31 |
[en] A LAGRANGIAN APPROACH FOR SIMULATING VISCOPLASTIC AND MULTIPHASE FLUIDS / [pt] UMA ABORDAGEM LAGRANGEANA PARA SIMULAÇÃO DE ESCOAMENTOS DE FLUIDOS VISCOPLÁSTICOS E MULTIFÁSICOSAFONSO PAIVA NETO 21 December 2007 (has links)
[pt] Na última década em Computação Gráfica, foram
desenvolvidas várias
técnicas de simulação do comportamento de objetos como
corpos rígidos,
água, fumaça, cabelo e tecido. Enquanto essas técnicas se
concentram em
modelos físicos simplificados através de fluidos
newtonianos monofásicos
ou de sólidos ideais, propomos aqui novas técnicas de
simulação de fluidos
viscoplásticos e multifásicos baseadas numa abordagem
lagrangeana
da equação de quantidade de movimento. Essas técnicas
consistem na
discretização do fluido através de um sistema de partículas
ao invés dos
tradicionais métodos baseados em malhas. A simulação
computacional da
dinâmica de fluidos é feita utilizando o método numérico
conhecido como
SPH (Smoothed Particle Hydrodynamics). Nessa tese,
representamos um
material viscoplástico como um fluido não-newtoniano que
varia entre os
estados sólido e líquido, e de alta para baixa viscosidade
dependendo de uma
força externa aplicada ao material ou de sua própria
temperatura. A simulação de fluidos multifásicos é
realizada através de um método híbrido
malha-partícula a fim de garantir a incompressibilidade no
método SPH. / [en] In the last decade in Computer Graphics, they were
development many
techniques to simulate the behavior of objects such as
rigid bodies, water,
smoke, hair and cloth. While these techniques focus in
simplifled physical
models through monophase fluids or ideal solids, we
propose new simulation
techniques of viscoplastic and multiphase fluids based on
a lagrangian
approach of momentum equation. These techniques consist in
the fluid
discretization through particles system instead of
traditional grid-based
methods. The computational fluid dynamics is performed
using the method
called SPH (Smoothed Particle Hydrodynamics). In this
thesis, we represent
a viscoplastic material like a non-newtonian fluid that it
varies between
solid and liquid states, and from low to high viscosity
depending on the
external force applied on material or its own temperature.
The simulation of
multiphase fluids is made using a hybrid method grid-
particle to guarantee
the incompressibility of SPH method.
|
32 |
[en] FORCED CONVECTION IN LAMINAR FLOWS OF VISCOPLASTIC LIQUIDS THROUGH TUBES AND ANNULI / [pt] CONVECÇÃO FORÇADA EM ESCOAMENTOS LAMINARES DE LÍQUIDOS VISCOPLÁSTICOS EM TUBOS E ESPAÇOS ANULARESMARIA HELENA FARIAS 05 January 2005 (has links)
[pt] Escoamentos de fluidos não Newtonianos são comumente
encontrados em processos industriais. Deste modo, é
importante conhecer bem o efeito dos processos sobre a
reologia desta classe de fluidos, assim como o inverso,
ou seja, conhecer o efeito da interferência da reologia de
tais fluidos sobre os processos. Fluidos não Newtonianos
exibem complexidade no seu comportamento mecânico, não
encontrada nos fluidos Newtonianos, como, por exemplo,
dependência da viscosidade com a taxa de cisalhamento
e a existência de uma tensão-limite de escoamento não nula.
