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HERMES-TP/SP (High Energy Rapid Modular Ensemble of Satellites Technologic and Scientific Pathfinder) is a constellation of six 3U nano-satellites hosting simple but innovative X-ray detectors, characterized by a large energy band and excellent temporal resolution, and thus optimized for the monitoring of Cosmic High Energy transients such as Gamma Ray Bursts and the electromagnetic counterparts of Gravitational Wave Events, and for the determination of their positions. The projects are funded by the Italian Ministry of University and Research and by the Italian Space Agency, and by the European Union Horizon 2020 Research and Innovation Program under Grant Agreement No. 821896. HERMES-TP/SP is an in-orbit demonstration, that should be tested starting from 2022. It is intrinsically a modular experiment that can be naturally expanded to provide a global, sensitive all sky monitor for high-energy transients., Comment: 15 pages, 3 figures, Proceedings of SPIE Astronomical Telescopes and Instrumentation 2020

The HERMES-TP/SP (High Energy Rapid Modular Ensemble of Satellites -- Technologic and Scientific Pathfinder) is an in-orbit demonstration of the so-called distributed astronomy concept. Conceived as a mini-constellation of six 3U nano-satellites hosting a new miniaturized detector, HERMES-TP/SP aims at the detection and accurate localisation of bright high-energy transients such as Gamma-Ray Bursts. The large energy band, the excellent temporal resolution and the wide field of view that characterize the detectors of the constellation represent the key features for the next generation high-energy all-sky monitor with good localisation capabilities that will play a pivotal role in the future of Multi-messenger Astronomy. In this work, we will describe in detail the temporal techniques that allow the localisation of bright transient events taking advantage of their almost simultaneous observation by spatially spaced detectors. Moreover, we will quantitatively discuss the all-sky monitor capabilities of the HERMES Pathfinder as well as its achievable accuracies on the localisation of the detected Gamma-Ray Bursts., 19 pages, 13 figures, Proceedings of SPIE Astronomical Telescopes and Instrumentation 2020

In this paper, the Lagrangian particle method δ - P l u s -SPH model is used to solve a series of benchmark test-cases of vortex induced vibrations (VIV). An Adaptive Particle Refinement (APR) technique is adopted to resolve correctly the boundary layer regions of the moving bodies, and to de-refine the particles that are transported far away. Furthermore, a switch correction on the pressure forces term is adopted in the momentum equation to completely remove the occurrence of the so-called Tensile Instability that leads to the development of numerical cavitation in negative pressure regions. Because of the Lagrangian nature of the method, difficulties arise when intense vortex wakes, typically developed in VIV problems, cross the outflow boundary. To this purpose, a damping zone is also implemented close to the outlet to improve the numerical stability of the scheme. The fluid–structure coupling technique is based on ghost particles properly generated inside the solid bodies. The validation is performed against test-cases for which reference solutions are available in the literature. Within those test-cases challenging benchmarks involving small mass ratios, large-amplitude body motions and multi-body interactions have been selected. For all the benchmark test-cases the δ - P l u s -SPH results are in good agreement with the reference solutions, demonstrating the ability of this particle method in solving complex VIV problems.

In the present work a consistent inclusion of a particle shifting technique (PST) in the weakly compressible Smoothed Particle Hydrodynamic (SPH) models is discussed. Recently, it has been shown that the use of PST can largely improve both the accuracy and the robustness of SPH models. In particular, the δ + -SPH model is a weakly-compressible SPH model where a PST is adopted along with a diffusive term in the continuity equation that helps removing the high-frequency noise on the pressure field. This specific SPH model is able to overcome the main drawbacks that afflict the standard weakly-compressible SPH model. In this work we demonstrate that a consistent introduction of the PST inside the SPH model leads to a new set of equations where some additional terms containing the particle shifting velocity δ u have to be taken into account. The effects of these δ u -terms become crucial for problems in confined or periodic domains, as well as for long-time simulations of free-surface flows. The proposed scheme is tested against challenging benchmark cases, highlighting when the δ u -terms play an important role or not. Further improvements of the PST algorithms for the numerical treatment of the scheme close to the free surface and along the solid boundaries are also discussed.

In two previous papers we presented an algorithm for coupling the Finite Volume (FV) method for the solution of 2D Navier–Stokes equations discretized on block structured Eulerian grids with the weakly-compressible Lagrangian Smoothed Particle Hydrodynamics (SPH) method. This coupling procedure exploits the SPH method to discretize flow regions close to free-surfaces and the Finite Volume approach to resolve both the bulk flow and the wall regions, where grid stretching can be favourably used. The information exchange between the two numerical schemes is established through overlapping zones. In the present paper this coupling paradigm is extended to a 3D framework. To this purpose, the extension of the algorithms for particle creation/deletion on the interfaces are described and issues related to free-surface intersection with each sub-domain boundary are addressed. Moreover, a new coupling procedure that simplifies the algorithm is proposed and tested. Effectiveness and accuracy achieved by the coupled solver are tested on challenging problems involving large free surface deformations and vorticity generation like the 3D flow past a cylinder below a free surface, the breaking wave generated by a ship bow in forward motion and the impact of a flat plate on the water surface.

