8,541 research outputs found
Direct numerical simulation of vortex ring evolution from the laminar to the early turbulent regime
No full textDirect numerical simulation is used to study the temporal development of single vortex rings at various Reynolds numbers and core thicknesses. Qualitative differences between the evolution of thin- and thick-core rings are observed leading to a correction factor to the classical equation for the ring translational velocity. We compare the obtained linear modal growth rates with previous work, highlighting the role of the wake in triply periodic numerical simulations. The transition from a laminar to a turbulent ring is marked by the rearrangement of the outer core vorticity into a clearly defined secondary structure. The onset of the fully turbulent state is associated with shedding of the structure in a series of hairpin vortices. A Lagrangian particle analysis was performed to determine the ring entrainment and detrainment properties and to investigate the possibility of an axial flow being generated around the circumference of the core region prior to the onset of turbulence
A study of the ring opening metathesis polymerization of polycyclic aromatic monomers and cyclopentenes with well defined initiators
No full textThis thesis describes studies into the ring opening metathesis polymerisation (ROMP) of polycyclic aromatic monomers and cyclopentenes. Chapter 1 reviews general aspects of ring opening metathesis polymerisation of relevance to the themes of this thesis. Chapter 2 describes the synthesis of polycyclic aromatic monomers, the endo and the exo Diels-Alder adducts of acenaphthylene and cyclopentadiene. Chapter 3 reports a study on the polymerisation of these monomers using well defined initiators and classical catalyst systems. Chapter 4 describes the synthesis of a substituted cyclopentene. Chapter 5 reports an investigation of polymerisation of cyclopentene using a variety of well defined initiators of general formula M(=NAr)(- OR)(_2)(=CHR) where M=Mo or W. Chapter 6 presents a study on the polymerisation of a substituted cyclopentene, 4-methylcyclopentene, using a series of well defined initiators, and the characterisations of the polymers obtained using infrared, (^1)H, and (^13)C n.m.r. spectroscopy, differential scanning calorimetry, and gel permeation chromatography and an analysis of detailed microstructure with respect to meso/racemic configurations in the polymer chain. Finally, Chapter 7 summarises the conclusions and makes some suggestions for future work
Numerical modelling of optical micro-cavity ring resonators for WDM networks
No full textAugmenting the level of integration for a lower cost and enhancing the performance of the
optical devices have turned out to be the focus of many research studies in the last few
decades. Many distinct approaches have been proposed in a significant number of researches
in order to meet these demands. Optical planar waveguides stand as one of vital employed
approach in many studies. Although, their low propagation loss, and low dispersion, they
suffers from high power losses at sharp bends. For this reason, large radius of curvature is
required in order to achieve high efficiency and compromise the high level of integration. For
the purpose of this research, in this thesis different ways to improve the performance of
optical microcavity ring resonators (MRRs) have been thoroughly investigated and new
configurations have been proposed.
The Multiresolution Time Domain (MRTD) technique was further developed and employed throughout this thesis as the main numerical modelling technique. The MRTD algorithm is
used as a computer code. This code is developed and enhanced using self built Compaq
Visual Fortran code. Creating the structure and Post-processing the obtained data is carried
out using self built MATLAB code. The truncating layers used to surround the computational
domain were Uniaxial Perfectly Matched Layers (UPML). The accuracy of this approach is
demonstrated via the excellent agreement between the results obtained in literature using
FDTD method and the results of MRTD.
This thesis has focused on showing numerical efficiency of MRTD where the mesh size
allowed or the total number of computed points is about half that used with FDTD.
Furthermore, the MRR geometry parameters such as coupling gap size, microring radius of curvature, and waveguide width have been thoroughly studied in order to predict and
optimise the device performance.
This thesis also presents the model analysis results of a parallel-cascaded double-microcavity
ring resonator (PDMRR). The analysis is mainly focus on the extraction of the resonant
modes where the effect of different parameters of the structure on transmitted and coupled
power is investigated.
Also, accurate analysis of 2D coupled microcavity ring resonator based on slotted
waveguides (SMRR) has been thoroughly carried out for the purpose of designing optical
waveguide delay lines based on slotted ring resonator (SCROW).