Verifica-se, atualmente, a existência de uma ampla lacuna
na literatura no que diz respeito µa compreensão da
interação de fluidos não newtonianos em diferentes
geometrias de escoamento, em particular sob o ponto de vista
térmico. Algumas geometrias, por serem mais comuns nas
linhas industriais, têm recebido maior atenção nas
investigações sobre o referido aspecto, como, por exemplo,
os casos do tubo circular e do espaço anular. Encontra-se
uma maior quantidade de trabalhos publicados de estudos
analíticos ou de simulação numérica, enquanto que são raros
os artigos baseados em investigações experimentais. No
presente trabalho, o qual teve como motivação a avaliação
do comportamento térmico de um poço de petróleo durante sua
perfuração, estudou-se experimentalmente o efeito da
reologia do fluido no processo de transferência de calor em
espaços anulares e, também, em tubos. O objetivo foi
determinar o coeficiente interno de transferência de calor
(Número de Nusselt) para o caso de parede interna com fluxo
de calor uniforme e parede externa adiabática para o anular
e fluxo de calor uniforme para o tubo. Utilizou-se um
fluido do tipo viscoplástico, que reproduz bem o
comportamento do fluido de perfuração, em diferentes
concentrações, no intuito de se observar a influência da
reologia do fluido no escoamento não isotérmico. Diferentes
razões de raios do espaço anular foram estudadas. Os
resultados experimentais mostram que, em escoamentos
laminares e completamente desenvolvidos, a reologia do
fluido não afeta a transferência de calor no espaço anular,
sendo esta, fundamentalmente, dependente da geometria.
Estes resultados estão de acordo com previsões teóricas
recentemente publicadas, e a principal contribuição do
presente trabalho é confirmar este resultado surpreendente,
que torna mais simples os projetos envolvendo o escoamento
de materiais viscoplásticos em espaços anulares sob as
condições de contorno investigadas. / [en] Non-Newtonian fluids flow are very common in industrial
processes, so it is important to know both the effect of
the process on the fluid and vice-versa. Non-Newtonian
Fluids exhibit complex mechanical behavior not found in
Newtonian fluids, such as shear-rate-dependent viscosity
and non-zero Yield stress. Nowadays there is a lack of
understanding in the literature of the interaction among
non-Newtonian fluids and different flow geometries,
particularly as far as heat transfer is concerned. Some
geometries are found more frequently in industrial
processes, being, accordingly, a more frequent subject of
research. Among these are the tubes and annuli. Most of the
published articles about this subject are analytical
studies or numerical simulations, while those based on
experimental investigations are rather scarce. This work is
focused in the evaluation of the thermal behavior of
oil wells during the flow of the drilling fluid. The effect
of fluid rheology on heat transfer in annular spaces and
circular tubes was investigated experimentally. The purpose
was to determine the convective heat transfer coefficient
(Nusselt number). The boundary conditions for the annuli
were uniform heat flux at the inner wall and adiabatic
outer wall, while, for the tube, the heat flux at the wall
was kept constant and uniform. To mimic the drilling fluid
mechanical behavior, the working fluids were viscoplastic
liquids at different concentrations. For the annuli,
different radius ratios were studied. The experimental
results showed that, for laminar and fully developed flow
in the annuli, the fluid rheology does not affect the
Nusselt number, which is governed by the radius ratio only.
These results are in agreement with recently published
theoretical predictions, and the main contribution of this
work is to confirm this surprising result, which renders
simpler the projects involving non-Newtonian fluids flowing
in annuli under the thermal boundary conditions
investigated.
|
33 |
Microstructural viscoplastic continuum model for asphalt concreteTashman, Laith 30 September 2004 (has links)
This dissertation presents the development of an anisotropic viscoplastic continuum damage model to describe the permanent deformation of asphalt pavements. The model is developed to account for several phenomena that influence the permanent deformation of Asphalt Concrete (AC) at high temperatures. These phenomena include strain rate dependency, confining pressure dependency, dilation, aggregate friction, anisotropy, and damage. The model is based on Perzyna's theory of viscoplasticity with Drucker-Prager yield function modified to account for the microstructure anisotropy and damage. A parametric study was conducted to study the effect of key factors such as inherent anisotropy and damage on the model response. A preliminary investigation was conducted to demonstrate the capabilities of the model and its sensitivity to changes in the microstructure distribution and loading conditions. The model was used to describe laboratory experimental measurements obtained from the Federal Highway Administration (FHWA) Accelerated Loading Facility (ALF). The model had a good match with these experimental measurements. In particular, using the damage parameter, the model was able to capture the point at which AC experienced tertiary creep in a static creep test. A comprehensive experiment was conducted to systematically determine the model parameters and the evolution laws that describe AC hardening, anisotropy, and damage. The experiment consisted of a set of compressive triaxial strength tests conducted at three confining pressures and five strain rates. Based on these experimental measurements, the model was modified to include a nonassociated flow rule. The model was shown to capture the experimental measurements very well. Furthermore, an experiment was conducted to capture and characterize damage evolution in AC due to permanent deformation. AC specimens were loaded using a triaxial compression setup to four predefined strain levels at three confining pressures. X-Ray computed tomography and image analysis techniques were used to capture and characterize the evolution of cracks and air voids in the deformed specimens. Damage was found to be a localized phenomenon in the sense that there exists a critical section in an AC specimen that is mainly responsible for failure. The results of the damage experiment supported the damage evolution function proposed in the viscoplastic model.
