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In this work, we develop an accurate and stable Updated Lagrangian particle hydrodynamics (ULPH) modeling to simulate complicated free-surface fluid flows. Leveraging its inherent properties as a Lagrangian particle method, the ULPH has natural advantages in modeling free-surface flows. However, similar to other meshfree methods, ULPH is subject to numerical instabilities and non-physical pressure fluctuations when solving the Navier–Stokes equation in the explicit numerical scheme. Within the framework of the ULPH method, several innovative enhanced treatment techniques have been proposed and combined with other previouly developed methods to establish an ULPH single-phase flow model. The main novelties of these techniques are the derivation of the density diffusive term in the continuum equation inspired by δ -SPH to eliminate pressure oscillations, and the proposal of a new free-surface search algorithm to determine the particles and their normal vectors at the free surface. The ULPH is a nonlocal fluid dynamics model, which is in fact a prototype of Peridynamics in fluid mechanics. Considering the nature of free-surface fluid flows, we strategically implement the diagonalization and renormalization of the shape tensor for particles located in close proximity to the free-surface region to improve the numerical stability of computations. Several complex free-surface flow benchmark examples have been simulated, which confirms that the enhanced treatment techniques can effectively capture the details of surface flow evolution and maintain long-term stability. Moreover, the qualitative and quantitative analyses of the results indicate that the proposed ULPH surface flow model is highly accurate and stable for simulating complex free-surface fluid flows. [ABSTRACT FROM AUTHOR]

The process of a damaged ship flooding is a complicated free surface flow problem. There is a complex coupling effect between the ship cabin and the flow inside and outside of the cabin. In this paper, a GPU (graphic processing unit)- δ + -SPH (smoothed particle hydrodynamics) numerical model for the cabin flooding in deep-water environments is developed based on GPU parallel acceleration technology and Nvidia's CUDA (compute unified device architecture). First, the computational accuracy and efficiency of this numerical model are verified by experiments results on the water flooding of a simple damaged cabin model. Furthermore, the flooding characteristics of a submarine cabin are analyzed, considering different numbers of damaged cabins, depths, and opening positions. Finally, the progressive flooding and the dynamic response characteristics of a full-scale submarine model are investigated. The results show that the process of progressive flooding in a submarine cabin is characterized by its rapidity and intensity. Different factors, for example, damaged cabin numbers, cabin depths, and opening positions, have great influences on the process of flooding and the motion of the submarine cabin. This study can offer valuable technical assistance in the post-damage remediation process. [ABSTRACT FROM AUTHOR]

High-speed water entry is a transient hydrodynamic process that is accompanied by strongly compressible flow, free surface splash, cavity evolution and other nonlinear hydrodynamic phenomena. To address these problems, a novel fluid–structure interaction (FSI) scheme based on the immersed boundary method is proposed which is suitable for strongly compressible multiphase flows. In this scheme, considering the multiphase interfaces at the immersed boundary, an improved immersed boundary method for effectively suppressing the non-physical force oscillation is proposed. Additionally, a quaternion-based six degrees of freedom motion system is used to describe rigid body motion, and the multiphase flow Eulerian finite element method is applied as the fluid solver. Using analytical solutions, experimental data and literature data, the accuracy and robustness of the FSI scheme are validated. Finally, the high-speed water entry of the slender body with different noses is investigated, and the hydrodynamic loads including the axial and normal drag forces and the bending moment are extensively discussed. The hydrodynamic load and motion trajectory are determined by the nose configuration. The tail slamming phenomenon is the primary focus, and it is revealed that its formation is primarily related to the pitch moment formed at the stage of crossing the free surface. Tail slamming also causes violent impact loads, especially bending moments, which may cause slender projectiles to break off. Finally, to combine the features of the flat and hemispherical noses, the water entry of the projectile with a truncated hemispherical nose is simulated and discussed. [ABSTRACT FROM AUTHOR]

This paper aims at presenting a general-purpose-oriented and fully parallelized meshless framework to simulate complex Fluid–Structure Interaction (FSI) problems in ocean engineering. In this framework, a Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) solver is combined with several advanced pre- and post-processing techniques. Based on the framework, we have been developing our in-house WCSPH-FSI package named SPHydro for solving hydrodynamic problems involving complex FSI processes in an accurate, efficient, and convenient manner. Three benchmarks are performed to qualitatively and quantitatively validate the accuracy and convergence of SPHydro. In addition, several practical applications are also provided to further highlight the generality and applicability of SPHydro in ocean engineering simulations. It is demonstrated that SPHydro holds satisfactory performance in solving complex FSI problems in ocean engineering and that the present framework can be further developed to tackle more complex FSI problems for general engineering applications due to its high flexibility and extensibility. [ABSTRACT FROM AUTHOR]

