82,869 research outputs found
Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America
Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling
A Phone Learning Model for Enhancing Productivity of Visually Impaired Civil Servants
Phone-based learning in civil service is the use of voice technologies to deliver learning and capacity building training services to
government employees. The Internet revolution and advancement in Information and Communications Technology (ICT) have given rise
to online and remote staff training for the purpose of enhancing workers productivity. The need for civil servants in Nigeria to develop
capacity that will enhance knowledge is a key requirement to having competitive advantage in the work place. Existing online learning
platforms (such as web-based learning, mobile learning, etc) did not consider the plight of the visually impaired. These platforms provide
graphical interfaces that require sight to access. The visually impaired civil servants require auditory access to functionalities that exist in
learning management system on the Internet. Thus a gap exist between the able-bodied and visually impaired civil servants on
accessibility to e-learning platform. The objective of this paper is to provide a personalized telephone learning model and a prototype
application that will enhance the productivity of the visually impaired workers in Government establishments in Nigeria. The model was
designed using Unified Modeling Language (UML) diagram. The prototype application was implemented and evaluated. With the
proposed model and application, the visually and mobility impaired worker are able to participate in routine staff training and
consequently enhances their productivity just like their able-bodied counterparts. The prototype application also serves as an alternative
training platform for the able-bodied workers. Future research direction for this study will include biometric authentication of learners
accessing the applicatio
New Formulation and Strong MISOCP Relaxations for AC Optimal Transmission Switching Problem
As the modern transmission control and relay technologies evolve,
transmission line switching has become an important option in power system
operators' toolkits to reduce operational cost and improve system reliability.
Most recent research has relied on the DC approximation of the power flow model
in the optimal transmission switching problem. However, it is known that DC
approximation may lead to inaccurate flow solutions and also overlook stability
issues. In this paper, we focus on the optimal transmission switching problem
with the full AC power flow model, abbreviated as AC OTS. We propose a new
exact formulation for AC OTS and its mixed-integer second-order conic
programming (MISOCP) relaxation. We improve this relaxation via several types
of strong valid inequalities inspired by the recent development for the closely
related AC Optimal Power Flow (AC OPF) problem. We also propose a practical
algorithm to obtain high quality feasible solutions for the AC OTS problem.
Extensive computational experiments show that the proposed formulation and
algorithms efficiently solve IEEE standard and congested instances and lead to
significant cost benefits with provably tight bounds
Localization of Macroscopic Object Induced by the Factorization of Internal Adiabatic Motion
To account for the phenomenon of quantum decoherence of a macroscopic object,
such as the localization and disappearance of interference, we invoke the
adiabatic quantum entanglement between its collective states(such as that of
the center-of-mass (C.M)) and its inner states based on our recent
investigation. Under the adiabatic limit that motion of C.M dose not excite the
transition of inner states, it is shown that the wave function of the
macroscopic object can be written as an entangled state with correlation
between adiabatic inner states and quasi-classical motion configurations of the
C.M. Since the adiabatic inner states are factorized with respect to each parts
composing the macroscopic object, this adiabatic separation can induce the
quantum decoherence. This observation thus provides us with a possible solution
to the Schroedinger cat paradoxComment: Revtex4,23 pages,1figur
Matrix Minor Reformulation and SOCP-based Spatial Branch-and-Cut Method for the AC Optimal Power Flow Problem
Alternating current optimal power flow (AC OPF) is one of the most
fundamental optimization problems in electrical power systems. It can be
formulated as a semidefinite program (SDP) with rank constraints. Solving AC
OPF, that is, obtaining near optimal primal solutions as well as high quality
dual bounds for this non-convex program, presents a major computational
challenge to today's power industry for the real-time operation of large-scale
power grids. In this paper, we propose a new technique for reformulation of the
rank constraints using both principal and non-principal 2-by-2 minors of the
involved Hermitian matrix variable and characterize all such minors into three
types. We show the equivalence of these minor constraints to the physical
constraints of voltage angle differences summing to zero over three- and
four-cycles in the power network. We study second-order conic programming
(SOCP) relaxations of this minor reformulation and propose strong cutting
planes, convex envelopes, and bound tightening techniques to strengthen the
resulting SOCP relaxations. We then propose an SOCP-based spatial
branch-and-cut method to obtain the global optimum of AC OPF. Extensive
computational experiments show that the proposed algorithm significantly
outperforms the state-of-the-art SDP-based OPF solver and on a simple personal
computer is able to obtain on average a 0.71% optimality gap in no more than
720 seconds for the most challenging power system instances in the literature
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A Preliminary Study on Using Multi-Nozzle Polymer Deposition System to Fabricate Composite Alginate/Carbon Nanotube Tissue Scaffolds
Three-dimensional composite alginate/single wall carbon nanotube (SWCNT) scaffolds
encapsulated with endothelial cells were fabricated by a multi-nozzle biopolymer freeform
deposition system. This system enables the converting of CAD designed scaffold pattern into
process toolpaths and the use of computer control program to guide the nozzle deposition at
spatial position for layered fabrication of 3D tissue scaffolds. The morphological, mechanical,
structural and biological properties of as-fabricated scaffolds were characterized by optical
microscope, SEM, Microtensile testing machine, Alamar Blue Assay, and Live-Dead Assay,
respectively. The multi-nozzle deposition system demonstrated a highly efficient and effective
process to build tissue scaffold or cell embedded constructs. Characterization results showed that
the incorporation of SWCNT into alginate not only enhanced the mechanical strength of the
scaffolds but also improved the cell affinity and the interaction with substrate. Further cell
culture experimental results also showed that the incorporation of SWCNT in alginate enhanced
endothelial cell proliferation compared with pure alginate scaffold.Mechanical Engineerin
Spin relaxation and decoherence of two-level systems
We revisit the concepts of spin relaxation and spin decoherence of two level
(spin-1/2) systems. From two toy-models, we clarify two issues related to the
spin relaxation and decoherence: 1) For an ensemble of two-level particles each
subjected to a different environmental field, there exists an ensemble
relaxation time which is fundamentally different from . When the
off-diagonal coupling of each particle is in a single mode with the same
frequency but a random coupling strength, we show that is finite while
the spin relaxation time of a single spin and the usual ensemble
decoherence time are infinite. 2) For a two-level particle under only a
random diagonal coupling, its relaxation time shall be infinite but its
decoherence time is finite.Comment: 5 pages, 2 figure
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