11,509 research outputs found
Extraction of coherent structures in a rotating turbulent flow experiment
The discrete wavelet packet transform (DWPT) and discrete wavelet transform
(DWT) are used to extract and study the dynamics of coherent structures in a
turbulent rotating fluid. Three-dimensional (3D) turbulence is generated by
strong pumping through tubes at the bottom of a rotating tank (48.4 cm high,
39.4 cm diameter). This flow evolves toward two-dimensional (2D) turbulence
with increasing height in the tank. Particle Image Velocimetry (PIV)
measurements on the quasi-2D flow reveal many long-lived coherent vortices with
a wide range of sizes. The vorticity fields exhibit vortex birth, merger,
scattering, and destruction. We separate the flow into a low-entropy
``coherent'' and a high-entropy ``incoherent'' component by thresholding the
coefficients of the DWPT and DWT of the vorticity fields. Similar thresholdings
using the Fourier transform and JPEG compression together with the Okubo-Weiss
criterion are also tested for comparison. We find that the DWPT and DWT yield
similar results and are much more efficient at representing the total flow than
a Fourier-based method. Only about 3% of the large-amplitude coefficients of
the DWPT and DWT are necessary to represent the coherent component and preserve
the vorticity probability density function, transport properties, and spatial
and temporal correlations. The remaining small amplitude coefficients represent
the incoherent component, which has near Gaussian vorticity PDF, contains no
coherent structures, rapidly loses correlation in time, and does not contribute
significantly to the transport properties of the flow. This suggests that one
can describe and simulate such turbulent flow using a relatively small number
of wavelet or wavelet packet modes.Comment: experimental work aprox 17 pages, 11 figures, accepted to appear in
PRE, last few figures appear at the end. clarifications, added references,
fixed typo
Village Baseline Study: Site Analysis Report for Rupandehi, Madhuri Village, Nepal
This is the report of the village baseline study of Madhuri Village in the CCAFS benchmark site of Rupandehi, Nepal conducted from June 5-9, 2011 to complement an earlier household baseline survey done in the same village.
Madhuri is located in the fertile area of the Indo-Gangetic Plain in Nepal yet faces challenges due to increasing populations, encroachment on forests, decreasing soil fertility, limited agriculture and animal productivity, lack of opportunities, and variable climatic conditions. Its circumstances present manageable opportunities to prevent an increase in food insecurity and further degradation natural resources. Madhuri has yet to incur any food or environmental crises
Non-perturbative orientifold transitions at the conifold
After orientifold projection, the conifold singularity in hypermultiplet
moduli space of Calabi-Yau compactifications cannot be avoided by geometric
deformations. We study the non-perturbative fate of this singularity in a local
model involving O6-planes and D6-branes wrapping the deformed conifold in Type
IIA string theory. We classify possible A-type orientifolds of the deformed
conifold and find that they cannot all be continued to the small resolution.
When passing through the singularity on the deformed side, the O-plane charge
generally jumps by the class of the vanishing cycle. To decide which classical
configurations are dynamically connected, we construct the quantum moduli space
by lifting the orientifold to M-theory as well as by looking at the
superpotential. We find a rich pattern of smooth and phase transitions
depending on the total sixbrane charge. Non-BPS states from branes wrapped on
non-supersymmetric bolts are responsible for a phase transition. We also
clarify the nature of a Z_2 valued D0-brane charge in the 6-brane background.
Along the way, we obtain a new metric of G_2 holonomy corresponding to an
O6-plane on the three sphere of the deformed conifold.Comment: 76 pages, references adde
Ultracold neutrons, quantum effects of gravity and the Weak Equivalence Principle
We consider an extension of the recent experiment with ultracold neutrons and
the quantization of its vertical motion in order to test the Weak Equivalence
Principle. We show that an improvement on the energy resolution of the
experiment may allow to establish a modest limit to the Weak Equivalence
Principle and on the gravitational screening constant. We also discuss the
influence of a possible new interaction of Nature.Comment: Revtex4, 4 pages. Discussion on the equivalence principle altered.