The smoothed particle hydrodynamics (SPH) method is used in this paper to model micropolar fluids, with emphasis on their dissipation mechanisms. To this aim, a dissipation function is defined at the particle level which depends on the relative velocity between particles but also on an additional spin degree of freedom, which modifies such relative velocity as well as introduces spin-related intrinsic dissipation mechanisms, comparable to those related to the rate of deformation tensor in Newtonian fluids. This dissipation function is then incorporated within the Lagrangian formalism, leading to a set of SPH particle equations to describe the dynamics. A continuous integral SPH version of the scheme is obtained with a bottom-up derivation which guarantees the consistency of the SPH term. The model is then enriched with two additional terms based exclusively on the spin derivatives, which grant it the maximal generality as an isotropic model for micropolar fluids. Finally, numerical verification and validation tests are documented that show that SPH is capable of accurately modeling this type of dynamics.

In Part I of this series [Marrone, Numerical study on the dissipation mechanisms in sloshing flows induced by violent and high-frequency accelerations. I. Theoretical formulation and numerical investigation, Phys. Rev. Fluids 6, 114801 (2021)10.1103/PhysRevFluids.6.114801], a theoretical formulation and the numerical model were developed in order to obtain a complete perspective of the energy balance of a violently accelerated flow confined inside a rectangular tank. The tank-fluid system was periodically excited with a predetermined law of motion and the force between the wall and the fluid and the global energy balance were computed. In this second part, the experimental validation of the previous formulation is presented. In order to make a comparison with a previous experimental campaign, where the tank moves along a single degree of freedom mechanical guide, two numerical problems have been studied: in the first, the decaying movement of the tank is prescribed according to the experimental measurements, and in the second the tank is coupled to a mass-spring-damper equation, and the sloshing force produced by the confined fluid acts as an external force. Both problems have been studied for two different fluids, water and oil, which implies a difference of two orders of magnitude in terms of Reynolds number. A complete description of the energy balance inside the fluid tank is performed and the complexity of the fluid dynamic behavior that takes place inside the tank is explained. The results are compared to the experimental measurements in terms of fluid-wall interaction and energy dissipation.

The sloshing motion of a confined liquid inside a vertically moving tank is analyzed in the present series of paper. The main objective of the study is to understand the multiple resulting energy dissipation mechanisms, namely wall-liquid impacts and free surface phenomena, among others. This analysis is connected to the damping effects on the aircraft wings caused by the liquid action inside the fuel tanks. Due to the complexity and nonlinearity of the flow generated inside the tank, only experiments or efficient numerical solvers are suitable for studying the problem. In this paper, the tank-fluid system is periodically excited with a prescribed law of motion and the nonlinear features are observed, the force between the wall and the fluid and the global energy balance are computed. A weakly compressible smoothed particle hydrodynamics model has been reformulated and adapted to this kind of violent and turbulent flow. The evolution of the different terms that appear in the energy conservation law are computed, and the formulation is compared to other alternatives where the advantages of the present formulation are indicated. The comparison with the experimental results and the fluid-structure interaction case is carried out in Part II [Marrone, Numerical study on the dissipation mechanisms in sloshing flows induced by violent and high-frequency accelerations. II. Comparison against experimental data, Phys. Rev. Fluids 6, 114802 (2021)10.1103/PhysRevFluids.6.114802] of this work.

Future space programmes pose some interesting research problems in the field of non-Keplerian dynamics, being the Moon and the cislunar space central in the proposed roadmap for the future space exploration. In these regards, the deployment of a cislunar space station on a non-Keplerian orbit in the lunar vicinity is a fundamental milestone to be achieved. The paper investigates the natural orbit-attitude dynamics and the attitude stabilisation of coupled motions for extended bodies in the Earth–Moon system. The discussion is carried out analysing the phase space of natural dynamics, constituted by both the orbital and the rotational periodic motions of a spacecraft in cislunar orbits. Floquet theory is applied to periodic orbit-attitude solutions in lunar proximity, to characterise their attitude stability properties and their attitude manifolds, which are discussed and analysed focusing on their dynamical features applicable to cislunar environment. Attitude stabilisation methods are proposed and developed, with particular attention to spin-stabilised solutions. Periodic orbit-attitude dynamics are studied to highlight possible favourable conditions that may be exploited to host a cislunar space station with a simplified control action. The focus of the analysis is dedicated to halo orbits and near-rectilinear halo orbit in the circular restricted three-body problem Earth–Moon system.

The study of an airfoil at low Reynolds number regime was found to be a typical problem where the inception of bifurcations leads the flow evolution from a stationary or periodic behaviour to a purely chaotic one. The present work extends the present literature where numerical investigations of the flow field past two-dimensional symmetric airfoils were performed by fixing the incidence and changing the Reynolds number (Re). Conversely, here the Reynolds number was fixed at Re = 1000 and the angle of attack (AoA) varied from 0∘ to 90∘. Different flow Modes and bifurcation phenomena are detected and, in addition, new non-linear phenomena are also identified. The latter consists of tripling period regimes and its bifurcations up to chaotic conditions. Moreover, for increasing AoA, an inverse transition from chaotic to periodic regimes is also observed. An in-depth study of these different flows is provided and new links with the time evolution of the forces acting on the airfoil, as well as with the wake structures observed, are offered. Phase portrait diagrams are evaluated through the time records of the lift force in order to highlight the eventual presence of limit cycles in the solutions. Accurate numerical simulations are performed with a Vortex Particle Method in a two-dimensional framework, using high spatial resolutions to solve in detail both the near-field and the far-field regions.

