18,312 research outputs found
A charging model for three-axis stabilized spacecraft
A charging model was developed for geosynchronous, three-axis stabilized spacecraft when under the influence of a geomagnetic substorm. The differential charging potentials between the thermally coated or blanketed outer surfaces and metallic structure of a spacecraft were determined when the spacecraft was immersed in a dense plasma cloud of energetic particles. The spacecraft-to-environment interaction was determined by representing the charged particle environment by equivalent current source forcing functions and by representing the spacecraft by its electrically equivalent circuit with respect to the plasma charging phenomenon. The charging model included a sun/earth/spacecraft orbit model that simulated the sum illumination conditions of the spacecraft outer surfaces throughout the orbital flight on a diurnal as well as a seasonal basis. Transient and steady-state numerical results for a three-axis stabilized spacecraft are presented
Bounces/Dyons in the Plane Wave Matrix Model and SU(N) Yang-Mills Theory
We consider SU(N) Yang-Mills theory on the space R^1\times S^3 with Minkowski
signature (-+++). The condition of SO(4)-invariance imposed on gauge fields
yields a bosonic matrix model which is a consistent truncation of the plane
wave matrix model. For matrices parametrized by a scalar \phi, the Yang-Mills
equations are reduced to the equation of a particle moving in the double-well
potential. The classical solution is a bounce, i.e. a particle which begins at
the saddle point \phi=0 of the potential, bounces off the potential wall and
returns to \phi=0. The gauge field tensor components parametrized by \phi are
smooth and for finite time both electric and magnetic fields are nonvanishing.
The energy density of this non-Abelian dyon configuration does not depend on
coordinates of R^1\times S^3 and the total energy is proportional to the
inverse radius of S^3. We also describe similar bounce dyon solutions in SU(N)
Yang-Mills theory on the space R^1\times S^2 with signature (-++). Their energy
is proportional to the square of the inverse radius of S^2. From the viewpoint
of Yang-Mills theory on R^{1,1}\times S^2 these solutions describe non-Abelian
(dyonic) flux tubes extended along the x^3-axis.Comment: 11 pages; v2: one formula added, some coefficients correcte
Fast Matrix Factorization for Online Recommendation with Implicit Feedback
This paper contributes improvements on both the effectiveness and efficiency
of Matrix Factorization (MF) methods for implicit feedback. We highlight two
critical issues of existing works. First, due to the large space of unobserved
feedback, most existing works resort to assign a uniform weight to the missing
data to reduce computational complexity. However, such a uniform assumption is
invalid in real-world settings. Second, most methods are also designed in an
offline setting and fail to keep up with the dynamic nature of online data. We
address the above two issues in learning MF models from implicit feedback. We
first propose to weight the missing data based on item popularity, which is
more effective and flexible than the uniform-weight assumption. However, such a
non-uniform weighting poses efficiency challenge in learning the model. To
address this, we specifically design a new learning algorithm based on the
element-wise Alternating Least Squares (eALS) technique, for efficiently
optimizing a MF model with variably-weighted missing data. We exploit this
efficiency to then seamlessly devise an incremental update strategy that
instantly refreshes a MF model given new feedback. Through comprehensive
experiments on two public datasets in both offline and online protocols, we
show that our eALS method consistently outperforms state-of-the-art implicit MF
methods. Our implementation is available at
https://github.com/hexiangnan/sigir16-eals.Comment: 10 pages, 8 figure
Striped Magnetic Ground State of the Kagome Lattice in Fe4Si2Sn7O16
We have experimentally identified a new magnetic ground state for the kagome
lattice, in the perfectly hexagonal Fe2+ (3d6, S = 2) compound Fe4Si2Sn7O16.
