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The technique of identifying Lagrangian Coherent Structures (LCSs) has emerged as a powerful tool for studying incompressible flows. Yet, the discrepancies arising from incompressibility assumptions often compromise the accuracy of LCSs constructed using Lagrangian particle methods. In this study, we introduce a modified framework to compute Finite-Time Lyapunov Exponents (FTLEs), addressing the misalignment between the fully incompressible assumption of LCS theory and the inherent incompressibility loss in simulations of particle methods. We begin by examining the correlation between the minimum and maximum FTLEs. By incorporating the deformation gradient and Cauchy-Green strain tensor which account for the time-advancing errors of incompressibility based on continuum theory, we enhance the computational accuracy of FTLEs. Moreover, we introduce the modified FTLE algorithm to the incompressible particle-in-cell (PIC) method for resolving free surface flows and fluid-structure interaction problems. Finally, numerical examples including the Tayler-Green vortices, water sloshing with baffles, an eccentric box sinking in water, and three-dimensional shear-driven cavity problems with high Reynolds numbers are tested to validate the effectiveness of the modified FTLE algorithm and the improved LCSs. These results demonstrate that the proposed modification scheme adeptly counteracts the errors caused by incompressibility loss, enabling accurate computation of FTLEs and detection of LCSs. [ABSTRACT FROM AUTHOR]

Unresolved and semi-resolved CFD-DEM coupling are frequently used for modeling particulate flows due to their high efficiency and satisfying accuracy. The grid size (in unresolved CFD-DEM) and smoothing distance (in semi-resolved CFD-DEM) are usually around three times of the particle diameter so as to properly reproduce the background flow field. An over-estimation of the grid size or smoothing distance can lead to low-fidelity flow fields, while under-estimations of these parameters can produce an inaccurate drag force. Therefore in practices, the size ratio is usually empirical, and how does it vary with cases remains unclear. In this paper, we provide an approach to identify the most suitable grid size or smoothing distance in coupled CFD-DEM. Firstly, resolved simulations based on immersed boundary method (IBM) are conducted to compute particles' drag forces. The obtained forces are then compared with those calculated by force models with certain smoothing lengths to identify an optimal smoothing length or effective grid size. During this process, integral scale of each case is also computed. Finally, linearity between integral length and the optimal smoothing distance or grid size is found. This correlation can offer guidance to CFD-DEM modeling in future and provide insights for novel drag force models. [Display omitted] • Optimal grid size in un-resolved CFD-DEM coupling is identified. • Optimal smoothing distance in semi-resolved CFD-DEM coupling is identified. • Optimal grid size and smoothing distance are determined by flow's integral scale. • Quantitative relation of the ratio of grid size to particle diameter is found. • Quantitative relation of ratio of smoothing distance to particle diameter is found. [ABSTRACT FROM AUTHOR]

Conductive particles greatly impair the insulation integrity of a gas insulated switchgear (GIS). Factors including applied voltage, frequency of the alternating current, size and coefficient of restitution of the metallic particle, pressurized gas, and sizes of the GIS dramatically influence the particle dynamics. Previous investigations basically focused on certain scenarios, seldom considered the effects from coefficient of restitution, and did not provide universal laws for the particle dynamics. This paper proposes a dimensionless number that considers all the above-mentioned factors, and presents the scaling law between dimensionless mean/maximum lifting height of a charged particle in GIS and the proposed dimensionless number. It is believed that the scaling law together with the dimensionless number can provide guidance and shrink the parameter window for relevant investigations on the GIS with sparsely distributed conductive particles. [ABSTRACT FROM AUTHOR]

In recent years, the investigations on Lagrangian Coherent Structures (LCS) in complex flows have attracted increasing attention due to their ability to accurately describe flow field details. In this study, we propose a novel and accurate numerical technique based on the Lagrangian–Eulerian Stabilized Collocation Method (LESCM) for computing the Finite Time Lyapunov Exponents (FTLEs), which is essential for extracting LCSs in viscous incompressible flows. LESCM, previously employed for simulating fluid–structure interaction problems, was developed based on the Material Point Method (MPM). The hybrid Lagrangian–Eulerian​ description in LESCM enables explicit tracking of fluid flow trajectories throughout the simulation and direct evaluation of the FTLE field. Furthermore. The errors in FTLEs caused by the Particle Shifting Technique (PST) are completely avoided due to the Eulerian characteristic of LESCM as the deformation gradient is calculated based on fixed Eulerian nodes rather than fluid particles with unphysical shifting. Consequently, the novel technique based on LESCM surpasses the accuracy of pure Lagrangian particle methods and provides an accurate way of detecting complex LCSs in flow fields. Additionally, By harnessing the remarkable efficiency of LESCM, MATLAB can now handle up to 16 million particles with ease, eliminating the need for parallel computation techniques. Several numerical examples, such as 2D Taylor-Green​ vortices, flow passing a circular cylinder and a 3-dimensional shear driven cavity problem, have been tested to demonstrate the effectiveness of this novel approach under various conditions. [ABSTRACT FROM AUTHOR]

Particulate flows with charged particles under electric flow field exist in many practical applications, including metal particles in Gas Insulated Switchgear (GIS) or manipulation of powders in fluidized bed. The motions of these particles are affected by the electrical field, ambient flow and collisions. This work further develops the four-way coupled CFD-DEM framework under electric field, which not only computes the coupling of flow field, electric field with particle motion and particle charge, the particle-occupied solid fractions on CFD cell, particle-particle interactions and particle-wall interactions, but also considers the dielectrophoretic force, particle-particle Coulomb force, virtual mass force and Magnus force. Particle electrification on its contact with electrode is computed as well. Besides, the multi-sphere model is also introduced into the present framework in order to simulate particles of irregular shapes. The model is validated with theoretic solutions and experiments, and is applied to GIS and fluidized bed. Based on the framework, statistic information from the particulate flow in GIS, e.g., time-averaged solid fraction, time-averaged electric field or particles' velocity and charge distribution are analyzed. A dimensionless number is proposed to predict the particle motion in GIS with Alternating Current (AC), which is further conducive to reducing the parameter window in relevant simulations and studies. • The four-way coupled CFD-DEM modeling framework for charged particulate flows is extended with more concerned forces. • Fluid fraction is counted in the standard four-way coupling. • Multi-sphere model is introduced for modeling irregular-shaped particles. • Interesting phenomena of charged particles in GIS are revealed and discussed. • A dimensionless number is proposed to predict the particle motion in GIS. [ABSTRACT FROM AUTHOR]

