84,918 research outputs found
Isotropic subbundles of
We define integrable, big-isotropic structures on a manifold as
subbundles that are isotropic with respect to the
natural, neutral metric (pairing) of and are closed by
Courant brackets (this also implies that ). We give the interpretation of such a structure by objects of
, we discuss the local geometry of the structure and we give a reduction
theorem.Comment: LaTex, 37 pages, minimization of the defining condition
On the geometry of double field theory
Double field theory was developed by theoretical physicists as a way to
encompass -duality. In this paper, we express the basic notions of the
theory in differential-geometric invariant terms, in the framework of
para-Kaehler manifolds. We define metric algebroids, which are vector bundles
with a bracket of cross sections that has the same metric compatibility
property as a Courant bracket. We show that a double field gives rise to two
canonical connections, whose scalar curvatures can be integrated to obtain
actions. Finally, in analogy with Dirac structures, we define and study
para-Dirac structures on double manifolds.Comment: The paper will appear in J. Math. Phys., 201
Comparison of gluon flux-tube distributions for quark-diquark and quark-antiquark hadrons
The distribution of gluon fields in hadrons is of fundamental interest in
QCD. Using lattice QCD we have observed the formation of gluon flux tubes
within 3 quark (baryon) and quark plus antiquark (meson) systems for a wide
variety of spatial distributions of the color sources. In particular we have
investigated three quark configurations where two of the quarks are close
together and the third quark is some distance away, which approximates a quark
plus diquark string. We find that the string tension of the quark-diquark
string is the same as that of the quark-antiquark string on the same lattice.
We also compare the longitudinal and transverse profiles of the gluon flux
tubes for both sets of strings, and find them to be of similar radii and to
have similar vacuum suppression.Comment: 6 pages, 9 figures and 1 tabl
Weak-Hamiltonian dynamical systems
A big-isotropic structure is an isotropic subbundle of ,
endowed with the metric defined by pairing. The structure is said to be
integrable if the Courant bracket ,
. Then, necessarily, one also has
, \cite{V-iso}. A weak-Hamiltonian dynamical system is a vector field
such that . We obtain the
explicit expression of and of the integrability conditions of under
the regularity condition We show that the
port-controlled, Hamiltonian systems (in particular, constrained mechanics)
\cite{{BR},{DS}} may be interpreted as weak-Hamiltonian systems. Finally, we
give reduction theorems for weak-Hamiltonian systems and a corresponding
corollary for constrained mechanical systems.Comment: 19 pages, minor improvement
Mechanical and electrochemical properties of multiple-layer diode laser cladding of 316L stainless steel
In the present investigation, a detailed mechanical and electrochemical properties of multiple-layer laser clad 316L stainless steel (from the powders produced by gas atomized route) has been carried out. Multiple-layer laser cladding of 316L stainless steel has been conducted using a diode laser. The mechanical property (rmcrohardness) of the fabricated product has been evaluated using a microhardness testing machine and correlated with the process parameters. The electrochemical property, mainly pitting corrosion resistance of the fabricated layer corresponding to maximum microhardness (in a 3.56% NaCl solution) has been evaluated using standard potentiodynamic polarization testing. The microhardness of the laser assisted fabricated layers was found to vary from 170 to 278 VHN, increased with decrease in applied power density and increase in scan speed and was higher than that of conventionally processed 316L (155 VHN). The superior microhardness value is attributed to grain refinement associated with laser melting and rapid solidification. The critical potential to pit formation (E-PP1) was measured to be 550 mV saturated calomel electrode (SCE) and superior to the conventionally processed 316L stainless steel (445 mV (SCE)). (c) 2005 Elsevier B.V. All rights reserved
Unsupervised Feature Selection with Adaptive Structure Learning
The problem of feature selection has raised considerable interests in the
past decade. Traditional unsupervised methods select the features which can
faithfully preserve the intrinsic structures of data, where the intrinsic
structures are estimated using all the input features of data. However, the
estimated intrinsic structures are unreliable/inaccurate when the redundant and
noisy features are not removed. Therefore, we face a dilemma here: one need the
true structures of data to identify the informative features, and one need the
informative features to accurately estimate the true structures of data. To
address this, we propose a unified learning framework which performs structure
learning and feature selection simultaneously. The structures are adaptively
learned from the results of feature selection, and the informative features are
reselected to preserve the refined structures of data. By leveraging the
interactions between these two essential tasks, we are able to capture accurate
structures and select more informative features. Experimental results on many
benchmark data sets demonstrate that the proposed method outperforms many state
of the art unsupervised feature selection methods
Synchronising Clock and Carrier Frequencies with Low and Coherent Phase Noise for 6G
Using radio over fiber techniques, we generate, transmit and receive a highly accurate 5-GHz spaced frequency comb, with carriers of extremely low phase noise. The carriers are used in an over the air transmission experiment, demonstrating lower phase noise than a commercially available PLL. The low phase noise and scalability of the design offer a new method of carrier and low-jitter clock distribution satisfying the stringent demands of future 6G systems. We demonstrate the efficacy of the method by over the air transmission of 128-QAM signal at 25 GHz. The recovered signal shows the advantage of low phase noise carriers even at received powers approaching -50 dB
Optimising the process parameters of selective laser melting for the fabrication of Ti6Al4V alloy
This is the author accepted manuscript. The final version is available from Emerald via the DOI in this recordPurpose- Surface roughness is an important evaluation index for industrial components and it strongly depends on the processing parameters for selective laser molten Ti6Al4V parts. This paper aims to obtain an optimum SLM parameter set to improve the surface roughness of Ti6Al4V samples.
Design/methodology/approach- A response surface methodology (RSM) based approach is proposed to improve the surface quality of selective laser molten Ti6Al4V parts and understand the relationship between the selective laser melting (SLM) process parameters and the surface roughness. The main SLM parameters (i.e. laser power, scan speed and hatch spacing) are optimised and Ti6Al4V parts are manufactured by the SLM technology with no post processes.
Findings- Optimum process parameters were obtained using the RSM method to minimise the roughness of the top and vertical side surfaces. Obtained parameter sets were evaluated based on their productivity and surface quality performance. The validation tests have been performed and the results verified the effectivity of the proposed technique. It was also shown that the top and vertical sides must be handled together to obtain better top surface quality.
Practical implications- The obtained optimum SLM parameter set can be used in the manufacturing of Ti6Al4V components with high surface roughness requirement.
Originality/value- RSM is used to analyse and determine the optimal combination of SLM parameters with the aim of improving the surface roughness quality of Ti6Al4V components, for the first time in the literature. Also, this is the first study which aims to simultaneously optimise the surface quality of top and vertical sides of titanium alloys.This research was supported by the National High Technology Research and Development Program of
China (863 Program: 2015AA042501)
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Melt conditioning by advanced shear technology (MCAST) for refining solidification microstructures
MCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently by BCAST at Brunel University for conditioning liquid metal prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence to the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys
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