31,808 research outputs found
Domain wall space-times with a cosmological constant
We solve vacuum Einstein's field equations with the cosmological constant in
space-times admitting 3-parameter group of isometries with 2-dimensional
space-like orbits. The general exact solutions, which are represented in the
advanced and retarded null coordinates, have two arbitrary functions due to the
freedom of choosing null coordinates. In the thin-wall approximation, the
Israel's junction conditions yield one constraint equation on these two
functions in spherical, planar, and hyperbolic domain wall space-times with
reflection symmetry. The remain freedom of choosing coordinates are completely
fixed by requiring that when surface energy density of domain walls
vanishes, the metric solutions will return to some well-known solutions. It
leads us to find a planar domain wall solution, which is conformally flat, in
the de Sitter universe.Comment: 9 pages. no figur
Dilute magnetic semiconductor and half metal behaviors in 3d transition-metal doped black and blue phosphorenes: a first-principles study
We present first-principles density-functional calculations for the
structural, electronic, and magnetic properties of substitutional 3d transition
metal (TM) impurities in two-dimensional black and blue phosphorenes. We find
that the magnetic properties of such substitutional impurities can be
understood in terms of a simple model based on the Hund's rule. The TM-doped
black phosphorenes with Ti, V, Cr, Mn, Fe and Ni impurities show dilute
magnetic semiconductor (DMS) properties while those with Sc and Co impurities
show nonmagnetic properties. On the other hand, the TM-doped blue phosphorenes
with V, Cr, Mn and Fe impurities show DMS properties, those with Ti and Ni
impurities show half-metal properties, whereas Sc and Co doped systems show
nonmagnetic properties. We identify two different regimes depending on the
occupation of the hybridized electronic states of TM and phosphorous atoms: (i)
bonding states are completely empty or filled for Sc- and Co-doped black and
blue phosphorenes, leading to non-magnetic; (ii) non-bonding d states are
partially occupied for Ti-, V-, Cr-, Mn-, Fe- and Ni-doped black and blue
phosphorenes, giving rise to large and localized spin moments. These results
provide a new route for the potential applications of dilute magnetic
semiconductor and half-metal in spintronic devices by employing black and blue
phosphorenes.Comment: 9 pages, 7 figure
A Unified Approximation Framework for Compressing and Accelerating Deep Neural Networks
Deep neural networks (DNNs) have achieved significant success in a variety of
real world applications, i.e., image classification. However, tons of
parameters in the networks restrict the efficiency of neural networks due to
the large model size and the intensive computation. To address this issue,
various approximation techniques have been investigated, which seek for a light
weighted network with little performance degradation in exchange of smaller
model size or faster inference. Both low-rankness and sparsity are appealing
properties for the network approximation. In this paper we propose a unified
framework to compress the convolutional neural networks (CNNs) by combining
these two properties, while taking the nonlinear activation into consideration.
