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See detailA Multiscale DEM-VOF Method for the simulation of three-phase flows
Pozzetti, Gabriele UL; Peters, Bernhard UL

in International Journal of Multiphase Flow (in press)

A novel multiscale approach for three-phase flows is presented. The goal of the proposed method is to solve arbitrary three-phase flow configurations in a computationally efficient way, and in particular ... [more ▼]

A novel multiscale approach for three-phase flows is presented. The goal of the proposed method is to solve arbitrary three-phase flow configurations in a computationally efficient way, and in particular taking into account the effects of different length scales while sharply reducing the computational burden. This is particularly important in chemical, environmental, and process engineering, where large-scale quantities are normally of interest, but small-scale dynamics cannot be neglected. The method is based on the definition of two different length scales: the bulk scale, and the fluid fine scale. A dual-grid approach is adopted in order to resolve the bulk scale with information from the fluid fine scale. It is shown that the described method succeeds in delivering more accuracy than a standard approach based on the volume averaging technique, still, it remains suitable for the solution of real interest problems. The method is shown to successfully satisfy experimental results presented in the literature. Examples of three-phase flows simulations are provided to show how the proposed numerical approach can describe the physics of particle-laden, free surface flows with competitive computational cost. It is shown how the proposed method can naturally extend the DEM-VOF method to the presence of complex interface dynamics. [less ▲]

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See detailOn the performance of an overlapping-domain parallelization strategy for Eulerian-Lagrangian Multiphysics software
Pozzetti, Gabriele UL; Besseron, Xavier UL; Rousset, Alban UL et al

in AIP Conference Proceedings ICNAAM 2017 (in press)

In this work, a strategy for the parallelization of a two-way CFD-DEM coupling is investigated. It consists on adopting balanced overlapping partitions for the CFD and the DEM domains, that aims to reduce ... [more ▼]

In this work, a strategy for the parallelization of a two-way CFD-DEM coupling is investigated. It consists on adopting balanced overlapping partitions for the CFD and the DEM domains, that aims to reduce the memory consumption and inter-process communication between CFD and DEM. Two benchmarks are proposed to assess the consistency and scalability of this approach, coupled execution on 252 cores shows that less than 1\% of time is used to perform inter-physics data exchange. [less ▲]

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See detailFlow characteristics of metallic powder grains for additive manufacturing
Peters, Bernhard UL; Pozzetti, Gabriele UL

in EPJ Web of Conferences (2017), 13001

Directed energy deposition technologies for additive manufacturing such as laser selective melting (SLM) or electron beam melting (EBM) is a fast growing technique mainly due to its flexibility in product ... [more ▼]

Directed energy deposition technologies for additive manufacturing such as laser selective melting (SLM) or electron beam melting (EBM) is a fast growing technique mainly due to its flexibility in product de- sign. However, the process is a complex interaction of multi-physics on multiple length scales that are still not entirely understood. A particular challenging task are the flow characteristics of metallic powder ejected as jets from a nozzle and shielded by an inert turbulent gas flow. Therefore, the objective is to describe numerically the complex interaction between turbulent flow and powder grains. In order to include both several physical processes and length scales an Euler-Lagrange technology is applied. Within this framework powder is treated by the Discrete-Element-Method, while gas flow is described by Euler approaches as found in classical Compu- tational Fluid Dynamics (CFD). The described method succeeded in delivering more accuracy and consistency than a standard approach based on the volume averaging technique and therefore, is suited for the solution of problems within an engineering framework. [less ▲]

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See detailEvaluating Erosion Patterns in an abrasive water jet cutting nozzle using XDEM
Pozzetti, Gabriele UL; Peters, Bernhard UL

in Advances in Powder Metallurgy & Particulate Materials (2017)

The objective of this work is to analyze particle-induced erosion within a nozzle for abrasive water jet cutting, through a Euler-Lagrange approach. In an abrasive water jet cutting device a high-speed ... [more ▼]