High-pressure bubble dynamics often involves many complex issues, including large deformations and inhomogeneities, strong compression, moving interfaces, and large discontinuities, that bring challenges to numerical simulations. In this work, an axisymmetric Riemann–smoothed particle hydrodynamics (SPH) method is used to simulate high-pressure bubbles near different boundaries. This Riemann–SPH can adopt the real sound speed instead of the artificial one for the air phase in the bubble. Therefore, the real compressibility of the air phase can be considered, and the corresponding time step is significantly increased. To avoid unphysical interface penetration and maintain relatively homogeneous particle distribution, a new and simple particle shifting scheme for multiphase flows is proposed. Additionally, to minimize the influence of the unphysical boundary on the bubble, a large fluid domain with an optimized initial particle distribution is adopted to reduce the particle number. Several high-pressure bubbles under different boundary conditions are considered, including in a free field, near a free surface, near a solid boundary, and near a rigid sphere. Numerical results show that these bubble dynamic behaviors can be reproduced with satisfactory accuracy. [ABSTRACT FROM AUTHOR]

Solid object water entry is a common problem in various natural, industrial and military applications, which involves large deformation of free surfaces and violent fluid–structure interactions. In computational fluid dynamics (CFD), this is a type of problem that tests the robustness and capacity of any CFD numerical algorithm. In this work, the newly-developed updated Lagrangian particle hydrodynamics (ULPH) method, which is a fluid version of peridynamics, is enhanced and applied to simulate solid object water entry problems. ULPH method is Lagrangian meshfree particle method that can ensure the specific free surface conditions automatically satisfied. The density filter and artificial viscosity diffusion are adopted in the ULPH scheme to stabilize and smooth the pressure field. In the process of a rigid body entering water, it may induce negative pressure in some areas of impact region, which can cause spurious tensile instability in some meshfree particle simulations, such as SPH and ULPH. A tensile instability control technique based on the ULPH framework has been developed to overcome the numerical instability. To validate the stability and accuracy of the ULPH approach in simulating water entry problems, several 2D and 3D examples of water entry have been carried out in this work. The necking and cavity pinch-off phenomena are visible in the numerical results. The simulation results of the ULPH method are well compared with experimental data and other numerical solutions. The water crown, cavity shapes and stream pattern formed around the rigid body entering the water can be well captured. The computation results show that the ULPH method has the ability to simulate the complex solid object water entry precess accurately. [ABSTRACT FROM AUTHOR]

During the process of wave slamming on a structure with sharp corners, the wave receding after wave impingement can induce strong negative pressure (relative to the atmospheric pressure) at the bottom of the structure, which is called the suction effect. From the practical point of view, the suction force induced by the negative pressure, coinciding with the gravity force, pulls the structure down and hence increases the risk of structural damage. In this work, the smoothed particle hydrodynamics (SPH) method, more specifically the δ+SPH model, is adopted to simulate the freak wave slamming on a fixed platform with the consideration of the suction effect, i.e., negative pressure, which is a challenging issue because it can cause the so-called tensile instability in SPH simulations. The key to overcome the numerical issue is to use a numerical technique named tensile instability control (TIC). Comparative studies using SPH models with and without TIC will show the importance of this technique in capturing the negative pressure. It is also found that using a two-phase simulation that takes the air phase into account is essential for an SPH model to accurately predict the impact pressure during the initial slamming stage. The freak wave impacts with different water depths are studied. All the multiphase SPH results are validated by our experimental data. The wave kinematics/dynamics and wave impact features in the wave-structure interacting process are discussed, and the mechanism of the suction effect characterized by the negative pressure is carefully analyzed. [ABSTRACT FROM AUTHOR]