The present work is dedicated to the improvement of the δ -SPH scheme. This is an enhanced weakly-compressible SPH model widely used in recent years thanks to its benefits to the standard SPH scheme, to its low CPU costs and to its ease of implementation. Nonetheless, the δ -SPH still presents some drawbacks as other SPH models. For example, in some critical conditions it does not prevent the tensile instability and the consequent numerical fragmentation. Furthermore, even if the use of a diffusive term in the SPH continuity equation is able to reduce numerical high frequencies on the pressure field, the velocity gradients are generally noisy because of the irregularities of the particle spatial configurations , which, in specific flow conditions, can induce also extra numerical-dissipation. For these reasons a particle shifting technique is used to improve the model and a special treatment has been developed for particles that are close to the free-surface region. The introduction of the particle-shifting procedure is generalized in the context of multi-resolutions for which a novel algorithm is formulated to handle the particle re-positioning in the different resolution levels. The proposed algorithms can be straightforwardly implemented in an SPH model without requiring cumbersome code modifications. The δ + -SPH is validated on seven different benchmarks giving a wide panorama on the improvements of this new SPH model.

An angular momentum conservative pure bulk viscosity term for smoothed particle hydrodynamics (SPH) is proposed in the present paper. This formulation permits independent modeling of shear and bulk viscosities, which is of paramount importance for fluids with large bulk viscosity in situations where sound waves or large Mach numbers are expected. With this aim a dissipative term proportional to the rate of change of the volume is considered at the particle level. The equations of motion are derived from the minimization of a Lagrangian combined with an appropriate dissipation function that depends on this rate of change of particle volume, in analogy with the corresponding entropy production contribution in fluids. Due to the Galilean invariance of the formulation, the new term is shown to exactly conserve linear momentum. Moreover, its invariance under solid-body rotations also ensures the conservation of angular momentum. Two verification cases are proposed: the one-dimensional propagation of a sound pulse and a two-dimensional case, modeling the time decay of an accelerating-decelerating pipe flow. The SPH solutions are compared to exact ones, showing that the newly proposed term behaves indeed as a viscosity associated only with the local expansion-compression of the fluid. In view of these considerations, we conclude that the method presented in this paper allows for setting up a bulk viscosity independently of the shear one and as large as any particular problem may require. At the same time, together with the prescribed momentum conservation to reproduce the Navier-Stokes equation, the new term also keeps the angular momentum conservation required to properly model free interfaces or overall rotations of the bulk fluid.

The High Energy Rapid Modular Ensemble of Satellites (HERMES) Technological and Scientific pathfinder is a space borne mission based on a constellation of LEO nanosatellites. The payloads of these CubeSats consist of miniaturized detectors designed for bright high-energy transients such as Gamma-Ray Bursts (GRBs). This platform aims to impact Gamma Ray Burst (GRB) science and enhance the detection of Gravitational Wave (GW) electromagnetic counterparts. This goal will be achieved with a field of view of several steradians, arcmin precision and state of the art timing accuracy. The localization performance for the whole constellation is proportional to the number of components and inversely proportional to the average baseline between them, and therefore is expected to increase as more. In this paper we describe the Payload Data Handling Unit (PDHU) for the HERMES-TP and HERMES SP mission. The PDHU is the main interface between the payload and the satellite bus. The PDHU is also in charge of the on-board control and monitoring of the scintillating crystal detectors. We will explain the TM/TC design and the distinct modes of operation. We also discuss the on-board data processing carried out by the PDHU and its impact on the output data of the detector., Comment: 11 pages, 10 figures. Proceedings of SPIE "Astronomical Telescopes and Instrumentation" 2020

The behaviour of a weakly-compressible SPH scheme obtained by rewriting the Navier-Stokes equations in an arbitrary Lagrangian-Eulerian (ALE) format is studied. Differently from previous works on ALE, which generally adopt conservative variables (i.e. mass and momentum) and rely on the use of Riemann solvers inside the spatial operators, the proposed model is expressed in terms of primitive variables (i.e. density and velocity) and is written by using the standard differential formulations of the weakly-compressible SPH schemes. Similarly to ALE-SPH models, the arbitrary velocity field is obtained by modifying the pure Lagrangian velocity of the material point through a velocity δ u → given by a Particle Shifting Technique (PST). We show that the above-mentioned ALE-SPH equations are, however, unstable when they are integrated in time. The instability appears in the form of large volume variations in those fluid regions characterised by high velocity strain rates. Nonetheless, the scheme can be stabilised if appropriate diffusion terms are included in both the equations of density and mass. This latter scheme, hereinafter called δ -ALE-SPH scheme, is validated against reference benchmark test-cases: the viscous flow around an inclined elliptical cylinder, the lid-driven cavity and a dam-break flow impacting a vertical wall.

In this work two particle methods are studied in the context of viscous flows. The first one is a Vortex Particle Method, called Diffused Vortex Hydrodynamics (DVH), recently developed to simulate complex viscous flows at medium and high Reynolds regimes. This method presents some similarities with the SPH model and its Lagrangian meshless nature, even if it is based on a different numerical approach. Advantages and drawbacks of the two methods have been previously studied in Colagrossi et al. [1] from a theoretical point of view and in Rossi et al. [2], where these particle methods have been tested on selected benchmarks. Further investigations are presented in this article highlighting analogies and differences between the two particle models.

The present work is dedicated to the detection of Lagrangian Coherent Structures (LCSs) in viscous flows through the Finite-Time Lyapunov Exponents (FTLEs) which have been addressed by several works in the recent literature. Here, a novel numerical technique is presented in the context of the Smoothed Particle Hydrodynamics (SPH) models. Thanks to the Lagrangian character of SPH, the trajectory of each fluid particle is explicitly tracked over the whole simulation. This allows for a direct evaluation of the FTLE field supplying a new way for the data analysis of complex flows. The evaluation of FTLE can be either implemented as a post-processing or nested into the SPH scheme conveniently. In the numerical results, three test-cases are presented giving a proof of concept for different conditions. The last test-case regards a naval engineering problem for which the present algorithm is successfully used to capture the submerged vortical tunnels caused by the splashing bow wave.

