Large eddy simulation of a tunnel fire

In this work a method known as large eddy simulation (LES) is tested to evaluate how appropriate it is to study fires in a road tunnel. The fire surface is modelled as an inlet of preheated fuel injected at a rate designed to produce the same energy as that produced by the fire. The results are validated by comparison with experimental tests carried out in the Ofenegg tunnel. This comparison shows that the simulations have good quantitative and qualitative agreement with the experiment

A 3D pseudospectral algorithm to simulate rotating flows in cylindrical cavities

International audienceWhen simulating flows in cylindrical rotating cavities a difficulty arises from the singularities appearing on the axis. Its singularities are due to the presence of terms 1/r^n (n = 1, 2) in the Navier-Stokes equations, where r is the radial dis-tance. To avoid evaluating differential equation coefficients which are infinite at that point, the grid must exclude the origin or specific pole conditions must be imposed. An efficient and accurate pseudo-spectral method has been here developed using collocation Chebyshev polynomials in the radial and axial directions and Fourier approximation in the azimuthal direction. To avoid the difficulty on the axis without prescribing any pole conditions, a new approach based on the work of Heinrichs [W. Heinrichs J. Comp. Phys. 199 (2004) 66-86] has been developed. The calculation domain is defined as (r, θ , z) ∈ [−1, 1] × [0, 2π] × [−1, 1] using an even number N of collocation points in the radial direction. Thus, r = 0 is not a collocation point. The clustering of collocation points around the rotation axis is also avoided due to the utilization of a Gauss-Lobatto distribution. The flow being indeed laminar close to the axis in most of the rotating flows. In the azimuthal direction, the overlap in the discretization is avoided by introducing a shift equal to π/2K (K the number of mesh points in θ -direction) for θ > π in the Fourier transform. The accuracy of the method was checked on the exact steady and unsteady analytical solutions and the capability of the method to simulate complex flows is illustrated considering the well documented case of the vortex breakdown phenomenon

NUMERICAL TREATMENT OF CYLINDRICAL COORDINATE SINGULARITIES

The present work proposes direct numerical simulations of some rotating disk flows using a pseudo-spectral method with collocation Chebyshev polynomials in the radial and axial directions and Fourier approximation in the periodic azimuthal direction. When using cylindrical coordinates to calculate the Navier-Stokes equations the singularity that appears on the axis (r = 0), because of the terms 1/r and 1/r2 is only apparent. To avoid evaluating differential equation coefficients which are infinite the spectral grid must exclude the origin. The interesting issue is how does one impose boundary conditions at the origin? With spectral methods, there are various ways to avoid this difficulty without prescribing any pole conditions. In this work, we have developed a method which consists in discretizing the whole diameter −R ≤ r ≤ R with an even number of radial Gauss-Lobatto nodes. In the azimuthal direction, the overlap in the discretization is avoided by introducing a shift equal to Pi /2K (K the number of mesh points in that direction) for Theta>Pi in the Fourier transform. Spectral convergence of the method is illustrated on an analytical solution. The ability of our numerical method to investigate complex unsteady flows is illustrated for three rotating flows where other reliable experimental and numerical results are available

Transition mechanisms to turbulence in a cylindrical rotor-stator cavity by pseudo-spectral simulations of Navier-Stokes equations

The flow above an infinite rotating disk is an example of 3D boundary-layers where crossflow instability can develop as over swept wings. With a second rotating disk parallel to the first, the configuration schematizes the cavity between the disks holding the blades of a turbine or compressor. Centrifugal and Coriolis forces produce a secondary flow in the meridian plane composed of two thin boundary-layers along the disks separated by a non-viscous geostrophic core. That produces adjacent coupled flow regions that are radically different in terms of flow stability and thickness scales involving very challenging simulations. Identify and characterize the transition mechanism is a necessity for developing future efficient control strategies of turbulent rotating boundary layers. The matter of the transition scenario is currently much debated (see in Viaud, Serre & Chomaz JFM 07, APS 2010) around the idea that a global instability might take place and lead to transition to turbulence. This work addresses the study of coherent structures related to the transition mechanisms in a rotor-stator cavity. An accurate pseudo-spectral algorithm has been developed dealing with the singularity at the centreline of the cylindrical coordinate system. The very unstable stator boundary is characterized at moderate Reynolds number by travelling axisymmetric structures that eventually damped close to the axis and localized rotating spiral at the periphery. The much more stable rotor layer seems governed by the same mechanisms than recently identified by Viaud et al 2010 of a cascade of global modes leading eventually to turbulence