The SCROW presented in this thesis are newly designed to function according to the
variation of the resonance coupling efficiency of a slotted ring resonators embedded between two parallel waveguides.
The slot of the structures is filled with SiO2 and Air that cause the coupling efficiency to vary
which in turn control both the group velocity and delay time of SCROW structures results
from the changing the properties of the bent slotted waveguide modes which strongly
depends on the slot’s position.
Significant improvements on the quality factor and greater delay time have been achieved by
introducing sub-wavelength-low-index slot into conventional waveguide
Leonard Kephart to Laura Kephart, November 11, 1948
No full textIn this letter of November 11, 1948, Leonard Kephart (1892-1988) writes his mother, Laura Kephart to thank her for the many letters he returned home to and send her some money. He says that George and Pauline have moved into an apartment and he again dreams about retirement and going to work for Buddy. He saw Barbara, Jane and her children while he was in Montgomery, Alabama. And he lets her know that he will be visiting Ithaca soon for the Seed Growers Association and will give her a ring as soon as he gets to town. He asks if she received the color pictures of the wedding from Barbara Bird
Effect of confinement on the decay of vortex ring
No full textThe effect of confinement on the decay of vortex ring is studied computationally using Lattice Boltzmann Method. An Initial vortex ring, introduced inside a wall bounded cubical domain, is let to evolve and its decay is noted in terms of maximum vorticity at the core and the total kinetic energy inside the domain. The study shows distinct regimes of decay in all cases of confinement ratios(ratio of vortex ring diameter to length of the cubical domain)
Ring Rotterdam als lab voor een integrale aanpak
No full textHet ministerie van IenW heeft de Leerstoel Gebiedsontwikkeling en de Vereniging Deltametropool gevraagd de mogelijkheden te verkennen voor een toekomstbestendige (her)ontwikkeling van de Ring Rotterdam en omgeving. Welke ontwerp- en sturingsaanpak hoort daarbij? “Niet de wil, maar de systematiek staat duurzame verstedelijking in de weg.”Urban Development ManagementPractice Chair Urban Area Developmen
Volcanic cooling signal in tree ring temperature records for the past millennium
No full textThe authors acknowledge the National Science Foundation for funding much of the research presented herein. RW's Scottish work is currently funded through the UK Leverhulme Trust and Natural Environment Research Council (NERC) projects, “RELiC: Reconstructing 8000 years of Environmental and Landscape change in the Cairngorms (F/00 268/BG)” and “SCOT2K: Reconstructing 2000 years of Scottish climate from tree rings (NE/K003097/1).”Tree rings are an important proxy for understanding the timing and environmental consequences of volcanic eruptions as they are precisely dated at annual resolution and, particularly in tree line regions of the world, sensitive to cold extremes that can result from climatically significant volcanic episodes. Volcanic signals have been detected in ring widths and by the presence of frost-damaged rings, yet are often most clearly and quantitatively represented within maximum latewood density series. Ring width and density reconstructions provide quantitative information for inferring the variability and sensitivity of the Earth's climate system on local to hemispheric scales. After a century of dendrochronological science, there is no evidence, as recently theorized, that volcanic or other adverse events cause such severely cold conditions near latitudinal tree line that rings might be missing in all trees at a given site in a volcanic year (stand-wide missing rings), resulting in misdating of the chronology. Rather, there is a clear indication of precise dating and development of rings in at least some trees at any given site, even under adverse cold conditions, based on both actual tree ring observations and modeling analyses. The muted evidence for volcanic cooling in large-scale temperature reconstructions based at least partly on ring widths reflects several factors that are completely unrelated to any misdating. These include biological persistence of such records, as well as varying spatial patterns of response of the climate system to volcanic events, such that regional cooling, particularly for ring widths rather than density, can be masked in the large-scale reconstruction average.Peer reviewe
Integral closures
No full textThe q^th-power algorithm from Leonard [3], [4], and Leonard and Pellikaan [1] is a
recent and efficient method of computing the integral closure of a ring in its field of fractions.
Previously, the q^th-power algorithm has been used to compute integral closures over finite
fields. In this dissertation, we use the q^th-power algorithm to compute integral closures over
the rationals and number elds
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