This work aims to clarify some of the dissipation properties observed in the weakly compressible SPH model with artificial diffusive terms, by specifically considering the δSPH formulation. The main features of the δ-SPH formulation are the use of two diffusive terms added in the continuity and momentum equations in order to stabilise the scheme. The action of the two artificial diffusive terms can be, in principle, arbitrarily tuned by two coefficients, namely δ and α. Thanks to the particular structure of the weakly-compressible SPH scheme it is possible to measure the energy dissipated by each term separately. Therefore, the effects on the dissipation process when changing δ and α are analysed. It is found that the two components are strictly related, and, surprisingly, different sets of coefficients lead in most of the cases to similar amount of total dissipated energy. Further, the results obtained by the δ-SPH formulation are compared with those obtained by a δLES-SPH. In the latter δ and α parameters are determined through a turbulence closure model and are, therefore, different for each particle. Within this new scheme the two parameters dynamically change depending on the local and instantaneous flow conditions and are no longer to be regarded as tunable parameters.

This paper presents a numerical analysis of the time behaviours of mechanical and internal fluid energies during generation, propagation and absorption of gravity waves. The analyses are performed through a Smoothed Particle Hydrodynamics (SPH) model in which the viscous and diffusive dissipations are determined through the turbulence closure model presented in Di Mascio et al. (2017) and therefore are a function of the local and instantaneous flow conditions. The wave generation is obtained through the action of a moving paddle. Particular attention is given to the analysis of the energy introduced in the fluid domain as the work made by the moving solid boundary on the fluid mass. In a first problem, a comparison on the time evolution of the energy components is presented for a wave reflecting case, obtained by a flat bottom flume which ends with a vertical wall, and a wave absorbing case, obtained through the generation of breaking waves induced by a sloping bottom. Successively, the energy analysis is applied to study the performances of waves interacting with a curtain breakwater for which reference data are available in the literature. The time evolutions of the single energy components are analyzed separately for two sub-regions of the domain: the first one is the wave propagation region, while the second one is the breakwater region. A specific analysis is thus devoted to determine the amount of energy dissipated, only due to the presence of the curtain breakwater.

In the present study, the sudden changes of the flow field past stalled airfoils for small variations of the Reynolds number are investigated numerically. A vortex particle method has been used for the simulations in a two-dimensional framework. The most critical configurations found with this solver are verified through the comparison with the solution given by a mesh-based finite volume solver. The airfoils considered are the NACA0010 and a narrow ellipse with the same thickness. The angle of attack is fixed to $\unicode[STIX]{x1D6FC}=30^{\circ }$ for which complex dynamics of the flow can take place in the different viscous regimes inspected. The Reynolds number ranges between $Re=100$ and $Re=3000$ and, within this interval, numerous bifurcations of the solution are observed in terms of mean lift and drag coefficients, Strouhal number and downstream wake. An analysis of these bifurcations is provided and links are made between the wake structures observed. On this base the flow patterns can be classified in different modes similarly to the analysis by Kurtulus (Intl J. Micro Air Vehicles, vol. 7(3), 2015, pp. 301–326; vol. 8(2), 2016, pp. 109–139). A discussion of the vortical evolution of the flow in the vicinity of the suction side of the airfoil is also provided.

A new Particle Vortex Method, called Diffused Vortex Hydrodynamics (DVH), is presented in this paper. The DVH is a meshless method characterized by the use of a regular distribution of points close to a solid surface to perform the vorticity diffusion process in the boundary layer regions. This redistribution avoids excessive clustering or rarefaction of the vortex particles providing robustness and high accuracy to the method. The generation of the regular distribution of points is performed through a packing algorithm which is embedded in the solver. The packing algorithm collocates points regularly around body of arbitrary shape allowing an exact enforcement on the solid surfaces of the no-slip boundary condition. The present method is tested and validated on different problems of increasing complexities up to flows with Reynolds number equal to 100,000 (without using any subgrid-scale turbulence model).

In this paper two dimensional free vorticity dynamics is studied using two different particle methods. The first one is a Diffused Vortex Hydrodynamics (DVH) and the second one is a Smoothed Particle Hydrodynamics (SPH) method. These two methods present some similarities linked to their meshless nature but they are based on different numerical approaches. In this work advantages and drawbacks are highlighted by testing the particle methods on selected test-cases and by performing heuristic convergence measurements. The DVH method discussed in this work is characterized by the use of a regular distribution of points to perform the vorticity diffusion process. This redistribution avoids excessive clustering or rarefaction of the vortex particles providing robustness and high accuracy of the method.

It is well known that the use of SPH models in simulating flow at high Reynolds numbers is limited because of the tensile instability inception in the fluid region characterized by high vorticity and negative pressure. In order to overcome this issue, the δ + -SPH scheme is modified by implementing a Tensile Instability Control (TIC). The latter consists of switching the momentum equation to a non-conservative formulation in the unstable flow regions. The loss of conservation properties is shown to induce small errors, provided that the particle distribution is regular. The latter condition can be ensured thanks to the implementation of a Particle Shifting Technique (PST). The novel variant of the δ + -SPH is proved to be effective in preventing the onset of tensile instability. Several challenging benchmark tests involving flows past bodies at large Reynolds numbers have been used. Within this a simulation characterized by a deforming foil that resembles a fish-like swimming body is used as a practical application of the δ + -SPH model in biological fluid mechanics.