A 3D pseudospectral method for cylindrical coordinates. Application to the simulations of rotating cavity flows

International audienceThe present work proposes a collocation spectral method for solving the three-dimensional Navier-Stokes equations using cylindrical coordinates. The whole diameter -R < r < R is discretized with an even number of radial Gauss-Lobatto collocation points and an angular shift is introduced in the Fourier transform that avoid pole and parity conditions usually required. The method keeps the spectral convergence that reduces the number of grid points with respect to lower-order numerical methods. The grid-points distribution densifies the mesh only near the boundaries that makes the algorithm well-suited to simulate rotating cavity flows where thin layers develop along the walls. Comparisons with reliable experimental and numerical results of the literature show good quantitative agreements for flows driven by rotating discs in tall cylinders and thin inter-disc cavities. Associated to a spectral vanishing viscosity [E. Séverac, E. Serre, A spectral vanishing viscosity for the LES of turbulent flows within rotating cavities, J. Comp. Phys. 226 (2007) 1234-1255], the method provides very promising LES results of turbulent cavity flows

Contribuição de uma abordagem CTSA para as aprendizagens no tópico aspetos quantitativos das reações químicas : um trabalho com alunos do 11º ano

Relatório da Prática de Ensino Supervisionada, Mestrado em Ensino de Física e de Química no 3º Ciclo do Ensino Básico e no Ensino Secundário, Universidade de Lisboa, Instituto de Educação, 2019Este trabalho tem como finalidade conhecer como é que uma abordagem CTSA contribui para as aprendizagens dos alunos na temática "aspetos quantitativos das reações químicas", da unidade "Equilíbrio Químico", do programa de Química da disciplina de Física e Química A, do 11º ano de escolaridade do Ensino Secundário. Para dar resposta a esta problemática pretende-se identificar as dificuldades que os alunos sentiram durante a realização das tarefas, recorrendo a uma abordagem CTSA, as aprendizagens realizadas e a avaliação que fazem do uso dessas tarefas. As intervenções foram realizadas numa turma do 11º ano constituída por 26 alunos. A proposta didática está organizada uma sequência de cinco aulas, duas de 90 minutos e três de 135 minutos. A totalidade das intervenções contempla o desenvolvimento de cinco tarefas de cariz investigativo, recorrendo a uma abordagem CTSA. Foi utilizada uma abordagem de investigação qualitativa, em que a recolha de dados foi efetuada recorrendo a entrevistas em grupo focado, aos registos escritos dos alunos e a notas de campo resultantes da observação das aulas. Os resultados demonstraram que os alunos sentiram dificuldades do tipo concetual, processual, social, atitudinal e axiológico. Os resultados revelaram também que os alunos superaram as dificuldades e aprenderam novos conceitos, desenvolveram novas competências e estabeleceram relações CTSA. As aprendizagens realizadas envolveram a mobilização de conhecimentos, o estabelecimento de relações entre conceitos, a seleção e resumo de informação, tirar conclusões e o trabalho em grupo. Os resultados mostraram ainda que os alunos avaliaram as tarefas como promotoras das aprendizagens e da literacia científica.This work aim is to know how a STSE approach, concerning quantitative aspects of chemical reactions, contributes to 11th grade students’ learning. To respond to this problem, we intend to identify the difficulties experienced by the students during the development of the tasks using STSE approach, the learning performed and their evaluation on the use of these tasks. The work was conducted in an 11th grade class involving 26 students. The didactic proposal is organized in a sequence of five lessons, two of 90 minutes and three of 135 minutes. The didactic proposal includes the development of five inquiry tasks, using a STSE approach. A qualitative research methodology was used. Data collection was performed using focused group interviews, written documents of the students and field notes of the teacher, concerning of observation of the lessons. The results showed that the students felt conceptual, procedural, social, attitudinal and axiological difficulties. The results also revealed that the students overcame the difficulties and learned new concepts, developed new competencies and established STSE relations. The learning carried out by the students involved knowledge mobilization, relationships between concepts, selection and summary information, draw conclusions and work in group. The results also showed that the students evaluated the inquiry tasks as promoters of learning and scientific literacy

