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[pt] MÁQUINAS BROWNIANAS NÃO LINEARES / [en] NONLINEAR BROWNIAN MACHINES06 April 2021 (has links)
[pt] Na última década temos visto grande interesse na física de motores microscópicos
de uma particula. Não só temos visto grandes avanços na descrição
teórica de como esses sistemas se comportam como também, graças aos
avanços na área de manipulação microscópica, somos capazes de reproduzir
esses sistemas experimentalmente. A literatura é vasta quando consideramos
máquinas onde uma partícula é sujeita a um potencial harmônico onde
podemos controlar sua rigidez e em contato com um banho térmico de temperatura
controlável. Motivados por esses resultados fascinantes, decidimos
investigar um mecanismo alternativo para o estudo de máquinas. Propomos
e investigamos uma configuração onde uma única partícula com potencial
interno não linear em contato com um banho térmico de temperatura T que
controlamos, em seguida introduzimos um potencial quadrático externo centrado
em uma posição L que quebrará a simetria criando uma direção onde
a partícula pode flutuar com maior facilidade. Podemos usar essa quebra
de simetria para converter calor em trabalho. Começando com uma correção
não linear ao potencial interno predominantemente linear, usamos a
teoria de perturbação para resolver a equação de Langevin do sistema até
a primeira ordem da não linearidade k4 e obtemos o trabalho esperado e
o calor absorvido. Então relaxamos a restrição de pequena não linearidade
impondo que o período de cada ciclo seja tão grande que, ao menos parcialmente,
o sistema possa ser considerado em equilíbrio com o banho térmico.
Usando mecânica estatística clássica obtemos resultados para um alcance
maior das não linearidades. Uma vez que a componente central de nossas
máquinas é a assimetria, extendemos o potencial interno para o mais geral,
embora nem sempre analítico V(i)(x) proporcional a (x) elevado à alfa, que chamamos de potencial
tipo-alfa. Usando principalmente técnicas numéricas investigamos as propriedades
e resultados para diferentes valores de alfa. Por fim estudamos o ciclo
de Carnot substituindo os ramos adiabáticos com isentrópicos, investigando
o relacionamento entre alfa e as trajetórias isentrópicas. Todos os resultados
são comparados com simulações numéricas. / [en] In the recent decade we have seen great interest in the physics of
single particle microscopic engines. Not only we have seen advances in the
theoretical understanding of how such systems behave but also, thanks
to the advanced level of microscopic manipulations, we are capable of
reproducing these systems in experimental situations. The literature is quite
large when considering machines where a single particle is subjected to a
harmonic potential where we can control the stiffness and in contact with
a heat bath of controllable temperature. Motivated by these outstanding
results, we have decided to investigate an alternative mechanism to studying
machines. We propose and investigate a setup where a single particle with
an internal nonlinear potential in contact with a heat bath of temperature
T that we can control, then we introduce an external quadratic potential
centered in a position L which will break the internal symmetry and create
a direction where the particle can fluctuate to with greater ease. We can
use this symmetry breaking to convert heat into work. Starting with a
nonlinear correction to a predominantly linear internal potential, we use
perturbation theory to solve the Langevin equation of the system up to the
first order o k4 and obtain the expected work and absorbed heat. We then
relax the restriction of a small nonlinear by imposing that the cycle periods
are so large that, at least to some extent, the system can be considered
in equilibrium with the heat bath. Using classical statistical mechanics we
obtain results for a wider range of nonlinearities. Since the key component of
our machines is the asymmetry, we extend the internal potential to the more
general but not always analytical form V(i)(x) proportional to (x) raised to alpha which we label alpha-typepotential. Using primarily numerical techniques investigate its properties
and outputs for different values of alpha. Lastly we study the Carnot cycle
by replacing the adiabatical branches with isentropic ones, investigating
the relationship between alpha and the isentropic trajectories. All results are
compared with numerical simulations.