|
34 |
Photoinduced material transport in amorphous azobenzene polymer filmsGrenzer, Marina January 2007 (has links)
The role played by azobenzene polymers in the modern photonic, electronic and opto-mechanical applications cannot be underestimated. These polymers are successfully used to produce alignment layers for liquid crystalline fluorescent polymers in the display and semiconductor technology, to build waveguides and waveguide couplers, as data storage media and as labels in quality product protection. A very hot topic in modern research are light-driven artificial muscles based on azobenzene elastomers.
The incorporation of azobenzene chromophores into polymer systems via covalent bonding or even by blending gives rise to a number of unusual effects under visible (VIS) and ultraviolet light irradiation. The most amazing effect is the inscription of surface relief gratings (SRGs) onto thin azobenzene polymer films. At least seven models have been proposed to explain the origin of the inscribing force but none of them describes satisfactorily the light induced material transport on the molecular level.
In most models, to explain the mass transport over micrometer distances during irradiation at room temperature, it is necessary to assume a considerable degree of photoinduced softening, at least comparable with that at the glass transition. Contrary to this assumption, we have gathered a convincing evidence that there is no considerable softening of the azobenzene layers under illumination. Presently we can surely say that light induced softening is a very weak accompanying effect rather than a necessary condition for the formation of SRGs.
This means that the inscribing force should be above the yield point of the azobenzene polymer. Hence, an appropriate approach to describe the formation and relaxation of SRGs is a viscoplastic theory. It was used to reproduce pulse-like inscription of SRGs as measured by VIS light scattering. At longer inscription times the VIS scattering pattern exhibits some peculiarities which can be explained by the appearance of a density grating that will be shown to arise due to the final compressibility of the polymer film.
As a logical consequence of the aforementioned research, a thermodynamic theory explaining the light-induced deformation of free standing films and the formation of SRGs is proposed. The basic idea of this theory is that under homogeneous illumination an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores.
Finally, some ideas about further development of this controversial topic will be discussed. / Azobenzenhaltige Polymere sind in modernen photonischen, elektronischen und opto-mechanischen Anwendungen nicht mehr wegzudenken. Diese Polymere werden erfolgreich in der Bildschirm- und Halbleitertechnologie eingesetzt, um Ausrichtungsschichten für flüssig-kristalline fluoreszierende Polymere zu produzieren sowie Wellenleiter und Wellenleiterkoppler herzustellen. Auch dienen sie als Medien für Datenspeicher oder der Sicherung von Qualitätsprodukten. Ein wichtiges Thema in der modernen Forschung sind lichtgetriebene künstliche Muskeln basierend auf azobenzenhaltigen Elastomeren.
Die Inkorporation von Azobenzene in Polymersysteme durch kovalente Bindungen oder durch Vermischung resultiert in einer Anzahl ungewöhnlicher Effekte, welche unter Bestrahlung mit sichtbarem und ultraviolettem Licht auftreten. Der erstaunlichste Effekt ist das Erzeugen von Oberflächengittern (Surface Relief Gratings - SRGs) auf dünnen azobenzenhaltigen Polymerfilmen. Es wurden mindestens sieben Modelle zur Erklärung der Herkunft der dieses Gitter erzeugenden Kraft vorgeschlagen aber keines von diesen kann befriedigend den lichtinduzierten Massetransport auf molekularem Niveau beschreiben.
