368 research outputs found
Topological Stability of Kinetic -Centers
We study the -center problem in a kinetic setting: given a set of
continuously moving points in the plane, determine a set of (moving)
disks that cover at every time step, such that the disks are as small as
possible at any point in time. Whereas the optimal solution over time may
exhibit discontinuous changes, many practical applications require the solution
to be stable: the disks must move smoothly over time. Existing results on this
problem require the disks to move with a bounded speed, but this model is very
hard to work with. Hence, the results are limited and offer little theoretical
insight. Instead, we study the topological stability of -centers.
Topological stability was recently introduced and simply requires the solution
to change continuously, but may do so arbitrarily fast. We prove upper and
lower bounds on the ratio between the radii of an optimal but unstable solution
and the radii of a topologically stable solution---the topological stability
ratio---considering various metrics and various optimization criteria. For we provide tight bounds, and for small we can obtain nontrivial
lower and upper bounds. Finally, we provide an algorithm to compute the
topological stability ratio in polynomial time for constant
Solid rocket booster thermal radiation model, volume 1
A solid rocket booster (SRB) thermal radiation model, capable of defining the influence of the plume flowfield structure on the magnitude and distribution of thermal radiation leaving the plume, was prepared and documented. Radiant heating rates may be calculated for a single SRB plume or for the dual SRB plumes astride the space shuttle. The plumes may be gimbaled in the yaw and pitch planes. Space shuttle surface geometries are simulated with combinations of quadric surfaces. The effect of surface shading is included. The computer program also has the capability to calculate view factors between the SRB plumes and space shuttle surfaces as well as surface-to-surface view factors
Geometrically-Complex Magnetic Field Distributions Enabled By Bulk, Laser-Micromachined Permanent Magnets At The Submillimeter Scale
High-energy-product permanent magnets (PM) are utilized in many industrial, research, consumer, and commercial applications. Indeed, there are many potential applications that can utilize sub-mm PM to create miniaturized versions of motors, generators, energy harvesters, undulators, sensors, actuators, and other microelectromechanical systems (MEMS) devices. Magnets in MEMS are both important and useful because they can provide a strong force at a distance within a compact package; however, there exists a gap in magnet technologies today where magnets have little to no presence between bottom-up microfabricated PM and top-down machined bulk PM. Thus, there is a need for a form of PM that can be 100–500��m thick with lateral dimensions of the same order to fill this gap and provide the advantageous magnetic properties of bulk PM at this scale. This dissertation presents the development of laser micromachining as a fabrication technology that enables the microfabrication of PM to generate geometrically complex magnetic fields at the sub-mm scale. Generating geometrically complex magnetic fields at the sub-mm scale opens up new possibilities in medical technology, energy generation, and many other applications. Models simulating magnetic properties and the effects of laser machining are presented and compared to measurements. The fabrication technology discussed here allows sub-mm, geometrically complex magnetic fields to be achieved while maintaining the characteristics of bulk PM. The utility of this advance in fabrication technology is demonstrated through multiple research vehicles, including undulators for radiation generation and multipole energy harvesters operable at low frequency. Such vehicles represent a small sample of the potential applications for this work
Vision-Based Autonomous Robotic Floor Cleaning in Domestic Environments
Fleer DR. Vision-Based Autonomous Robotic Floor Cleaning in Domestic Environments. Bielefeld: Universität Bielefeld; 2018
Atomic Scale Investigation of Pressure Induced Phase Transitions in the solid State
In this work, atomic scale investigation of pressure-induced transformations in the solid state have been carried out. A series of compounds including GaN, ZnO, CaF2, and AgI, in addition
to elemental phosphorus have been studied. The corresponding transition mechanisms have been
elucidated with a clear description of atomic displacements and intermediate structures involved
therein.