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Caractéristiques d'une butée de turbocompresseur : Approches numérique et expérimentale / Characteristics of a turbocharger thrust bearing : Numerical and experimental approachesRemy, Benoit 09 December 2015 (has links)
En raison de la tendance grandissante du downsizing, des hautes performances, des réductions de la consommation et des émissions des véhicules modernes, la turbocompression des moteurs thermiques s’avère de plus en plus innovante et technologique. Leader dans ce domaine, Honeywell Turbo Technologies s’intéresse à deux axes de recherche majeurs des systèmes lubrifiés. Un délai temporel est couramment observé entre l’appui sur la pédale d’accélérateur et l’augmentation de la pression de l’air ambiant du côté de la roue compresseur. Il s’agit du décalage turbo ou turbo lag. Celui-ci a pour origine principale l’existence d’un couple de frottement parasite au sein de la butée. En ce qu’il pénalise les performances du turbo et limite le confort de conduite, la réduction du turbo lag constitue un enjeu majeur de l’industrie des turbocompresseurs automobiles. En tant que phénomène présent à chaque instant du fonctionnement du véhicule, les pulsations moteurs constituent également un axe de recherche primordial. Celles-ci proviennent des ouvertures et fermetures successives des soupapes des cylindres et résultent en d’importantes variations de la charge axiale appliquée à la butée du turbocompresseur. Or le dimensionnement actuel de la butée se basant sur l’étude stationnaire des cas critiques de la vie réelle du moteur, l’influence de ces pulsations sur le comportement de la butée doit être déterminée. Ce manuscrit présente une modélisation thermohydrodynamique du contact de la butée appliquée au cas des turbocompresseurs. Une équation de Reynolds modifiée est établie et intègre le modèle Modifié de Phan-Thien et Tanner. Elle relie les caractéristiques rhéologiques des huiles de moteurs actuelles au comportement macro-moléculaire des additifs à longues chaînes de polymères qu’elles contiennent. La nécessité de considérer les effets inertiels du fluide ainsi que la recirculation inter-patin est démontrée. L’influence de l’élasticité du lubrifiant sur la dissipation d’énergie au sein du contact est analysée numériquement. Il est montré qu’un compromis sur l’élasticité du lubrifiant peut mener à une réduction du turbo lag. De plus, un banc d’essai est développé en régime transitoire afin de mettre en évidence l’influence des pulsations moteurs sur les performances de la butée. Les résultats obtenus expérimentalement corroborent les effets dynamiques observés par les prédictions numériques. / With increasing focus on downsizing, high performances, low fuel consumption and emission reduction of modern vehicles, turbochargers of combustion engines have become more and more innovative and technological. Leader in this field, Honeywell Turbo Technologies is interested in two major lines of research regarding lubricated contacts. A delay in response time is frequently observed between the gas pedal push and the pressure rise of ambient air at the compressor wheel. It is called turbo lag. This phenomenon is mainly induced by a parasitic frictional torque existing in the thrust bearing contact. Penalizing turbochargers performances and drivability, the reduction of turbo lag constitutes a major concern for the automotive turbocharger industry. As an event constantly occurring on working engines, exhaust gas pulsations represent an essential line of research. Due to successive openings and closings of engine valves, these pulsations result in significant axial load variations applied to the turbocharger thrust bearing. Current sizing of thrust bearing design resting on steady state studies of critical on-engine cases, the influence of exhaust gas pulsations on the thrust bearing behavior needs to be determined. This thesis presents a thermohydrodynamic model of the thrust bearing contact applied to turbochargers. A Modified Reynolds equation is established and uses the Modified Phan-Thien and Tanner model. It connects rheological characteristics of current engine oils to the behavior of long polymeric chain additives they contain. The necessity for fluid inertia and oil recirculation to be considered is demonstrated. The influence of oil elasticity on energy dissipation within the contact is numerically analyzed. A compromise on the extent of oil elasticity can lead to a turbo lag reduction. In addition, a thrust bearing rig is developed to highlight the influence of exhaust gas pulsations on thrust bearing performances. Experimental results support dynamic effects predicted by simulations.
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