Um einen Massetransport über Mikrometerabstände zu erklären, ist es in den meisten Modellen notwendig, eine deutliche lichtinduzierte Erweichung, die mit der Erweichung bei dem Glasübergang vergleichbar ist, anzunehmen. Entgegen dieser Annahme wurden in dieser Arbeit überzeugende Beweise gesammelt, dass es keine signifikante Erweichung in azobenzenhaltigen Schichten bei homogener Bestrahlung gibt. Deshalb kann man davon ausgehen, dass die lichtinduzierte Erweichung eher ein schwacher begleitender Effekt als eine notwendige Voraussetzung für die Bildung derartiger Oberflächengitter ist.
Aus dieser Beobachtung muss geschlussfolgert werden, dass die erzeugende Kraft oberhalb der Fließgrenze azobenzenhaltiger Polymer liegt. Deshalb ist die viskoplastische Theorie ein geeigneter Ansatz zur Beschreibung der Formation von Oberflächengittern und deren Relaxation. Dieser Ansatz wurde genutzt, um die beim pulsartigen Erzeugen von Oberflächengittern gemessene Lichtstreuung mit einem Modell zu beschreiben. Bei längeren Bestrahlungszeiten weist das Streumuster einige Besonderheiten auf, die durch die Bildung eines Dichtegitters erklärt werden können. Dieses Gitter entsteht infolge der begrenzten Kompressibilität des Polymerfilms.
Als logische Konsequenz der oben genannten Erkenntnisse wird eine thermodynamische Theorie, die die lichtinduzierte Deformation des freistehenden Filmes und die Oberflächen-gitterbildung wiedergeben kann, vorgeschlagen. Die Hauptidee ist, dass eine ursprünglich isotrope Probe sich unter homogener Bestrahlung entlang der Polarisationsrichtung ausdehnen muss, um die Entropieabnahme, die durch die lichtinduzierte Reorientierung der Azobenzene entstanden ist, zu kompensieren.
In der Folge werden einige Ideen über die weitere Entwicklung dieses interessanten Themas diskutiert.
|
35 |
Effect of twinning on texture evolution of depleted uranium using a viscoplastic self-consistent modelHo, John 20 August 2012 (has links)
Texture evolution of depleted uranium is investigated using a viscoplastic self-consistent model. Depleted uranium, which has the same structure as alpha-uranium, is difficult to model as it has an orthorhombic symmetry structure, as well as many twin systems which must be addressed in order to properly simulate the textural evolution.
The VPSC method allowed for a flexible model which could not only encompass the low symmetry component but also the twinning components of depleted uranium. The model focuses on the viscoplastic regime only, neglecting the elastic regime of deformation, and uses a self-consistent method to solve the model.
Different deformation processes, such as torsion, rolling, and swaging, are simulated and the theoretical textures, plotted as pole figures or inverse pole figures, are compared with previous experimental textures found for alpha uranium from previous experimental sources.
A specific twin system, the (176)[512] system, is also given special consideration. This twin system is a dominant deformation mode for alpha uranium at high strain rates, but is quite elusive in general. Different deformation processes are compared where this twin system is active and not active. This allows comparison on the effect of this twin on the overall texture of depleted uranium. In addition, a sample of depleted uranium from Y12 which was analyzed for (176)[512] twins is compared to theoretical results from a VPSC simulation where the (176)[512] twin is active.
|
36 |
A constraint based viscoplastic model of granular materialNordberg, John January 2011 (has links)
The goal of this thesis is to develop a constraint based viscoplastic fluid model suitable for time-efficient dynamics simulation in 3D of granular matter. The model should be applicable to both the static and dense flow regime and at large pressures. The thesis is performed for UMIT Research Lab at Umeå University. It is a part of the research at UMIT connected to LKAB and Volvo CE and its applications can be in simulating industrial processes or training simulators. My work is based on previous work done by Claude Lacoursière, Martin Servin and Kenneth Bodin. They have created a constraint fluid model based on {\sph} and Claude's PhD. thesis. This model is extended with additional constraints to handle shear forces, which is necessary to model granular material. Some test cases are specified and compared visually to each other and to the results of other work. The model seems to work visually but more analysis and larger systems are needed to be certain. The model should scale well and is well suited for parallellization.
