Iterative Solvers for Physics-based Simulations and Displays

La génération d’images et de simulations réalistes requiert des modèles complexes pour capturer tous les détails d’un phénomène physique. Les équations mathématiques qui composent ces modèles sont compliquées et ne peuvent pas être résolues analytiquement. Des procédures numériques doivent donc être employées pour obtenir des solutions approximatives à ces modèles. Ces procédures sont souvent des algorithmes itératifs, qui calculent une suite convergente vers la solution désirée à partir d’un essai initial. Ces méthodes sont une façon pratique et efficace de calculer des solutions à des systèmes complexes, et sont au coeur de la plupart des méthodes de simulation modernes. Dans cette thèse par article, nous présentons trois projets où les algorithmes itératifs jouent un rôle majeur dans une méthode de simulation ou de rendu. Premièrement, nous présentons une méthode pour améliorer la qualité visuelle de simulations fluides. En créant une surface de haute résolution autour d’une simulation existante, stabilisée par une méthode itérative, nous ajoutons des détails additionels à la simulation. Deuxièmement, nous décrivons une méthode de simulation fluide basée sur la réduction de modèle. En construisant une nouvelle base de champ de vecteurs pour représenter la vélocité d’un fluide, nous obtenons une méthode spécifiquement adaptée pour améliorer les composantes itératives de la simulation. Finalement, nous présentons un algorithme pour générer des images de haute qualité sur des écrans multicouches dans un contexte de réalité virtuelle. Présenter des images sur plusieurs couches demande des calculs additionels à coût élevé, mais nous formulons le problème de décomposition des images afin de le résoudre efficacement avec une méthode itérative simple.Realistic computer-generated images and simulations require complex models to properly capture the many subtle behaviors of each physical phenomenon. The mathematical equations underlying these models are complicated, and cannot be solved analytically. Numerical procedures must thus be used to obtain approximate solutions. These procedures are often iterative algorithms, where an initial guess is progressively improved to converge to a desired solution. Iterative methods are a convenient and efficient way to compute solutions to complex systems, and are at the core of most modern simulation methods. In this thesis by publication, we present three papers where iterative algorithms play a major role in a simulation or rendering method. First, we propose a method to improve the visual quality of fluid simulations. By creating a high-resolution surface representation around an input fluid simulation, stabilized with iterative methods, we introduce additional details atop of the simulation. Second, we describe a method to compute fluid simulations using model reduction. We design a novel vector field basis to represent fluid velocity, creating a method specifically tailored to improve all iterative components of the simulation. Finally, we present an algorithm to compute high-quality images for multifocal displays in a virtual reality context. Displaying images on multiple display layers incurs significant additional costs, but we formulate the image decomposition problem so as to allow an efficient solution using a simple iterative algorithm

Numerical investigation of acoustic solitons

Acoustic solitons can be obtained by considering the propagation of large amplitude sound waves across a set of Helmholtz resonators. The model proposed by Sugimoto and his coauthors has been validated experimentally in previous works. Here we examine some of its theoretical properties: low-frequency regime, balance of energy, stability. We propose also numerical experiments illustrating typical features of solitary waves

Agricultural, forest and rural policy sectors' receptiveness to agroforestry intercropping systems in Quebec (Canada)

PosterIn Quebec (Canada), intercropping trees and crops is a new practice in the agricultural landscape dominated by conventional monocropping systems. As research goes on and slowly reveals the potential of these systems to address some key issues in agriculture, forestry and rural development, and as farmers’ awareness increases, there is a pressing need to find public support for these systems. In order to help the promoters of agroforestry intercropping systems in their search of support in the political arena, we conducted a comparative study of the receptiveness of the agricultural, forest and rural policy sectors to these systems. As the literature on public policy processes has stressed the importance of policy stakeholders’ ideas in the implementation of new policies, we used a conceptual framework based on cognitive schemes to compare the ideas driving these policy sectors with the ideas supporting agroforestry intercropping systems. Results based on the analysis of formal publications and semi-directed interviews conducted with agroforestry experts and policy stakeholders underline that agroforestry intercropping systems are mostly featured by their promoters as sustainable and multifunctional systems. The rural policy sector has proven to be the most receptive and the forest sector the less receptive, while the agricultural sector lies in-between, offering small-scale support to agroforestry intercropping systems. The analysis highlights that policy sectors integrating multifunctionality in their core ideas are supportive of agroforestry intercropping systems, while sectors remaining largely focused on specialization and productivity are less receptive. Our study also shows that public incentives might not be sufficient to drive the adoption of agroforestry intercropping systems since major barriers to their implementation remain in some policy sectors. Featuring these systems as productive might be a necessary argument shift to tear down policy barriers and increase public support

