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• Numerical simulation of flame retardant polymers using a combined Eulerian–Lagrangian finite element formulation

  J. Marti, J. Vega, D. Wang, E. Oñate

  Applied Sciences (2021). Vol. 11(13), 5952

  
  

  Abstract

  Many polymer-made objects show a trend of melting and dripping in fire, a behavior that may be modified by adding flame retardants (FRs). These affect materials properties, e.g., heat absorption and viscosity. In this paper, the effect of a flame retardant on the fire behavior of polymers in the UL 94 scenario is studied. This goal is achieved essentially by applying a new computational strategy that combines the particle finite element method for the polymer with an Eulerian formulation for air. The sample selected is a polypropylene (PP) with magnesium hydroxide at 30 wt.%. For modelling, values of density, conductivity, specific heat, viscosity, and Arrhenius coefficients are obtained from different literature sources, and experimental characterization is performed. However, to alleviate the missing viscosity at a high temperature, three viscosity curves are introduced on the basis of the viscosity curve provided by NIST and the images of the test. In the experiment, we burn the specimen under the UL 94 condition, recording the process and measuring the temperature evolution by means of three thermocouples. The UL 94 test is solved, validating the methodology and quantifying the effect of FR on the dripping behavior. The numerical results prove that well-adjusted viscosity is crucial to achieving good agreement between the experimental and numerical results in terms of the shape of the polymer and the temperature evolution inside the polymer.

  Abstract
  Many polymer-made objects show a trend of melting and dripping in fire, a behavior that may be modified by adding flame retardants (FRs). These affect materials properties, [...]

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• High-fidelity simulation of pathogen propagation, transmission and mitigation in the built environment

  R. Lohner, H. Antil, A. Srinivasan, S. Idelsohn, E. Oñate

  Archives of Computational Methods in Engineering (2021). Special Issue: Modeling and Simulation of Infectious Diseases

  
  

  Abstract

  An overview of high-fidelity modeling of pathogen propagation, transmission and mitigation in the built environment is given. In order to derive the required physical and numerical models, the current understanding of pathogen, and in particular virus transmission and mitigation is summarized. The ordinary and partial differential equations that describe the flow, the particles and possibly the UV radiation loads in rooms or HVAC ducts are presented, as well as proper numerical methods to solve them in an expedient way. Thereafter, the motion of pedestrians, as well as proper ways to couple computational fluid dynamics and computational crowd dynamics to enable high-fidelity pathogen transmission and infection simulations is treated. The present review shows that high-fidelity simulations of pathogen propagation, transmission and mitigation in the built environment have reached a high degree of sophistication, offering a quantum leap in accuracy from simpler probabilistic models. This is particularly the case when considering the propagation of pathogens via aerosols in the presence of moving pedestrians.

  Abstract
  An overview of high-fidelity modeling of pathogen propagation, transmission and mitigation in the built environment is given. In order to derive the required physical and [...]

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• Effects of expandable graphite on char morphology and pyrolysis of epoxy based intumescent fire-retardant coating

  S. Ullah, F. Ahmad, A. Al-Sehemi, M. Raza, M. Assiri, A. Irfan, E. Oñate, G. Yeoh

  Journal of Applied Polymer Science (2021). Vol. 138(41), 51206

  
  

  Abstract

  The current study was designed to investigate the effects of expandable graph-ite (EG) on fire protection properties of intumescent fire-retardant coating forsteel structures. Several formulations of intumescent coating were preparedand tested according to ISO 834 for char expansion. The chars were foundwithout cracks and bonded with the steel substrate. The results showed thatthe coating slowly degraded during the test and char remained in contact withvertically tested coated substrate. The coated substrates were also tested forweather resistance using humid and ultraviolet environment. The char wascharacterized by using FESEM, XRD, FTIR, TGA, and XPS analysis. FESEMexamined char morphology of the coatings after furnace fire test. XRD andFTIR showed the presence of graphite, borophosphate; boron oxide andsassolite in the char. TGA and DTGA results disclosed that EG improved theresidual mass of coating. XPS analysis showed the percentages of carbon andoxygen are 48.50 and 43.45 in char of formulation with 12.8% EG. The resultsof weathering test coatings showed decreased in char expansion because of ahumidity and UV light. The formulation with 9.8% EG showed the maximumchar expansion and high residual mass among the formula tions investigated inthis study. The weathering tested coated samples showed their capability offire protection.

  Abstract
  The current study was designed to investigate the effects of expandable graph-ite (EG) on fire protection properties of intumescent fire-retardant coating forsteel structures. [...]

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• Finite element methods for advection-diffusion-absorption and fluid flow problems

  A. Puigferrat, E. Oñate, I. Pouplana

  Monograph CIMNE (2021). M193, 114 pp.