Representational symmetry analysis of neutron diffraction data shows that below
T_N = 3.5 K, the spins on 2/3 of the magnetic ions order into canted
antiferromagnetic chains, separated by the remaining 1/3 which are
geometrically frustrated and show no long-range order down to at least T = 0.1
K. Moessbauer spectroscopy confirms that there is no static order on the latter
1/3 of the magnetic ions - i.e., they are in a liquid-like rather than a frozen
state - down to at least 1.65 K. A heavily Mn-doped sample
Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation
vector q = (0, 1/2 , 1/2 ) breaks hexagonal symmetry, we see no evidence for
magnetostriction in the form of a lattice distortion within the resolution of
our data. We discuss the relationship to partially frustrated magnetic order on
the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict
symmetry breaking ground states for perfect kagome lattices.Comment: 5 pages, 5 figure
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Multiplexed model predictive control of interconnected systems
A Multiplexed Model Predictive Control (MMPC) scheme with Quadratic Dissipativity Constraint (QDC) for interconnected systems is presented in this paper. A centralized MMPC is designed for the global system, wherein the controls of subsystems are updated sequentially to reduce the computational time. In MMPC, the global state vector of the interconnected system is required by the optimization. The QDC is converted into an enforced stability constraint for the MMPC as an alternative to the terminal constraint and terminal cost in this approach. The nominal recursive feasibility for the global system and the iterative feasibility for the local subsystems are obtained via set operations on the invariant sets. The admissible sets for the control inputs are obtained and employed in this approach for the QDC-based stability constraint. The set operations are speed up by multiple magnitudes thanks to the implementation of multiplexed inputs in MMPC. Numerical simulations with Automatic Generation Control (AGC) in power systems having tie-lines demonstrate the theoretical development.The authors acknowledge the support by the Singapore National Research Foundation (NRF) under its Campus for Research Excellence And Technological Enterprise (CREATE) programme and the Cambridge Centre for Advanced Research in Energy Efficiency in Singapore (Cambridge CARES), C4T project.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/CDC.2015.740256
A momentum-conserving, consistent, Volume-of-Fluid method for incompressible flow on staggered grids
The computation of flows with large density contrasts is notoriously
difficult. To alleviate the difficulty we consider a consistent mass and
momentum-conserving discretization of the Navier-Stokes equation.
Incompressible flow with capillary forces is modelled and the discretization is
performed on a staggered grid of Marker and Cell type. The Volume-of-Fluid
method is used to track the interface and a Height-Function method is used to
compute surface tension. The advection of the volume fraction is performed
using either the Lagrangian-Explicit / CIAM (Calcul d'Interface Affine par
Morceaux) method or the Weymouth and Yue (WY) Eulerian-Implicit method. The WY
method conserves fluid mass to machine accuracy provided incompressiblity is
satisfied which leads to a method that is both momentum and mass-conserving. To
improve the stability of these methods momentum fluxes are advected in a manner
"consistent" with the volume-fraction fluxes, that is a discontinuity of the
momentum is advected at the same speed as a discontinuity of the density. To
find the density on the staggered cells on which the velocity is centered, an
auxiliary reconstruction of the density is performed. The method is tested for
a droplet without surface tension in uniform flow, for a droplet suddenly
accelerated in a carrying gas at rest at very large density ratio without
viscosity or surface tension, for the Kelvin-Helmholtz instability, for a
falling raindrop and for an atomizing flow in air-water conditions
Instantons and the singlet-coupling in the chiral quark model
Chiral quark model with a broken-U(3) flavor symmetry can be interpreted as
the effective theory of the instanton-dominated non-perturbative QCD. This
naturally suggests the possibility of a negative singlet/octet coupling ratio,
which has been found, in a previous publication, to be compatible with the
phenomenological description of the nucleon spin-flavor structure.Comment: 9 page
Exploring metastability via the third harmonic measurements in single crystals of - showing anomalous Peak effect
We explore the metastability effects across the order-disorder transition
pertaining to the peak effect phenomenonon in critical current density ()
via the first and the third harmonic ac susceptibility measurements in the
weakly pinned single crystals of -. An analysis of our data
suggests that an imprint of the limiting (spinodal) temperature above which
is path independent can be conveniently located in the third harmonic
data ().Comment: Accepted for publication in Phys. Rev.
Experimental Falsification of Leggett's Non-Local Variable Model
Bell's theorem guarantees that no model based on local variables can
reproduce quantum correlations. Also some models based on non-local variables,
if subject to apparently "reasonable" constraints, may fail to reproduce
quantum physics. In this paper, we introduce a family of inequalities, which
allow testing Leggett's non-local model versus quantum physics, and which can
be tested in an experiment without additional assumptions. Our experimental
data falsify Leggett's model and are in agreement with quantum predictions.Comment: 5 pages, 3 figures, 1 tabl
Calculations of Resonance Coupling Constants in the Scalar Sector of the ENJL Model
We derive the scalar resonance coupling constants of resonance chiral theory
from the Extended Nambu Jona-Lasinio model by using heat-kernel expansion.Comment: 7 page
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