Les structures d'impact sur Europe : implications sur la rhéologie et la thermique de la croûte de glace. Impact features on Europa : rheological and thermal states of the icy crust.

texte intégral en français English abridged versionInternational audienceThe depth of the brittle/ductile transition has been estimated under each graben rounding two ancient impact basins on Europa, Callanish and Tyre. Surface thermal gradients have been computed and are respectively equal to 4.3 +/- 0.4 and 2.4 +/- 0.25 K/km. These results differ strongly from previous works obtained on other Europa's features. This can be explained by the variation of both the icy grain size and the chemical composition of the crust, but also by the variation of the local deformation rate

Paraglacial evolution of Conway glacier complex foreland, Northwestern Spitsbergen, Svalbard

International audienceThis study aims to provide information concerning paraglacial evolution of landforms and sediments in a proglacial area in the Kongsfjorden area. Several subjects are emphasized:1- chronology of deglaciation2 - morphology of landforms3 - sedimentology of deposits4 - disintegration of the moraine complex by contemporary processe

Observation of a phononic Mollow triplet in a hybrid spin-nanomechanical system

Reminiscent of the bound character of a qubit's dynamics confined on the Bloch sphere, the observation of a Mollow triplet in the resonantly driven qubit fluorescence spectrum represents one of the founding signatures of Quantum Electrodynamics. Here we report on its observation in a hybrid spin-nanomechanical system, where a Nitro-gen Vacancy spin qubit is magnetically coupled to the vibrations of a Silicon Carbide nanowire. A resonant microwave field turns the originally parametric hybrid interac-tion into a resonant process, where acoustic phonons are now able to induce transitions between the dressed qubit states, leading to synchronized spin-oscillator dynamics. We further explore the vectorial character of the hybrid coupling to the bidimensional de-formations of the nanowire. The demonstrated microwave assisted synchronization of the spin-oscillator dynamics opens novel perspectives for the exploration of spin-dependent forces, the key-ingredient for quantum state transfer

La mobilisation de la violence à des fins politiques : la crise zimbabwéenne au regard du droit international des droits humains (2008-2013)