• A numerical model is developed to analyze underwater explosion of slender explosive and structure responses. • Slender explosive will generate a directional pressure field in the surrounding water. • The shape of explosive strongly influences shock wave dynamics in the near-field. • The direction effect of pressure and energy flux is significant. • The response of a plate subjected to near-field underwater explosion of a slender explosive is studied. The shape of a high-explosive is an important factor to consider in the near-field and contact explosion, but this has been rarely studied previously. In this paper, we first conduct experiments to study the directional effects of the underwater explosion shock wave of slender explosives. A numerical model based on the multi-material arbitrary Lagrangian Eulerian (MM–ALE) technique is then developed to study the influence of the explosive slender ratio on the generated shock waves. The numerical model is validated by experiment data. Then, tests are conducted with constant-mass cylindrical explosives with slender ratios ranging from 2.0 to 9.2. By comparing with the center-detonated spherical explosive, the peak pressure, pulse duration, impulse and energy flux of the underwater explosion of slender explosives are analyzed in detail. Furthermore, the effectiveness of the fluid-structure interaction algorithm is verified through the experimental data of a plate subjected to underwater contact explosion. Finally, the response of plates subjected to the underwater explosion of a slender explosive at different azimuths is studied. The interaction between the plate and the directional loads is analyzed. We conclude that the loads and structure responses of the near-field underwater explosion of slender explosives are different from those of spherical explosives. The analyses and results provide a reference for the near-field underwater explosion loads and structural response studies. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The rigid motions of powders widely exist in powder-based laser additive manufacturing, like powder deposition, entrainment and spattering. These phenomena simultaneously happen as the laser melts these powders, and can have significant impacts on the quality of the printed products. However, existing models can hardly efficiently reproduce the co-existence of melting and rigid motion which greatly influences the molten pool and molten track evolution. The bottleneck therein lies in the demand that unresolved Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) has on the CFD grid size, that is, larger than 3 times of the particle diameter. However, this large size ratio (CFD grid cell vs particle diameter) handicaps the description of the particle deformation during melting. Hence in this paper, a kernel approximation-based semi-resolved CFD-DEM numerical model that allows the refinement of CFD grid is employed, and then coupled with the Volume of Fluid (VOF) method to reproduce the molten metal-gas interface. On this basis, the rigid motions of the powders, like entrainment and spattering, are finally simultaneously realized as the molten pool evolves at high efficiency. Powders remained in the unclosed molten pool trail defect are well explained with the proposed model as well. This model can also be applied to simulate the particle-fluid interactions in Directed Energy Deposition. It is believed that this semi-resolved VOF-DEM Additive Manufacturing model will serve as a good tool for future investigations into the particle behaviors in powder-based additive manufacturing. [Display omitted] • Semi-resolved VOF-DEM Additive Manufacturing model is developed. • Particle rigid motion is simultaneously modeled with molten pool evolution. • Spattered powders left in the defect are well explained. • Modeling of complex particulate flows in Directed Laser Deposition is realized. [ABSTRACT FROM AUTHOR]

An improved coupling method of smoothed particle hydrodynamics (SPH) and element bending group (EBG) is developed for modeling the interaction of viscous flows with free surface and flexible structures with free and fixed ends. SPH and EBG are both particle methods which are appealing in modeling problems with free surfaces, moving interfaces and large deformations. SPH is used to model viscous fluid, while EBG is used to model flexible structure. Structure particles are also used as moving boundary for SPH, and the interaction of flexible structure with fluid is therefore modeled through the interaction of structure particles and fluid particles. A fixed-end treatment is introduced for flexible structures. A free surface treatment and a surface tension model are used for free surface flow. The improved SPH–EBG method is applied to simulate problems of dam break flow on flexible structures. The good agreement of presented numerical results with existing experimental and numerical results clearly demonstrates the effectiveness of the SPH and EBG coupling approach in modeling fluid–flexible structure interactions. [ABSTRACT FROM AUTHOR]

Abstract: Multiphase fluid motion in unsaturated fractures and fracture networks involves complicated fluid dynamics, which is difficult to model using grid-based continuum methods. In this paper, the application of dissipative particle dynamics (DPD), a relatively new mesoscale method to simulate fluid motion in unsaturated fractures is described. Unlike the conventional DPD method that employs a purely repulsive conservative (non-dissipative) particle–particle interaction to simulate the behavior of gases, we used conservative particle–particle interactions that combine short-range repulsive and long-range attractive interactions. This new conservative particle–particle interaction allows the behavior of multiphase systems consisting of gases, liquids and solids to be simulated. Our simulation results demonstrate that, for a fracture with flat parallel walls, the DPD method with the new interaction potential function is able to reproduce the hydrodynamic behavior of fully saturated flow, and various unsaturated flow modes including thin film flow, wetting and non-wetting flow. During simulations of flow through a fracture junction, the fracture junction can be fully or partially saturated depending on the wetting property of the fluid, the injection rate and the geometry of the fracture junction. Flow mode switching from a fully saturated flow to a thin film flow can also be observed in the fracture junction. [Copyright &y& Elsevier]

In this article, three novel algorithms are developed and successfully applied to investigate the stable solutions of time‐fractional nonlinear telegraph equations. Firstly, we presented the shifted Gegenbauer polynomials through appropriate transformations. The approximation of a function u(||x||, t) is defined via shifted Gegenbauer polynomials (SGPs) and then developed the operational matrices of a positive integer and non‐integer derivatives assisted by multi‐dimensional shifted Gegenbauer polynomials vectors. Linearized fully spectral method (LFSM) is proposed by means of function approximations and developed operational matrices whilst Picard iterative scheme is used to linearize the discussed problem. To introduce a semi‐discrete method (SDM), the temporal derivative is approximated through a central difference scheme whereas spatial derivatives are approximated assisted by novel operational matrices. A fully numerical method (FNM) is proposed by introducing the central difference for temporal and spatial variables. Proposed computational methods transform the time‐fractional nonlinear telegraph equation into a system of linear algebraic equations that are easier to tackle. An inclusive comparative study is witnesses that suggested schemes are effective, accurate and well‐matched to investigate the approximate solutions of the problems udnerstudy. Convergence and error bound of the suggested methods are investigated theoretically whereas stability analysis is performed numerically. Moreover, the developed methods can be used conveniently to examine the numerical solution of other multi‐dimensional highly nonlinear fractional or variable order problems of physical nature. [ABSTRACT FROM AUTHOR]

• Semi-resolved CFD–DEM is extended to particulate flows with thermal convection. • Background fluid properties are corrected using kernel-based approximation. • Model A is adopted to address fluid-particle interaction with more force models. • Turbulence model is incorporated into the semi-resolved CFD–DEM. • The semi-resolved CFD–DEM provides better predictions and more details in modeling thermal particulate flows in fluidized bed. • Several phenomena observed in thermal fluidized bed are discussed. Particulate flows with heat transfer widely exist in industry including fluidized bed reactor and powder-based 3D printing, and are frequently modeled with CFD–DEM coupled approaches. Recently, we developed a semi-resolved CFD–DEM approach, which bridges the simulation gap between the resolved and unresolved CFD–DEM (Wang et al., 2019). The semi-resolved CFD–DEM is as efficient as the conventional unresolved CFD–DEM and as accurate as the resolved CFD–DEM, while it is validated for particulate flows with momentum exchange with Model B without heat exchange. In this paper, we further extend the semi-resolved CFD–DEM approach to model particulate flows with momentum exchange and thermal convection. Firstly, similar to background fluid velocity, background fluid temperature is corrected through kernel-based approximations on the neighboring fluid cells rather than simply taking values in the local cell containing the concerned particle. Secondly, Model A is adopted in the semi-resolved CFD–DEM, leading to more powerful capabilities in handling complicated flows. Force models like the Magnus force, virtual mass force are also included. Thirdly, turbulence model is incorporated into the semi-resolved CFD–DEM approach to address possible turbulence effects. Finally, the developed semi-resolved CFD–DEM is applied to modeling the complex particulate flows in fluidized bed. The obtained numerical results are compared with experimental data and results from other sources. It is demonstrated that the present semi-resolved CFD–DEM is effective in modeling particulate flows with thermal convection. Compared to the conventional unresolved CFD–DEM, this semi-resolved CFD–DEM can provide better predictions and more details on particle temperature distribution, mean temperature, solid/fluid fraction, velocity autocorrelation and vorticity field. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) is a promising additive manufacturing (AM) technique that has been attracting more and more attention both in academia and industry. The complex physics inherent in SLM processes, such as fluid flow, heat transfer, and phase change, pose great challenges to the quality control of AM products and a profound understanding of the mechanism of SLM is, therefore, significant. The existing literature mainly focuses on experimental investigations or numerical simulations where the provided results are usually limited and not applicable to general cases. To tackle this problem, we present a dimensionless analysis on SLM in this paper. Firstly, a theoretical analysis through normalization of the energy equation of the SLM process is conducted. Secondly, four key dimensionless numbers are proposed to characterize the thermo-dynamical behaviors of SLM including welding parameter, melting and vaporization efficiency, and track size parameter. Thirdly, a set of commonly-used processing parameters are employed to estimate these dimensionless numbers that provides a qualitative understanding of their physical significances. Finally, the presented dimensionless numbers are employed to analyze the SLM process and to reveal that these dimensionless numbers are effective in describing and predicting the physics of SLM for continuous track forming, porosity evolution, and track size. [ABSTRACT FROM AUTHOR]

