermal non-equilibrium conditions(LTNE). Three sets of distribution function were used to solve the coupled momentum and heat transfer equations. One set was used to compute the flow field based on the generalized non-Darcy model; the other two sets were used to solve the temperature fields of fluid and solid under the LTNE. To describe the anisotropy of flow field of the porous media, a permeability tensor and a Forchheimer coefficient tensor were introduced into the model. Additionally, a heat conductivity tensor and a special relaxation matrix with some off-diagonal elements were selected for the thermal anisotropy. Furthermore, by selecting an appropriate equilibrium moments and discrete source terms accounting for the local thermal non-equilibrium effect, as well as choosing an off-diagonal relaxation matrix with some specific elements, the presented model can recover the exact governing equations for natural convection under LTNE with anisotropic permeability and thermal conductivity with no deviation terms through the Chapman-Enskog procedure. Finally, the proposed model was adopted to simulate several benchmark problems. Good agreements with results in the available literatures can be achieved, which indicate the wide practicability and the good accuracy of the present model.

Solid particles and liquid droplets widely coexist in many industrial processes. Particle-droplet interactions critically influence the dynamics of those processes. In this study, the interactions between a freely moving solid particle and a freely moving liquid droplet are numerically investigated using the lattice Boltzmann method. Until now the open literature on such topic is quite sparse. Through the present numerical investigation, two regimes to classify the interactions between a freely moving solid particle and a freely moving liquid droplet are proposed. In addition, it is found that the particle-to-droplet size ratio and particle's wettability play critical roles in such interactions. (C) 2018 Elsevier Ltd. All rights reserved.

An accurate evaluation of lightning electromagnetic pulse (LEMP) using the finite-difference time-domain (FDTD) method in 2-D cylindrical coordinates is studied, which takes the soil dispersion into account. The parameters of engineering soil models are reformed by the vector-fitting (VF) scheme, for an efficient handling in FDTD. The FDTD updating equations for the dispersive soil are developed with the semi-analytical recursive convolution (SARC) algorithm. The cylindrical CPML is also developed for truncating the dispersive soil. The efficiency of the proposed method is validated by comparing the numerical results with the Cooray-Rubinstein (CR) approximation. The proposed method provides an accurate FDTD evaluation of LEMP considering the soil dispersion and can be further incorporated into the simulations of more complicated LEMP problems.

To improve the modeling accuracy of radiative transfer, the scattering properties of aerosol particles with irregular shapes and inhomogeneous compositions should be simulated accurately. To this end, a light-scattering model for nonspherical particles is established based on the pseudo-spectral time domain（PSTD） technique. In this model, the perfectly matched layer with auxiliary differential equation（ADE-PML）, an excellent absorption boundary condition（ABC） in the finite difference time domain generalized for the PSTD, and the weighted total field/scattered field（TF/SF） technique is employed to introduce the incident light into 3 D computational domain. To improve computational efficiency, the model is further parallelized using the Open MP technique. The modeling accuracy of the PSTD scheme is validated against Lorenz–Mie, Aden–Kerker, T-matrix theory and DDA for spheres, inhomogeneous particles and nonspherical particles, and the influence of the spatial resolution and thickness of ADE-PML on the modeling accuracy is discussed as well. Finally, the parallel computational efficiency of the model is also analyzed. The results show that an excellent agreement is achieved between the results of PSTD and well-tested scattering models, where the simulation errors of extinction efficiencies are generally smaller than 1%, indicating the high accuracy of our model. Despite its low spatial resolution, reliable modeling precision can still be achieved by using the PSTD technique, especially for large particles. To suppress the electromagnetic wave reflected by the absorption layers, a six-layer ADE-PML should be set in the computational domain at least.

