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• Transmisión de Calor Apuntes

  E. Oñate, F. Zarate

  Personal Repository of Francisco Zarate (2000). 3

  
  

  Abstract
  Apuntes de Clase

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• P1/P0+ elements for incompressible flows with discontinuous material properties

  E. Oñate, P. Nadukandi, S. Idelsohn

  Comput. Methods Appl. Mech. Engrg., (2015). Vol. 293, pp. 191-206

  
  

  Abstract

  We present a 3-noded triangle and a 4-noded tetrahedra with a continuous linear velocity and a discontinuous linear pressure field formed by the sum of an unknown ''constant pressure field'' and ''a prescribed linear field'' that satisfies the steady state momentum equations for a constant body force. The elements are termed P1/P0+ as the “effective” pressure field is linear, although the unknown pressure field is piecewise constant within each element. The elements have an excellent behaviour for incompressible viscous flow problems with discontinuous material properties formulated in either Eulerian or Lagrangian descriptions. The necessary numerical stabilization for dealing with the inf-sup condition imposed by the incompressibility constraint and high convective effects (in Eulerian flows) is introduced via the Finite Calculus (FIC) approach. For the sake of clarity, the element derivation is presented first for the simpler Stokes equations written in the standard Eulerian frame. The extension of the formulation to the Navier-Stokes equations written in the Eulerian and Lagrangian frameworks is straightforward and is presented in the second part of the paper. The efficiency and accuracy of the new P1/P0+ triangle is verified by solving a set of incompressible multifluid flow problems using a Lagrangian approach and a classical Eulerian description. The excellent performance of the new triangular element in terms of mass conservation and general accuracy for analysis of fluids with discontinuous material properties is highlighted.

  Abstract
  We present a 3-noded triangle and a 4-noded tetrahedra with a continuous linear velocity and a discontinuous linear pressure field formed by the sum of an unknown ''constant [...]

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• Advances in the modelling of railway ballast using the Discrete Element Method (DEM)

  J. Irazábal González, F. Salazar, E. Oñate

  (2018). Fourth International Conference on Railway Technology: Research, Development and Maintenance (RAILWAYS 2018))

  
  

  Abstract

  The development of high-speed train lines has increased significantly during the last decades leading to more demanding loads in railway infrastructures. Most of these infrastructures were constructed using railway ballast, whose main roles are resisting to vertical and horizontal loads and facing climate action. Moreover, new challenges are arising in the railway industry, such as the development of high-speed train lines in locations with extreme weather. For these reasons, the implementation of a numerical code able to represent ballast behaviour, including its interaction with other structures, has become very attractive. Among a wide range of numerical methods, the Discrete Element Method (DEM) was found to be effective for the calculation of engineering problems with granular materials. This approach considers the discontinuous nature of these materials and has proven to be a very useful tool to obtain complete qualitative information on calculations of groups of particles. The code used in this work is developed within DEMPack, a specific software tool for modelling physical problems using the DEM. The computer program is adapted to meet the needs for representing the behaviour of railway ballast.

  Abstract
  The development of high-speed train lines has increased significantly during the last decades leading to more demanding loads in railway infrastructures. Most of these infrastructures [...]

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• Shape characterization of railway ballast stones for discrete element calculations

  J. Irazábal González, F. Salazar, E. Oñate

  (2017). V International Conference on Particle-Based Methods (PARTICLES 2017)

  
  

  Abstract

  Railway ballast is a layer of granular material that resists to vertical and horizontal loads, produced by the passing train over the rail. The calculation of this kind of complex geomechanic problems has been traditionally addressed using refined constitutive models, based in continuum assumptions. Although these models may be suitable in the evaluation of the critical state of soils, or in the calculation of bulk material masses flowing, they are not appropriate to represent the local discontinuities of granular materials, which induce special features such as anisotropy or instabilities. The Discrete Element Method (DEM) is an alternative approach that considers the discontinuous nature of granular materials, which has proven to be a very useful tool to obtain complete qualitative information on calculations of groups of particles. However, the computational cost of contact evaluation between Discrete Elements (DEs) is high and limits the calculation capability. In this regard, it should be noted that particle shape greatly affects contact calculation computational cost, being spherical DEs the less computational demanding type of particles. From the point of view of micro-scale analysis, it is essential to represent the exact geometry of the grains. By contrast, if the interest lies in the behaviour of the granular material as a whole, particles geometry is not a determining factor. Therefore, for efficiency purposes, a trade-off between particle shape accuracy and computational cost needs to be achieved. In this work, different approaches to represent ballast stones are assessed: spheres with rolling friction, sphere clusters, polyhedrons and superquadrics. The first two were chosen for further analysis. Rolling friction allows avoiding excessive rotation when irregular shaped materials are simulated as spherical particles. This work presents a new insight for its application called the Bounded Rolling Friction model. Regarding sphere clusters, there is a key point in the friction between elements. As they reproduce irregular particles using clumps of spheres rigidly joined, the cavities between those spheres introduce interlocks that increase friction. To overcome this drawback, a new contact model is proposed. Finally, results of the application of both approaches are displayed, and conclusions are drawn as regards the convenience of using more accurate and computational demanding geometries.