In the first group of compounds, the long standing debate on the transition path of the
wurtzite(WZ)-to-rocksalt(RS) transition in semiconductors, GaN and ZnO was resolved using geometrical
modeling combined with molecular dynamics (MD) simulations conducted in the frame
of transition path sampling (TPS) method. In GaN, a two-step mechanism through a metastable
intermediate phase with a tetragonal structure iT has been revealed from simulations. In ZnO,
the tetragonal intermediate structure was kinetically less stable, although still part of the real
transition mechanism. It appeared at the interface between WZ and RS as consequence of a layers
shearing. The transition regime in ZnO was characterized by a competition between iT structure
and another hexagonal intermediate with hexagonal symmetry iH. Although possible, the latter
is not functional for the transition.
In both cases, GaN and ZnO, two points of agreement with experiments have been revealed.
The tilting of structures after transition, and the phonon mode softening associated with atomic
displacements leading to the tetragonal structure iT
In the second group of compounds, the investigation of transitions in superionic conductors,
CaF2 and AgI, demonstrated a different and particular behavior of atomic motion under pressure.
The solid-solid reconstruction of CaF2 structure was shown to be initiated and precedented by high
disorder of the anionic sublattice. The percolation of fluoride ions through voids in the fluorite
structure created a thin interface of liquid like state. The sparce regions caused by the departure
of anions facilitates the cation sublattice reconstruction.
In AgI, ion diffusion during the wurtzite/zincnlende(ZB)$rocksalt transition was more pronounced
due to the extended stacking disorder WZ/ZB. The Ag+ ions profited not only from the
structure of the interface but used the combination of interstitial voids offered by both phases,
WZ and ZB, to achieve long diffusion paths and cause the cation sublattice to melt. Clearly, a
proper account for such phenomena cannot be provided by geometry-designed mechanisms based
on symmetry arguments.
In phosphorus, the question of how the stereochemically active lone pairs are reorganized during
the orthorhombic (PI) to trigonal (PV) structural transition was answered by means of simulations.
Computation was performed at different levels theory.
First, the mechanism of the transition was obtained from TPS MD simulations. MD runs
were performed within density functional tight binding method (DFTB). The analysis of atomic
displacements along the real transformation path indicated a fast bond switching mechanism.
In a second step, the nature of the interplay between orbitals of phosphorus during the bond
switching was investigated. A simultaneous deformation of lone pair and P−P bond showed
a mutual switching of roles during the transformation. This interplay caused a low dimensional
polymerization of phosphorus under pressure. The corresponding structure formed as zigzag linear
chain of fourfold coordinated phosphorus atoms (· · ·(P(P2))n · · ·) at the interface between PI and
PV phases.
A further result of this work was the development of a simulation strategy to incorporate
defects and chemical doping to structural transformations. On top of the transition path sampling
iterations, a Monte Carlo like procedure is added to stepwise substitute atoms in the transforming
system. Introducing a chemically different dopant to a pure system represents a perturbation to
the energy landscape where the walk between different phases is performed. Therefore, any change
in the transition regime reflects the kinetic preference of a given structural motif at times of phase
formation.
This method was applied to the elucidation of WZ-RS transition mechanism in the series of
semiconducting compounds AlN, GaN, and InN. Simulations showed that In atoms adopt the
same transformation mechanism as in GaN and favor it, while Al atoms demonstrated a significant
reluctance to the path going through tetragonal intermediate iT. The difference between transition
regime in mixed systems InxGa1−xN and AlxGa1−xN is in agreement with experiments on high pressure
behavior of AlN, GaN, and InN. While transitions in GaN and InN are reversible down
to ambient conditions, AlN is stable.
The work presented in this thesis constitutes the seed of new perspectives in the understanding
of pressure-induced phase transformations in the solid state, where the physics and the chemistry
are brought together by means of computer simulations
Structured meshes: composition and remeshing guided by the Curve-Skeleton
Virtual sculpting is currently a broadly used modeling metaphor with rising
popularity especially in the entertainment industry. While this approach
unleashes the artists' inspiration and creativity and leads to wonderfully
detailed and artistic 3D models, it has the side effect, purely technical,
of producing highly irregular meshes that are not optimal for subsequent
processing. Converting an unstructured mesh into a more regular and struc-
tured model in an automatic way is a challenging task and still open prob-
lem.