The objective of this work is to analyze particle-induced erosion within a nozzle for abrasive water jet cutting, through a Euler-Lagrange approach. In an abrasive water jet cutting device a high-speed water jet is used to accelerate abrasive particles forming a turbulent mixture of water, entrained air and abrasive powders traveling at hundreds of meters per second. The focusing tube represents a key component, whose primary scope is to focus and stabilize the flow forming in the mixing chamber, in order to ensure optimal cutting performances of the device. Nevertheless, this nozzle often happens to be the first target of the erosive action of the flow. This phenomenon significantly shortens the operational life of a nozzle. The numerical approach proposed in this work aims to provide an insight to this very fast and disruptive phenomena that are difficult and expensive to be captured by purely experimental studies. [less ▲]

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See detailMultiscale-multiphysics approaches for engineering applications
Pozzetti, Gabriele UL; Peters, Bernhard UL

in AIP Conference Proceedings (2017), 1863(1), 180001

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See detailOn the choice of a phase interchange strategy for a multiscale DEM-VOF Method
Pozzetti, Gabriele UL; Peters, Bernhard UL

in AIP Conference Proceedings (2017), 1863

In this work a novel Multiscale DEM-VOF method is adopted to study three phase flows. It consists in solving the fluid momentum, mass conservation and the phase advection at a different scale with respect ... [more ▼]

In this work a novel Multiscale DEM-VOF method is adopted to study three phase flows. It consists in solving the fluid momentum, mass conservation and the phase advection at a different scale with respect to the fluid-particle coupling problem. This allows the VOF scheme to resolve smaller fluid structures than a classic DEM-VOF method, and opens the possibility of adopting different volume interchange techniques. Two different volume interchange techniques are here described and compared with reference to high and low particle concentration scenarios. Considerations about the respective computational costs are also proposed. [less ▲]

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See detailON THE INFLUENCE OF DIFFERENT MAPPING TECHNIQUES FOR A MULTISCALE APPROACH TO TURBULENT THREE-PHASE FLOWS
Pozzetti, Gabriele UL; Peters, Bernhard UL

Scientific Conference (2016, June 06)

In this work we investigate a multiscale approach for high Stokes number, turbulent three phase flows. It is widely proven that a straightforward application of Galerkin's method to problems characterized ... [more ▼]

In this work we investigate a multiscale approach for high Stokes number, turbulent three phase flows. It is widely proven that a straightforward application of Galerkin's method to problems characterized by multiscale phenomena does not generally lead to robust numerical solutions. In this optic, multiscale methods are commonly adopted in order to provide solutions for complex problems in an highly efficient way. In certain problems it is convenient to identify multiple scales (more than 2), each characterized by its own characteristic spatial and temporal length. For this kind of problems a possible approach consists in completely resolving the coarse scales, partially resolving the middle scales, while analytically modeling the smallest. In turbulent three phase flows with high Stokes number, those can be identified respectively as the particle characteristic scale, the interface dynamic scale, and the turbulent fine scale. The coarse scale is here resolved through an Eulerian-Lagrangian approach that enables us to track the particle motion in a Lagrangian way. We partially resolve the middle-scale through the usage of a supporting domain where semi local variables are resolved. The solution of the middle-scale is based on the Volume of Fluid (VOF) technique in order to capture the dynamic interface, while turbulent phenomena are solved with a Large Eddy Simulation (LES) approach. The coarse-scale domain and the middle-scale domain must exchange informations and this process is obtained by mapping variables between the two fields. We will here show how the choice of the mapping technique largely affect the solution in therms of both accuracy and efficiency. A thoughtful study about the optimal mapping strategy could therefore be extremely beneficial in order to discover the most suitable scale-linking technique. The aim of this work is to investigate the effect of the adopted mapping technique on the resolved scale. Simulations with different Reynolds and Stokes number are proposed and compared, and conclusions about the consistency of the mapping technique are drawn. [less ▲]

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See detailA DEM-LES-VOF METHOD FOR TURBULENT THREE PHASE FLOWS
Pozzetti, Gabriele UL; Peters, Bernhard UL

Scientific Conference (2016, May 26)

In this work a robust Computational Fluid Dynamic (CFD) - Discrete Element Method(DEM) coupling that can predict free-surface, turbulent flows is presented. A correct prediction of multiphase turbulent ... [more ▼]