|
37 |
Viscoelastic{Viscoplastic Damage Model for Asphalt ConcreteGraham, Michael A. 2009 August 1900 (has links)
This thesis presents a continuum model for asphalt concrete incorporating non-
linear viscoelasticity, viscoplasticity, mechanically-induced damage and moisture-
induced damage. The Schapery single-integral viscoelastic model describes the
nonlinear viscoelastic response. The viscoplastic model of Perzyna models the time-
dependent permanent deformations, using a Drucker-Prager yield surface which is
modified to depend on the third deviatoric stress invariant to include more complex
dependence on state of stress. Mechanically-induced damage is modeled using continuum damage mechanics, using the same modified Drucker-Prager law to determine
damage onset and growth. A novel moisture damage model is proposed, modeling
moisture-induced damage using continuum damage mechanics; adhesive moisture-
induced damage to the asphalt mastic-aggregate bond and moisture-induced cohesive
damage to the asphalt mastic itself are treated separately.
The analytical model is implemented numerically for three-dimensional and plane
strain finite element analyses, and a series of simulations is presented to show the
performance of the model and its implementation. Sensitivity studies are conducted
for all model parameters and results due to various simulations corresponding to
laboratory tests are presented.
In addition to the continuum model, results are presented for a micromechanical
model using the nonlinear-viscoelastic-viscoplastic-damage model for asphalt mastic and a linear elastic model for aggregates. Initial results are encouraging, showing the
strength and stiffness of the mix as well as the failure mode varying with moisture
loading. These initial results are provided as a an example of the model's robustness
and suitability for modeling asphalt concrete at the mix scale.
|
38 |
Effect of electro-mechanical loading in metallic conductorsGallo, Federico Guido 09 February 2011 (has links)
The development of high powered electro-magnetic devices has generated interest in the effect of combined electromagnetic and mechanical loading of such structures. Materials used in high-current applications – aluminum alloys and copper – are subjected to heat pulses of short duration (in the range of a few hundred microseconds to a few milliseconds); immediately following or along with such heat pulses, these materials are also subjected to large mechanical forces. In previous work reported in the literature, ejection of material from the vicinity of preexisting defects such as cracks, notches or discontinuities have been observed resulting from short-duration high-intensity current pulses; after a series of pulses, permanent deformation and weakening of intact material has also been reported. But a lack of complete understanding of the effects of short duration current pulses hinders the assessment of the reliability of such conductors in high energy applications. Therefore, an investigation was undertaken to examine the behavior of electromagnetically and mechanically loaded conductors. This work investigates the effects of short-duration, high-current-density pulses in combination with
viii
mechanical loading. The aim is to develop a theoretical model to describe the resulting mechanical response. The model is to provide a characterization of the possible effects of thermally-induced plastic strains on metals loaded beyond or just below their yield strength or below the critical stress intensity factor.
