235,664 research outputs found
Towards the Standard Model spectrum from elliptic Calabi-Yau
We show that it is possible to construct supersymmetric three-generation
models of Standard Model gauge group in the framework of non-simply-connected
elliptically fibered Calabi-Yau, without section but with a bi-section. The
fibrations on a cover Calabi-Yau, where the model has 6 generations of SU(5)
and the bundle is given via the spectral cover description, use a different
description of the elliptic fibre which leads to more than one global section.
We present two examples of a possible cover Calabi-Yau with a free involution:
one is a fibre product of rational elliptic surfaces ; another example is
an elliptic fibration over a Hirzebruch surface. There we give the necessary
amount of chiral matter by turning on in the bundles a further parameter,
related to singularities of the fibration and the branching of the spectral
cover.Comment: 31 pages, 1 eps-figure, reference added, bundle parameters for dP9
model change
A web-based teaching/learning environment to support collaborative knowledge construction in design
A web-based application has been developed as part of a recently completed research which proposed a conceptual framework to collect, analyze and compare different design experiences and to construct structured representations of the emerging knowledge in digital architectural design. The paper introduces the theoretical and practical development of this application as a teaching/learning environment which has significantly contributed to the development and testing of the ideas developed throughout the research. Later in the paper, the application of BLIP in two experimental (design) workshops is reported and evaluated according to the extent to which the application facilitates generation, modification and utilization of design knowledge
Best-First Surface Realization
Current work in surface realization concentrates on the use of general,
abstract algorithms that interpret large, reversible grammars. Only little
attention has been paid so far to the many small and simple applications that
require coverage of a small sublanguage at different degrees of sophistication.
The system TG/2 described in this paper can be smoothly integrated with deep
generation processes, it integrates canned text, templates, and context-free
rules into a single formalism, it allows for both textual and tabular output,
and it can be parameterized according to linguistic preferences. These features
are based on suitably restricted production system techniques and on a generic
backtracking regime.Comment: 10 pages, LaTeX source, one EPS figur
Relation between the grafting density of liquid crystal macromolecule and the symmetry of self-assembled bulk phase: coarse-grained molecular dynamics study
I consider a generic coarse-grained model suitable for the study of bulk
self-assembly of liquid crystal (LC) macromolecules. The cases include LC
dendrimers, gold nanoparticles modified by polymer chains with terminating LC
groups and others. The study is focused on the relation between a number of
grafted chains, , and the symmetry of the self-assembled bulk
phases. Simple space-filling arguments are used first to estimate stability
intervals for a rod-like, disc-like and spherulitic conformations in terms of
. These are followed by coarse-grained molecular dynamics
simulations for both spontaneous and aided self-assembly of LC macromolecules
into bulk phases. In spontaneous self-assembly runs, essential coexistence of
rod-like and disc-like conformations is observed (via analysis of the
histograms for the molecular asphericity) in a broad interval of ,
which prevents the formation of defect-free structures. The use of uniaxial and
planar aiding fields is found to improve self-assembly into monodomain phases
by promoting conformations of respective symmetry. Strong shape-phase relation,
observed experimentally, is also indicated by the simulations by the
coincidence of the stability intervals for the respective conformations with
those for the bulk phases.Comment: 12 pages, 8 figure
Linear Solar Models
We present a new approach to study the properties of the sun. We consider
small variations of the physical and chemical properties of the sun with
respect to Standard Solar Model predictions and we linearize the structure
equations to relate them to the properties of the solar plasma. By assuming
that the (variation of) the present solar composition can be estimated from the
(variation of) the nuclear reaction rates and elemental diffusion efficiency in
the present sun, we obtain a linear system of ordinary differential equations
which can be used to calculate the response of the sun to an arbitrary
modification of the input parameters (opacity, cross sections, etc.). This new
approach is intended to be a complement to the traditional methods for solar
model calculation and allows to investigate in a more efficient and transparent
way the role of parameters and assumptions in solar model construction. We
verify that these Linear Solar Models recover the predictions of the
traditional solar models with an high level of accuracy.Comment: 29 pages, 10 figure
Efficient prediction of broadband trailing edge noise and application to porous edge treatment
Trailing edge noise generated by turbulent flow traveling past an edge of an
airfoil is one of the most essential aeroacoustic sound generation mechanisms.
It is of great interest for noise problems in various areas of industrial
application. First principle based CAA with short response time are needed in
the industrial design process for reliable prediction of spectral differences
in turbulent-boundary-layer trailing-edge noise due to design modifications. In
this paper, an aeroacoustic method is studied, resting on a hybrid CFD/CAA
procedure. In a first step RANS simulation provides a time-averaged solution,
including the mean-flow and turbulence statistics such as length-scale,
time-scale and turbulence kinetic energy. Based on these, fluctuating sound
sources are then stochastically generated by the Fast Random Particle-Mesh
Method to simulate in a second CAA step broadband aeroacoustic sound. From
experimental findings it is well known that porous trailing edges significantly
lower trailing edge noise level over a large range of frequencies reaching up
to 8dB reduction. Furthermore, sound reduction depends on the porous material
parameters, e.g. geometry, porosity, permeability and pore size. The paper
presents first results for an extended hybrid CFD/CAA method including porous
materials with prescribed parameters. To incorporate the effect of porosity, an
extended formulation of the Acoustic Perturbation Equations with source terms
is derived based on a reformulation of the volume averaged Navier-Stokes
equations into perturbation form. Proper implementation of the Darcy and
Forchheimer terms is verified for sound propagation in homogeneous and
anisotropic porous medium. Sound generation is studied for a generic symmetric
NACA0012 airfoil without lift to separate secondary effects of lift and camber
on sound from those of the basic edge noise treatments.Comment: 37 page
Dynamic whole-body motion generation under rigid contacts and other unilateral constraints
The most widely used technique for generating wholebody motions on a humanoid robot accounting for various tasks and constraints is inverse kinematics. Based on the task-function approach, this class of methods enables the coordination of robot movements to execute several tasks in parallel and account for the sensor feedback in real time, thanks to the low computation cost.
To some extent, it also enables us to deal with some of the robot constraints (e.g., joint limits or visibility) and manage the quasi-static balance of the robot. In order to fully use the whole range of possible motions, this paper proposes extending the task-function approach to handle the full dynamics of the robot multibody along with any constraint written as equality or inequality of the state and control variables. The definition of multiple objectives is made possible by ordering them inside a strict hierarchy. Several models of contact with the environment can be implemented in the framework. We propose a reduced formulation of the multiple rigid planar contact that keeps a low computation cost. The efficiency of this approach is illustrated by presenting several multicontact dynamic motions in simulation and on the real HRP-2 robot
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