• We extend a third-order weighted vartiational rDG(P 1 P 2) to solve the incompressible flows on unstructured grids. • High-order DoFs are reconstructed by minimizing a weighted interfatial jump integration function using variational method. • This new rDG(P 1 P 2) method is able to achieve the designed optimal third order of accuracy. • The computational costs of this new rDG(P 1 P 2) method are significantly reduced compared to the standard DG(P 2) method. In this paper, a third-order reconstructed discontinuous Galerkin (DG) method based on a weighted variational minimization principle, which is denoted as P 1 P 2 (WVr) method, is presented for solving the incompressible flows on unstructured grids. In this method, the first-order degrees of freedom (DoFs) are obtained directly from the underlying second-order DG method, while the second-order DoFs are reconstructed through the weighted variational reconstruction. Specifically, we first introduce a weighted interfacial jump integration (WIJI) function which represents a measure of the jump between the reconstructed polynomial solutions from two neighboring cells. Then, we build the constitutive relations by minimizing this WIJI function using the variational method. A number of incompressible flow problems in both steady and unsteady forms are presented to assess the performance of the proposed P 1 P 2 (WVr) method. The numerical results demonstrate that the P 1 P 2 (WVr) method is able to achieve the designed optimal third-order accuracy at a significantly reduced computational costs. Moreover, when a suitable value of the weight parameter is chosen to be used, the P 1 P 2 (WVr) method outperforms the reconstructed DG methods based on either least-squares or Green-Gauss reconstruction for the simulations of incompressible flows. [ABSTRACT FROM AUTHOR]

Hypervelocity impact (HVI) on carbon fiber reinforced plastics (CFRPs) is associated with extreme impulse response of material as well as complex characteristics of the composites. It is a challenging task to predict the physical process of CFRP-HVI problems accurately. We apply a GPU-accelerated smoothed particle hydrodynamics (SPH) method which incorporates the composite material model and the decoupled finite particle method to simulate the CFRP-HVI processes. The presented SPH model is validated by simulating the processes of aluminum spheres impacting metallic plates. The simulation results agree well with available experimental and numerical results, and the GPU parallelization technique significantly improves the simulation efficiency (350 times faster than an equivalent serial SPH model). With such computational accuracy and efficiency, the model is extended to simulate CFRP laminate impact problems, and a particle convergence analysis is performed. It is shown that the model can obtain convergent results when modeling each CFRP layer with three SPH particles in the thickness direction and correspondingly using about twenty million particles in total. The simulation can be completed within several hours, and the dominant mechanisms of the CFRP-HVI can be captured quantitatively. To further investigate the protective performance of CFRP structures, HVI on a well-known shielding structure Whipple bumper is investigated numerically. The dynamic response of the structure is well reproduced, and the results show that the CFRP pressure wall is more effective than the metallic one. These simulation results demonstrate that the presented SPH model can model the CFRP-HVI problems accurately and efficiently. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to develop a corrected unresolved CFD-DEM method that can reproduce the wake effects in modeling particulate flows at moderate Reynolds number. Design/methodology/approach: First, the velocity field in the wake behind a settling particle is numerically investigated by a resolved method, in which the finite volume method (FVM) is applied to model the fluid flow, discrete element method (DEM) is applied to simulate the motion of particles and immersed boundary method (IBM) is used to tackle fluid solid interaction. Second, an analytical scaling law is given, which can effectively describe the velocity field in the wake behind the settling particle at low and middle Reynolds numbers. Third, this analytical expression is incorporated into unresolved modeling to correct the relative velocity between the particle and its surrounding fluid and enable the influence of the wake of the particle on its neighboring particles. Findings: Two numerical examples, the sedimentation of dual particles, a list of particles and even more particles are provided to show the effectiveness of the presented velocity corrected unresolved method (VCUM). It is found that, in both examples simulated with VCUM, the relative positions of the particles changed, and drafting & kissing phenomenon and particle clustering phenomenon were clearly observed. Practical implications: The developed VCUM can be highly beneficial for modeling industrial particulate flows with DKT and particle clustering phenomena. Originality/value: VCUM innovatively incorporates the wake effects into unresolved CFD-DEM method. It improves the computational accuracy of conventional unresolved methods with comparable results from resolved modeling, while the computational cost is greatly reduced. [ABSTRACT FROM AUTHOR]

The continuum surface force (CSF) model in conjunction with the smoothed particle hydrodynamics (SPH) method is commonly used in the simulation of multi-phase flows with surface tension effect. However, it cannot be directly applied for modeling free-surface flows since the normalization requirement, which states that the integral of the surface delta function through the interface should be equal to one, is violated due to the particle deficiency issue. In this work, we propose an improved CSF model based on the SPH method with the aim of recovering the normalization requirement. The particle deficiency issue at the free surface is tackled by introducing the jump in color function into the calculation of the surface delta function. As a result, the accuracy of the surface tension measurement is significantly improved without using the virtual particles. Besides, this improved CSF model also allows an accurate and efficient implementation of the Robin boundary condition which is frequently encountered in heat transfer problems. Several benchmark problems are investigated to validate the performance of the proposed method. The numerical results verify that the improved CSF model in SPH method is well capable of simulating free-surface flows and heat transfer problems. [ABSTRACT FROM AUTHOR]

Abstract Particulate flow has a wide range of industrial applications and is frequently modeled with coupled CFD–DEM approaches. Herein, we first identified a simulation gap between the resolved CFD–DEM and unresolved CFD–DEM through a size effect study. Then we analyzed the error sources of the conventional unresolved CFD–DEM when modeling particulate flows with comparable mesh size and particle diameter. We finally developed a semi-resolved CFD–DEM model, which combines the advantages of both resolved and unresolved CFD–DEM models. The semi-resolved CFD–DEM uses a drag force model to characterize particle–fluid interaction, while the relative velocity in the drag force model is corrected through kernel-based approximations on the neighboring fluid cells rather than simply taking values in the local cell containing the concerned particle, and the void fraction in the force model is corrected as well. In order to improve the computational efficiency, a Hilbert curve based searching strategy is used to identify the fluid cells covered by the influencing area of the kernel function. A number of numerical simulations have been conducted and numerical results from different CFD–DEM approaches are compared together with experimental data. It is shown that the presented semi-resolved CFD–DEM bridges the simulation gap between the resolved CFD–DEM and unresolved CFD–DEM while it is as efficient as the conventional unresolved CFD–DEM and as accurate as the resolved CFD–DEM. [ABSTRACT FROM AUTHOR]

We present a weakly‐compressible two‐phase poroviscoelastic biofilm model based on Smoothed Particle Hydrodynamics (SPH) method. The biofilm is modeled as a mixture of solid and fluid phases interacting via drag forces. Benefiting from the Lagrangian feature of the SPH method, large deformations of the solid structure can be modeled straightforwardly. [ABSTRACT FROM AUTHOR]