  Abstract
  Railway ballast is a layer of granular material that resists to vertical and horizontal loads, produced by the passing train over the rail. The calculation of this kind of [...]

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• Geometric representation of railway ballast using the Discrete Element Method (DEM)

  J. Irazábal González, F. Salazar, E. Oñate

  (2016). European Congress on Computational Methods in Applied Sciences and Engineering

  
  

  Abstract

  The development of high-speed train lines has increased during the last twenty years, leading to more demanding loads in railway infrastructures. For these reasons, the implementation of a numerical tool for the calculation of railway ballast behaviour has been found useful, as it will enables design optimization. Regarding the numerical method, the DEM is considered effective and powerful for the calculation of engineering problems with granular and discontinuous materials. Due to the fact that railroad ballast layer consists of discrete aggregate particles, the DEM is considered suitable for the simulation of particulate ballast material. However, the computational cost of contact calculation between irregular particles is high and limits the calculation capability. From the point of view of micro-scale analysis, it is essential to represent the exact geometry of the particle. On the other hand, if the interest lies in the behaviour of the granular material as a whole, the geometry is not a determining factor. Besides that, setting up a simulation of granular material taking care of the exact geometry of each particle will not be efficient. Current work presents different geometrical approaches for the representition of ballast stones: spheric particles with rolling friction, sphere clusters, polyhedrons and superquadrics; showing their advantages and drawbacks. Finally, some simulation results, using spheric particles and sphere clusters, are displayed in order to evaluate the convenience or not of using more accurate and computational demanding geometries in each case.

  Abstract
  The development of high-speed train lines has increased during the last twenty years, leading to more demanding loads in railway infrastructures. For these reasons, the implementation [...]

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• Numerical modelling with discrete elements of rockfall protection systems

  J. Irazábal González, S. Latorre, F. Salazar, M. Celigueta, E. Oñate

  (2017). V International Conference on Particle-Based Methods (PARTICLES 2017)

  
  

  Abstract

  Some important infrastructures like roads, railway tracks or dams were constructed in places threatened by natural hazards. With the purpose of preserving these infrastructures from landslides and rock-falls, different containment systems are installed, and one of the most popular are the flexible metallic fences. The development of full-scale laboratory tests to evaluate the behaviour of flexible metallic fences is unfunctional, accounting to the huge magnitude of the event. On the other hand, small-scale testing may lead to inaccurate results, due to the distortion in the contours (e.g. anchors of the metallic fences). These problems in laboratory testing have led to the popularization of the use of numerical methods. In this study, the bonded Discrete Element Method (DEM) is used for the analysis of the behaviour of flexible metallic fences for rockfall protection. The bonded DEM is a modification of the classical DEM which assumes that bonds exist between particles, resisting their separation. In this case, the net cables are represented using rigid spheres joined by bond elements that are deformed according to an elasto-plastic law. Calculations were carried out using the DEMPack program, a specific software developed in CIMNE for modelling with the bonded DEM. This software allows considering the inter-action between discrete and finite elements, which can be useful to represent the boundaries of the domain, such as the surface of the slope. The code is firstly validated reproducing benchmark tests available in the literature. Finally, full-scale tests are computed in order to evaluate the energy dissipation capacity of the fence during a rockfall event.

  Abstract
  Some important infrastructures like roads, railway tracks or dams were constructed in places threatened by natural hazards. With the purpose of preserving these infrastructures [...]

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• A unified Lagrangian formulation for solid and fluid dynamics and its possibility for modelling submarine landslides and their consequences

  X. Zhang, E. Oñate, S. Galindo, J. Bleyer

  Comput. Methods Appl. Mech. Engrg., (2018). (prepint) Vol. 343, pp. 314-338

  
  