Traitement numérique de la singularité à l'axe pour les écoulements 3D en cavités tournantes

Dans le cadre des méthodes spectrales, une nouvelle approche est développée pour traiter la singularité à l'axe des équations de Navier-Stokes en coordonnées cylindriques. La solution est recherchée sur le diamètre en utilisant un nombre pair de points et décalage angulaire dans la transformée de Fourier est introduit au-delà de pi afin d'éviter de résoudre deux fois les mêmes points. La méthode est validée sur des solutions analytiques puis des simulations 3D dans des cavités cylindriques seront présentées montrant l'efficacité de cette méthode à appréhender des écoulements en rotation

A 3D pseudospectral method for cylindrical coordinates. Application to the simulations of rotating cavity flows

La simulation d'écoulements dans des cavités cylindriques en rotation présente une difficulté particulière en raison de l'apparition de singularités sur l'axe. Le présent travail propose une méthode collocative pseudospectrale suffisamment efficace et précise pour surmonter cette difficulté et résoudre les équations 3D de Navier-Stokes écrites en coordonnées cylindriques. Cette méthode a été développée dans le cadre des différentes études menées au laboratoire M2P2, utilisant une méthode collocative de type Chebychev dans les directions radiale et axiale et Fourier-Galerkin dans la direction azimutale [thêta]. Pour éviter de prescrire des conditions sur l'axe, une nouvelle approche a été développée. Le domaine de calcul est défini par (r,[thêta],z)&#8712;[-1,1]×[0,2π]×[-1,1] avec un nombre N pair de points de collocation dans la direction radiale. Ainsi, r=0 n'est pas un point de collocation. La distribution de points de type Gauss-Lobatto selon r et z densifie le maillage seulement près des parois ce qui rend l'algorithme bien adapté pour simuler les écoulements dans des cavités cylindriques en rotation. Dans la direction azimutale, le chevauchement des points dû à la discrétisation est évitée par l'introduction d'un décalage égal à π/2K à [thêta]>π dans la transformée de Fourier. La méthode conserve la convergence spectrale. Des comparaisons avec des résultats expérimentaux et numériques de la littérature montrent un très bon accord pour des écoulements induits par la rotation d'un disque dans des cavités cylindriques fermées.When simulating flows in cylindrical rotating cavities, a difficulty arises from the singularities appearing on the axis. In the same time, the flow field itself does not have any singularity on the axis and this singularity is only apparent. The present work proposes an efficient and accurate collocation pseudospectral method for solving the 3D Navier-Stokes equations using cylindrical coordinates. This method has been developed in the framework of different studies of rotor-stator flows, using Chebyshev collocation in the radial and axial directions and Fourier-Galerkin approximation in the azimuthal periodic direction [thêta]. To avoid the difficulty on the axis without prescribing any pole and parity conditions usually required, a new approach has been developed. The calculation domain is defined as (r,[thêta];,z)&#8712;[-1,1]×[0,2π]×[-1,1] using an even number N of collocation points in the radial direction. Thus, r=0 is not a collocation point. The method keeps the spectral convergence. The grid-point distribution densifies the mesh only near the boundaries that makes the algorithm well-suited to simulate rotating cavity flows where thin layers develop along the walls. In the azimuthal direction, the overlap in the discretization is avoided by introducing a shift equal to π/2K for [thêta]>π in the Fourier transform. Comparisons with reliable experimental and numerical results of the literature show good quantitative agreements for flows driven by rotating discs in cylindrical cavities. Associated to a Spectral Vanishing Viscosity, the method provides very promising LES results of turbulent cavity flows with or without heat transfer

Numerical investigations on wake control using synthetic microjets,

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Micro-jets synthétiques et contrôle de sillages: Modélisation et simulation

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