As a cutting-edge additive manufacturing (AM) technique, powder bed fusion (PBF) has gained much attention in both academic research and practical applications. Revealing the mechanisms of PBF in a simple and physically interpreting way is imperative but also challenging, as PBF is a multi-scale process with comprehensive material properties and process parameters. Dimensional groups can shed light on the underlying physics involved in PBF, while traditional dimensional analysis approaches are limited in extracting unique dimensionless groups and measuring their importance. In this work, we integrated dimensional analysis with the active subspace method (ASM), a data-driven dimension reduction method, and developed a novel data-driven dimensional analysis (DDDA) framework for predicting the melt pool morphology and porosity evolution in PBF. Different from the conventional dimensional analysis, DDDA can explore more potential dimensionless groups while measuring their relative importance. Hence we identified those dimensionless numbers of most significant importance and calibrated the scaling laws for predicting melt pool morphology and porosity evolution. Furthermore, the correlation between track size and porosity formation, as well as how single-track keyhole can contribute to the bulk porosity, is also presented. With its performance demonstrated through good agreement with experimental results, it is believed that this proposed DDDA framework will be a useful tool for transforming our knowledge in the evolution of melt pool morphology, keyhole defects and porosity. [Display omitted] • A novel data-driven dimensional analysis (DDDA) framework for powder bed fusion (PBF) process is established. • Active subspace method combined with dimensional analysis is introduced into PBF process for the first time. • Dimensionless groups and scaling laws for melt pool morphology and porosity evolution are found by the DDDA framework. • Mechanisms behind the porosity evolution for both single/multi-tracks are identified. [ABSTRACT FROM AUTHOR]

Elliptical particles settling in Bingham fluids in a confined, the vertical channel is studied using the immersed boundary-lattice Boltzmann method to understand their sedimentation characteristics. The present elliptical particles-Bingham fluids two-phase systems involve a single particle, a pair of particles, a chain of particles, and a cluster of particles. The graphics processing unit (GPU) is used to improve computational efficiency. Special attention is placed on the influence of yield stress on sedimentation behavior. For a pair of elliptic particles, alongside the drafting–kissing–tumbling (DKT) mode or non-DKT sedimentation mode, a periodic sedimentation mode is found. The results indicate that two elliptic particles keep a kissing state and their trajectories and orientations periodically change during settling in the confined channel. For the elliptic particles chain, the first kissing occurs between the middle and lower particles regarding any Bingham number. Moreover, particles form the cluster for different inclination angles of the particle chain at the Bingham number Bn = 0.02. For the cluster of particles (15 × 15) , the effect of vortex interaction around particles in Newtonian fluids is revealed, and the rheological property of the fluid–particle interaction in Bingham fluids is displayed. Overall, the role of yield stress on the sedimentation of elliptic particles is significant. With the acceleration of GPU, the present method has great potential for modeling large numbers of particles settling in non-Newtonian fluids. [ABSTRACT FROM AUTHOR]

The weakly compressible SPH (WCSPH) method has been widely used for solving hydrodynamic problems, in which the fluid is assumed to be weakly compressible through limiting the density deviation to a small value. Despite its simplicity and robustness in implementation, the WCSPH method is generally hindered by its poor accuracy and spurious oscillations in the pressure field which may lead to numerical instability. In order to dampen the spurious oscillations without introducing excessive dissipation, a Roe-type approximate Riemann solver with a low-dissipation limiter was proposed and applied to the WCSPH method by Zhang et al. (2017). The present paper aims to extend this low-dissipation Riemann solver to the finite particle method (FPM), which can be referred to an enhanced or modified version of the conventional SPH method, for reducing spurious pressure oscillations and meanwhile achieving higher order of accuracy. Numerical results demonstrate that the Riemann solver based FPM is effective in modeling incompressible flows and it produces a more accurate flow field than the conventional WCSPH and Riemann solver based WCSPH methods. [ABSTRACT FROM AUTHOR]

As a typical fluid–structure interaction (FSI) problem, water entry involves violent fluid flows and changing free surfaces, which presents great challenges for numerical modeling. Smoothed Particle Hydrodynamics (SPH) is a Lagrangian particle method that has natural advantages in modeling free surfaces and moving interfaces. However, SPH is computationally expensive due to the search of particle–particle interactions, and it causes great difficulties for performing large-scale simulations of 3D FSI problems. In this work, we present an accelerated SPH framework based on the Graphics Processing Unit (GPU) techniques to study water entry problems. The multi-threading programmed by Compute Unified Device Architecture (CUDA) is applied to enhance the computational performance in terms of efficiency and scale. Compared to the Single-CPU-based strategy, the newly presented GPU-accelerated SPH method is computationally more efficient with a speedup over hundreds times and enables a larger memory available for large-scale simulations of around ten million particles for three-dimensional cases. With the GPU-accelerated SPH method, the 3D water entry of a circular cylinder is investigated with some kinematic and dynamic characteristics explained. The results demonstrate that the rotational characteristic of a 3D cylinder in water entry is related to the dimensionless number γ defined as the ratio of the initial inclination angle to the initial velocity angle. The rotation of a cylinder changes from anticlockwise to clockwise with the increase in γ. A transition value of γ exists between the anticlockwise to clockwise rotation, which focuses on the range from 1.0 to 6.0. Meanwhile, the water entry of a 3D circular cylinder leads to a violent impact on the bottom of the cylinder, which causes a peak value of pressure being a maximum value at the early stage of the water entry. It is also indicated that the selection of the initial inclination angle has a great effect on the maximum pressure. • A GPU-accelerated SPH method for fluid–structure interaction problems is developed. • The GPU-accelerated SPH method can greatly improve the computational ability compared to CPU-based SPH. • The water entry characteristics of a 3D cylinder relate to the ratio of the initial inclination angle to velocity angle. • A cross-region that the trajectory of the centroid always passes through is determined by the initial velocity angle. [ABSTRACT FROM AUTHOR]

The drafting, kissing, and tumbling (DKT) of a pair of particles in Newtonian fluids are frequently observed in particle sedimentations and have been well studied. However, sedimentation shows distinct behaviors when the particle pair is released in non-Newtonian fluids, and dynamics of the DKT have still not been completely understood. In this paper, the immersed boundary-lattice Boltzmann method with a hybrid multiple-relaxation-time scheme is used to examine the dynamics of the DKT of a pair of particles settling in non-Newtonian fluids. The particle pair is released in a confined channel filled with non-Newtonian fluids, in which the non-Newtonian behaviors of shear-thinning/shear-thickening and yield stress are considered. The results demonstrate that the horizontal migration direction at the initial stage of the separation state in a shear-thinning fluid is totally different from that happening in a Newtonian fluid. It is attributed to the asymmetrical fluid viscosity on both sides of the pair of particles. Our findings also show that yield stress plays a dominant role in the sedimentation velocity and separation distance between the particles forming this pair. Under the effect of yield stress, three sedimentation structures, which correspond to different dynamic behaviors of the pair of particles, are also identified during the sedimentation. Furthermore, some critical conditions, regarding horizontal migration toward the right or left side, DKT/non-DKT and kissing/non-kissing states are established through the Bingham number (B n)-power-law index (n) diagrams and tables. Also, the effect of the diameter ratio between two particles on the DKT is studied. [ABSTRACT FROM AUTHOR]

In this work, we propose an efficient smoothed particle hydrodynamics (SPH) method for simulating laser powder bed fusion (LPBF). The multi-physics process of LPBF, including the heat transfer and phase change with complex boundaries, is accurately resolved by a novel heat source model and a modified continuous surface force based on a corrected surface delta function. Moreover, we also develop an efficient tensile instability control algorithm for preventing the pressure oscillations. The present method is implemented in a GPU-accelerated framework, and its performance is well demonstrated by simulating the LPBF processes with both single-layer and multi-layer powder beds (with the help of surface reconstruction). The numerical results are compared well with the experimental ones which clearly verify the ability of the present method in capturing the complex physical phenomenon of LPBF. [ABSTRACT FROM AUTHOR]

