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Multiscale approach for modeling hot mix asphaltDessouky, Samer Hassan 29 August 2005 (has links)
Hot mix asphalt (HMA) is a granular composite material stabilized by the presence
of asphalt binder. The behavior of HMA is highly influenced by the microstructure
distribution in terms of the different particle sizes present in the mix, the directional
distribution of particles, the distribution of voids, and the nucleation and propagation of
cracks. Conventional continuum modeling of HMA lacks the ability to explicitly account
for the effect of microstructure distribution features. This study presents the development of
elastic and visco-plastic models that account for important aspects of the microstructure
distribution in modeling the macroscopic behavior of HMA.
In the first part of this study, an approach is developed to introduce a length scale to
the elasticity constitutive relationship in order to capture the influence of particle sizes on
HMA response. The model is implemented in finite element (FE) analysis and used to
analyze the microstructure response and predict the macroscopic properties of HMA. Each
point in the microstructure is assigned effective local properties which are calculated using
an analytical micromechanical model that captures the influence of percent of particles on
the microscopic response of HMA. The moving window technique and autocorrelation
function are used to determine the microstructure characteristic length scales that are usedin strain gradient elasticity. A number of asphalt mixes with different aggregate types and
size distributions are analyzed in this paper.
In the second part of this study, an elasto-visco-plastic continuum model is
developed to predict HMA response and performance. The model incorporates a Drucker-
Prager yield surface that is modified to capture the influence of stress path direction on the
material response. Parameters that reflect the directional distribution of aggregates and
damage density in the microstructure are included in the model. The elasto-visco-plastic
model is converted into a numerical formulation and is implemented in FE analysis using a
user-defined material subroutine (UMAT). A fully implicit algorithm in time-step control is
used to enhance the efficiency of the FE analysis. The FE model used in this study
simulates experimental data and pavement section.
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Charecterization of inertial and pressure effects in homogeneous turbulenceBikkani, Ravi Kiran 01 November 2005 (has links)
The objective of the thesis is to characterize the linear and nonlinear aspects of inertial
and pressure effects in turbulent flows. In the first part of the study, computations of
Navier-Stokes and 3D Burgers equations are performed in the rapid distortion (RD) limit
to analyze the inviscid linear processes in homogeneous turbulence. By contrasting the
results of Navier- Stokes RD equations and Burgers RD equations, the effect of pressure
can be isolated. The evolution of turbulent kinetic energy and anisotropy components
and invariants are examined. In the second part of the thesis, the velocity gradient
dynamics in turbulent flows are studied with the help of inviscid 3D Burgers equations
and restricted Euler equations. The analytical asymptotic solutions of velocity gradient
tensor are obtained for both Burgers and restricted Euler equations. Numerical
computations are also performed to identify the stable solutions. The results are
compared and contrasted to identify the effect of pressure on nonlinear velocity gradient
dynamics. Of particular interest are the sign of the intermediate principle strain-rate and
tendency of vorticity to align with the intermediate principle strain-rate. These aspects of
velocity gradients provide valuable insight into the role of pressure in the energy cascade
process.
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Scattered neutron tomography based on a neutron transport problemScipolo, Vittorio 01 November 2005 (has links)
Tomography refers to the cross-sectional imaging of an object from either transmission or reflection data collected by illuminating the object from many different directions. Classical tomography fails to reconstruct the optical properties of thick scattering objects because it does not adequately account for the scattering component of the neutron beam intensity exiting the sample.
We proposed a new method of computed tomography which employs an inverse problem analysis of both the transmitted and scattered images generated from a beam passing through an optically thick object.
This inverse problem makes use of a computationally efficient, two-dimensional forward problem based on neutron transport theory that effectively calculates the detector readings around the edges of an object. The forward problem solution uses a Step-Characteristic (SC) code with known uncollided source per cell, zero boundary flux condition and Sn discretization for the angular dependence. The calculation of the uncollided sources is performed by using an accurate discretization scheme given properties and position of the incoming beam and beam collimator. The detector predictions are obtained considering both the collided and uncollided components of the incoming radiation.
The inverse problem is referred as an optimization problem. The function to be minimized, called an objective function, is calculated as the normalized-squared error between predicted and measured data. The predicted data are calculated by assuming a uniform distribution for the optical properties of the object. The objective function depends directly on the optical properties of the object; therefore, by minimizing it, the correct property distribution can be found. The minimization of this multidimensional function is performed with the Polack Ribiere conjugate-gradient technique that makes use of the gradient of the function with respect to the cross sections of the internal cells of the domain.
The forward and inverse models have been successfully tested against numerical results obtained with MCNP (Monte Carlo Neutral Particles) showing excellent agreements.