Each layer in the network is approximated by the sum of a structured sparse
component and a low-rank component, which is formulated as an optimization
problem. Then, an extended version of alternating direction method of
multipliers (ADMM) with guaranteed convergence is presented to solve the
relaxed optimization problem. Experiments are carried out on VGG-16, AlexNet
and GoogLeNet with large image classification datasets. The results outperform
previous work in terms of accuracy degradation, compression rate and speedup
ratio. The proposed method is able to remarkably compress the model (with up to
4.9x reduction of parameters) at a cost of little loss or without loss on
accuracy.Comment: 8 pages, 5 figures, 6 table
The Transformation of Trust in Chinaâs Alternative Food Networks: Disruption, Reconstruction, and Development
Food safety issues in China have received much scholarly attention, yet few studies systematically examined this matter through the lens of trust. More importantly, little is known about the transformation of different types of trust in the dynamic process of food production, provision, and consumption. We consider trust as an evolving interdependent relationship between different actors. We used the Beijing County Fair, a prominent ecological farmersâ market in China, as an example to examine the transformation of trust in Chinaâs alternative food networks. We argue that although there has been a disruption of institutional trust among the general public since 2008 when the melamine-tainted milk scandal broke out, reconstruction of individual trust and development of organizational trust have been observed, along with the emergence and increasing popularity of alternative food networks. Based on more than six months of fieldwork on the emerging ecological agriculture sector in 13 provinces across China as well as monitoring of online discussions and posts, we analyze how various social factorsâincluding but not limited to direct and indirect reciprocity, information, endogenous institutions, and altruismâhave simultaneously contributed to the transformation of trust in Chinaâs alternative food networks. The findings not only complement current social theories of trust, but also highlight an important yet understudied phenomenon whereby informal social mechanisms have been partially substituting for formal institutions and gradually have been building trust against the backdrop of the food safety crisis in China
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Computational and experimental study of air hybrid engine concepts
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel UniversityThe air hybrid engine absorbs the vehicle kinetic energy during braking, stores it in an air tank in the form of compressed air, and reuses it to start the engine and to propel a vehicle during cruising and acceleration. Capturing, storing and reusing this braking energy to achieve stop-start operation and to give additional power can therefore improve fuel economy, particularly in cities and urban areas where the traffic conditions involve many stops and starts. In order to reuse the residual kinetic energy, the vehicle operation consists of 3 basic modes, i.e. Compression Mode (CM), Expander Mode (EM) and normal firing mode, as well as stop-start operation through an air starter. A four-cylinder 2 litre diesel engine has been modelled to operate in four air hybrid engine configurations so that the braking and motoring performance of each configuration could be studied. These air hybrid systems can be constructed with production technologies and incur minimum changes to the existing engine design. The regenerative engine braking and starting capability is realised through the employment of an innovative simple one-way intake system and a production cam profile switching (CPS) mechanism. The hybrid systems will allow the engine to be cranked by the compressed air at moderate pressure without using addition starters or dedicated valves in the cylinder head. Therefore, the
proposed air hybrid engine systems can be considered as a cost-effective regenerative hybrid powertrain and can be implemented in vehicles using existing production technologies. A novel cost-effective pneumatic regenerative stop-start hybrid system, Regenerative Engine Braking Device (RegenEBD), for buses and commercial vehicles is presented. RegenEBD is capable of converting kinetic energy into pneumatic energy in the compressed air saved in an air tank using a production engine braking device and other production type automotive components and a proprietary intake system design. The compressed air is then used to drive an air starter to achieve regenerative stop-start operations. The proposed hybrid system can work with the existing vehicle transmission system and can be implemented with the retro-fitted valve actuation device and a sandwich block mounted between the cylinder head and the production intake manifold. Compression mode operation is achieved by keeping the intake valves from fully closed throughout the four-strokes through a production type variable valve exhaust brake (VVEB) device on the intake valves. As a result, the induced air could be compressed through the opening gap of intake valves into the air tank through the intake system of proprietary design. The compressed air can then be used to crank the engine directly through the air expander operation or indirectly through the action of an air starter in production. A single cylinder camless engine has been set up and operated to evaluate the compression mode performance of two air hybrid concepts. The experimental results are then compared with the computational output with excellent agreement. In order to evaluate the potential of the air hybrid engine technologies, a new vehicle driving cycle simulation program has been developed using Matlab Simulink. An air hybrid engine sub-model and methodology for modelling the air hybrid engineâs performance have been proposed and implemented in the vehicle driving cycle simulation. The NEDC analysis of a Ford Mondeo vehicle shows that the vehicle can achieve regenerative stop-start operations throughout the driving cycle when it is powered by a 2.0litre diesel engine with air hybrid operation using a 40litre air tank of less than 10bar pressure. The regenerative stop-start operation can lead to 4.5% fuel saving during the NEDC. Finally, the Millbrook London Transport Bus (MLTB) driving cycle has been used to analyse the effectiveness of RegenEBD on a double deck bus powered by a Yuchai diesel engine. The results show that 90% stop-starts during the MLTB can be accomplished by RegenEBD and that a significant fuel saving of 6.5% can be obtained from the regenerative stop-start operations
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