The present work is dedicated to the enhancement and application of the recently developed δ + -SPH scheme for two and three dimensional water entry problems. The cylinder surface presents a circular shape and therefore the position where the flow separation happens is tightly related to the implementation of the boundary condition. A special treatment for the particle shifting between fluid and solid wall is highlighted. Adaptive particle refinement (APR) is applied in this work to reduce the computational cost. It is found that, APR and the δ + -SPH scheme benefit to each other. That is to say the former reduces the computing cost of the latter while in return the latter solves the problem of particle disorder of the inactive particles of the former. Thanks to the combination of APR and δ + -SPH especially in three dimensional (3D) cases, the overall computational cost is significantly reduced while sufficiently fine particle resolution can be obtained in the flow region characterized by large pressure gradient close to the structure surface. The fairly good agreements between the SPH results and the experimental data prove the present SPH model to be a reliable tool in accurately solving the fluid–structure interacting problems in ocean engineering. [ABSTRACT FROM AUTHOR]

In numerical simulations of fluid dynamics based on the smoothed particle hydrodynamics (SPH) method, the formula for the viscosity term is always complicated since it is a second-order derivative term. In this paper, for Newtonian fluids, a novel formula for the viscous force satisfying the Stokes hypothesis is proposed in the weakly compressible framework. Because of the ease of adopting only a single layer of particles to enforce the boundary condition, the normal flux method is implemented for a solid wall boundary, which should be suitable for complex geometries and thin structures. The normal flux method can mitigate the decrease in numerical accuracy caused by the lack of neighbouring particles at the boundary. Meanwhile, the kernel function at the boundary is modified to satisfy the normalization condition. The accuracy and robustness of the proposed algorithm are verified by a series of cases of numerical fluid simulations, paving the way for SPH simulations coupled with complex thin shell structures for fluid–structure interaction problems. • A novel viscous force that satisfies Stokes hypothesis is derived. • The viscous force near the single-layer boundary is presented. • Kernel function correction technique is proposed to ensure consistence near the single-layer boundary. [ABSTRACT FROM AUTHOR]

In the current study, an improved numerical model is proposed in the compressible fields based on Smoothed Particle Hydrodynamics (SPH), which is comprised of MUSCL interpolation in multiphase flow, enhanced particle regeneration technique (PRT) and the particle shifting technique (PST) in compressible flows. The PRT is specially proposed to deal with compressible problems, in which the volume of particles have large variation during the whole simulation process. Different to the conventional PRT [1] , an interface control (IC) method is proposed to deal with the mass conservation problem which may result in the unphysical movement of interface when modeling multifluids. The multiphase MUSCL interpolation aims at dealing with over-dissipation problem that exists in the Godunov-type SPH which may result in the wrong detection of wave front. To avoid large discontinuity between different fluids, two kinds of extrapolation schemes (constant extrapolation and isentropic extrapolation) are discussed and compared at the inspiration of the ghost fluid method (GFM). The proposed MUSCL-based compressible SPH model is validated and discussed in several challenging test cases, such as multiphase shock tube like problem, the shock wave impacting on multifluids interface problem and Richtmyer-Meshkov instability etc., in which good agreements are obtained. • An enhanced particle regeneration technique is proposed to simulate compressible multiphase flows based on SPH. • The mass conservation property is improved leading to the accurate detection of multiphase interface. • The wave front can be better predicted and higher resolution results are obtained combined with MUSCL scheme. • The strongly compressible shock tube problem and the shock wave impacting on the multiphase interface problem is simulated. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of charge parameters of the underwater contact explosion based on the axisymmetric smoothed particle hydrodynamics (SPH) method. The dynamic boundary particle is proposed to improve the pressure fluctuation and numerical accuracy near the symmetric axis. An in-depth study is carried out over the influence of charge shapes and detonation modes on the near-field loads in terms of the peak pressure and impulse of shock waves. For different charge shapes, the cylindrical charge with different length-diameter ratios may cause strong directivity of peak pressure and impulse in the near field. Compared with spherical charge, the peak pressure of cylindrical charge may be either weakened or enhanced in different directions. Within a certain range, the greater the length-diameter ratio is, the more obvious the effect will be. The weakened ratio near the detonation end may reach 25% approximately, while the enhanced ratio may reach around 20% in the opposite direction. However, the impulse in different directions seems to be uniform. For different detonation modes, compared with point-source explosion, the peak pressure of plane-source explosion is enhanced by about 5%. Besides, the impulse of plane-source explosion is enhanced by around 5% near the detonation end, but close to those of the point-source explosion in other directions. Based on the material constitutive relation in the axisymmetric coordinates, a simple case of underwater contact explosion is simulated to verify the above conclusions, showing that the charge parameters of underwater contact explosion should not be ignored. [ABSTRACT FROM AUTHOR]