The present work is dedicated to the application of the recently developed (δ+-SPH) scheme to the self-propulsive fishlike swimming hydrodynamics. In the numerical method, a particle shifting technique (PST) is implemented in the framework of δ-SPH, combining with an adaptive particle refinement (APR) which is a numerical technique adopted to refine the particle resolution in the local region and de-refine particles outside that region. This comes into being the so-called δ+-SPH scheme which contributes to higher numerical accuracy and efficiency. In the fishlike swimming modeling, a NACA0012 profile is controlled to perform a wavy motion mimicking the fish swimming in water. Thanks to the mesh-free characteristic of SPH method, the NACA0012 profile can undergo a wavy motion with large amplitude and move forward freely, avoiding the problem of mesh distortion. A parallel staggered algorithm is adopted to perform the fluid-structure interaction between the foil and the surrounding fluid. Two different approaches are adopted for the fishlike swimming problem. In Approach 1, the foil is fixed and flaps in a free stream and in Approach 2, the wavy foil can move forward under the self-driving force. The numerical results clearly demonstrate the capability of the δ+-SPH scheme in modeling such kind of self-propulsive fishlike swimming problems. Keywords: Smoothed particle hydrodynamics, δ+-SPH, Fishlike swimming, Wavy foil, Swimming propulsion

A space station in the vicinity of the Moon can be exploited as a gateway for future human and robotic exploration of the solar system. The natural location for a space system of this kind is about one of the Earth–Moon libration points. The study addresses the dynamics during rendezvous and docking operations with a very large space infrastructure in an EML2 Halo orbit. The model takes into account the coupling effects between the orbital and the attitude motion in a circular restricted three-body problem environment. The flexibility of the system is included, and the interaction between the modes of the structure and those related with the orbital motion is investigated. A lumped parameter technique is used to represents the flexible dynamics. The parameters of the space station are maintained as generic as possible, in a way to delineate a global scenario of the mission. However, the developed model can be tuned and updated according to the information that will be available in the future, when the whole system will be defined with a higher level of precision.

DOI: 10.1103/PhysRevE.96.023101Filiació URV: SI Stokes' hypothesis, the zeroing of the bulk viscosity in a Newtonian fluid, is discussed in this paper. To this aim, a continuum macroscopic fluid domain is initially modeled as a Hamiltonian system of discrete particles, for which the interparticle dissipative forces are required to be radial in order to conserve the angular momentum. The resulting system of particles is then reconverted to the continuum domain via the framework of the smoothed particle hydrodynamics (SPH) model. Since an SPH-consistent approximation of the Newtonian viscous term in the momentum equation incorporates interparticle radial as well as nonradial terms, it is postulated that the latter must be null. In the present work it is shown that this constraint implies that first and second viscosities are equal, resulting in a positive value for the bulk viscosity, in contradiction to the cited Stokes' hypothesis. Moreover, it is found that this postulate leads to bulk viscosity coefficients close to values found in the experimental literature for monoatomic gases and common liquids such as water.

The Smoothed Particle Hydrodynamics (SPH) method, often used for the modelling of the Navier- Stokes equations by a meshless Lagrangian approach, is revisited from the point of view of Large Eddy Simulation (LES). To this aim, the LES filtering procedure is recast in a Lagrangian framework by defining a filter that moves with the positions of the fluid particles at the filtered velocity. It is shown that the SPH smoothing procedure can be reinterpreted as a sort of LES Lagrangian filter- ing, and that, besides the terms coming from the LES convolution, additional contributions (never accounted for in the SPH literature) appear in the equations when formulated in a filtered fashion. Appropriate closure formulas are derived for the additional terms and a preliminary numerical test is provided to show the main features of the proposed LES-SPH model.

In the present work we describe a numerical algorithm which gives a measure of the disorder in particle distributions in two and three dimensions. This applies to particle methods in general, disregarding the fact they use topological connections between particles or not. The proposed measure of particle disorder is tested on specific configurations obtained through the perturbation of a regular lattice. It turns out that the disorder measure may be qualitatively related to the mean absolute value of the perturbation. Finally, some applications of the proposed algorithm are shown by using the Smoothed Particle Hydrodynamics (SPH) method. (C) 2014 Elsevier ay. All rights reserved.

In the present paper the study of the two-dimensional flow field past a circular cylinder for Reynolds number up to 5 · 10 5 is addressed. A Lagrangian particle method approach has been exploited and the simulations have been performed with high spatial resolutions in order to resolve all the main vortical scales. Long simulation time evolutions have been performed in order to get the vortex shedding dynamics as well as the Fourier analysis of the loads. The adopted numerical method allows to discuss both local (boundary layer and near wake dynamics) and global (far wake dynamics) aspects of the problem.The fundamental aspects related to the different identified flow states as well as the drag crisis mechanism are investigated.

In the present work, the energy conservation properties of the Smoothed Particle Hydrodynamics (SPH) are investigated in the presence of fluid–solid interactions. Similarly to the fluid phase, the solid bodies are modeled through solid particles so that the whole solid–fluid domain can be described as a unique particle system. The pressure and velocity fields are, then, extended over the solid particles in different ways, taking into account consistency issues related to the SPH differential operators and using the projection of the Navier–Stokes equations on the solid boundary. It is shown that, when solid particles are considered, the energy equation of the particle system contains some extra terms that depend on the pressure–velocity field extensions. The presence of these extra terms does not affect the consistency of the SPH model, since they tend to vanish when the spatial resolution is increased. Three prototypical numerical test cases are considered in order to provide a quantitative perspective of the matter and confirm the theoretical developments of the paper. These test cases show that the nature of these terms is globally dissipative, being thus in accordance with the Second Law of Thermodynamics. In particular, the viscous extra term displays a remarkably slow rate of convergence, this being a relevant outcome for those SPH practitioners that deal with these types of flows.