Bound is improve
Detailed Balance and Intermediate Statistics
We present a theory of particles, obeying intermediate statistics ("anyons"),
interpolating between Bosons and Fermions, based on the principle of Detailed
Balance. It is demonstrated that the scattering probabilities of identical
particles can be expressed in terms of the basic numbers, which arise naturally
and logically in this theory. A transcendental equation determining the
distribution function of anyons is obtained in terms of the statistics
parameter, whose limiting values 0 and 1 correspond to Bosons and Fermions
respectively. The distribution function is determined as a power series
involving the Boltzmann factor and the statistics parameter and we also express
the distribution function as an infinite continued fraction. The last form
enables one to develop approximate forms for the distribution function, with
the first approximant agreeing with our earlier investigation.Comment: 13 pages, RevTex, submitted for publication; added references; added
sentence
Isolation and characterization of few-layer black phosphorus
Isolation and characterization of mechanically exfoliated black phosphorus
flakes with a thickness down to two single-layers is presented. A modification
of the mechanical exfoliation method, which provides higher yield of atomically
thin flakes than conventional mechanical exfoliation, has been developed. We
present general guidelines to determine the number of layers using optical
microscopy, Raman spectroscopy and transmission electron microscopy in a fast
and reliable way. Moreover, we demonstrate that the exfoliated flakes are
highly crystalline and that they are stable even in free-standing form through
Raman spectroscopy and transmission electron microscopy measurements. A strong
thickness dependence of the band structure is found by density functional
theory calculations. The exciton binding energy, within an effective mass
approximation, is also calculated for different number of layers. Our
computational results for the optical gap are consistent with preliminary
photoluminescence results on thin flakes. Finally, we study the environmental
stability of black phosphorus flakes finding that the flakes are very
hydrophilic and that long term exposure to air moisture etches black phosphorus
away. Nonetheless, we demonstrate that the aging of the flakes is slow enough
to allow fabrication of field-effect transistors with strong ambipolar
behavior. Density functional theory calculations also give us insight into the
water-induced changes of the structural and electronic properties of black
phosphorus.Comment: 11 main figures, 7 supporting figure
Supersymmetric Three-Form Flux Perturbations on
We consider warped type IIB supergravity solutions with three-form flux and
supersymmetry, which arise as the supergravity duals of confining
gauge theories. We first work in a perturbation expansion around , as in the work of Polchinski and Strassler, and from the
conditions and the Bianchi identities recover their first-order solution
generalized to an arbitrary superpotential. We find the second
order dilaton and axion by the same means. We also find a simple family of
exact solutions, which can be obtained from solutions found by Becker and
Becker, and which includes the recent Klebanov--Strassler solution.Comment: 19 pages. reference added, minor clarifications. v3: reference to
non-Abelian BPS monopole solution corrected (Chamseddine-Volkov
Homogeneous heterotic supergravity solutions with linear dilaton
I construct solutions to the heterotic supergravity BPS-equations on products
of Minkowski space with a non-symmetric coset. All of the bosonic fields are
homogeneous and non-vanishing, the dilaton being a linear function on the
non-compact part of spacetime.Comment: 36 pages; v2 conclusion updated and references adde
No-scale supersymmetry breaking vacua and soft terms with torsion
We analyze the conditions to have no-scale supersymmetry breaking solutions
of type IIA and IIB supergravity compactified on manifolds of SU(3)-structure.
The supersymmetry is spontaneously broken by the intrinsic torsion of the
internal space. For type IIB orientifolds with O9 and O5-planes the mass of the
gravitino is governed by the torsion class W_1, and the breaking is mediated
through F-terms associated to descendants of the original N=2 hypermultiplets.
For type IIA orientifolds with O6-planes we find two families of solutions,
depending on whether the breaking is mediated exclusively by hypermultiplets or
by a mixture of hypermultiplets and vector multiplets, the latter case
corresponding to a class of Scherk-Schwarz compactifications not dual to any
geometric IIB setup. We compute the geometrically induced mu-terms for D5, D6
and D9-branes on twisted tori, and discuss the patterns of soft-terms which
arise for pure moduli mediation in each type of breaking. As for D3 and
D7-branes in presence of 3-form fluxes, the effective scalar potential turns
out to possess interesting phenomenological properties.Comment: 44 pages; several minor corrections and added reference
Stabilizing All Kahler Moduli in Type IIB Orientifolds
We describe a simple and robust mechanism that stabilizes all Kahler moduli
in Type IIB orientifold compactifications. This is shown to be possible with
just one non-perturbative contribution to the superpotential coming from either
a D3-instanton or D7-branes wrapped on an ample divisor. This
moduli-stabilization mechanism is similar to and motivated by the one used in
the fluxless G_2 compactifications of M-theory. After explaining the general
idea, explicit examples of Calabi-Yau orientifolds with one and three Kahler
moduli are worked out. We find that the stabilized volumes of all two- and
four-cycles as well as the volume of the Calabi-Yau manifold are controlled by
a single parameter, namely, the volume of the ample divisor. This feature would
dramatically constrain any realistic models of particle physics embedded into
such compactifications. Broad consequences for phenomenology are discussed, in
particular the dynamical solution to the strong CP-problem within the
framework.Comment: RevTeX, 24 pages, 2 tables, 1 figure
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