In this paper, we propose a targeted essentially non-oscillatory (TENO) SPH scheme. In this scheme, a 5-point stencil consisting of 3 candidate sub-stencils is adopted to implement the TENO reconstruction as originally proposed in the mesh-based methods. In the original TENO reconstruction, these five points are fixed and equidistant. However, it is difficult for SPH to determine these five points due to the random particle distribution. To remedy this issue, for two interacting particles, we first consider them as two existing stencil points. Then, for the other three points, we search for the closest particles to their exact positions. Subsequently, considering the gradient of these particles, we can finally obtain the values of these target points. Afterwards, the primitive values of points are reconstructed by TENO reconstruction where the polynomial interpolations are modified. Numerical results show that the proposed TENO-SPH scheme can be applied to reproduce some compressible flows involving shocks and small-scale structures, some incompressible vortex flows and free surface flows with superior accuracy. • A targeted essentially non-oscillatory (TENO) SPH scheme is proposed. • The accuracy of SPH is improved by TENO reconstruction. • TENO-SPH can be applied to resolve compressible, vortex and free surface flows. • TENO-SPH shows sharp shock-capturing capacity and low-dissipation property. [ABSTRACT FROM AUTHOR]

There are many compressible flows involving strong discontinuities in nature and engineering applications. Their behaviors are complicated by the existence of shock waves, rarefaction waves and contact discontinuities. To investigate these flows, a shock-capturing scheme based on smoothed particle hydrodynamics (SPH) is proposed. In this scheme, Roe's approximate Riemann solver cooperated with a novel limiter is embedded into the SPH governing equations to capture shocks. This limiter is simple and effective to control numerical dissipations, and the use of this limiter eliminates the tunable artificial viscosity as required by the conventional SPH. Additionally, to restore the accuracy limited by heterogeneous particle distribution that is usually encountered in the simulation of strongly-compressible flows, the gradient operator in the continuity equation is corrected by a renormalization procedure. Last but not least, for initial particle distribution, unlike the equal mass particle distribution commonly used in compressible SPH simulations, the equal spacing particle distribution is adopted in this scheme, which makes it much easier to model multidimensional problems. The present scheme has been verified by a set of benchmark tests involving contact discontinuities, extreme shock waves and strong rarefaction waves, some of which are firstly simulated by the SPH method. • An accurate shock-capturing SPH scheme is proposed to simulate compressible flows. • A novel dissipation limiter is designed for Riemann-SPH to adjust dissipations. • The proposed scheme can capture sharp shock waves. [ABSTRACT FROM AUTHOR]

Abstract: Damaged ship at sea will be a direct threat to lives and property, and it has a great significance of studying ship's remaining buoyancy, stability, sinking time and other important parameters. The process of a damaged ship sinking into water is a complex motion involving ship hull, inner and outer fluid coupled with waves and many other factors. It is featured by high nonlinearity and hard to establish a precise theoretical model to study. Yet SPH (smoothed particle hydrodynamics) as a meshfree method has a great advantage in solving such problems because of the nature of self-adaptive and Lagrangian. Firstly, the experiments of two scaled ship models with different openings sinking into water are carried out, through the sinking processes of broadside opening and bottom opening models, the conclusion is drawn that although the serious loss of stability of broadside opening model, the sinking time and other parameters are more conducive to rescue after maritime distress. Secondly, the parallel program of three dimensional SPH is developed to simulate the above more complex model, broadside opening model. The coupled process of sloshing is compared with that of experiment, and the results show good agreement with each other which verify the accuracy and feasibility of three dimensional parallel program. [Copyright &y& Elsevier]