  
  

  Abstract

  The objective of the work is to develop a numerical tool to describe how the concentration of one or more substances distributed in a fluid environment changes under the effect of three transport processes: advection, diffusion and absorption. For that purpose, it is essential to know the interaction of the transported substance with the fluid medium. The work aims to develop stabilized numerical methods for solving the transport and fluid flow equations in a coupled manner for greater accuracy, efficiency and speed when predicting the motion of the transported substances in the fluid. Emphasis is put in the transport of substances in fluids at high P\'eclet numbers. The practical motivation of the work is predicting the transport of a pollutant in air in urban environments. The work document summarizes the research published in three papers published in JCR journals of high impact. The author of the work is also the first author in the three papers. The papers are attached to the document in the corresponding chapters. The description of the work developments has been organized as follows. First, we present the research carried out in the work for the development of a generalized stabilized Finite Increment Calculus-Finite Element Method (FIC--FEM) formulation for solving the multidimensional transient advection-diffusion-absorption equation. The starting point of the developments are the governing equations for the multidimensional steady advection-diffusion-absorption and the unidimensional transient advection-diffusion-absorption problems obtained via the FIC procedure. The good behaviour of the new FIC--FEM formulation is shown in several examples of application. This work was published in the first of the three papers mentioned. In the following chapter we present an innovative numerical method for solving transport problems with high values of advection and / or absorption. A Lagrangian approach based on the updated version of the classical Particle Finite Element Method (PFEM) has been developed to calculate the advection of substances in fluids, while a Eulerian strategy based on the stabilized FIC--FEM formulation is adopted to compute diffusion and absorption effects. The new semi-Lagrangian approach has been validated in its application of a series of academic examples of transport of substances for different values of the P\'eclet and Damk\"ohler numbers. Finally, we derive a procedure for coupling the fluid and transport equations to model the distribution of a pollutant in a street canyon. In our case, we have considered black carbon (BC) as the pollutant. The evolution of the fluid flow is calculated with a standard stabilized finite element method using the Quasi-Static Variational Multiscale (QS-VMS) technique. For the temperature and pollutant transport we use the semi-Lagrangian procedure developed in the work. Several examples of application have been solved to illustrate the accuracy and practicability of the proposed numerical tool for predicting the transport of a pollutant in air in urban environments. One of the examples are presented in the third paper, while another academic one is presented in the appendix of this document.

  Abstract
  The objective of the work is to develop a numerical tool to describe how the concentration of one or more substances distributed in a fluid environment changes under the effect [...]

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• Topology Optimization using the UNsmooth VARiational Topology OPtimization (UNVARTOP) method: an educational implementation in Matlab (preprint)

  D. Yago, J. Cante, O. Lloberas-Valls, J. Oliver

  Structural and Multidisciplinary Optimization (2021). Vol. 63, pages 955–981

  
  

  Abstract

  This paper presents an efficient and comprehensive MATLAB code to solve two-dimensional structural topology optimization problems, including minimum mean compliance, compliant mechanism synthesis and multi-load compliance problems. The Unsmooth Variational Topology Optimization (UNVARTOP) method, developed by Oliver et al. [22], is used in the topology optimization code, based on the finite element method (FEM), to compute the sensitivity and update the topology. The paper also includes instructions to improve the bisection algorithm, modify the computation of the Lagrangian multiplier by using an Augmented Lagrangian to impose the constraint, implement heat conduction problems and extend the code to three-dimensional topology optimization problems. The code, intended for students and newcomers in topology optimization, is included as an appendix (AppendixA) and it can be downloaded fromhttps://github.com/DanielYagotogether with supplementary material.

  Abstract
  This paper presents an efficient and comprehensive MATLAB code to solve two-dimensional structural topology optimization problems, including minimum mean compliance, compliant [...]

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• Multiresonant Layered Acoustic Metamaterial (MLAM) solution for broadband low-frequency noise attenuation through double-peak sound transmission loss response (preprint)

  D. Roca, J. Cante, O. Lloberas-Valls, T. Pàmies, J. Oliver

  Extreme Mechanics Letters (2021). Volume 47, 101368

  
  

  Abstract

  The problem of noise control and attenuation is of interest in a broad range of applications, especially in the low-frequency range, below 1000 Hz. Acoustic metamaterials allow us to tackle this problem with solutions that do not necessarily rely on high amounts of mass, however most of them still present two major challenges: they rely on complex structures making them difficult to manufacture, and their attenuating capabilities are limited to narrow frequency bandwidths. Here we propose the Multiresonant Layered Acoustic Metamaterial (MLAM) concept as a novel kind of acoustic metamaterial based on coupled resonances mechanisms. Their main advantages hinge on providing enhanced sound attenuation capabilities in terms of a double-peak sound transmission loss response by means of a layered configuration suitable for large scale manufacturing.

  Abstract
  The problem of noise control and attenuation is of interest in a broad range of applications, especially in the low-frequency range, below 1000 Hz. Acoustic metamaterials [...]