The reconstructions of several objects were successful. In the case of a single intrusion, TNTs (Tomography Neutron Transport using Scattering) was always able to detect the intrusion. In the case of the double body object, TNTs was able to reconstruct partially the optical distribution. The most important defect, in terms of gradient, was correctly located and reconstructed. Difficulties were discovered in the location and reconstruction of the second defect.
Nevertheless, the results are exceptional considering they were obtained by lightening the object from only one side. The use of multiple beams around the object will significantly improve the capability of TNTs since it increases the number of constraints for the minimization problem.
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Top hole drilling with dual gradient technology to control shallow hazardsElieff, Brandee Anastacia Marie 30 October 2006 (has links)
Currently the "Pump and Dump" method employed by Exploration and
Production (E&P) companies in deepwater is simply not enough to control increasingly
dangerous and unpredictable shallow hazards. "Pump and Dump" requires a heavy
dependence on accurate seismic data to avoid shallow gas zones; the kick detection
methods are slow and unreliable, which results in a need for visual kick detection; and it
does not offer dynamic well control methods of managing shallow hazards such as
methane hydrates, shallow gas and shallow water flows. These negative aspects of
"Pump and Dump" are in addition to the environmental impact, high drilling fluid (mud)
costs and limited mud options.
Dual gradient technology offers a closed system, which improves drilling simply
because the mud within the system is recycled. The amount of required mud is reduced,
the variety of acceptable mud types is increased and chemical additives to the mud
become an option. This closed system also offers more accurate and faster kick
detection methods in addition to those that are already used in the "Pump and Dump"
method. This closed system has the potential to prevent the formation of hydrates by
adding hydrate inhibitors to the drilling mud. And more significantly, this system successfully controls dissociating methane hydrates, over pressured shallow gas zones
and shallow water flows.
Dual gradient technology improves deepwater drilling operations by removing
fluid constraints and offering proactive well control over dissociating hydrates, shallow
water flows and over pressured shallow gas zones. There are several clear advantages for
dual gradient technology: economic, technical and significantly improved safety, which
is achieved through superior well control.
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Supersonic turbulent boundary layers with periodic mechanical non-equilibriumEkoto, Isaac Wesley 25 April 2007 (has links)
Previous studies have shown that favorable pressure gradients reduce the
turbulence levels and length scales in supersonic flow. Wall roughness has been shown to
reduce the large-scales in wall bounded flow. Based on these previous observations new
questions have been raised. The fundamental questions this dissertation addressed are: (1)
What are the effects of wall topology with sharp versus blunt leading edges? and (2) Is it
possible that a further reduction of turbulent scales can occur if surface roughness and
favorable pressure gradients are combined? To answer these questions and to enhance the
current experimental database, an experimental analysis was performed to provide high
fidelity documentation of the mean and turbulent flow properties along with surface and
flow visualizations of a high-speed ( 2.86 M = ), high Reynolds number (Re 60,000 q û )
supersonic turbulent boundary layer distorted by curvature-induced favorable pressure
gradients and large-scale ( 300 s k + û ) uniform surface roughness. Nine models were tested
at three separate locations. Three pressure gradient models strengths (a nominally zero, a
weak, and a strong favorable pressure gradient) and three roughness topologies
(aerodynamically smooth, square, and diamond shaped roughness elements) were used.
Highly resolved planar measurements of mean and fluctuating velocity components were
accomplished using particle image velocimetry. Stagnation pressure profiles were acquired with a traversing Pitot probe. Surface pressure distributions were characterized
using pressure sensitive paint. Finally flow visualization was accomplished using
schlieren photographs.
Roughness topology had a significant effect on the boundary layer mean and
turbulent properties due to shock boundary layer interactions. Favorable pressure
gradients had the expected stabilizing effect on turbulent properties, but the
improvements were less significant for models with surface roughness near the wall due
to increased tendency towards flow separation. It was documented that proper roughness
selection coupled with a sufficiently strong favorable pressure gradient produced regions
of âÂÂnegativeâ production in the transport of turbulent stress. This led to localized areas of
significant turbulence stress reduction. With proper roughness selection and sufficient
favorable pressure gradient strength, it is believed that localized relaminarization of the
boundary layer is possible.
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Microfluidic chamber arrays for testing cellular responses to soluble-matrix and gradient signalsPark, Edward S. 20 January 2011 (has links)
This work develops microfluidic technologies to advance the state-of-the-art in living cell-based assays. Current cell-based assay platforms are limited in their capabilities, particularly with respect to spatial and temporal control of external signaling factors, sample usage, and throughput. The emergence of highly quantitative, data-driven systems approaches to studying biology have added further challenges to develop assay technologies with greater throughput, content, and physiological relevance. The primary objectives of this research are to (i) develop a method to reliably fabricate 3-D flow networks and (ii) apply 3-D flow networks to the development and testing of microfluidic chamber arrays to query cellular response to soluble-matrix signal combinations and gradient signaling fields. An equally important objective is for the chamber arrays to be scaled efficiently for higher-throughput applications, which is another reason for 3-D flow networks.