PSTD (Pseudo Spectral Time Domain) is an excellent model for the light scattering simulation of non spherical aerosol particles. However, due to the particularity of its discretization form of the Maxwell's equations, the traditional Total Field/Scattering Field (TF/SF) technique for FDTD (Finite Differential Time Domain) is not applicable to PSTD, and the time-consuming pure scattering field technique is mainly applied to introduce the incident wave. To this end, the weighted TF/SF technique proposed by X. Gao is generalized and applied to the 3D-PSTD scattering model. Using this technique, the incident light can be effectively introduced by modifying the electromagnetic components in an inserted connecting region between the total field and the scattering field region with incident terms, where the incident terms are obtained by weighting the incident field by a window function. To optimally determine the thickness of connection region and the window function type for PSTD calculations, their influence on the modeling accuracy is firstly analyzed. To further verify the effectiveness and advantages of the weighted TF/SF technique, the improved PSTD model is validated against the PSTD model equipped with pure scattering field technique in both calculation accuracy and efficiency. The results show that, the performance of PSTD seems to be not sensitive to variation of window functions. The number of the connection layer required decreases with the increasing of spatial resolution, where for spatial resolution of 24 grids per wavelength, a 6-layer region is thick enough. The scattering phase matrices and integral scattering parameters obtained by the improved PSTD show an excellent consistency with those well-tested models for spherical and nonspherical particles, illustrating that the weighted TF/SF technique can introduce the incident precisely. The weighted TF/SF technique shows higher computational efficiency than pure scattering technique. (C) 2018 Elsevier Ltd. All rights reserved.

Due to the inadequate understanding of the scattering properties of nonspherical aerosols, considerable uncertainties still exist in the radiative transfer numerical simulation. To this end, a new scattering model for nonspherical aerosols is established based on Multi-Resolution Time-Domain (MRTD) scheme. The model is comprised of three modules: near field calculation module, near-to-far transformation module and scattering parameters computation module, in which, the near electromagnetic field is calculated by MRTD technique, the near-to-far transformation scheme is performed by volume integral method, and the calculation models for extinction and absorption cross section are directly derived from Maxwell's curl equations in the frequency domain. To achieve higher computational efficiency, the model is further parallelized by MPI non-blocking repeated communication technique. The accuracy of the scattering model is validated against Lorenz-Mie, Aden-Kerker and T-matrix theories for spherical particles, particles with inclusions and nonspherical particles. At last, the parallel computational efficiency of the MRTD scattering model is quantitatively discussed as well. The results obtained by parallel MRTD scattering model show an excellent agreement with those of the well-tested scattering theories, where the relative simulation errors of the phase function are less than 5% for most scattering angles. In backward directions, the simulation errors are much larger than that in forward scattering directions due to the stair approximation in particle construction. The computational accuracy of the integral scattering parameters like extinction and absorption efficiencies is higher than phase matrix, where the simulation errors of extinction and absorption efficiencies for the particle with a size parameter of 10 achieve -0.4891% and -1.6933%, respectively.

The research on double diffusive convection in porous media is important to deepen our insights into sustainable development and environment protection. A lattice Boltzmann (LB) model for REV (representative elementary volume) scale simulation of double diffusive convection in fluid-saturated porous media is proposed in the present work. It can work well not only for porous media with uniform porosity but also for non-uniform porous media. Several benchmark tests are adopted to validate its capability. The good agreement with previous publications demonstrates its applicability. It can provide an alternative numerical tool for modelling complex heat and mass transfer in fluid-saturated porous media beyond double diffusive convection, such as heat and moisture transfer in multi-layer building materials. (C) 2017 Elsevier Ltd. All rights reserved.

Gas-solid flows are commonly found in nature, as well as in industries. In such flows the size of the solid particles generally is not uniform. In addition, usually there is heat transfer between solid particles and gas flows. The hydrodynamics and heat transfer both make the behavior of gas-solid flows extremely complicated. In order to reveal these effects, in this paper three cases: (1) two isothermal, (2) two hot and (3) two cold spherical particles with various size ratios are investigated using lattice Boltzmann method-immersed boundary (LB-IB). It is observed that, for the first time, the tumbling duration of both two hot particles and two cold particles settling in vertical channel, is prolonged with size ratio increasing. The differences of threshold size ratio among the three cases are significant and the threshold size ratio of two hot particles is the largest one. Especially, it is found that heat transfer affects critically the interaction of two hot particles with low size ratios. In addition, against particle size ratio increasing, heat transfer effects on the interaction between two non-identical particles become weak. (C) 2017 Elsevier Ltd. All rights reserved.