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• High performance reduction technique for multiscale finite element modeling (HPR-FE2): Towards industrial multiscale FE software (preprint)

  M. Raschi, O. Lloberas-Valls, A. Huespe, X. Oliver

  Computer Methods in Applied Mechanics and Engineering (2021). Volume 375, 113580

  
  

  Abstract

  The authors have shown in previous contributions that reduced order modeling with optimal cubature applied to finite element square (FE2) techniques results in a reliable and affordable multiscale approach, the HPR-FE2technique. Such technique is assessed here for an industrial case study of a generic 3D reinforced composite whose microstructure is represented by two general microcells accounting for different deformation mechanisms, microstrucural phases and geometry arrangement. Specifically, in this approach the microstrain modes used for building the reduced order model (ROM) are obtained through standard proper orthogonal decomposition (POD) techniques applied over snapshots of a representative sampling strain space. Additionally, a reduced number of integration points is obtained by exactly integrating the main free energy modes resulting from the sampling energy snapshots. The outcome consists of a number of dominant strain modes integrated over a remarkably reduced number of integration points which provide the support to evaluate the constitutive behavior of the microstructural phases. It is emphasized that stresses are computed according to the selected constitutive law at the reduced integration points and, therefore, the strategy inherits advantageous properties such as model completeness and customization of material properties. Overall results are discussed in terms of the consistency of the multiscale analysis, customization of the microscopic material parameters and speedup ratios compared to high-fidelity finite element (HF) simulations.

  Abstract
  The authors have shown in previous contributions that reduced order modeling with optimal cubature applied to finite element square (FE2) techniques results in a reliable [...]

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• A fully Lagrangian formulation for fluid-structure interaction problems with free-surface flows and fracturing solids (preprint)

  A. Cornejo, A. Franci, F. Zárate, E. Oñate

  Computers & Structures (2021). Vol. 250, 106532

  
  

  Abstract

  This paper presents a novel coupled formulation for fluid-structure interaction (FSI) problems involving free-surface fluid flows, fracture phenomena, solid mutual contact and large displacements. The numerical formulation combines three different Lagrangian computational methods. The Particle Finite Element Method (PFEM) is used to solve the free-surface fluid flow, a Finite Element Method (FEM) with smoothed isotropic damage model is employed for the solution of solid structures and debris, finally, the Discrete Element Method (DEM) is used to manage the contact interaction between different solid boundaries, including the new ones generated by propagating cracks. The proposed method has a high potential for the prediction of the structural damages on civil constructions caused by natural hazards, such as floods, tsunami waves or landslides. Its application field can also be extended to fracture phenomena in structures and soils/rocks arising from explosions or hydraulic fracking processes. Several numerical examples are presented to show the validity and accuracy of the numerical technique proposed.

  Abstract
  This paper presents a novel coupled formulation for fluid-structure interaction (FSI) problems involving free-surface fluid flows, fracture phenomena, solid mutual contact [...]

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• Semi-Lagrangian formulation for the advection–diffusion–absorption equation (preprint)

  A. Puigferrat, M. Masó, I. de-Pouplana, G. Casas, E. Oñate

  Computer Methods in Applied Mechanics and Engineering (2021). Vol. 380, 113807

  
  

  Abstract

  We present a numerical method for solving advective–diffusive–absorptive problems with high values of advection and absorption. A Lagrangian approach based on the updated version of the classical Particle Finite Element Method (PFEM) is used to calculate advection, while a Eulerian strategy based on the Finite Element Method (FEM) is adopted to compute diffusion and absorption. The Eulerian FEM procedure is based on a Finite Increment Calculus (FIC) stabilized formulation recently developed by the authors. The most relevant features of each computational approach are outlined and the coupling scheme is explained. Several problems are solved to validate the method: the evolution of a localized concentration field in two dimensions (2D), the evolution of a spherical field in 3D and three benchmark problems from the literature with high absorption.

  Abstract
  We present a numerical method for solving advective–diffusive–absorptive problems with high values of advection and absorption. A Lagrangian approach based [...]

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• The Pseudo-Direct Numerical Simulation method for multi-scale problems in mechanics

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

  Computer Methods in Applied Mechanics and Engineering (2021). Vol. 380, 113774

  
  

  Abstract

  We present an overview of the Pseudo-Direct Numerical Simulation (P-DNS in short) method for the solution of multi-scale phenomena. The method can be seen as an adaptation of the variational multi-scale (VMS) method, where the fine solution is solved numerically instead of analytically. Also, from the point of view of homogenization methods it can be seen as an evolution of Finite Element square (FE2) methods, where the most expensive part of the computations is performed offline. The name of P-DNS arises from the premise that in all multi-scale simulations the numerical result obtained with a very fine discretization is correct without the need to introduce any additional model (i.e. turbulence models) or stabilization procedures for transport equation terms (as in residual-based VMS methods). What is intended here is that the P-DNS solution tends to the DNS solution of the problem, accepting as a premise that the DNS solution is a reliable result. In this paper we present first an overview of the P-DNS methodology in the context of an abstract Dirichlet problem involving a second order differential operator that could be non-symmetric and non-necessarily positive definite. Next, the P-DNS approach is applied to the fluid mechanics equations accounting for turbulent phenomena. Examples showing the applicability of the P-DNS method for solving transport problems involving different scales are then presented.

  Abstract
  We present an overview of the Pseudo-Direct Numerical Simulation (P-DNS in short) method for the solution of multi-scale phenomena. The method can be seen as an adaptation [...]

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