  Abstract

  Consequences of submarine landslides include both their direct impact on offshore infrastructure, such as subsea electric cables and gas/oil pipelines, and their indirect impact via the generated tsunami. The simulation of submarine landslides and their consequences has been a long-standing challenge majorly due to the strong coupling among sliding sediments, seawater and infrastructure as well as the induced extreme material deformation during the complete process. In this paper, we propose a unified finite element formulation for solid and fluid dynamics based on a generalised Hellinger–Reissner variational principle so that the coupling of fluid and solid can be achieved naturally in a monolithic fashion. In order to tackle extreme deformation problems, the resulting formulation is implemented within the framework of the particle finite element method. The correctness and robustness of the proposed unified formulation for single-phase problems (e.g. fluid dynamics problems involving Newtonian/Non-Newtonian flows and solid dynamics problems) as well as for multi-phase problems (e.g. two-phase flows) are verified against benchmarks. Comparisons are carried out against numerical and analytical solutions or experimental data that are available in the literature. Last but not least, the possibility of the proposed approach for modelling submarine landslides and their consequences is demonstrated via a numerical experiment of an underwater slope stability problem. It is shown that the failure and post-failure processes of the underwater slope can be predicted in a single simulation with its direct threat to a nearby pipeline and indirect threat by generating tsunami being estimated as well.

  Abstract
  Consequences of submarine landslides include both their direct impact on offshore infrastructure, such as subsea electric cables and gas/oil pipelines, and their indirect [...]

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• Dynamic modelling of retrogressive landslides with emphasis on the role of clay sensitivity

  X. Zhang, S. Sloan, E. Oñate

  Int. Journal for Numerical and Analytical Methods in Geomechanics (2018). Vol. 42, pp. 1806-1822 (preprint)

  
  

  Abstract

  This paper presents a detailed numerical study of the retrogressive failure of landslides in sensitive clays. The dynamic modelling of the landslides is carried out using a novel continuum approach, the particle finite element method, complemented with an elastoviscoplastic constitutive model. The multiwedge failure mode in the collapse is captured successfully, and the multiple retrogressive failures that have been widely observed in landslides in sensitive clays are reproduced with the failure mechanism, the kinematics, and the deposition being discussed in detail. Special attention has been paid to the role of the clay sensitivity on each retrogressive failure as well as on the final retrogression distance and the final run‐ out distance via parametric studies. Moreover, the effects of the viscosity of sensitive clays on the failure are also investigated for different clay sensitivities.

  Abstract
  This paper presents a detailed numerical study of the retrogressive failure of landslides in sensitive clays. The dynamic modelling of the landslides is carried out [...]

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• Lagrangian versus Eulerian integration errors

  S. Idelsohn, E. Oñate, N. Nigro, P. Becker

  Comput. Methods Appl. Mech. Engrg., (2015). Vol. 293, pp. 191–206; (preprint)

  
  

  Abstract

  The possibility to use a Lagrangian frame to solve problems with large time-steps was successfully explored previously by the authors for the solution of homogeneous incompressible fluids and also for solving multi-fluid problems [28-30]. The strategy used by the authors was named Particle Finite Element Method second generation (PFEM-2). The objective of this paper is to demonstrate in which circumstances the use of a Lagrangian frame with particles is more accurate than a classical Eulerian finite element method, and when large time-steps and/or coarse meshes may be used.

  Abstract
  The possibility to use a Lagrangian frame to solve problems with large time-steps was successfully explored previously by the authors for the solution of homogeneous incompressible [...]

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• Surface tension problems solved with the Particle Finite Element Method using large time-steps

  J. Giménez, N. Nigro, S. Idelsohn, E. Oñate

  Computers and Fluids (2016). (preprint) Vol. 141, pp. 90-104

  
  

  Abstract

  In previous works [1,2], the authors have presented a highly efficient extension of the Particle Finite Element Method, called PFEM-2, to solve two-phase flows. The methodology which uses X-IVS [3] to treat convection terms allowing large time-steps was validated for problems where the gravity forces and/or the inertial forces dominate the flow. Although that is the target range of problems to solve with PFEM-2, most of real problems that fall in these categories also includes other flow regimes in certain regions of the domain. Maybe the most common secondary regime is when the surface tension dominates, as an example when drops or bubbles are released from the main flow, and this feature must be taken into account in any complete numerical strategy. Attending to that, in this work the treatment of the surface tension to PFEM-2 is included. An implicit CSF methodology is employed together with a coupling between the marker function with a Level Set function to obtain a smooth representation of the normal of the interface which allows an accurate curvature calculation. Examples for curvature calculation and isolated bubbles and drops are presented where the accuracy and the computational efficiency are analyzed and contrasted with other numerical methodologies. Finally, a simulation of a jet atomization is analyzed. This case presents the above mentioned features: it is a inertia-dominant flow with a surface tension phenomena on drops and ligaments break up that can not be neglected.

  Abstract
  In previous works [1,2], the authors have presented a highly efficient extension of the Particle Finite Element Method, called PFEM-2, to solve two-phase flows. The methodology [...]

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