In ocean engineering, the applications are usually related to a free surface which brings so many interesting physical phenomena (e.g. water waves, impacts, splashing jets, etc.). To model these complex free surface flows is a tough and challenging task for most computational fluid dynamics (CFD) solvers which work in the Eulerian framework. As a Lagrangian and meshless method, smoothed particle hydrodynamics (SPH) offers a convenient tracking for different complex boundaries and a straightforward satisfaction for different boundary conditions. Therefore SPH is robust in modeling complex hydrodynamic problems characterized by free surface boundaries, multiphase interfaces or material discontinuities. Along with the rapid development of the SPH theory, related numerical techniques and high-performance computing technologies, SPH has not only attracted much attention in the academic community, but also gradually gained wide applications in industrial circles. This paper is dedicated to a review of the recent developments of SPH method and its typical applications in fluid-structure interactions in ocean engineering. Different numerical techniques for improving numerical accuracy, satisfying different boundary conditions, improving computational efficiency, suppressing pressure fluctuations and preventing the tensile instability, etc., are introduced. In the numerical results, various typical fluid-structure interaction problems or multiphase problems in ocean engineering are described, modeled and validated. The prospective developments of SPH in ocean engineering are also discussed.

The present paper is dedicated to the post-processing analysis of the time pressure signals when a weakly-compressible Smoothed Particle Hydrodynamics (SPH) model is used to simulate free-surface flows. Indeed, it is well known in literature that this particle model is characterized by the occurrence of high-frequency acoustic noise making the pressure signals unusable for engineering applications. This non-physical pressure noise is both linked to the weakly-compressible approach and to numerical inaccuracies. To reduce the latter, different enhanced SPH models have been developed in recent years. Nonetheless, even using accurate and stable SPH schemes when simulating water impacts, acoustic signals can be generated and, since the speed of sound used in the model is not the real one, these acoustic components need to be filtered in a suitable way. Indeed, for low Mach number regime the solution of the compressible Navier-Stokes equation can be decomposed in an incompressible solution plus an acoustic perturbation. In this work, a filtering technique based on Wavelet Transform is presented. In the proposed procedure the acoustic frequencies are individuated through a preliminary analysis which takes into account: the adopted speed of sound, the spatial resolutions and the fluid domain configuration. The filtering technique to remove the acoustic components is finally applied to several test-cases ranging from prototype problems to more practical applications such as a violent sloshing flow.

Single-phase two-dimensional flow past a circular cylinder intersecting, or close to, a free surface at a Reynolds number of 180 is numerically investigated in this paper series using the Smoothed Particle Hydrodynamics (SPH) method. The wake behavior for Froude numbers between 0.3 and 2.0, based on the diameter, and for submergence-diameter ratios between − 0.5 and 2.5 is examined. This range significantly extends existing literature on the topic. Vorticity shed by the cylinder, vortex generation due to free-surface breaking, and mixing processes are discussed. Regarding the submergence dependence, it has been found that for small gap ratios, the classical von Karman vortex shedding from the cylinder does not take place. In turn, vortex shedding originates from wave-breaking at the free surface, occurring simultaneously with the transport of free-surface fluid elements into the bulk of the fluid. It has been also found that for even smaller depth ratios, a vorticity layer remains spatially localized between the cylinder and the free surface, and a large stagnation recirculating area develops behind the cylinder. In some of these cases, the whole mass of fluid in that area eventually gets detached after several shedding cycles and it is advected downstream. According to the authors’ knowledge, this is a previously unreported form of wake instability. It has been also found that as the Froude number is increased, the classical von Karman vortex street shed from the cylinder is blocked only to be recovered at very high Froude numbers, in agreement with linear stability predictions. Regarding the challenging and not previously investigated half-submerged configuration, flows in which the cylinder acts as a barrier, flows with alternation of dry and wet cylinder top surface, and flows with cavities have been described.

The study of energetic free-surface flows is challenging because of the large range of interface scales involved due to multiple fragmentations and reconnections of the air-water interface with the formation of drops and bubbles. Because of their complexity the investigation of such phenomena through numerical simulation largely increased during recent years. Actually, in the last decades different numerical models have been developed to study these flows, especially in the context of particle methods. In the latter a single-phase approximation is usually adopted to reduce the computational costs and the model complexity. While it is well known that the role of air largely affects the local flow evolution, it is still not clear whether this single-phase approximation is able to predict global flow features like the evolution of the global mechanical energy dissipation. The present work is dedicated to this topic through the study of a selected problem simulated with both single-phase and two-phase models. It is shown that, interestingly, even though flow evolutions are different, energy evolutions can be similar when including or not the presence of air. This is remarkable since, in the problem considered, with the two-phase model about half of the energy is lost in the air phase while in the one-phase model the energy is mainly dissipated by cavity collapses.

The Diffused Vortex Hydrodynamics (DVH) is a recent numerical model born as an evolution of the classical vortex particle methods, where an improvement of the solution quality has been achieved through a regularization of the particles spatial distribution during the vorticity diffusion process. The DVH method is a pure meshless method which adopts a body-fitted approach to enforce no-slip boundary condition on solid surfaces. In the present work it is exploited to perform an accurate analysis of the vorticity field generated by the incompressible flow around bodies with geometrical singularities. In common academic or technological applications, such singularities are quite frequent (e.g. trailing edges of wing profiles or of propeller blades) and the classical Euler mesh-based methods can suffer in modeling these geometries. In the recent years a wide literature was produced in order to describe enhanced numerical methods that could overcome such challenging problems. The potentialities of the DVH approach to problems where bodies with geometric singularities are involved are deeply discussed and some examples are finally offered.