In this work a robust Computational Fluid Dynamic (CFD) - Discrete Element Method(DEM) coupling that can predict free-surface, turbulent flows is presented. A correct prediction of multiphase turbulent flows should ideally be able to capture the discrete dynamics of a dispersed phase (solid particles), and at the same time to take into account the evolution of possible fluid-dynamic instabilities. In this optic a CFD-DEM approach seems promising as it is able to combine the well developed CFD techniques for the study of free-surface flows with the accuracy of the Discrete Particle Method(DPM). A key point of the CFD-DEM method is the coupling between the discrete and the continuous phases. In particular the volume replacement between phases, and the interaction between the discrete phase and the continuous interface must be taken into account in order to perform accurate three phase simulations. In this work two different approaches to simulate the volume replacement between phases are presented and compared within a four way coupling with a Large Eddy Simulation(LES)-Volume Of Fluid(VOF) solver. The four-way coupled equations for the solid and the fluids are then presented, and some test cases provided in order to evaluate the accuracy of the new solver. Particular emphasis is posed to study the effects of the coupling on the interface dynamics and stability. The continuous two-phase solver used for the simulations is based on the OpenFoam® architecture, while the discrete phase solver is built on the XDEM code. [less ▲]

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See detailA PRELIMINARY STUDY ON THE STABILITY OF PARTICLE LADEN JETS THROUGH A FULLY COUPLED CFD-DEM SOLVER
Peters, Bernhard UL; Pozzetti, Gabriele UL

Poster (2016, May 22)

Jets are widely used in engineering applications. In material machinery, hydro-transportation systems as well as in chemical industry it is common to deal with a dispersed solid phase interacting with the ... [more ▼]

Jets are widely used in engineering applications. In material machinery, hydro-transportation systems as well as in chemical industry it is common to deal with a dispersed solid phase interacting with the jet, and therefore creating a so-called slurry-jet or particle-laden jet. The stability of a jet is a key issue for many of these processes, still the underlying physics of this turbulent multiphase flow is highly complicated. Conventional CFD approaches have been proven satisfying for the study of the stability of two-phase jets. When a solid dispersed phase is present in the system, the stability problem gets more complicated and dependent on the solid phase dynamic. A possible solution for the problem is to extend the CFD solver capability through a correct coupling with a DEM solver. In this work a preliminary investigation on the potentialities of this kind of approach is presented and compared with a pure CFD approach. In particular the effect of the presence of differently sized particles in the jet is outlined and the influence of particle properties and concentration is investigated. Finally some considerations about the computational cost of different methods are proposed. The fluid phases are solved through an Eulerian finite volume (FV) multiphase solver based on the OpenFoam® libraries, and coupled with the XDEM code in order to treat the dispersed phase in a Lagrangian way. [less ▲]

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See detailBerechnung des Transportes von Treibgut bei Hochwasser
Peters, Bernhard UL; Pozzetti, Gabriele UL; Liao, Yu-Chung UL

in 39. DRESDNER WASSERBAUKOLLOQUIUM (2016)

Hochwasser hervorgerufen durch natürliche Ursachen wie Schneeschmelze oder durch bauliche Maßnahmen wie Flussbegradigung verursacht häufig eine Flutkatastrophe mit verheerenden Überschwemmungen. Zu den ... [more ▼]

Hochwasser hervorgerufen durch natürliche Ursachen wie Schneeschmelze oder durch bauliche Maßnahmen wie Flussbegradigung verursacht häufig eine Flutkatastrophe mit verheerenden Überschwemmungen. Zu den schon katastrophalen Folgen von Hochwasser addieren sich häufig noch die Schäden von gefährliche Treibgut, das mit den Fluten mitgerissen wird und unter Umständen über weite Strecken transportiert wird. Mitgerissenes Treibgut kann zur Verklausung von Brücken führen oder auch Bauwerke zerstören. Um die Folgen eines Hochwassers einschließlich Transport von Treibgut abschätzen zu können, sind numerische Werkzeuge eine adäquate Ergänzung zu experimentellen Methoden, die oft mit einem sehr hohen Aufwand verbunden sind. Deshalb wird im vorliegenden Beitrag eine neue und innovative numerischer Ansatz vorgestellt, der den Transport von Treibgut bei Hochwasser aber auch bei Normalwasser beschreibt. Dazu werden die beiden numerischen Methoden beruhend auf einem diskreten und kontinuierlichem Ansatz gekoppelt. Letzterer beinhaltet die Euler Methoden, mit denen die Strömung des Wassers im Rahmen von bewährten Rechenmethoden der Computational Fluid Dynamik (CFD) bestimmt wird. Treibgut wird diskret betrachtet, in dem es mit der Diskreten Element Methode (DEM) beschreiben wird. Damit kann sowohl jedes einzelne Element des Treibgutes berücksichtigt werden als auch seine Eigenschaften wie Größe, Form und Gewicht. Innerhalb dieses Ansatzes können die Kontaktkräfte zwischen den einzelnen Elementen des Treibgutes berechnet werden, mit denen sich Geschwindigkeit, Position und Orientierung des Treibgutes bestimmen lassen. Zusätzlich wird über eine Kopplung zur fluiden Phase der Einfluss sowohl der Wassergeschwindigkeit als des Auftriebs mit berücksichtigt. [less ▲]