Water entry problem is common in ocean engineering, including complex fluid–structure interaction (FSI) phenomena at the interfaces. As a Lagrangian and mesh-free particle method, the smoothed particle hydrodynamics (SPH) method owns great advantages in dealing with FSI problems. However, solid boundary treatment has always been one of the main issues affecting the accuracy of the SPH method. Through combining the dummy particle boundary and the normal flux method, an improved solid boundary treatment technology is proposed in this paper. After fully validated by a series of 2D and 3D water entry cases, the boundary treatment method is applied to simulate the water entry of a lifeboat. The results indicate that the SPH model in this paper can provide a reliable numerical prediction for water entry problems with 3D complex geometries. • An improved solid boundary treatment technology is proposed. • Strict numerical validations are carried out using both 2D and 3D water entry cases. • The improved boundary treatment is applied to the water entry of a lifeboat. [ABSTRACT FROM AUTHOR]

In the past few decades, the weighted essentially non-oscillatory (WENO) scheme has been widely used in mesh-based methods and has achieved great success, but its application to the smoothed particle hydrodynamics (SPH) is still limited. In this paper, a simple and accurate implementation of the WENO scheme to the SPH method is proposed for solving compressible flows with discontinuities and small-scale structures. In the proposed scheme, several equal spacing points along the line joining two interacting particles are selected to constitute candidate stencils. However, due to the Lagrangian characteristic of SPH, some points may be lost. To solve this problem, we first search the particles closest to these missing points, and then the first-order Taylor series expansion is used to obtain the corresponding primitive variables of these points. After that, through the WENO strategy, the left and right states at the interface between two interacting particles are reconstructed. Finally, the inter-particle interactions are determined by using Roe's approximate Riemann solver. Several numerical tests show that the proposed WENO-SPH method is robust and able to accurately capture shockwaves, and benefiting from the low-dissipation property, it also has a good performance in resolving small-scale structures in flows. • A new WENO-SPH method is proposed for compressible flows with discontinuities. • The convergence of the WENO-SPH is studied by reconstructing a simple function. • The accuracy of the WENO-SPH is validated through several numerical tests. [ABSTRACT FROM AUTHOR]

In this work, a particle regeneration technique is developed for Smoothed Particle Hydrodynamics (SPH). In traditional SPH, the particle disorder phenomenon will occur when dealing with the strongly compressible flow problem. To solve this, in the present work, uniformly distributed background particles filled in the computational domain are adopted. The particle regeneration technique is that the fluid particles replaced by the background particles when the fluid density changes to a specific limitation. The fluid variables of the background particles are approximated by the fluid variables of the initial particles in their support domain. For the multiphase flow, the multiphase interface is calculated by an interface reproducing algorithm, in which, we defined an indicator variable, and set the indicator value discontinuity between different materials. By setting the threshold value, the multiphase interface is reconstructed. Meanwhile, the momentum equation for the Riemann SPH is modified to eliminate the instability in the light phase. Several numerical examples are studied to verify the present algorithm. [ABSTRACT FROM AUTHOR]

A 2D fluid–structure interaction (FSI) solver based on Smoothed Particle Hydrodynamics (SPH) and RKPM is established in this paper. Since we focus on the incompressible flow problem, the WCSPH is adopted to simulate the fluid. Meanwhile, the structure is modeled by the RKPM beam formulation, in which the plane strain hypothesis is adopted, and only a column of particles is needed to discretize the beam. Since the support domain of the fluid particle near the FS interface is truanted, the so-called normal-flux method is adopted to consider surface integration terms. Moreover, we propose a new excess pressure for the incompressible fluid flow based on Bernoulli's principle, which is more accurate for the simulation of incompressible FSI problems. Besides, the particle shifting scheme near the fluid–structure interface is modified. Finally, the accuracy and stability of the present FSI solver are verified by several numerical examples. [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]

This paper presents an improved particle shifting technology (IPST) for smoothed particle hydrodynamics (SPH). It allows particles in the vicinity of the free-surface to be shifted in all the directions in case they are not uniformly distributed, rather than just shifted tangentially to the free-surface in some conventional particle shifting technologies. Therefore, with the new technique, the imprecise shifting due to the inaccurate evaluations of the local normal and tangential vectors of the free-surface can be avoided, and a more uniform particle distribution near the free-surface can be obtained even for long-term simulations, which improves the accuracy and stability of the SPH method. Besides, by combining the advantages of two existing free-surface detection methods, an optimized detection approach is provided. It is capable of detecting the free-surface particles accurately, and meanwhile, it is easier to implement and requires lower computational costs, especially for three-dimensional problems. Several numerical tests show that the proposed IPST and the optimized free-surface detection method are robust and accurate for the simulation of a variety of free-surface flows. • An improved particle shifting technology (IPST) in the SPH method is proposed. • An optimized free-surface detection method in SPH is introduced. • The accuracy and robustness of the proposed IPST is validated on several long-term simulations. [ABSTRACT FROM AUTHOR]