Au pouvoir de 1980 à 2017, le président zimbabwéen Robert Mugabe a non seulement repoussé les limites de la longévité politique, mais le régime politique qu’il a présidé pendant près de 38 ans a également su défié certaines idées reçues en maintenant une légitimité certaine sur le plan international en dépit de violations massives et bien documentées des droits humains lui étant attribuées. En effet, malgré un bilan catastrophique sur le plan du droit international des droits humains, le régime Mugabe a su, dès les années 2000, se positionner en rempart contre l’impérialisme sur le continent africain en se dotant d’une légitimité idéologique renouvelée. Cette légitimité idéologique renouvelée a su être habilement utilisée contre ses opposants politiques, accusés d’être à la solde de forces impérialistes extérieures afin de banaliser, voire de justifier, les violations des droits humains commises à leur encontre, en particulier des droits civils et politiques, taxés de « non-africains » En plaçant au centre de son objet d’étude la mobilisation de la violence à des fins politiques à grande échelle lors de l’année électorale de 2008 et lors des quatre années subséquentes au Zimbabwe, ce mémoire s’intéresse au paradoxe de la légitmité certaine ayant permis au régime Mugabe de demeurer en place en dépit de violations massives très bien documentées des droits humains protégés par le droit international. Si l’ampleur des violences a été à l’origine de la mise en place d’un gouvernement d’unité nationale avec l’opposition, le résultat a plutôt bénéficié au régime Mugabe en lui permettant de demeurer au pouvoir sans trop le partager. En effet, les critiques lui étant adressées à propos de son traitement des droits civils et politiques ont accrédité son message de victime d’acharnement impérialiste auprès de certains acteurs régionaux, affaiblissant du même coup le rapport de force de l’opposition.In power from 1980 to 2017, Zimbabwean President Robert Mugabe not only personally pushed the limits of political longevity, but the political regime he presided over for nearly 38 years also defied preconceived ideas about political survival by maintaining a certain level of international legitimacy, despite massive and well-documented human rights violations. Indeed, despite a catastrophic human rights record from the perspective of international human rights law, since the 2000s, the Mugabe regime was able to position itself as a bulwark against imperialism and neo-colonialism on the African continent with renewed ideological legitimacy. This renewed ideological legitimacy has been skilfully used against political opponents who were accused of serving external imperialist forces, in order to trivialize or even justify the human rights abuses committed against them; in particular abuses of civil and political rights, presented as being "non-African". By focusing on the large-scale mobilization of violence for political purposes in the 2008 election year and in the four subsequent years in Zimbabwe, this master's thesis focuses on the paradox of the legitimacy that allowed the Mugabe regime to remain in place despite massive and well-documented violations of human rights that are protected by international law. While the scale of the violence was at the origin of the establishment of a government of national unity with the opposition, the result benefited the Mugabe regime by allowing it to govern without genuinely sharing power. Indeed, criticism of his treatment of civil and political rights has accredited his stature as a victim of imperialist vilification to certain regional actors, thereby weakening the opposition's balance of power within the powersharing deal

Promoting tree-based intercropping systems in the political arena : a cognitive analysis of public policies in agriculture, natural resources and rural development in Quebec

Paper presented at the 13th North American Agroforesty Conference, which was held June 19-21, 2013 in Charlottetown, Prince Edward Island, Canada.In Poppy, L., Kort, J., Schroeder, B., Pollock, T., and Soolanayakanahally, R., eds. Agroforestry: Innovations in Agriculture. Proceedings, 13th North American Agroforestry Conference, Charlottetown, Prince Edward Island, Canada, June 19-21, 2013.In Quebec, tree-based intercropping (TBI) systems are considered as potential contributors to climate change adaptation as well as ecological goods and services providers. TBI systems are consequently promoted by stakeholders as complex systems accurately addressing issues related to environmental deterioration, landscape degradation and rural devitalization. However, financing the implementation of these systems on farms remains a challenge. In fact, no clear policy on agroforestry or TBI systems has been developed and implemented yet in the province, complicating the work of both receptive farmers and experts. Thus, we could wonder how the arguments put forward by TBI systems promoters are or can be integrated at all in the mainstream ideas now driving three specific public policies (agriculture, natural resources and rural development). Our research uses qualitative methods and a content analysis based on Muller�s notion of referentials to understand how TBI systems can be integrated in the pre-existing referentials of actors in the agriculture, natural resources and rural development public policies. Based on the analysis of formal publications and semi-directed interviews, our preliminary results show that the referentials driving agriculture, natural resources and rural development policies are slowly shifting from strict economic development to multifunctionality and sustainable development. TBI systems may then have the opportunity to be integrated in these policies� referentials, especially when they are promoted as multifunctional and sustainable systems. Nonetheless, the integration of TBI systems is insufficient to produce large-scale policies given the present policies referentials, but may lead to small-scale initiatives support.Genevi�ve Laroche (1), Jean Mercier (2) and Alain Olivier (1) ; 1. D�partement de phytologie, Facult� des sciences de l'agriculture et de l'alimentation, Universit� Laval, Qu�bec, Canada. 2. D�partement de sciences politiques, Facult� des sciences humaines et sociales, Universit� Laval, Qu�bec, Canada.Includes bibliographical references