Méthodes rapides et efficaces pour la résolution numérique d'équations de type Hamilton-Jacobi avec application à la simulation de feux de forêt

Cette thèse est divisée en trois chapitres. Le premier explique comment utiliser la méthode «level-set» de manière rigoureuse pour faire la simulation de feux de forêt en utilisant comme modèle physique pour la propagation le modèle de l'ellipse de Richards. Le second présente un nouveau schéma semi-implicite avec une preuve de convergence pour la solution d'une équation de type Hamilton-Jacobi anisotrope. L'avantage principal de cette méthode est qu'elle permet de réutiliser des solutions à des problèmes «proches» pour accélérer le calcul. Une autre application de ce schéma est l'homogénéisation. Le troisième chapitre montre comment utiliser les méthodes numériques des deux premiers chapitres pour étudier l'influence de variations à petites échelles dans la vitesse du vent sur la propagation d'un feu de forêt à l'aide de la théorie de l'homogénéisation.

Hypersonic phonon propagation in mesoscopic systems by Brillouin spectroscopy

Phononic crystals, capable to block or direct the propagation of elastic/acoustic waves, have attracted increasing interdisciplinary interest across condensed matter physics and materials science. As of today, no generalized full description of elastic wave propagation in phononic structures is available, mainly due to the large number of variables determining the band diagram. Therefore, this thesis aims for a deeper understanding of the fundamental concepts governing wave propagation in mesoscopic structures by investigation of appropriate model systems. The phononic dispersion relation at hypersonic frequencies is directly investigated by the non-destructive technique of high-resolution spontaneous Brillouin light scattering (BLS) combined with computational methods. Due to the vector nature of the elastic wave propagation, we first studied the hypersonic band structure of hybrid superlattices. These 1D phononic crystals composed of alternating layers of hard and soft materials feature large Bragg gaps. BLS spectra are sensitive probes of the moduli, photo-elastic constants and structural parameters of the constituent components. Engineering of the band structure can be realized by introduction of defects. Here, cavity layers are employed to launch additional modes that modify the dispersion of the undisturbed superlattice, with extraordinary implications to the band gap region. Density of states calculations in conjunction with the associated deformation allow for unambiguous identication of surface and cavity modes, as well as their interaction with adjacent defects. Next, the role of local resonances in phononic systems is explored in 3D structures based on colloidal particles. In turbid media BLS records the particle vibration spectrum comprising resonant modes due to the spatial confinement of elastic energy. Here, the frequency and lineshapes of the particle eigenmodes are discussed as function of increased interaction and departure from spherical symmetry. The latter is realized by uniaxial stretching of polystyrene spheres, that can be aligned in an alternating electric field. The resulting spheroidal crystals clearly exhibit anisotropic phononic properties. Establishing reliable predictions of acoustic wave propagation, necessary to advance, e.g., optomechanics and phononic devices is the ultimate aim of this thesis.

Predication and propagation : a method for analyzing evaluative meanings in technology policy

In this article I outline and demonstrate a synthesis of the methods developed by Lemke (1998) and Martin (2000) for analyzing evaluations in English. I demonstrate the synthesis using examples from a 1.3-million-word technology policy corpus drawn from institutions at the local, state, national, and supranational levels. Lemke's (1998) critical model is organized around the broad 'evaluative dimensions' that are deployed to evaluate propositions and proposals in English. Martin's (2000) model is organized with a more overtly systemic-functional orientation around the concept of 'encoded feeling'. In applying both these models at different times, whilst recognizing their individual usefulness and complementarity, I found specific limitations that led me to work towards a synthesis of the two approaches. I also argue for the need to consider genre, media, and institutional aspects more explicitly when claiming intertextual and heteroglossic relations as the basis for inferred evaluations. A basic assertion made in this article is that the perceived Desirability of a process, person, circumstance, or thing is identical to its 'value'. But the Desirability of anything is a socially and thus historically conditioned attribution that requires significant amounts of institutional inculcation of other 'types' of value-appropriateness, importance, beauty, power, and so on. I therefore propose a method informed by critical discourse analysis (CDA) that sees evaluation as happening on at least four interdependent levels of abstraction.

Damage propagation in reinforced concrete frames under external blast loading

Iconic and significant buildings are the common target of bombings by terrorists causing large numbers of casualties and extensive property damage. Recent incidents were external bomb attacks on multi-storey buildings with reinforced concrete frames. Under a blast load circumstance, crucial damage initiates at low level storeys in a building and may then lead to a progressive collapse of whole or part of the structure. It is therefore important to identify the critical initial influence regions along the height, width and depth of the building exposed to blast effects and the structure response in order to assess the vulnerability of the structure to disproportionate and progressive collapse. This paper discusses the blast response and the propagation of its effects on a two dimensional reinforced concrete (RC) frame, designed to withstand normal gravity loads. The explicit finite element code, LS DYNA is used for the analysis. A complete RC portal frame seven storeys by six bays is modelled with reinforcement details and appropriate materials to simulate strain rate effects. Explosion loads derived from standard manuals are applied as idealized triangular pressures on the column faces of the numerical models. The analysis reports the influence of blast propagation as displacements and material yielding of the structural elements in the RC frame. The effected regions are identified and classified according to the load cases. This information can be used to determine the vulnerability of multi-storey RC buildings to various external explosion scenarios and designing buildings to resist blast loads.

Predication, propagation, and mediation SFL, CDA, and the inculcation of evaluative meaning systems

In this chapter I propose a theoretical framework for understanding the role of mediation processes in the inculcation, maintenance, and change of evaluative meaning systems, or axiologies, and how such a perspective can provide a useful and complementary dimension to analysis for SFL and CDA. I argue that an understanding of mediation—the movement of meaning across time and space—is essential for the analysis of meaning. Using two related texts as examples, I show how an understanding of mediation can aid SFL and CDA practitioners in the analysis of social change.

Predictions of angle dependent tortuosity and elasticity effects on sound propagation in cancellous bone

The anisotropic pore structure and elasticity of cancellous bone cause wave speeds and attenuation in cancellous bone to vary with angle. Previously published predictions of the variation in wave speed with angle are reviewed. Predictions that allow tortuosity to be angle dependent but assume isotropic elasticity compare well with available data on wave speeds at large angles but less well for small angles near the normal to the trabeculae. Claims for predictions that only include angle-dependence in elasticity are found to be misleading. Audio-frequency data obtained at audio-frequencies in air-filled bone replicas are used to derive an empirical expression for the angle-and porosity-dependence of tortuosity. Predictions that allow for either angle dependent tortuosity or angle dependent elasticity or both are compared with existing data for all angles and porosities.

Truth, obligation and ethics in the propagation of democracy

In this paper I present an analysis of the language used by the National Endowment for Democracy (NED) on its website (NED, 2008). The specific focus of the analysis is on the NED's high usage of the word “should” revealed in computer assisted corpus analysis using Leximancer. Typically we use the word “should” as a term to propose specific courses of action for ourselves and others. It is a marker of obligation and “oughtness”. In other words, its systematic institutional use can be read as a statement of ethics, of how the NED thinks the world ought to behave. As an ostensibly democracy-promoting institution, and one with a clear agenda of implementing American foreign policy, the ethics of NED are worth understanding. Analysis reveals a pattern of grammatical metaphor in which “should” is often deployed counter intuitively, and sometimes ambiguously, as a truth-making tool rather than one for proposing action. The effect is to present NED's imperatives for action as matters of fact rather than ethical or obligatory claims.