Spacecraft ditching on the water surface is a classic and widely concerned problem in aerospace and marine engineering. Researchers have carried out many experimental and numerical investigations to explore the slamming load and the stability of the ditching trajectory. The δ +-SPH model is adopted in this study to simulate and analyze the load and motion characteristics of a capsule after entering the water with different attitude angles and impact velocities. A particle shifting method is included in the δ +-SPH model to increase accuracy. For determining the ideal particle spacing for this issue, a convergence analysis is done. Based on that, the slamming loads of the capsule ditching at different pitch angles are simulated and explored. Then, a simulation of the Titan capsule ditching is performed, and the results are compared to the experimental data. It is verified that large inclination angles and vertical velocities of the capsule tend to cause overturning, which should be carefully considered and avoided in the engineering design. Finally, the ditching of a capsule on Titan's hydrocarbon lake is also simulated. • A 3D SPH model is presented to investigating the ditching of space capsule. • The pitch angle exerts significant effects on the ditching load. • The overturning of the capsule during ditching is studied. • The superiority of SPH method is demonstrated for the study of capsule ditching. [ABSTRACT FROM AUTHOR]

This work presents a multi-resolution fluid–structure interaction (FSI) solver for hydroelastic issues by coupling the Weakly Compressive Smoothed Particle Hydrodynamics (WCSPH) method and the Finite Element Method (FEM). The fluid is simulated by the SPH method, and the structure response is considered by the FEM beam. The normal flux method is employed to calculate the coupled force between the fluid and structure by considering the surface integration term in SPH approximation equations. A multi-resolution technology is proposed by discretizing the fluid and structure with different resolutions. The resolution of beam elements can be either coarser or finer than that of SPH particles. Finally, the accuracy, robustness and efficiency of the current FSI solver are strictly validated by several typical benchmarks, including the hydrostatic, free-surface flow and water entry problems. • We developed a coupled SPH-FEM method for fluid–structure interactions. • Different spatial–temporal resolution is employed for fluid and solid structure. • The accuracy, efficiency, and robustness of the FSI solver are strictly validated. [ABSTRACT FROM AUTHOR]

Lifeboats can be launched through free-fall in extreme maritime accidents to fast leave the hazardous area, which deserves particular attention because it is tightly related to the safety of lifeboats and the carried passengers as well. This paper, within the framework of a fully 3D and multi-resolution Smoothed Particle Hydrodynamics (SPH) model, aims at investigating the water entry dynamics of a free-fall lifeboat with different water entry angles. First of all, two rigorous model validations are performed to demonstrate the accuracy, stability and convergence of the present SPH model. Furthermore, the water entry dynamics of the lifeboat with respect to different water entry angles, including the motion characteristics and cavity evolutions, is investigated and discussed in detail based on the SPH results. It is clearly demonstrated that the present SPH model possesses (1) satisfactory accuracy for predicting the water entry dynamics of a lifeboat, and (2) a better ability to capture the splashing jets and scattering droplets than mesh-based solvers thanks to its entirely Lagrangian nature. In addition, it is also demonstrated that the water entry angle exerts significant effects on the water entry dynamics of a lifeboat, which should be taken into account in the design stage to provide safer productions. • A multi-resolution and fully 3D SPH model for water entry problems is presented. • A simple procedure to generate uniform particles is applied for complex geometries. • The present SPH model is shown to be superior in simulating water entry problems. • The water entry angle exerts significant effects on lifeboat's water entry dynamics. [ABSTRACT FROM AUTHOR]