A square with a thermal square column is a simple but nontrivial research prototype for nanofluid research. However, until now, the effects of the temperature of the square column on the heat and mass transfer of nanofluids have not been revealed comprehensively, especially on entropy generation. To deepen insight into this important field, the natural convection of the SiO2-water nanofluid in a square cavity with a square thermal column is studied numerically in this study. The effects of the thermal column temperature (T = 0.0, 0.5, 1.0, 1.5), the Rayleigh number (ranging from 103 to 106), and the volume fraction of the nanoparticle (varying from 0.01 to 0.04) on the fluid flow, heat transfer, and entropy generation are investigated, respectively. It is found that, no matter at a low or high Rayleigh number, the volume fraction of the nanoparticle shows no considerable effects on the flow field and temperature field for all the temperatures of the thermal column. With an increase in the volume fraction, the mean Nusselt number increases slightly. At the same time, it is found that, with an increase in the temperature of the thermal column, the average Nusselt number gradually decreases at all values of the Rayleigh number. Meanwhile, it is found that, at a high Rayleigh number, the heat transfer mechanism is the main parameter affecting the increase in the total entropy generation rather than the volume fraction. In addition, no matter at a high or low Rayleigh number, when T = 0.5, the total entropy generation is the minimum.

In order to improve the computational efficiency of multi-resolution time domain (MRTD) scattering model, a multi-size synchronous-computational scheme (MSCS) is proposed. By using MSCS, the scattering properties of the particles with different sizes can be simultaneously calculated by MRTD model in one wave-particle interaction simulation. In this model, the pulse plane wave with a wide spectrum is taken as the incident light, and the light scattering simulation for particles with different sizes is transformed into the scattering calculation for a size-fixed particle at different wavelengths. To guarantee the stability and precision of the improved MRTD (IMRTD) model, the method to design model's input parameters, such as the spatial resolution, discrete time interval and pulse width, is proposed. To validate the accuracy of IMRTD model, its results are compared with those of Mie and T-Matrix theory, and the influence of spatial resolution on the precision of IMRTD is analyzed as well. At last, model's computational efficiency is also discussed. The simulation results show that, IMRTD method can calculate the scattering parameters of particles with different sizes simultaneously and accurately, where, in case that the pulse width is 5.56 × 10-8ns, and the radius of the size-fixed particle is 0.5μm (its size parameter is 6.28), light scattering process by particles with size parameters up to 12.56 can be successfully simulated. With the increasing of spatial resolution, the simulation accuracy is improved for all particles, and the improvement for large particles is more notable than that for small ones. It can also be found that the computational efficiency of IMRTD is much higher than that of traditional version.

To study the lightning electromagnetic pulse (LEMP) coupling and protection problems of shielding enclosure with penetrating wire, we adopt the model with proper size which is close to the practical engineering and the two-step finite-difference time-domain (FDTD) method is used for calculation in this paper. It is shown that the coupling voltage on the circuit lead inside the enclosure increases about 34 dB, when add 1.0 m long penetrating wire at the aperture, comparing with the case without penetrating wire. Meanwhile, the waveform, has the same wave outline as the lightning current source, shows that the penetrating wire brings a large number of low frequency component into the enclosure. The coupling effect in the enclosure will reduce greatly when penetrating wire has electrical connection with the enclosure at the aperture and the coupling voltage increase only about 12 dB than the case without penetrating wire. Moreover, the results show that though the waveguide pipe can reduce the coupling effect brought by the penetrating wire, the exposing part of penetrating wire can increase the coupling when the penetrating wire outside the enclosure is longer than the waveguide pipe and the longer the exposing part is, the stronger the coupling is.

The performance of absorbing boundary condition (ABC) is an important factor influencing the simulation accuracy of MRTD (Multi-Resolution Time-Domain) scattering model for non-spherical aerosol particles. To this end, the Convolution Perfectly Matched Layer (CPML), an excellent ABC in FDTD scheme, is generalized and applied to the MRTD scattering model developed by our team. In this model, the time domain is discretized by exponential differential scheme, and the discretization of space domain is implemented by Galerkin principle. To evaluate the performance of CPML, its simulation results are compared with those of BPML (Berenger's Perfectly Matched Layer) and ADE-PML (Perfectly Matched Layer with Auxiliary Differential Equation) for spherical and non-spherical particles, and their simulation errors are analyzed as well. The simulation results show that, for scattering phase matrices, the performance of CPML is better than that of BPML; the computational accuracy of CPML is comparable to that of ADE-PML on the whole, but at scattering angles where phase matrix elements fluctuate sharply, the performance of CPML is slightly better than that of ADE-PML. After orientation averaging process, the differences among the results of different ABCs are reduced to some extent. It also can be found that ABCs have a much weaker influence on integral scattering parameters (such as extinction and absorption efficiencies) than scattering phase matrices, this phenomenon can be explained by the error averaging process in the numerical volume integration. (C) 2017 Elsevier Ltd. All rights reserved.