Since structured meshes are useful in different applications, it is of in-
terest to be able to guarantee such property also in scenarios of part based
modeling, which aim to build digital objects by composition, instead of
modeling them from a scratch.
This thesis will present methods for obtaining structured meshes in two
different ways. First is presented a coarse quad layout computation method
which starts from a triangle mesh and the curve-skeleton of the shape. The
second approach allows to build complex shapes by procedural composition
of PAM's. Since both quad layouts and PAMs exploit their global struc-
ture, similarities between the two will be discussed, especially how their
structure has correspondences to the curve-skeleton describing the topology
of the shape being represented. Since both the presented methods rely on
the information provided by the skeleton, the difficulties of using automat-
ically extracted curve-skeletons without processing are discussed, and an
interactive tool for user-assisted processing is presented
Structured meshes: composition and remeshing guided by the Curve-Skeleton
Virtual sculpting is currently a broadly used modeling metaphor with rising
popularity especially in the entertainment industry. While this approach
unleashes the artists' inspiration and creativity and leads to wonderfully
detailed and artistic 3D models, it has the side effect, purely technical,
of producing highly irregular meshes that are not optimal for subsequent
processing. Converting an unstructured mesh into a more regular and struc-
tured model in an automatic way is a challenging task and still open prob-
lem.
Since structured meshes are useful in different applications, it is of in-
terest to be able to guarantee such property also in scenarios of part based
modeling, which aim to build digital objects by composition, instead of
modeling them from a scratch.
This thesis will present methods for obtaining structured meshes in two
different ways. First is presented a coarse quad layout computation method
which starts from a triangle mesh and the curve-skeleton of the shape. The
second approach allows to build complex shapes by procedural composition
of PAM's. Since both quad layouts and PAMs exploit their global struc-
ture, similarities between the two will be discussed, especially how their
structure has correspondences to the curve-skeleton describing the topology
of the shape being represented. Since both the presented methods rely on
the information provided by the skeleton, the difficulties of using automat-
ically extracted curve-skeletons without processing are discussed, and an
interactive tool for user-assisted processing is presented
Cultural relationships in southern Ecuador 300 BC- AD 300: Excavations at the Guarumal and Punta Brava sites
Based upon an analysis of pottery excavated from the Guarumal and Punta Brava archaeological sites in south coastal Ecuador, the author seeks to place these sites into an overall cultural and chronological framework within the timescale 300 BC - AD 300, and to use some of the issues deriving from a discussion of the material and the occupation of the sites to assess the validity of the Jambeli culture, as defined by Estrada, Keggers and Evans (1964).
"All Jambeli Phase sites are shell middens..." (ibld: 486) is one of the assertions questioned in this thesis, together with the question of using white-on-red decorated pottery as a distinguishing feature of the Ecuadorian Regional Developmental Period. Some of the material described as being of the Jambeli culture is likely to have been misidentified and wrongly ascribed pottery deriving from late Formative period cultures in the area, of which the most important is the Guayaquil phase, from the Gulf of Guayaquil.
A clear sequence of development of pottery forms and styles can be demonstrated for the site Guarumal, from late Formative period Chorrera-like antecedents, exhibiting certain similarities with the Pechiche culture, through to those more typical of the Regional Developmental period - which is the Jambeli culture in this area - in the later phases of occupation.
Stylistic parallels with several contemporary cultures in southern
Ecuador (and parts of northern Peru) are also examined, for the
insights or challenges they offer to questions of cultural relationships and interactions over a wider geographical area.
It is increasingly clear that a re-evaluation of the Jambeli culture is necessary to take account of archaeological research of the last twenty years, research which has shown that the Jambeli culture was not merely a coastal adaptation of shell-fishing communities, but that it strecthed well into the interior and had Formative period roots
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