A 2D+t approach is applied to study the wave pattern generated by high speed slender ships with a sharp stem. This allows approximating the ship motion through a set of equations which are mathematically equivalent to those governing the unsteady 2D free-surface flow generated by a deformable body in the vertical plane transverse to the ship. To describe the body deformation, a proper modeling of the solid boundaries is needed. To this purpose the fixed ghost particle technique recently developed by the authors for 2D SPH schemes is adopted. Then, an in-depth analysis of the bow breaking wave evolution is provided focusing on the different features of the plunging jet as function of the ship forward velocity. The comparison with experimental data proves the proposed SPH scheme to be robust and accurate.

The global exploration roadmap suggests, among other ambitious future space programmes, a possible manned outpost in lunar vicinity, to support surface operations and further astronaut training for longer and deeper space missions and transfers. In particular, a Lagrangian point orbit location - in the Earth- Moon system - is suggested for a manned cis-lunar infrastructure; proposal which opens an interesting field of study from the astrodynamics perspective. Literature offers a wide set of scientific research done on orbital dynamics under the Three-Body Problem modelling approach, while less of it includes the attitude dynamics modelling as well. However, whenever a large space structure (ISS-like) is considered, not only the coupled orbit-attitude dynamics should be modelled to run more accurate analyses, but the structural flexibility should be included too. The paper, starting from the well-known Circular Restricted Three-Body Problem formulation, presents some preliminary results obtained by adding a coupled orbit-attitude dynamical model and the effects due to the large structure flexibility. In addition, the most relevant perturbing phenomena, such as the Solar Radiation Pressure (SRP) and the fourth-body (Sun) gravity, are included in the model as well. A multi-body approach has been preferred to represent possible configurations of the large cis-lunar infrastructure: interconnected simple structural elements - such as beams, rods or lumped masses linked by springs - build up the space segment. To better investigate the relevance of the flexibility effects, the lumped parameters approach is compared with a distributed parameters semi-analytical technique. A sensitivity analysis of system dynamics, with respect to different configurations and mechanical properties of the extended structure, is also presented, in order to highlight drivers for the lunar outpost design. Furthermore, a case study for a large and flexible space structure in Halo orbits around one of the Earth-Moon collinear Lagrangian points, L1 or L2, is discussed to point out some relevant outcomes for the potential implementation of such a mission.

Flow past a circular cylinder close to a free surface at low Reynolds number is investigated numerically in this paper extending the work done in previous 2014 and 2015 OMAE papers [1, 2]. In the former, the dependence of the flow with the submergence was discussed and in the latter the flow at high Froude numbers was investigated. It was found that shedding is blocked as the cylinder approaches de free surface with an increasing value of the net lift coefficient and a reduction of the lift alternate oscillations due to such shedding. This variation on the lift pattern suggests that if a VIVACE device [3] is set under these flow characteristics, its performance will significantly differ from that in deep water conditions. These devices take advantage of flow induced motions in bluff bodies (such as the cylinder) to generate electric power. Singh & Mittal [4] coupled VIV system is studied in present paper in the presence of a free surface, selecting a configuration with large response in unbounded flow and adding a free-surface to such case. Results are presented and discussed.

A novel system of equations has been defined which contains diffusive terms in both the continuity and energy equations and, at the leading order, coincides with a standard weakly-compressible SPH scheme with artificial viscosity. A proper state equation is used to associate the internal energy variation to the pressure field and to increase the speed of sound when strong deformations/compressions of the fluid occur. The increase of the sound speed is associated to the shortening of the time integration step and, therefore, allows a larger accuracy during both breaking and impact events. Moreover, the diffusive terms allows reducing the high frequency numerical acoustic noise and smoothing the pressure field. Finally, an enhanced formulation for the second-order derivatives has been defined which is consistent and convergent all over the fluid domain and, therefore, permits to correctly model the diffusive terms up to the free surface. The model has been tested using different free surface flows clearly showing to be robust, efficient and accurate. An analysis of the CPU time cost and comparisons with the standard SPH scheme is provided.

This work is dedicated to the energy decomposition analysis of a viscous free-surface flow. In the presence of a free surface, the viscous dissipation for a Newtonian liquid can be decomposed into two terms: an enstrophy component and a free-surface deformation component. The latter requires the evaluation of volume and surface integrals in the meshless framework. The analysis is based on the weakly compressible smoothed particle hydrodynamics formalism. The behavior of the energy terms is studied in standing wave problems by changing the viscosity and the wave amplitude. Finally, an analysis of a complex shallow water breaking wave case is provided. It is shown that in presence of intense breaking phenomena the two energy components are always comparable, whereas generally the free surface component is dominant on the viscous dissipation of gravity waves.

For the past few years, heave plates have been used in the offshore industry due to their favorable hydrodynamic characteristics in oscillating flows, i.e., increased added mass and damping. The hydrodynamic coefficients of heave plates are strongly influenced by the proximity of solid boundaries. Assessing how this variation depends on the proximity to the seabed and on the amplitude of the oscillation is the main aim of this paper. In this process, a new model describing the work done by damping in terms of the flow enstrophy is described herein. This new approach is able to provide a direct correlation between the vortex shedding processes and the linear damping coefficient. Numerical simulations have been performed using the finite volume open source solver OpenFOAM. Simulations have been conducted on a solid circular disc oscillating axially in water at various elevations above an impermeable seabed boundary. Results for hydrodynamic coefficients are validated against previously published experimental data. At low KC numbers, a systematic increase in added mass and damping, corresponding to an increase in the seabed proximity, is observed. As seen in experiments, a critical KC where the monotonic trend of the hydrodynamic coefficients with KC is disrupted and that depends on the seabed distance exists. The physical behavior of the flow around the critical KC is explained through an analysis of the flow enstrophy.