In this paper, we set a metal layer to surround the lightning rod system to reduce the lightning electromagnetic fields when lightning strokes to the lightning rod All the underground fields amplitude decrease with the metal layer existing. Three influence factors are considered including metal layer height and grounding depth and distance between metal layer and lightning channel. The effect of metal layer height on underground electromagnetic fields is more notable than two other factors and mainly for electric fields. The metal layer grounding depth only affects the vertical electric field amplitude for the time more than 2 μs. The distance between metal layer and lightning channel can affect the vertical electric field slightly and hardly on two other kinds of fields. The horizontal electric field underground appears to be little affected by the metal layer. For comparison, we also discuss the effect of metal layer on the field aboveground and the results show that the reducing effect of metal layer on the fields aboveground is a little more obvious than that on underground fields.

Porous media are commonly found not only in the nature but also in industries. Furthermore, porous media is an important research prototype for a diversity of disciplines. So far a REV (representative elementary volume) scale lattice Boltzmann (LB) model has been proposed and popularly used for investigation on heat transfer in porous media. Unfortunately, such model suffers from a serious drawback that it can not address an investigated domain where the heat capacitance (the product of density and specific heat capacity) of porous media varies spatially obviously. Such deficit restricts dramatically its applicable range. The purpose of the present work is to remedy such serious shortcoming in a simple way. Numerical validation demonstrates the capability and reliability of the present model. In order to clearly show the advantage of the present model, here a single-relaxation-time LB model is taken as an example to illustrate how to remedy the shortcoming of previous models. Its multiple-relaxation-time counterpart can be established straightforwardly in the same way. (C) 2017 Elsevier Ltd. All rights reserved.

Scattering process of aerosol particles plays an important role in atmospheric radiative transfer since it can modify the transmission, reflection and absorption ability of atmospheric system. Owning to the uncertainty of aerosol particles' scattering properties, which results from their complicated geometries and inhomogeneous compositions, there still exists a considerable uncertainty in the radiative transfer numerical simulation, and simulating the scattering properties of aerosol with irregular shapes has become a hotspot in meteorological study. To this end, a new aerosol scattering model is developed based on multi-resolution time-domain (MRTD), by which the scattering processes of nonspherical and inhomogeneous particles can be simulated. In this model, the near electromagnetic field is calculated by MRTD technique. Considering the particularity of aerosol medium, a transformation technique from near field to far field is derived based on volume integration method, and then the scattering amplitude matrix and Meller matrix can be calculated by the obtained far electric field as well. The models for particle extinction and absorption cross section are derived from Maxwell's curl equations in the frequency domain, by which the integration scattering properties can be simulated accurately. The MRTD scattering model is validated by comparing with Mie theory and T matrix method for spherical particle, ellipsoidal particle and cylindrical particle, and the influence of grid size on the simulation accuracy is analyzed subsequently. In the last part, the efficiency of the MRTD scattering model is quantitatively discussed. The simulation results show that the relative errors of scattering phase function simulated by our model are less than 8%, and the errors in forward scattering direction are much smaller, which are less than 4%. The precisions for extinction and absorption efficiency are much higher than the results from the scattering phase function, and the relative errors can reduce to 0.1% for particles with their radii comparable to the wavelength of incident light. The gird size has a significant influence on model precision; to achieve the same accuracy, the grid size first increases with increasing particle radius, and then decreases as a function of particle size for particles with size parameter less than 20. In the next step, we will try to establish the scattering property database of nonspherical particles based on the MRTD scattering model developed here.

To reduce the electromagnetic radiation, we set a metal layer to surround lightning rod for shielding. Then we use the finite-difference time-domain (FDTD) method to analyze the field distribution laws of this situation. Moreover, we also calculate the corresponding field distribution without metal layer for comparison. It is shown that the metal layer affects the electromagnetic field distribution laws, especially for the vertical electric field E-z above the ground. The field value decrease as the distance from the lightning channel increases and the extent reduces except the Ez above the ground with metal layer exists. No matter the metal layer existing or not, the field distribution laws below the ground are the same. In addition, there exist many differences between electromagnetic fields obtained above and below the ground with metal layer exists, including wave shape and value. The finite conductivity of metal layer and discharge occurring between the rod and the coaxial metal layer should be considered.