On-Shell Description of Unsteady Flames

The problem of non-perturbative description of unsteady premixed flames with arbitrary gas expansion is solved in the two-dimensional case. Considering the flame as a surface of discontinuity with arbitrary local burning rate and gas velocity jumps given on it, we show that the front dynamics can be determined without having to solve the flow equations in the bulk. On the basis of the Thomson circulation theorem, an implicit integral representation of the gas velocity downstream is constructed. It is then simplified by a successive stripping of the potential contributions to obtain an explicit expression for the vortex component near the flame front. We prove that the unknown potential component is left bounded and divergence-free by this procedure, and hence can be eliminated using the dispersion relation for its on-shell value (i.e., the value along the flame front). The resulting system of integro-differential equations relates the on-shell fuel velocity and the front position. As limiting cases, these equations contain all theoretical results on flame dynamics established so far, including the linear equation describing the Darrieus-Landau instability of planar flames, and the nonlinear Sivashinsky-Clavin equation for flames with weak gas expansion.Comment: 21 pages, 3 figures; extended discussion of causality, new references adde

Rotating strings

Analytical expressions are provided for the configurations of an inextensible, flexible, twistable inertial string rotating rigidly about a fixed axis. Solutions with trivial radial dependence are helices of arbitrary radius and pitch. Non-helical solutions are governed by a cubic equation whose roots delimit permissible values of the squared radial coordinate. Only curves coplanar with the axis of rotation make contact with it.Comment: added to discussion and made small revisions to tex

Dynamical Renormalization Group Study for a Class of Non-local Interface Equations

We provide a detailed Dynamic Renormalization Group study for a class of stochastic equations that describe non-conserved interface growth mediated by non-local interactions. We consider explicitly both the morphologically stable case, and the less studied case in which pattern formation occurs, for which flat surfaces are linearly unstable to periodic perturbations. We show that the latter leads to non-trivial scaling behavior in an appropriate parameter range when combined with the Kardar-Parisi-Zhang (KPZ) non-linearity, that nevertheless does not correspond to the KPZ universality class. This novel asymptotic behavior is characterized by two scaling laws that fix the critical exponents to dimension-independent values, that agree with previous reports from numerical simulations and experimental systems. We show that the precise form of the linear stabilizing terms does not modify the hydrodynamic behavior of these equations. One of the scaling laws, usually associated with Galilean invariance, is shown to derive from a vertex cancellation that occurs (at least to one loop order) for any choice of linear terms in the equation of motion and is independent on the morphological stability of the surface, hence generalizing this well-known property of the KPZ equation. Moreover, the argument carries over to other systems like the Lai-Das Sarma-Villain equation, in which vertex cancellation is known {\em not to} imply an associated symmetry of the equation.Comment: 34 pages, 9 figures. Journal of Statistical Mechanics: Theory and Experiments (in press

UK Large-scale Wind Power Programme from 1970 to 1990: the Carmarthen Bay experiments and the Musgrove Vertical-Axis Turbines

This article describes the development of the Musgrove Vertical Axis Wind Turbine (VAWT) concept, the UK ‘Carmarthen Bay’ wind turbine test programme, and UK government’s wind power programme to 1990. One of the most significant developments in the story of British wind power occurred during the 1970s, 1980s, and 1990s, with the development of the Musgrove vertical axis wind turbine and its inclusion within the UK Government’s wind turbine test programme. Evolving from a supervisor’s idea for an undergraduate project at Reading University, the Musgrove VAWT was once seen as an able competitor to the horizontal axis wind systems that were also being encouraged at the time by both the UK government and the Central Electricity Generating Board, the then nationalised electricity utility for England and Wales. During the 1980s and 1990s the most developed Musgrove VAWT system, along with three other commercial turbine designs was tested at Carmarthen Bay, South Wales as part of a national wind power test programme. From these developmental tests, operational data was collected and lessons learnt, which were incorporated into subsequent wind power operations.http://dx.doi.org/10.1260/03095240677860621

