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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]

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]

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]

• 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]

• 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]

Numerical simulation of fluid structure interaction (FSI) problems is one of the most challenging topics in computational fluid dynamics. In this paper, coupling edge-based smoothed finite element method (ES-FEM) and smoothed particle hydrodynamics (SPH) method (ES-FEM-SPH) is proposed for solving FSI problems, where the edge-based smoothed finite element method is used to model the movement and deformation of structures, and the smoothed particle hydrodynamics is used to model the fluid flow. In ES-FEM, the gradient smoothing technique is applied over the smoothing domain and it can effectively overcome the "overly-stiff" effect in conventional FEM model. Some correction algorithms including density correction, kernel gradient correction and particle shift technique are integrated into the SPH method to improve computational stability and accuracy. A virtual particle coupling scheme is used to implement the coupling of ES-FEM and SPH with complex geometry interface. As ES-FEM is more accurate than conventional FEM, and it is expected that this ES-FEM-SPH coupling approach should be superior than existing FEM-SPH coupling approaches. A number of test examples with FSI are investigated with the presented ES-FEM-SPH, and compared with results from other approaches including FEM-SPH. From the obtained numerical results, we can conclude that the ES-FEM-SPH coupling approach is effective to simulate FSI problems. • Coupling edge-based smoothed finite element method (ES-FEM) with SPH is proposed for solving FSI problems. • The coupling strategy is based on the automatically generated virtual particles. • The gradient smoothing technique in ES-FEM overcomes the "overly-stiff" effect in conventional FEM. • The SPH method with integrated algorithms is effective and robust in modeling fluid flows. • ES-FEM-SPH is demonstrated to be effective in modeling diversified FSI problems. [ABSTRACT FROM AUTHOR]

• Four integrated SPH models are presented and compared for simulating natural convection. • The most suitable SPH model for natural convection is provided. • Natural convection for R a = 10 9 and Pr = 0.71 is successfully modeled for the first time by using the SPH method. • Mechanisms of the natural convection in a square cavity at different Rayleigh numbers are discussed. This paper conducts the simulation of natural convection in a differentially heated square cavity at high Rayleigh numbers by using the smoothed particle hydrodynamics (SPH) method. Due to the decrease of the accuracy and stability, it is challenging for the SPH method to simulate natural convection at high Rayleigh numbers, and there are few reported SPH literatures of natural convection at R a > 10 6 for air (Pr = 0.71). In this study, four integrated SPH models are presented to simulate the natural convection and their accuracy and stability are assessed. These four SPH models are associated with Kernel Gradient Correction (KGC) to improve approximation accuracy and Particle Shifting Technology (PST) to regularize particle distribution while they are different in treating density diffusion and calculating the pressure term. The numerical results show that SPH model_4 (KGC, PST, δ -SPH and asymmetric pressure approximation) is the most suitable for simulating the closed natural convection problems, especially at high Rayleigh numbers. Good agreements with reference solutions are obtained by SPH model_4 for the natural convection at 10 4 ≤ R a ≤ 10 8. Furthermore, the simulation of natural convection at R a = 10 9 is conducted by SPH model_4. The evolutions of thermal convection are described in detail. It is found that dynamics characteristic reveals that the dominant force is the pressure gradient, rather than the buoyancy force before the quasi-steady state. In addition, the chaotic motion at R a = 10 9 has significant influence to the heat transfer characteristic in the vertical boundary layers. [ABSTRACT FROM AUTHOR]

Predicting the propagation and transformation of the solitary wave is very important in coastal engineering. This paper presents a numerical investigation of the solitary wave breaking over a slope by using a finite particle method (FPM), which is an enhanced smoothed particle hydrodynamics (SPH). We firstly conduct a comparative study on SPH and FPM in modelling solitary wave and it is demonstrated that FPM performs better than SPH qualitatively and quantitatively. A modified particle shifting technique (PST) is integrated into FPM to avoid possible ill corrective matrix due to extremely disordered particle distribution. We find that the artificial viscous coefficient can greatly influence the wave run-up and propose an empirical equation to quickly determine the optimal value of the artificial viscous coefficient used in the FPM simulations. The solitary wave breaking is then modelled for scenarios with relative wave heights and slopes, and three typical breaking types, including surging breaker (SU), plunging breaker (PL) and spilling breaker (SP), are analyzed. The result indicates that SP affects the pressure field more extensive than PL and SU. When the breaker type transits from SP to SU, the breaker tends to break at a narrower region close to the shoreline, the breaking depth decreases, the breaking index gradually turns into an unstable value, and the relative run-up height of breaking waves becomes larger. Although the wave celerity at breaking has no clear relationship with breaker types, the wave celerity at breaking increases with the increase of the relative wave height and slightly decreases with a steeper slope. • For simulations of the solitary wave breaking over slopes, FPM performs better than SPH in terms of stability and accuracy. • An empirical equation is proposed to decide the artificial viscous coefficient for simulating wave transformation by FPM. • The spilling breaker affects the pressure field more extensive than plunging breaker and surging breaker. • The wave celerity at breaking has no clear relationship with breaker types. • The wave dissipation turns into lower when wave breaking types change from spilling breaker to surging breaker. [ABSTRACT FROM AUTHOR]