Réductionnisme

Résumé

Au sens le plus large, on appelle réductionnisme la position selon laquelle une théorie, un domaine de discours ou un concept peut être expliqué, défini ou subsumé sous un autre. Dans le champ des sciences, le réductionnisme est une position philosophique, opposée à l'idée d'holisme, qui s'appuie sur une stratégie de simplification. Le réductionnisme consiste à réduire une notion en d'autres notions plus fondamentales. Toutefois, dans la pratique scientifique, le réductionnisme est moins une position philosophique sur l'ensemble du réel qu'une stratégie explicative. L'approche réductionniste considère tout système réel comme la résultante agrégative d'un ensemble de sous-systèmes ou d'éléments qui le composent, l'explication des propriétés d'un système physique ne pouvant s'effectuer qu'à partir de propriétés plus élémentaires. Les propriétés holistiques d'un système qui ne peuvent être ainsi expliquées à partir des propriétés constitutives doivent être ou bien éliminées de l'expl

Source officielle

https://fr.wikipedia.org/wiki/R%C3%A9ductionnisme

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The work addresses research by design as a subject and a method. It explores theorizations, experimentations, and verifications, all of them engaged in writing, drawing and building. It is discovery-led, and starts from a blurry "state of the arts". It oscillates between architectural processes, products and performance without considering any of them isolated as research. Its relevance emerges. It is the design process that constructs, delimits the research corpus and produces architectural insight and knowledge. The first part addresses architectural meaning. It engages interdisciplinary connections: structuralism in linguistics and philosophy, color theory, and a displacement into deconstruction. It concludes that, although an architectural design process can deliberately try to produce and uphold determinate architectural meaning, such (objective) meaning is immediately deviated, notably by the appearance of architectural usages, scale, and possibilities of appropriation. The second part investigates deviation as a substantial part of architectural creation. If noise is irreducible, it must also structure architectural creation. It connects to domains resisting to reductionist thought: complexity sciences, literary critics, archaeological processes, and theorizations dealing with the secondary such as psychoanalysis and gender studies. The work is marked by a dilemma related to empowerment: the originality and authenticity of design and its delivery at a certain moment always seem to prohibit inhabitation, appropriation. The secondary, subject to dominant, single authorship, becomes primary. Architecture seems resistant to modernity in the Kantian sense. The work re-centers on psychiatry, gender studies and (health) care: the understanding of natures rather than appearance, structure or essence. In the third part, it moves from the production of difference to inscriptions of memories and future possibilities; from design as making to design as the initiation and care of a shared transformation process, accompaniment of differance. The understanding of architectural reality shifts from the finality of object to a process beyond control, co-construction between different authors/users occurring in minor events, in a very local and specific context. The work proposes a design process driven by care, encounter of authors and users in quality, accompanying different and specific emerging natures of (territorial) identity.