This paper is dedicated to (1) emphasizing the importance of removing the Tensile Instability (TI) induced by negative pressures in Smoothed Particle Hydrodynamics (SPH) simulations, and (2) proposing new modifications for the Tensile Instability Control (TIC) and Particle Shifting Technique (PST) to simulate Fluid–Structure Interaction (FSI) phenomena with violent free-surface evolutions. Four numerical benchmarks associated with classical FSI problems characterized by negative pressures are simulated to show how the tensile instability destroys an SPH simulation without any TI prevention strategies. Therefore, PST and TIC are adopted into the SPH simulations for the sake of suppressing the tensile instability. However, different combinations of TI prevention strategies, i.e. different combinations of PST and TIC, lead to different results, and hence detailed investigations and discussions are provided. It is demonstrated that when negative pressures are moderate, solely using either PST or TIC works well. Nonetheless, it is in favor of using both PST and TIC if an SPH simulation is characterized by extremely strong negative pressures. Furthermore, for the problems with a convex-shaped structure penetrating a free-surface, special modifications are necessary for both PST and TIC in order to obtain accurate SPH results. • Importance of removing the tensile instability in WCSPH simulations is stressed. • New modifications for PST and TIC at boundaries are presented for FSI problems. • Solely using PST could fail to suppress the tensile instability. • Solely using TIC is hard to achieve homogenous particle distribution. • PST, TIC and their modifications at boundaries should be used simultaneously. [ABSTRACT FROM AUTHOR]

Abstract: In order to evaluate the strength and comparability of impulsive environment of model and practical structure in the water when subjected to underwater explosion, a new shock factor based on energy acting on the structure is presented to describe the loading of underwater explosion. To test the validity of this new factor, numerical experiments of double stiffened cylindrical shells are carried out a series of cases designed by the new factor and two other standard shock factors respectively. The results of the cases designed by the new factor indicate that the kinetic energy, potential energy and shock response spectrums of the structures agree well with each other in different cases designed by the equal new shock factor. However, the results of the cases designed by the two other standard shock factors are rather diverse. The analysis considers that the old shock factors do not take the spherical characteristics of shock wave front and relative position between detonation and structure into account, which can hardly reflect the similarity of underwater explosion loadings. The new shock factor can make up for such limitations. [Copyright &y& Elsevier]

In this work, we present a higher-order nonlocal continuum theory of a recently developed updated Lagrangian particle hydrodynamics (ULPH) (see: Tu and Li, 2017 and Yan et al., 2019) and its applications to multiphase flows. The original nonlocal differential operators used in ULPH have only the first-order accuracy, and they may not be able to simulate some complicated and delicate three-dimensional flow problems. In order to improve the computational accuracy and stability, we adopt the nonlocal synchronized differential operator from the reproducing kernel particle method (RKPM), which is regarded as the higher-order nonlocal differential operator, and we apply them to build higher-order ULPH formulations. The main advantage of the higher-order nonlocal differential operator over the original one is its high accuracy in unstructured particle distribution, and it is fully controlled by the approximation polynomial basis. Numerical verifications have been carried out to validate the accuracy of the proposed approach by using various polynomial bases. Several challenging and delicate three-dimensional multiphase flow benchmark problems are solved to demonstrate the capability of the proposed method. The numerical results of the higher-order ULPH show good agreement with theoretical and numerical solutions in the literature, demonstrating the promising potential of higher-order nonlocal ULPH formulation in modeling and simulating multiphase flows with substantial topological variations of interfaces and high density ratios. • We have shown in the first time that the particle nonlocal differential operators are originated from RKPM. • We have developed a higher-order nonlocal ULPH theory and its computation formulations. • We have employed the higher-order nonlocal ULPH to model and simulate 3D multiphase flow computations. [ABSTRACT FROM AUTHOR]

• A robust WCSPH model is applied to deal with the water entry of the bow-flare ship section. • The evolution laws of the free surface and spry jet are analyzed. • The water entry of a three dimensional ship hull is simulated. In the marine and ocean engineering area, the water-entry process is a typical problem of fluid-solid interaction, which involves complex phenomenon such as the large deformation of free surface and considerable structure movement. In this process, the predicting impact load and summarizing the evolution law of the free surface deformation have important reference significance for the hull structure design. To address this issue, numerical simulations are carried out using a robust Smoothed Particle Hydrodynamics (SPH) model. The accuracy of the present SPH model is firstly validated by simulating the water entry of bow-flare section in two and three dimensions, respectively. Then, we investigate the variation characteristics of impact load and the evolution of free surface during the entry of the bow-flare ship section with different roll angles (θ = 0 ∘ ∼ 20. 3 ∘ ). Attention is focused on the flow separating phenomenon and the spray jet on the free surface. Finally, the water entry of a three dimensional ship hull is simulated. The velocity and pressure of flow field are analyzed and some conclusions are drawn. [ABSTRACT FROM AUTHOR]