439 research outputs found
A Logarithmic Conformal Field Theory Solution For Two Dimensional Magnetohydrodynamics In Presence of The Alf'ven Effect
When Alf`ven effect is peresent in magnetohydrodynamics one is naturally lead
to consider conformal field theories, which have logarithmic terms in their
correlation functions. We discuss the implications of such logarithmic terms
and find a unique conformal field theory with centeral charge
, within the border of the minimal series, which satisfies
all the constraints. The energy espectrum is found to be \newline .Comment: Latex, 9 page
Explicit force control V.S. impedance control for micromanipulation.
International audienceThis paper presents a study of different force control schemes for controlling contact during manipulation tasks at the microscale. Explicit force control and impedance control are compared in a contact transition scenario consisting of a compliant microforce sensor mounted on a microrobotic positioner, and a compliant microstructure fabricated using Silicon MEMS. A traditional double mass-spring-damper model of the overall robot is employed to develop the closed-loop force controllers. Specific differences between the two control schemes due to the microscale nature of contact are highlighted in this paper from the experimental results obtained. The limitations and tradeoffs of the two control laws at the microscale due to the presence of backlash are discussed. A simple method to deal with the pull-off force effects specific to the microscale is proposed. Future improvements of the impedance control schemes to include adaptation are discussed in order to handle objects with unknown stiffness
Subcritical Superstrings
We introduce the Liouville mode into the Green-Schwarz superstring. Like
massive supersymmetry without central charges, there is no kappa symmetry.
However, the second-class constraints (and corresponding Wess-Zumino term)
remain, and can be solved by (twisted) chiral superspace in dimensions D=4 and
6. The matter conformal anomaly is c = 4-D < 1. It thus can be canceled for
physical dimensions by the usual Liouville methods, unlike the bosonic string
(for which the consistency condition is c = D <= 1).Comment: 9 pg., compressed postscript file (.ps.Z), other formats (.dvi, .ps,
.ps.Z, 8-bit .tex) available at
http://insti.physics.sunysb.edu/~siegel/preprints/ or at
ftp://max.physics.sunysb.edu/preprints/siege
Ultrawide phononic band gap for combined in-plane and out-of-plane waves
We consider two-dimensional phononic crystals formed from silicon and voids,
and present optimized unit cell designs for (1) out-of-plane, (2) in-plane and
(3) combined out-of-plane and in-plane elastic wave propagation. To feasibly
search through an excessively large design space (10e40 possible realizations)
we develop a specialized genetic algorithm and utilize it in conjunction with
the reduced Bloch mode expansion method for fast band structure calculations.
Focusing on high symmetry plain-strain square lattices, we report unit cell
designs exhibiting record values of normalized band-gap size for all three
categories. For the combined polarizations case, we reveal a design with a
normalized band-gap size exceeding 60%.Comment: 4 pages, 1 figure, submitted for journal publicatio
Blended Multi-Modal Deep ConvNet Features for Diabetic Retinopathy Severity Prediction
Diabetic Retinopathy (DR) is one of the major causes of visual impairment and
blindness across the world. It is usually found in patients who suffer from
diabetes for a long period. The major focus of this work is to derive optimal
representation of retinal images that further helps to improve the performance
of DR recognition models. To extract optimal representation, features extracted
from multiple pre-trained ConvNet models are blended using proposed multi-modal
fusion module. These final representations are used to train a Deep Neural
Network (DNN) used for DR identification and severity level prediction. As each
ConvNet extracts different features, fusing them using 1D pooling and cross
pooling leads to better representation than using features extracted from a
single ConvNet. Experimental studies on benchmark Kaggle APTOS 2019 contest
dataset reveals that the model trained on proposed blended feature
representations is superior to the existing methods. In addition, we notice
that cross average pooling based fusion of features from Xception and VGG16 is
the most appropriate for DR recognition. With the proposed model, we achieve an
accuracy of 97.41%, and a kappa statistic of 94.82 for DR identification and an
accuracy of 81.7% and a kappa statistic of 71.1% for severity level prediction.
Another interesting observation is that DNN with dropout at input layer
converges more quickly when trained using blended features, compared to the
same model trained using uni-modal deep features.Comment: 18 pages, 8 figures, published in Electronics MDPI journa
Bio-inspired non self-similar hierarchical elastic metamaterials
Hierarchy provides unique opportunities for the design of advanced materials with superior properties that arise
from architecture, rather than from constitutive material response. Contrary to the quasi-static regime, where
the potential of hierarchy has been largely explored, its role in vibration mitigation and wave manipulation
remains elusive.
So far, the majority of the studies concerning hierarchical elastic metamaterials have proposed a selfsimilar
repetition of a specific unit cell at multiple scale levels, leading to the activation of the same bandgap
mechanism at different frequencies. On the contrary, here, we show that by designing non self-similar
hierarchical geometries allows us to create periodic structures supporting multiple, highly attenuative and
broadband bandgaps involving (independently or simultaneously) different scattering mechanisms, namely,
Bragg scattering, local resonance and/or inertial amplification, at different frequencies. The type of band
gap mechanism is identified and discussed by examining the vibrational mode shapes and the imaginary
component of the wavenumber in the dispersion diagram of the unit cell. We also experimentally confirm
this by performing measurements in the lowest frequency regime on a 3D printed structure.
Hierarchical design strategies may find application in vibration mitigation for civil, aerospace and
mechanical engineering
The No-Hair Conjecture in 2D Dilaton Supergravity
We study two dimensional dilaton gravity and supergravity following
hamiltonian methods. Firstly, we consider the structure of constraints of 2D
dilaton gravity and then the 2D dilaton supergravity is obtained taking the
squere root of the bosonic constraints. We integrate exactly the equations of
motion in both cases and we show that the solutions of the equation of motion
of 2D dilaton supergravity differs from the solutions of 2D dilaton gravity
only by boundary conditions on the fermionic variables, i.e. the black holes of
2D dilaton supergravity theory are exactly the same black holes of 2D bosonic
dilaton gravity modulo supersymmetry transformations. This result is the
bidimensional analogue of the no-hair theorem for supergravity.Comment: Plain Tex, 19pp, IPNO-TH 93/2
Integrable models and degenerate horizons in two-dimensional gravity
We analyse an integrable model of two-dimensional gravity which can be
reduced to a pair of Liouville fields in conformal gauge. Its general solution
represents a pair of ``mirror'' black holes with the same temperature. The
ground state is a degenerate constant dilaton configuration similar to the
Nariai solution of the Schwarzschild-de Sitter case. The existence of
solutions and their relation with the solution given by the 2D
Birkhoff's theorem is then investigated in a more general context. We also
point out some interesting features of the semiclassical theory of our model
and the similarity with the behaviour of AdS black holes.Comment: Latex, 16 pages, 1 figur
Non-Commutative Gauge Theories and the Cosmological Constant
We discuss the issue of the cosmological constant in non-commutative
non-supersymmetric gauge theories. In particular, in orbifold field theories
non-commutativity acts as a UV cut-off. We suggest that in these theories
quantum corrections give rise to a vacuum energy \rho, that is controlled by
the non-commutativity parameter \theta, \rho ~ 1/theta^2 (only a soft
logarithmic dependence on the Planck scale survives). We demonstrate our claim
in a two-loop computation in field theory and by certain higher loop examples.
Based on general expressions from string theory, we suggest that the vacuum
energy is controlled by non-commutativity to all orders in perturbation theory.Comment: 11 pages, RevTex. 4 eps figures. v2: Typos corrected. To appear in
Phys.Rev.
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