EPFL2012

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Austenitic stainless steels is used in many components of nuclear power plants, particularly in the pipes of cooling systems. Owing to power transients and to start-ups and shutdowns, these components are subjected to thermo-mechanical loadings (low-cycle fatigue LCF & high cycle fatigue HCF) and flow-induced vibration (HCF). The corresponding fatigue design curves were established with solid smooth specimens tested in air at room temperature under strain-control. However, these curves do not consider the influence of LWR water environments that were shown to reduce fatigue life. US NRC Regulatory Guide 1.207 or other national equivalents (JSME Code in Japan) then were established taking strain rate, dissolved oxygen and temperature into consideration. Besides, a significant number of issues have been recently identified, which may have an impact on fatigue life but have not been sufficiently investigated, e.g., the potential negative effects of mean stress, mean strain, surface finish, long-term static hold, specimen geometry, multiaxial stress state were sensitivities not explicitly addressed. This study was launched to assess the effect of mean stress on fatigue behavior of a 316L austenitic steel in boiling water reactor environment with hydrogen water chemistry (BWR/HWC) at 288°C. Load-controlled fatigue tests were selected as the easiest experimental technique to impose a pre-defined mean stress. The tests were carried out on hollow specimens at different stress amplitudes and mean stresses in BWR/HWC environment and in air at 288°C, the latter serving as reference environment to evaluate the life reduction induced by the BWR/HWC medium. Few tests in strain-controlled were performed to derive a consistent way to represent the data obtained with the two control modes together. The test results reported in the form of stress-life curves showed that, in LCF regime (< 1e5 cycles), positive and negative mean stresses increase and BWR/HWC environment decreases fatigue life. In the HCF regime, negative mean stress is always beneficial for fatigue life but positive mean stress decreases fatigue life in BWR/HWC. The beneficial effect of mean stress is attributed to its enhanced cyclic hardening on material, which leads to smaller strain amplitude at given stress amplitude. The load-controlled data were converted into strain-life presentation by considering the average strain amplitude over the whole loading cycles. Additionally, a modified Smith-Watson-Topper mean stress correction method was successfully used to correlate the results with and without mean stress. Finally we showed that strain energy density criteria are good alternatives to correlate all data. Crack growth rates were determined from the striation spacing on the fracture surfaces with high resolution scanning electron microscopy (HRSEM); crack initiation sites were also studied with HRSEM, and the microstructures were observed with transmission electron microscopy and electron channeling contrast imaging. From the striation spacing measurement, we concluded that BWR/HWC environment essentially reduces the number cycles needed to initiate a physical crack (crack depth < 50 microns) but modifies slightly the crack growth rate, which correlates well with a strain intensity factor and J-integral method. Life predictions were also done by a well-trained artificial neuron network that considers mechanical, environmental and material properties factors.

EPFL2020

Chargement

Numerical characterization and engineering of transport in morphologically complex heterogeneous media

Xiaoyu Dai

A multi-scale numerical methodology for the assessment of radiation and optical characteristics of complex structured soot-contaminated snow layers was investigated first. The methodology accounted for the exact morphology at the various scales and utilizes volume-averaging approaches with the corresponding effective properties to couple the scales. At the continuum level, the volume-averaged coupled radiative transfer equations were solved utilizing i) effective radiative transport properties obtained by direct Monte Carlo simulations at the pore level, and ii) averaged bulk material properties obtained at particle level by discrete dipole approximation calculations. The contribution of soot additives to the radiative forcing was quantitatively estimated by numerical simulation and qualitatively validated with the published experimental data.Additional coupled heat and mass transfer phenomena were then investigated for a two-step solar thermochemical water/carbon dioxide splitting reactor. It was composed of two distinct scales: porous reacting materials at the mesoscale and a batch reactor at the macro-scale. A transient multi-physics volume-averaged model of the reactive porous media undergoing the reduction and oxidation steps was developed. The effective heat and mass transport properties of reticulated porous foams were incorporated through porosity-dependent empirical correlations. Porosity-dependent reduction, oxidation, and cyclic performance for a uniform and homogenous porous structure were assessed and followed by the investigation of diverse structures with non-uniform porosities along the reactor. Subsequently, the effect of structural anisotropy on the system performance was assessed by extending the previous model into 2-dimensions. The porous medium was assumed to exhibit anisotropic transport behaviors in terms of heat transfer (radiation, conduction, convection) and mass transfer. Additionally, two operational configurations were investigated: a conventional concurrent flow configuration of fluid and irradiation and a scenario where the principal flow axis was perpendicular to the irradiation direction. In terms of radiative properties, a structure with a complete forward scattering phase function and low scattering albedo ensured the largest absorption, increasing the production rate. Anisotropic structures with a higher degree of anisotropy in permeability were more favored when the incoming radiation was perpendicular to the fluid flow direction, where the interphase heat convection was enhanced.Lastly, the engineering of the porous structure was explored by establishing a direct relationship between the local morphological characteristics and the reduction performance. This was implemented by direct-pore simulations on the discrete scale that were directly coupled to the volume-averaged finite volume simulations on the continuum scale. A large number of porous structures were artificially generated and its transport characteristics calculated by the pore-levels simulations. These calculated results were used to train a machine-learning algorithm that predicted the oxygen yield solely from based on the morphology. The learning algorithm identified the most significant morphological descriptors for a specific performance target and provided guidelines for the optimal porous structure design.

EPFL2021

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Numerical characterization and engineering of transport in morphologically complex heterogeneous media

Xiaoyu Dai

EPFL2021

EPFL2012

EPFL2020