Two prototype chamber arrays are designed, modeled, fabricated, and characterized. Furthermore, tests are performed wherein cells are introduced into the chambers and microenvironments are presented to elicit complex responses. Specifically, soluble-matrix signaling combinations and soluble signal gradients are presented. The study of complex biological processes necessitates improved assay techniques to control the microenvironment and increase throughput. Quantitative morphological, migrational, and fluorescence readouts, along with qualitative observations, suggest that the chamber arrays elicit responses; however further experiments are required to confirm specific phenotypes. The experiments provide initial proof-of-concept that the developed arrays can one day serve as effective and versatile screening platforms.
Understanding the integration of extracellular signals on complex cellular behaviors has significance in the study of embryonic development, tissue repair and regeneration, and pathological conditions such as cancer. The microfluidic chamber arrays developed in this work could form the basis for enhanced assay platforms to perform massively parallel interrogation of complex signaling events upon living cells. This could lead to the rapid identification of synergistic and antagonistic signaling mechanisms that regulate complex behaviors. In addition, the same technology could be used to rapidly screen potential therapeutic compounds and identify suitable candidates to regulate pathological processes, such as cancer and fibrosis.
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Approche calculatoire pour la déconvolution en aveugle application à l'imagerie SIMS /Letierce, François Delosme, Jean-Marc. January 2007 (has links) (PDF)
Thèse de doctorat : Informatique : Evry-Val d'Essonne : 2007. / Titre provenant de l'écran-titre.
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Learning gradients and canonical correlation by kernel methods /Cai, Jia. January 2009 (has links) (PDF)
Thesis (Ph.D.)--City University of Hong Kong, 2009. / "Submitted to Department of Mathematics in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves [52]-58)
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Gradient formation in cemented carbides with 85Ni:15Fe-binder phase / Gradientbildning i hårdmetall med 85Ni:15Fe-bindefasLarsson, Niklas January 2015 (has links)
In today’s inserts used for metal cutting the binder phase consists of cobalt (Co).However, EU’s REACH programme and the U.S’s National Toxicity Programme(NTP) classified Co as toxic/carcinogenic. Therefore, there is a strong need toinvestigate alternative binder phases. This thesis covers sintering and characterisationof cemented carbide with a binder phase consisting of nickel (Ni) and iron (Fe) withthe composition of 85Ni:15Fe. The aim was to study the gradient formation of turninginsert and find sintering processes to achieve a gradient structure with the targetedthickness of 26 microns. Simulations in ThermoCalc provided a suitable composition and a starting point forsintering parameters. The influences of sintering process parameters, such as holdingtime, temperature and counter pressure on the formation of the gradient zone wereinvestigated. Hot isostatic pressing (HIP) sintering was done in order to study thegradient formation as well as to reduce the porosity when needed. Sintered insertswere analysed by light optical microscopy. It was found that there are at least three possible ways to control the formation ofthe gradient: sintering in vacuum with a holding time of 20 min at 1450°C, sintering at1450°C with a counter pressure of 5 mbar nitrogen, and sintering with a counterpressure of 11.5 mbar followed by a double sinter-hip with 55 bar argon atmosphere.However, only the last process fulfilled the microstructure criteria in terms ofporosity and binder phase distribution. It is clear that the formation of gradient zonesin 85Ni:15Fe can be predicted, however calculations and simulations need to beoptimized in order to get more accurate results.
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Experimental characterization of drift-wave turbulence in the sheared, cylindrical slabLee, Kevin Michael 24 March 2011 (has links)
Plasma turbulence on a uniform density gradient with unfavorable magnetic curvature is investigated extensively in the Helimak device. The turbulence is strong with density and electrostatic potential fluctuation levels in excess of 40%. Measurements of the dispersion relation, k[subscript z], and k[subscript parallel lines] identify the the fluctuations with drift-waves, which propagate in the poloidal direction at the diamagnetic drift velocity and have a small, but nite parallel wavenumber. A non-zero phase shift between the density and potential fluctuations gives rise to turbulent cross-eld particle transport, which is measured using spectral techniques. In addition, the electrostatic drift-wave fluctuations have a small magnetic component that is driven by the turbulent parallel current [scientific symbols]. An examination of nonlinear processes associated with the plasma turbulence uncovers high levels of intermittency near the plasma edge and long-time persistence of the density fluctuations on the order of the parallel confinement time. An analysis of the bispectrum conrms the existence local and nonlocal three-wave interactions between unstable drift-waves although the turbulent saturation of the density fluctuation spectrum is likely due nonlinear processes acting in the time domain. / text
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