Type Ia Supernova Explosion Models

Because calibrated light curves of Type Ia supernovae have become a major tool to determine the local expansion rate of the Universe and also its geometrical structure, considerable attention has been given to models of these events over the past couple of years. There are good reasons to believe that perhaps most Type Ia supernovae are the explosions of white dwarfs that have approached the Chandrasekhar mass, M_ch ~ 1.39 M_sun, and are disrupted by thermonuclear fusion of carbon and oxygen. However, the mechanism whereby such accreting carbon-oxygen white dwarfs explode continues to be uncertain. Recent progress in modeling Type Ia supernovae as well as several of the still open questions are addressed in this review. Although the main emphasis will be on studies of the explosion mechanism itself and on the related physical processes, including the physics of turbulent nuclear combustion in degenerate stars, we also discuss observational constraints.Comment: 38 pages, 4 figures, Annual Review of Astronomy and Astrophysics, in pres

Resolvent methods for steady premixed flame shapes governed by the Zhdanov-Trubnikov equation

Using pole decompositions as starting points, the one parameter (-1 =< c < 1) nonlocal and nonlinear Zhdanov-Trubnikov (ZT) equation for the steady shapes of premixed gaseous flames is studied in the large-wrinkle limit. The singular integral equations for pole densities are closely related to those satisfied by the spectral density in the O(n) matrix model, with n = -2(1 + c)/(1 - c). They can be solved via the introduction of complex resolvents and the use of complex analysis. We retrieve results obtained recently for -1 =< c =< 0, and we explain and cure their pathologies when they are continued naively to 0 < c < 1. Moreover, for any -1 =< c < 1, we derive closed-form expressions for the shapes of steady isolated flame crests, and then bicoalesced periodic fronts. These theoretical results fully agree with numerical resolutions. Open problems are evoked.Comment: v2: 29 pages, 6 figures, some typos correcte

Architecture and communication : building the values of authors and their works in the 21st century