In the experiments reported here, two types of specimens, undamaged and damaged, were subjected to combined electromechanical loads. Undamaged specimens were used to observe thermally-induced plastic strains - strains not caused by an increase in mechanical loading, but rather resulting from the reduction of yield strength and post-yield stiffness due to the increase in temperature. The experiments were conducted such that it would be possible to develop a model that would conclusively account for the observed material behavior. The second sets of specimens were weakened a priori by the introduction of a crack in order to study the influence of such crack-like defects on the electrical and mechanical fields, and to produce a safe design envelope with respect to the loading conditions. Failure was found to occur due to melting triggered by joule heating; a quantitative criterion based on current concentration and heat accumulation near the crack tip has been developed based on these experimental results. / text
|
39 |
Microstructural viscoplastic continuum model for asphalt concreteTashman, Laith 30 September 2004 (has links)
This dissertation presents the development of an anisotropic viscoplastic continuum damage model to describe the permanent deformation of asphalt pavements. The model is developed to account for several phenomena that influence the permanent deformation of Asphalt Concrete (AC) at high temperatures. These phenomena include strain rate dependency, confining pressure dependency, dilation, aggregate friction, anisotropy, and damage. The model is based on Perzyna's theory of viscoplasticity with Drucker-Prager yield function modified to account for the microstructure anisotropy and damage. A parametric study was conducted to study the effect of key factors such as inherent anisotropy and damage on the model response. A preliminary investigation was conducted to demonstrate the capabilities of the model and its sensitivity to changes in the microstructure distribution and loading conditions. The model was used to describe laboratory experimental measurements obtained from the Federal Highway Administration (FHWA) Accelerated Loading Facility (ALF). The model had a good match with these experimental measurements. In particular, using the damage parameter, the model was able to capture the point at which AC experienced tertiary creep in a static creep test. A comprehensive experiment was conducted to systematically determine the model parameters and the evolution laws that describe AC hardening, anisotropy, and damage. The experiment consisted of a set of compressive triaxial strength tests conducted at three confining pressures and five strain rates. Based on these experimental measurements, the model was modified to include a nonassociated flow rule. The model was shown to capture the experimental measurements very well. Furthermore, an experiment was conducted to capture and characterize damage evolution in AC due to permanent deformation. AC specimens were loaded using a triaxial compression setup to four predefined strain levels at three confining pressures. X-Ray computed tomography and image analysis techniques were used to capture and characterize the evolution of cracks and air voids in the deformed specimens. Damage was found to be a localized phenomenon in the sense that there exists a critical section in an AC specimen that is mainly responsible for failure. The results of the damage experiment supported the damage evolution function proposed in the viscoplastic model.
|
40 |
A Viscoelastic-Viscoplastic Analysis of Fiber Reinforced Polymer Composites Undergoing Mechanical Loading and Temperature ChangesJeon, Jaehyeuk 16 December 2013 (has links)
This study presents a combined viscoelastic (VE)-viscoplastic (VP) analysis for Fiber Reinforced Polymer (FRP) composites subject to simultaneous mechanical load and conduction of heat. The studied FRP composites consist of unidirectional fibers, which are considered as linearly elastic with regards to their mechanical response, and isotropic polymeric matrix, which shows viscoelastic-viscoplastic response under various stresses and temperatures. Due to the viscoelastic and viscoplastic behavior of the polymeric matrix, the overall FRP composites exhibit a combined time-dependent and inelastic behavior. A simplified micromechanical model, consisting of a unit-cell with four fiber and matrix subcells, is formulated to homogenize the overall heat conduction and viscoelastic-viscoplastic responses of the FRP composites. The micromechanical model is compatible with a displacement based finite element (FE) and is implemented at the Gaussian integration points within the continuum finite elements, which is useful for analyzing the overall time-dependent response of FRP composite structures under various boundary conditions. The Schapery nonlinear integral model combined with the Perzyna viscoplastic model is used to describe the viscoelastic-viscoplastic response of the polymer constituents. An integrated time integration algorithm is formulated at the micromechanics level in order to solve the nonlinear viscoelastic-viscoplastic constitutive model at the matrix subcells and obtain the overall nonlinear response of the FRP. The viscoelastic-viscoplastic micromechanical model is validated usingexperimental data on off-axis glass/epoxy FRP composites available in literature. The overall response of the FRP composites determined from the simplified micromechanical model is also compared with the ones generated from microstructures of FRP with various fiber arrangements dispersed in homogeneous polymer matrix. The microstructural models of the FRP with detailed fiber arrangements are generated using FE. The effects of thermal stresses, due to the mismatches in the coefficient of thermal expansions of the fibers and polymeric matrix, and stress concentrations/discontinuities near the fiber and matrix interfaces on the overall thermo-mechanical deformation of FRP composites are studied using the two micromechanical models discussed above. Finally, an example of structural analysis is performed on a polymeric smart sandwich composite beam, having FRP skins and polymeric foam core with piezoelectric sensors integrated to the FRP skins, undergoing three point bending at an elevated temperature. The creep displacement is compared to experimental data available in literature.
|
Page generated in 0.0765 seconds