A numerical method based on smoothed particle hydrodynamics with adaptive spatial resolution (SPH-ASR) was developed for simulating free surface flows. This method can reduce the computational demands while maintaining the numerical accuracy. In this method, the spatial resolution changes adaptively according to the distance to the free surface by numerical particle splitting and merging. The particles are split for refinement when they are near the free surface, while they are merged for coarsening when they are away from the free surface. A search algorithm was implemented for identifying the particles at the free surface. A particle shifting technique, considering variable smoothing length, was introduced to improve the particle distribution. The presented SPH-ASR method was validated by simulating various free surface flows, and the results were compared to those obtained using SPH with uniform spatial resolution (USR) and experimental data. [ABSTRACT FROM AUTHOR]

• An indirect ALE DG model on moving triangular meshes with mesh topological adaptability is developed. • The best features of the Lagrangian description, DG method and adaptive topology optimization strategy are incorporated. • The present scheme can handle hydrodynamics problems which involve complex deformations, large distortions and strong shock. • The new indirect ALE DG scheme is more accurate and stable than the conventional Lagrangian DG scheme. In this paper, we present a novel cell-centered indirect Arbitrary-Lagrangian-Eulerian (ALE) discontinuous Galerkin (DG) scheme on moving unstructured triangular meshes with mesh topological adaptability, aiming to deal with the strong distortions and large deformation flow problems. The scheme combines the explicit time marching Lagrangian DG methodology with the adaptive mesh topology optimization technique. The scheme consists of the following three steps. Firstly, we utilize the Runge-Kutta DG method to solve the compressible Euler equation in Lagrangian framework, and employ a nodal solver to obtain the nodal velocity and numerical fluxes across element boundaries. The physical variable and nodal position are updated in this step. Secondly, the adaptive mesh topology optimization technique, which includes the mesh refinement, edge collapse operation and mesh regularization, is implemented to eliminate the highly distorted elements and improve the mesh quality. Thirdly, the conservative remapping algorithm is employed, which can maintain the conservative interpolation of the Lagrangian solution onto the remeshed grid. The present indirect ALE DG scheme can ensure the high quality of the mesh by optimizing the topology connectivity, so that the present scheme can successfully simulate complex vortical flow problems for a sufficient simulation time. Due to the inherent Lagrangian nature, the present scheme can naturally track the multi-material flow interface, rather than using algorithms with interface reconstruction or diffuse interfaces. The scheme is validated with several benchmark flow problems. It is demonstrated that the present indirect ALE DG scheme with topological adaptability can accurately simulate flow problems with large fluid deformations and distortions. It can achieve remarkable improvements compared with the conventional Lagrangian DG method with fixed topological connectivity. [ABSTRACT FROM AUTHOR]

Finding analytical and semi-analytical solutions of two-dimensional nonlinear fractional-order problems arising in mathematical physics is a challenging task for research community. In this work, an innovative scheme is proposed based on the shifted Gegenbauer wavelets. For this purpose first, we introduce shifted Gegenbauer polynomials via suitable transformation. Then, we present three-dimensional shifted Gegenbauer wavelets using shifted Gegenbauer polynomials. We illustrate function approximation of three variables, for example, u (x , y , t) through shifted Gegenbauer wavelets. To compute the novel operational matrices of positive integer and non-integer order derivative of shifted Gegenbauer wavelets vector in one, two and three dimensionals piecewise functions are utilized. Moreover, we describe associated theorems to validate our newly proposed scheme mathematically. The proposed algorithm is innovative because of the incorporation of Picard iterative scheme to tackle highly nonlinear problems of fractional-order. The current computational scheme coverts a mathematical model to a system of linear algebraic equations that are easier to solve. To validate the accuracy, credibility, and reliability of the present method, we analyze various fractional-order problems, including Bloch–Torrey, Burgers, Schrödinger, Rayleigh–Stokes and sine-Gordon. We also conduct a detailed comparative study, which demonstrates that the proposed computational scheme is effective to find the analytical and semi-analytical solutions of the aforementioned problems. Moreover, the proposed computational method can be utilized to analyze the solutions of other higher dimensional nonlinear fractional or variable order problems of physical nature. [ABSTRACT FROM AUTHOR]

Abrasive air jet (AAJ) machining is attractive for micromachining hard and brittle materials, while it is usually a big challenge to numerically investigate the particle velocity and concentration distribution in the particle erosion process in the AAJ. In this work, a recently developed semi-resolved CFD-DEM approach which bridges the simulation gap between the resolved and unresolved CFD-DEM, is further improved to reconstruct the background information with a double cosine kernel function. The semi-resolved CFD-DEM is then employed to numerically investigate the gas-particle two-phase flow in the AAJ and numerical results show that this semi-resolved CFD-DEM is more accurate in modeling particulate flow in the fine AAJ nozzle than the conventional unresolved CFD-DEM. We further conduct mechanism investigations on the AAJ micromachining process including particle flow characteristics inside the cylindrical nozzle, velocity, and concentration distribution over the nozzle exit, which are essential jet characteristic features. We identified the particle flow patterns, analyzed the particle distribution, and its correlation with air pressure, abrasive mass flow rate, and turbulence effects. The present simulation results and analyses can be great helpful in understanding the erosion mechanism and optimizing the setting parameters to improve the cutting performance of AAJ. The semi-resolved CFD-DEM approach is used to investigate the gas-particle two-phase flow in AAJ. Unlabelled Image • A double cosine kernel function (SCKF) was introduced into a recently developed semi-resolved CFD-DEM. • The semi-resolved CFD-DEM with SCKF was shown to be effective in investigating abrasive air jet (AAJ). • The particle-scale distribution information in a fine AAJ nozzle was studied. • The effects of process parameters and turbulence on particle distribution were analyzed. [ABSTRACT FROM AUTHOR]

Understanding the connection between physiology and kinematics of natural swimmers is of great importance to design efficient bio-inspired underwater vehicles. This study looks at high-fidelity three-dimensional numerical simulations for flows over an undulating American eel with prescribed anguilliform kinematics. Particularly, our work focuses on why natural anguilliform swimmers employ wavelengths shorter than their bodylengths while performing wavy kinematics. For this purpose, we vary the undulatory wavelength for a range of values generally observed in different aquatic animals at Strouhal numbers 0.30 and 0.40. We observe that our anguilliform swimmer is able to demonstrate more suitable hydrodynamic performance for wavelengths of 0.65 and 0.80. For longer wavelengths, the swimmer experiences large frictional drag, which deteriorates its performance. The wake topology was dominated by hairpin-like structures, which are closely linked with the underlying physics of anguilliform swimming found in nature. [ABSTRACT FROM AUTHOR]