Pourquoi ce projet plaît-il plus qu’un autre ? Pourquoi est-il plus diffusé dans les revues spécialisées en architecture que l’immeuble d’à côté ? Et, finalement, pourquoi son architecte est plus connu que son confrère ? Si la clef du succès réside dans la valeur de celui qui en est le récipiendaire, identifier les traits qui distinguent une architecture et son architecte de la masse des projets livrés quotidiennement en France permettrait de comprendre les raisons de leur consécration. Fruit d'une expérience d'objectivitation participante au long court au sein d'une rédaction de revue spécialisée, cette recherche vise à décortiquer la fabrique des valeurs au sein du champ de l'architecture au XXIe siècle.Dans notre société postmoderne, mondialisée et dématérialisée, la concurrence est rude pour les architectes et l’injonction, toujours plus pressante : il faut séduire pour construire et avoir construit pour séduire. Si, en tant que média de communication, l’architecture est au service de la reconnaissance de son maître d’ouvrage, les architectes contemporains ne sont pas en reste. Conscients du potentiel de transmission de leurs réalisations, ils les mobilisent au service de la construction de leur propre image de marque. De même qu'ils investissent le temps de leur conception, la médiation qui les fait advenir, pour y bâtir les images, les représentations et les discours qui serviront leur propre reconnaissance, en tant qu'auteur. Émerge alors, en réponse aux exigences de notre société et sa « culture hégémonique du temps réel* », une architecture icône, à la fois œuvre d’art et objet de culte. Dans cette course à la signature, l’architecture est mobilisée pour façonner la figure d’auteur de son concepteur. Au fond, n’est-elle pas que la pétrification d’un moment de communication ? Basée sur l’hypothèse que c’est avant tout les images qu’ils convoquent, les représentations et les discours qu’ils stimulent, qui légitiment une œuvre – de la conception à la mise en service du bâtiment – et son auteur, à condition de savoir les transmettre, cette recherche propose de dresser un état des lieux fourni des règles du jeu qui influencent aujourd’hui la conception du projet d’architecture. Dans la continuité des études sociologiques menées sur le champ de l’architecture, notamment sur les stratégies d’accès à la commande et les modes de co-conception de l'espace, il s’agit d’esquisser l’idéal-type du jeune architecte contemporain à partir de l’étude du parcours et de la production intellectuelle et bâtie d’architectes distingués au sein de cet univers, notamment les lauréats des Albums des jeunes architectes et paysagistes. Ces réflexions sur les jeunes professionnels s’accompagnent de l’exploration des pratiques d'acteurs qui les accompagnent dans la construction de leur réputation, les rédacteurs des revues d'architecture et ceux, jusque-là peu étudiés car relativement émergents dans le monde de l’architecture : les professionnels de la communication, qui facilitent l’accès à la commande des architectes, à l’influence peu visible mais incontestable et dont il s’agit de dresser le portrait et d’objectiver les comportements.* BAUDRILLARD Jean, NOUVEL Jean, Les objets singuliers, Paris : Arléa, 2013 [1e éd. 2000], p.105What makes one project more appealing than another ? What accounts for the fact that this building, rather than the one next door, appears in the pages of specialized architecture journals ? And why is its architect better known than another ? If the key to success resides in the value of the person who is the object of such esteem, identifying the features that make an architectural work and its architect stand out from the mass of projects delivered daily in France would allow us to understand the reasons for their reputation. The aim of this research – the result of a long-term participant objectivation experience in a specialized journal – is to examine the making of values in the field of architecture in the 21st century.In our globalized and dematerialized postmodern society, competition between architects is fierce and the following reality has become increasingly crucial : to build you have to win over and to win over you have to have built. Architects are perfectly aware that their architecture serves as a medium of communication. Conscious of the message transmission potential of their accomplishments, they mobilize them in the service of building their own brand image. Likewise they devote the time of their conception, the mediation that brings them about, to building the images, representations, and discourses that will serve their own recognition as authors. What emerges then, in response to the demands of our society and its “hegemonic culture of real time*”, is iconic architecture, at once artwork and object of adoration. In this race for signature architecture, the work is mobilized to forge the figure of designer as author.This research is based on the assumption that what legitimates a work (from conception to commissioning) and its author are the images it elicits and the representations and discourses that it stimulates, provided one knows how to transmit them. We therefore propose to draw up a detailed description of the game rules that influence the design of architectural projects today. Expanding on sociological studies conducted in the field of architecture, in particular with regard to strategies of access to commissions and modes of co-designing space, we will describe the ideal model of the young contemporary architect, based on a study of the careers and intellectual and built output of award-winning architects, notably the winners of AJAP/Albums des Jeunes Architectes et Paysagistes. We will also look at the practices of those who accompany these young professionals in building their reputation – architecture journal editors but also communication specialists who facilitate access to commissions and who have not been the subject of much study because their role in the world of architecture is relatively new and their influence, though incontestable, has not been very visible. The goal here is to sketch their portrait and objectify their behavior.* BAUDRILLARD Jean, NOUVEL Jean, Les objets singuliers, Paris : Arléa, 2013 [1e éd. 2000], p.10

Sur une forme remarquable des équations de Maxwell-Lorentz dans l'univers à 5 dimensions

En admettant, suivant une suggestion de M. L. de Broglie, que l'équation dite complémentaire de Lorentz [FORMULE] doive être abandonnée, ajoutons au 1er membre un nouveau terme ∂b/∂x° où x° représente la 5e dimension de l'univers de Kaluza et Klein, et b, une 5e composante du potentiel d'univers, dont les 4 autres composantes sont le potentiel scalaire ψ et le potentiel vecteur a. L'hypothèse [FORMULE] où U représente le courant d'univers généralisé par l'adjonction, aux 4 composantes que donnent la densité d'électricité p et le vecteur courant C'c, d'une 5e composante β = -Δb + 1/c² . ∂²b/∂t² réduit les 5 équations de propagation telles que : Ψ = ΔΨ - 1/c² . ∂²Ψ/∂t² à l'unique équation [FORMULE]