In the present work the simulation of water impacts is discussed. The investigation is mainly focused on the energy dissipation involved in liquid impacts in both the frameworks of the weakly compressible and incompressible models. A detailed analysis is performed using a weakly compressible Smoothed Particle Hydrodynamics (SPH) solver and the results are compared with the solutions computed by an incompressible mesh-based Level-Set Finite Volume Method (LS-FVM). Impacts are numerically studied using single-phase models through prototypical problems in 1D and 2D frameworks. These problems were selected for the conclusions to be of interest for, e.g., the numerical computation of the flow around plunging breaking waves. The conclusions drawn are useful not only to SPH or LS-FVM users but also for other numerical models, for which accurate results on benchmark test-cases are provided.

Two-dimensional monophasic flow past a circular cylinder intersecting or close to a free surface at Reynolds 180 is numerically investigated using the Smoothed Particle Hydrodynamics (SPH) method. The wake behaviour for a range of conditions with Froude numbers between 0.3 and 2.0, a gap ratio of 0.55 and for a half-submerged cylinder are studied. The SPH technique allows for a non-diffusive computation of the free surface evolution, even while breaking and fragmentation may occur. Vorticity shed by the cylinder, vortex generation due to free surface breaking, mixing processes, and drag and lift coefficients behaviour are discussed. It has been found that, for certain Froude numbers and for the analysed submergence ratio, a vorticity layer remains spatially localized between the cylinder and the free surface and a large recirculating wake area develops, which eventually gets detached after several shedding cycles, being advected downstream, a previously unreported form of wake instability, to the authors’ knowledge. This behaviour is blocked when the cylinder is intersecting the free surface.Copyright © 2015 by ASME

In literature, it is well know that the Smoothed Particle Hydrodynamics method can be affected by numerical noise on the pressure field when dealing with liquids. This can be highly dangerous when an SPH code is dynamically coupled with a structural solver. In this work a simple procedure is proposed to improve the computation of the pressure distribution in the dynamics of liquids. Such a procedure is based on the use of a density diffusion term in the equation for the mass conservation. This diffusion is a pure numerical effect, similar to the well known artificial viscosity originally proposed in SPH method to smooth out the shock discontinuities. As the artificial viscosity, the density diffusion used here goes to zero increasing the number of particles recovering consistency and convergence of the final numerical scheme adopted. Different artificial density diffusion formulas have been studied, paying attention to prevent unphysical changes of the flows. To show the improvements of the new scheme proposed here, a suitable set of examples, for which reference solutions or experimental data are available, has been tested.

Flow past a circular cylinder close to a free surface at low Reynolds and large Froude numbers is investigated numerically using the Smoothed Particle Hydrodynamics model. This meshless method allows for a non-diffusive computation of the free surface evolution, even while breaking and fragmentation may occur. The distance of the cylinder to the free surface, submergence, is varied in order to investigate the detached flow patterns dependence with this factor. Vorticity shed by the cylinder, vortex generation due to free surface breaking, mixing processes, and drag and lift coefficients behavior are discussed. It has been found that, for small submergences, the classical Von Karman vortex shedding from the cylinder does not take place. In turn, moderate vortex shedding occurs, departing not from the cylinder but from vorticity generated at the free surface. This shedding takes places simultaneously with the transport of free surface fluid elements into the bulk of the fluid. It has been also found that for even smaller depth ratios, a vorticity layer remains spatially localized between the cylinder and the free surface, and a stagnation recirculating area develops behind the cylinder. Results are compared with literature finding reasonable qualitatively agreement with experimental works conducted with similar geometrical configuration but larger Reynolds number.Copyright © 2014 by ASME

We review the basic ideas underlying two popular particle methods for fluids: smoothed-particle hydrodynamics and the vortex method. We discuss conver- gence problems, numerical schemes, and examples of practical simulations.

The smoothed-particle-hydrodynamics (SPH) method has been used to study the evolution of free-surface Newtonian viscous flows specifically focusing on dissipation mechanisms in gravity waves. The numerical results have been compared with an analytical solution of the linearized Navier-Stokes equations for Reynolds numbers in the range 50-5000. We found that a correct choice of the number of neighboring particles is of fundamental importance in order to obtain convergence towards the analytical solution. This number has to increase with higher Reynolds numbers in order to prevent the onset of spurious vorticity inside the bulk of the fluid, leading to an unphysical overdamping of the wave amplitude. This generation of spurious vorticity strongly depends on the specific kernel function used in the SPH model.

A weakly compressible SPH solver is presented for applications involving nonlinear interaction between water waves and floating bodies. A complete algorithm able to compute fully coupled viscous Fluid–Solid interactions is described. No slip boundary condition on the solid surface is enforced through a ghost–fluid technique and the global loads are evaluated through the momentum exchange between fluid and ghost particles. A dedicated algorithm is developed to manage the intersection between the free surface and the solid profile. An explicit synchronous algorithm is proposed for the full coupling between fluid and rigid bodies. Stability, convergence and conservation properties are tested on several freely floating test cases and a final validation of the full algorithm is performed for the interaction between a 2-D box and a wave packet.