• The CFD-DEM method combined with micro-scale reconstruction is used to study proppant transport in hydraulic fracture. • Typical flow behaviors of proppant particles and fracturing fluid in different fracture models are analyzed. • The surface area ratio is utilized to characterize the cross-scale relation between proppant distribution and fracturing parameters. • The parameter effects of fluid, proppant, and fracture on proppant migration in fracture are discussed. A comprehensive investigation of the dynamical characteristics of proppant-laden flow systems in hydraulic fractures is important for the development of unconventional oil and gas resources. In this paper, we combine the micro-scale reconstruction technique and CFD-DEM method to numerically investigate the mobility and distribution rules of proppant-laden fluid in rough hydraulic fractures. To characterize realistic fracture surface morphology, six synthetic fracture models were assembled based on optical scanning data of real shale from three sets of hydraulic fracturing experiments. In order to accurately simulate the fluid-particle two-phase flow in rough fractures, the lift forces and particle rolling effect are considered in the CFD-DEM model. First, the effectiveness of the used CFD-DEM solver was validated by comparing it to the experimental data. We then examined mechanisms for the typical flow behaviors of proppant particles and fracturing fluid in smooth and rough fractures. Subsequently, the surface area ratio of the synthetic fracture model is adopted to characterize the cross-scale relationship between the micro-scale proppant distribution and fracture's geometric features induced by the macroscale fracturing parameters (i.e., confining stress, fluid viscosity, flow rate, and sand ratio). Lastly, the parameter effects of the fracturing fluid, proppant particle, and fracture aperture on the dynamic characteristics of particulate flow in the hydraulic fracture are further discussed. In summary, the presented numerical model and findings can help better understand particle transport and distribution in real rock fractures and bridge the connectivity of the micro-scale particle flow and macroscopic fracturing parameter, which is vital for hydraulic fracturing process optimization. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel approach based on shifted Gegenbauer wavelets to attain approximate solutions of some classed of time‐fractional nonlinear problems. First, we present the approximation of a function of two variables u(x,t) with help of shifted Gegenbauer wavelets and then some novel operational matrices are proposed with the help of piecewise functions to investigate the positive integer derivative (Dx and Dt), fractional‐order derivative (Pxα and Ptβ), fractional‐order integration (Jxα and Jtβ) and delay terms (Dba and Ddc) of approximated function u(x,t). In order to transform the discussed nonlinear problem into linear problem Picard iterative scheme has been adopt. The current scheme converts the discussed highly nonlinear time‐fractional problem into system of linear algebraic equation the help of developed operational matrices and Picard idea. Analysis on the error bound and convergence to authenticate the mathematical formulation of the computational algorithm. We solve various test problems, such as the van der Pol oscillator model, generalized Burger–Huxley, neutral delay parabolic differential equations, sine‐Gordon, parabolic integro‐differential equation and nonlinear Schrödinger equations to show the efficiency and accuracy of linearized shifted Gegenbauer wavelets method. A comprehensive comparative examination shows the credibility, accuracy, and reliability of the presently proposed computational approach. Also, this scheme can be extended conveniently to other multi‐dimensional physical problems of highly nonlinear fractional or variable order of complex nature. [ABSTRACT FROM AUTHOR]

• A novel coupling approach of ES-FEM-SPH is developed for solving thermal-fluid-structure interaction (TFSI) problems. • The updated Lagrangian ES-FEM is developed for solving the coupled thermal elastic problems. • The SPH method, after integrating with particle shifting technique and kernel gradient correction, is robust and effective in modeling thermal fluid flows. • The ghost particle coupling algorithm is developed for treating fluid structure conjugate heat transfer. Thermal-fluid-structure interaction (TFSI) problems are significant in science and engineering, and usually pose great challenges for numerical simulations due to the coupled effects of thermal convection, fluid flow and structure deformation. In this paper, a novel coupling approach of smoothed finite element method (ES-FEM) with an improved smoothed particle hydrodynamic (SPH) method is developed for TFSI problems. In the coupling approach, the edge based ES-FEM is used to model solid domain and the Lagrangian SPH is used to model fluid flow. In ES-FEM, the temperature and velocity gradient smoothing technique are applied over the edge-based smoothing domain for thermal structure coupling problems. In SPH, some state-of-art algorithms including kernel gradient correction (KGC) and particle shift technique (PST) are integrated to ensure computational accuracy for simulating thermal fluid flows. A ghost particle coupling algorithm is developed to handle fluid-structure interaction and fluid-structure conjugate heat transfer, and the kinematic condition, dynamics conditions and conservation of energy are satisfied. Four numerical examples are tested to demonstrate the effectiveness of the present coupling approach of ES-FEM-SPH for TFSI problems. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this paper, a direct numerical simulation technique, the Finite Element Fictitious Boundary Method (FBM), is used to simulate fluid–solid two-phase flows of different general shaped particles. The momentum interactions between solid and fluid phases are handled by using the FBM. The continuity and momentum equations are solved on a fixed Eulerian grid that is independent of flow features by using a discrete projection scheme inside a multi-grid finite element approach. A detailed description is presented for the geometric representation and modeling of two-dimensional particles of different general shapes, i.e., circular, elliptical, square, rectangular, triangular, and pentagonal shapes inside the fluid. We discussed the effects of particle shapes and the influences on the settling behavior of the particles. A comparison of the settling trajectories of the particles of the same mass but with different shapes is presented. Moreover, depending upon the particle's shape, some interesting facts are discovered, which have a great influence on the particles' trajectory and settling velocity. Some very important correlations between the drag force coefficient and particle's Reynolds numbers with different density ratios of particles are obtained. Furthermore, we also studied the settling behavior of elliptical and rectangular particles with different axis ratios and a boomerang particle with different concave angles. [ABSTRACT FROM AUTHOR]

In this paper, we establish a fluid-structure interaction (FSI) model to investigate the dynamic interactions between the underwater explosion (UNDEX) shock wave and a movable sphere near the free surface. We utilize the local discontinuous Galerkin (LDG) method to capture the propagation of the shock wave in the fluid domain and employ the pressure cutoff model to calculate cavitation effects. The fluid elements at the fluid-structure interface are directly coupled to the structural dynamic model, and the structural transient dynamic responses are coupled with fluid acoustic pressure at the fluid-structure interface in the governing equation. The validity of the present FSI model is verified by comparing with the continuous Galerkin method. Due to the advantage of the LDG method in capturing the discontinuous wave, the present model shows better properties than the traditional coupled acoustic-structural model. With the present FSI model, we investigate the interaction between the UNDEX shock wave and a submerged and floated sphere. Under the combined effects of the free surface and structure, the UNDEX shock and cavitation loading characteristics are analyzed, and the influences of complicated cavitation effects on dynamic responses of the sphere are discussed. [ABSTRACT FROM AUTHOR]

• Natural thermal convection induces fluid motion. • Particle shape-effects do play an important role in the particle and overall system dynamics. • Thermal heat exchange significantly affects the motion of the two-phase flow. • Multi-grid FEM-FBM is robust and potentially powerful tool for real particulate flow simulations. In particulate flows, particle-particle collisions play a very important role in determining the flow behavior of the fluid-particle two-phase systems. Thus, it is very important in the numerical simulations of particulate flows to treat the particle-particle interactions with a felicitous method. In this paper, a recently developed direct numerical simulation (DNS) technique, the Finite Element Fictitious Boundary Method (FEM-FBM), for thermal convective particulate flows is used for the simulations of dense particulate suspensions of elliptic shaped particles. The momentum and temperature flow fields are coupled with the aid of Boussinesq approximation. The thermal and momentum interactions between solid and fluid phases are handled by using the Fictitious boundary method (FBM). The continuity, momentum, and energy equations are solved on a fixed Eulerian mesh which is independent of flow features by using a multi-grid finite element scheme. A modified collision model is proposed that can handle not only the interactions between the circular particles but also effectively treat the collisions between the elliptic shaped particles. Firstly, we validate the newly developed collision model for isothermal circular particles together with a comparative study of "drafting, kissing and tumbling" (DKT) motion of both elliptic and circular shaped isothermal particles. Then we investigated the DKT motions of the hot and cold elliptic particles with energy exchange and studied the lateral behavior of the particles in numerous settings. Further, we studied the DKT motion of catalyst particles and investigate the particles behavior at different Grashof numbers. Numerical tests are performed to show that the present method is robust and provides an efficient approach for the simulations of particulate flows with a large number of elliptic particles. [ABSTRACT FROM AUTHOR]