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See detailA preliminary investigation of the growth of an aneurysm with a multiscale monolithic Fluid-Structure interaction solver
Cerroni, D.; Manservisi, S.; Pozzetti, Gabriele UL

in Journal of Physics: Conference Series (2015), 655(1), 012050

In this work we investigate the potentialities of multi-scale engineering techniques to approach complex problems related to biomedical and biological fields. In particular we study the interaction ... [more ▼]

In this work we investigate the potentialities of multi-scale engineering techniques to approach complex problems related to biomedical and biological fields. In particular we study the interaction between blood and blood vessel focusing on the presence of an aneurysm. The study of each component of the cardiovascular system is very difficult due to the fact that the movement of the fluid and solid is determined by the rest of system through dynamical boundary conditions. The use of multi-scale techniques allows us to investigate the effect of the whole loop on the aneurysm dynamic. A three-dimensional fluid-structure interaction model for the aneurysm is developed and coupled to a mono-dimensional one for the remaining part of the cardiovascular system, where a point zero-dimensional model for the heart is provided. In this manner it is possible to achieve rigorous and quantitative investigations of the cardiovascular disease without loosing the system dynamic. In order to study this biomedical problem we use a monolithic fluid-structure interaction (FSI) model where the fluid and solid equations are solved together. The use of a monolithic solver allows us to handle the convergence issues caused by large deformations. By using this monolithic approach different solid and fluid regions are treated as a single continuum and the interface conditions are automatically taken into account. In this way the iterative process characteristic of the commonly used segregated approach, it is not needed any more. [less ▲]

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See detailNumerical validation of a κ-ω-κ θ -ω θ heat transfer turbulence model for heavy liquid metals
Cerroni, D.; Vià, R. Da; Manservisi, S. et al

in Journal of Physics: Conference Series (2015), 655(1), 012046

The correct prediction of heat transfer in turbulent flows is relevant in almost all industrial applications but many of the heat transfer models available in literature are validated only for ordinary ... [more ▼]

The correct prediction of heat transfer in turbulent flows is relevant in almost all industrial applications but many of the heat transfer models available in literature are validated only for ordinary fluids with Pr ≃ 1. In commercial Computational Fluid Dynamics codes only turbulence models with a constant turbulent Prandtl number of 0.85 — 0.9 are usually implemented but in heavy liquid metals with low Prandtl numbers it is well known that these models fail to reproduce correlations based on experimental data. In these fluids heat transfer is mainly due to molecular diffusion and the time scales of temperature and velocity fields are rather different, so simple turbulence models based on similarity between temperature and velocity cannot reproduce experimental correlations. In order to reproduce experimental results and Direct Numerical Simulation data obtained for fluids with Pr ≃ 0.025 we introduce a κ-ε-κ θ -ε θ turbulence model. This model, however, shows some numerical instabilities mainly due to the strong coupling between κ and ε on the walls. In order to fix this problem we reformulate the model into a new four parameter κ-ω-κ θ -ω θ where the dissipation rate on the wall is completely independent on the fluctuations. The model improves numerical stability and convergence. Numerical simulations in plane and channel geometries are reported and compared with experimental, Direct Numerical Simulation results and with results obtained with the κ-ε formulation, in order to show the model capabilities and validate the improved κ-ω model. [less ▲]

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