A liquid bridge between two neighboring particles is commonly observed in nature and various industrial processes. An accurate prediction of the profile of a liquid bridge is significantly important in particulate flows, while it is an analytically challenging task as well. In this paper, we develop an asymptotic solution for a catenoid liquid bridge profile, which is the minimal surface ensuring the minimum total surface energy. Our asymptotic solution is based on a rapid convergent predictor-corrector algorithm that considers different factors including boundary conditions, volume conservation, and geometrical relations while providing the relationship between the liquid bridge profile, bridge radius, half-filling angles, and creeping distances. Therefore, this asymptotic solution of the catenoid of the liquid bridge is applicable to general scenarios of any two neighboring particles of either equal or different sizes having identical or different contact angles. In order to validate the proposed asymptotic solution, we performed comprehensive experiments where the observed and predicted liquid bridge profiles and the resultant capillary forces from both the approaches are found closely matching. Moreover, we also investigate and report the influence of the radii ratio, contact angles, particle distances, and the liquid bridge volumes on its profiles. [ABSTRACT FROM AUTHOR]

Abstract Composites are considered as effective materials in absorbing the energy of blast and impact, and have been often used in complex loading environments. A numerical model is developed to analyze the response and damage of S-2 glass/SC-15 epoxy composite laminate subjected to combined loadings with blast and fragments. A composite progressive failure model is used to investigate the damage and the failure of the material. The proposed numerical model is validated by its comparison with the experimental data obtained from the literature. Three different types of loading combinations: (1) ammunition fragments impact loading, (2) bare blast loading, and (3) the combinations of blast and fragments loading, correspond to the scenarios of the near-field and the far-field explosions. Moreover, the response and the damage evaluation of the composite are analyzed here. It is concluded that the damage modes of the laminate are correlated to the type of the loading. Furthermore, the synergetic effects for the composite laminate subjected to the combined blast loading and the fragments impact are verified and evaluated. By comparing the energy absorption and the damage zone in the laminate, the synergetic effect is attributed to the conjunction of damage and the continuous pressure of the detonation products. [ABSTRACT FROM AUTHOR]

As a promising powder-based additive manufacturing technology, selective laser melting (SLM) has gained great popularity in recent years. However, experimental observation of the melting and solidification process is very challenging. This hinders the study of the physical mechanisms behind a variety of phenomena in SLM such as splashing and balling effects, and further poses challenges to the quality control of the products. Powder-scale computational models can reproduce the multi-physics process of SLM. In this study, we couple the Finite Volume Method (FVM) and Discrete Element Method to model the deposition of powder particles, and use the FVM to model the melting process, both with ambient air. In particular, a cutting-edge sharp surface capturing technique (iso-Advector) is incorporated into the Volume of Fluid Model to reconstruct the interface between different phases during the melting process. Iso-Advector is then used to capture and reconstruct the interface between molten material and ambient air, which is further used as a solid boundary for spreading the next powder layer. As such, 3D geometrical data is exchanged between these two stages repeatedly to reproduce the powder spreading-melting process of SLM incorporating different scan paths on multiple powder layers. To demonstrate the effectiveness of the powder-scale multi-physics modeling framework, typical scenarios with different fabrication parameters (Ti-6Al-4V powder) are simulated and compared with experimental observations available in literature. [ABSTRACT FROM AUTHOR]

Computer modeling of complex fluid flows usually presents great challenges for conventional grid-based numerical methods. Smoothed particle hydrodynamics (SPH) is a meshfree Lagrangian particle method and has special advantages in modeling complex fluid flows, especially those with large fluid deformations, fluid-structure interactions, and multi-scale physics. In this paper, we review the recent developments of SPH in methodology and applications for modeling complex fluid flows. Specifically, in methodology, some important issues including modified SPH particle approximation schemes for improving discretization accuracy, different particle regularization techniques, and various boundary treatment algorithms for solid boundary, free surface, or multiphase interface are described. More importantly, the SPH method with ideas from the dissipative particle dynamics for complex fluids in macro- or meso-scales is discussed. In applications, different complex fluid flows, including biological flows, microfluidics and droplet dynamics, non-Newtonian fluid flows, free surface flows, multiphase flows, and flows with fluid-structure interaction, are reviewed. Some concluding remarks in SPH modeling of complex fluid flows are provided. [ABSTRACT FROM AUTHOR]

This paper presents a direct numerical simulation (DNS) technique, the Finite Element Fictitious Boundary Method (FEM-FBM) for the simulation of fluid-solid two-phase flows with heat transfer. The heat transfer equation is introduced to study thermal convection in fluid-solid two-phase flows. The Boussinesq approximation is considered for the coupling of momentum and temperature flow fields. Multi-grid finite element solver is used to compute flow equations of mass, momentum, and energy on a fixed Eulerian mesh which is independent of time and the solid particles are allowed to move freely in the whole computational domain. Fictitious boundary method (FBM) is used to treat the particles inside the fluid, FBM takes account of the thermal and momentum interaction between the fluid and the particles. The accuracy and stability of presented method are validated by comparing our test cases results with the results reported in the available literature. Numerical tests are performed to show that this method is potentially powerful and provides an efficient approach to simulate complex thermal convective particulate flows with a large number of particles. [ABSTRACT FROM AUTHOR]

In this paper, a recently developed direct numerical simulation technique, the Finite Element Fictitious Boundary Method (FEM-FBM) [K. Walayat et al., "An efficient multi-grid finite element fictitious boundary method for particulate flows with thermal convection," Int. J. Heat Mass Transfer 126, 452–465 (2018)], is used to simulate sedimentation of an elliptic particle with thermal convection. The momentum and temperature flow fields are coupled with the aid of Boussinesq approximation. The thermal and momentum interactions between solid and fluid phases are handled by using the fictitious boundary method (FBM). The continuity, momentum, and energy equations are solved on a fixed Eulerian mesh which is independent of flow features by using a multi-grid finite element scheme. Two validation tests are conducted to show the accuracy of the present method, and then the effects of thermal properties of fluid on the sedimentation of an elliptic particle are studied. It is demonstrated that the dynamics of hot elliptic particle sedimentation depend on the thermal diffusivity and thermal expansion of the fluid. A comparative study of the forces and torque acting on the hot, cold, and isothermal particle is reported. Moreover, different sedimentation modes of hot and cold elliptic particles are identified in an infinitely long channel. The mechanism of transitions of particle settling modes from tumbling to inclined and then to the horizontal mode is discovered. Also, we discovered a new sedimentation mode of the hot elliptic particle in cold fluid, i.e., the vertical mode. Furthermore, buoyancy effects for the catalyst particle are studied at different initial orientations. [ABSTRACT FROM AUTHOR]

Flow through arrays of micropillar embedded inside microfluidic chip systems is important for various microfluidic devices. It is critical to accurately predict the mass flow rate through pillar arrays based on the pillar design. This work presents a dissipative particle dynamics (DPD) model to simulate a problem of flow across periodic arrays of circular micropillar and investigates the permeability of two types of micropillar arrays. The flow fields including horizontal and vertical velocity fields, the number density field, and the streamline of the flow are analyzed. The predicted solid volumes by the presented DPD simulation of both types of arrays are quite close to the actual counterparts. These quantitative agreements show usefulness and effectiveness of the DPD model in simulating arrays of micropillar. By comparing two types of micropillar arrangement patterns, we find that the arrangement pattern of micropillar does not have significant influence on the permeability of the array. [ABSTRACT FROM AUTHOR]

• The semi-resolved CFD-DEM with a multi-smoothing distance scheme is further developed for the polydispersed particle system. • Grain-scale reconstruction by μCT images is employed to quantitatively characterize the heterogeneous porous media. • Particle-scale migration process and distribution characteristics of fine particles in porous media are examined. • The influences of powder properties and anisotropy effects on migration processes and heat transfer are studied. A comprehensive understanding of the migrating phenomena of microscale particles associated with fluid flows and heat exchange in porous media is essential for developing unconventional geo-resources. In this work, we further develop the kernel-based semi-resolved CFD-DEM and combine it with grain-scale reconstruction to numerically investigate particulate flows and associated heat transfer processes in porous media. To characterize wide grain size distributions and heterogeneity of rock skeleton, a 3D spherical-packed model is quantitatively assembled based on CT images of real rock. After a series of validations, it is found that the results from the improved semi-resolved CFD-DEM, including motion, heat transfer, and pressure drop behaviors of particulate systems, match better with experimental observations and analytical solutions than that obtained from the unresolved CFD-DEM. On this basis, the particle-scale migration and distribution characteristics of fine particles are further studied. It is found that the penetration distance, temperature, and energy distributions of particle systems rely on the variation of powder properties (i.e., concentration and size) and the effects of matrix anisotropy. In a higher powder concentration system, more deposited particles at the upper of the packed bed prevent the downward migration of powder particles and lower the quantity of heat exchange between the fine and matrix particles. In addition, this can also lead to a more pronounced jamming phenomenon and reduce the seepage capacity of the packed bed. As the powder diameter increases, the heat transfer capacity of fine particles increases and gradually dominates the temperature of particulate systems. The deposition patterns and temperature variation behaviors of fine particles along the migration direction caused by the anisotropy of the porous medium are also identified. Therefore, the developed computational framework can be a powerful tool to reproduce the dynamics of fine particles in heterogeneous porous materials and reveal the underlying mechanisms. [ABSTRACT FROM AUTHOR]

The underwater explosion (UNDEX) phenomenon involves hydrodynamic behaviors with a wide range of spatial scales, from macroscopic cavitation regions to refined areas with fluid-structure interactions around the shock wavefront. Correct description of the refined shock wavefront is important for determining the shock wave position and pressure peak. In this work, the local discontinuous Galerkin (LDG) method for the two-dimensional wave equation, which allows for the local triangular meshes refinement and merging during the shock wave propagation process, is presented to model the UNDEX shock hydrodynamics. This article is an extension of our previous work, where we discretize the UNDEX total wave formulation in the LDG framework and simulate the UNDEX shock wave and cavitation phenomenon. In order to save computational cost (including CPU time and computational storage) and capture the shock wavefront more accurately, the LDG method is extended to be in combination with the mesh adaption technique. The troubled-cell indicator, depending on the local jump of the pressure, is adopted to identify elements which require to be refined or coarsened. Typical numerical examples are tested to assess the capability of the present h -adaptive LDG method for capturing the UNDEX shock wave. The numerical results clearly demonstrate that the mesh adaption technique enables us to dynamically increase the mesh resolution in close proximity to the flow discontinuities with a large pressure jump. The local increase of the mesh resolution results in a substantial increase in the accuracy of the pressure peak. In terms of the computational efficiency, the h -adaptive LDG method can save nearly half of the calculation time in comparison to the non-adaptive LDG method on uniform meshes, whose mesh resolution is the same as that of the maximum level of refinement of the h -adaptive LDG method. • An h -adaptive LDG method is presented for simulating the UNDEX shock hydrodynamics. • The mesh resolution is adaptive with the evolution of the UNDEX shock wavefront. • The present h -adaptive LDG method can well capture the UNDEX pressure peak at a relative small computational cost. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this paper is to investigate different baffles on mitigating liquid sloshing in a rectangular tank due to a horizontal excitation and to find out the optimal selection of sloshing mitigation for practical applications. Design/methodology/approach – The numerical study is conducted by using a proven improved smoothed particle hydrodynamics (SPH), which is convenient in tracking free surfaces and capable of obtaining smooth and correct pressure field. Findings – Liquid sloshing effects in a rectangular tank with vertical middle baffles, horizontal baffles, T-shape baffles and porous baffles are investigated together with those without any baffles. It is found that the existence of baffles can mitigate sloshing effects and the mitigation performance depends on the shape, structure and location of the baffles. Considering the balance of sloshing mitigation performance and the complexity in structure and design, the I shaped and T shaped baffles can be good choices to mitigate sloshing effects. Practical implications – The presented methodology and findings can be helpful in practical engineering applications, especially in ocean engineering and problems with large sloshing effects. Originality/value – The SPH method is a meshfree, Lagrangian particle method, and therefore it is an attractive approach for modeling liquid sloshing with material interfaces, free surfaces and moving boundaries. In most previous literature, only simple baffles are investigated. In this paper, more complicated baffles are investigated, which can be helpful in practical applications and engineering designs. [ABSTRACT FROM AUTHOR]

• A new weakly-compressible SPH model is proposed to simulate the multi-phase flows with large density ratios. • The SPH model removes unnecessary contributions from different phases for immiscible multi-phase flows. • The SPH model improves both the stability and accuracy across the multi-phase interfaces. • The SPH model allows the use of larger CLF numbers to greatly improve computational efficiency. The discontinuity across interface of multi-phase flows with large density ratios usually poses great challenges for numerical simulations. The smoothed particle hydrodynamics (SPH) is a meshless method with inherent advantages in dealing with multi-phase flows without the necessity of tracking the moving interfaces. In this paper, we develop a new weakly-compressible SPH model for multi-phase flows with large density ratios while allowing large CFL numbers. In the present SPH model, the continuity equation is first modified by eliminating the influence from particles of different phases based on the simple fact that different phases will not contribute when calculating the density for immiscible multi-phase flows; thus, the modified continuity equation will only consider the influence from neighboring particles of the same phase. The pressure and density of the particles of other phases are then re-initialized by using the Shepard interpolation function. The present multi-phase SPH model has been tested by four numerical examples, including the two-phase hydrostatic water, standing waves, liquid sloshing, and dam breaking. It has been demonstrated that the present multi-phase SPH model can obtain satisfactory results stably, even at large CFL numbers, and this means that large time steps can be employed. Therefore, the present multi-phase SPH model can significantly save computational cost through using large time steps, especially for large-scale problems with a large number of particles. [ABSTRACT FROM AUTHOR]

Existing studies of fluid-structure interaction (FSI) in ocean engineering mainly focus on the interaction between Newtonian fluids and structure. The FSI problems involving non-Newtonian fluids, especially viscoplastic fluids, have rarely been studied while the inherent dynamic behavior is not clear. In this paper, an immersed boundary-lattice Boltzmann method (IB-LBM) is developed for numerical investigations on FSI problems involving viscoplastic fluids. The present IB-LBM is integrated with a hybrid multiple relaxation times (MRT) scheme where different diagonal relaxation matrices are used for modeling Newtonian and non-Newtonian fluids, and are combined in a hybrid manner using a step function to achieve smooth transition for Newtonian to non-Newtonian fluid behavior at the FSI area. Four benchmark problems are used to validate the IB-LBM with hybrid MRT scheme. It is demonstrated that the numerical model can avoid numerical instability when modeling viscoplastic fluid flow and reduce the numerical boundary slip in the IB-LBM. The numerical model is further used to study the viscoplastic fluid flow around a fixed and moving cylinder (or particle). We show that the present IB-LBM with the hybrid MRT scheme is effective in modeling FSI involving viscoplastic fluids while the obtained phenomena are quite different from those with Newtonian fluids. [ABSTRACT FROM AUTHOR]