86,644 research outputs found
The benefit of high-conductivity materials in film cooled turbine nozzles
This study presents an experimental and numerical investigation of the beneficial effect of higher conductivity materials in HP turbine nozzles. Most of the literature studies focus on the maximum temperature that a nozzle can withstand, whereas the effect of thermal gradients is often neglected. However thermal gradients have higher influence on the life of the components and they have to be given careful consideration. In this work it is shown that thermal gradients are reduced by using high conductivity materials and, as a consequence, the nozzles life is appreciably increased. A representative film cooled leading edge with an internal impingement plate was studied experimentally at Texas AM University. Two materials were used, namely polycarbonate and stainless steel, in order to highlight the impact of conduction on coolant effectiveness. Numerically conjugate heat transfer simulations have been carried out with an in house solver to analyse in detail the impact of conduction and internal convection. Both experimental and numerical results show that by increasing the conductivity in the solid region, the thermal gradients are strongly reduced. Numerically it is shown that using inserts of nickel-aluminide alloys in nozzles may reduce the thermal gradients from 3 to 4 times if compared to nowadays design. © 2012 Elsevier Inc
Behavior of large-scale rectangular columns confined with FRP composites
This paper focuses on axially loaded, large-scale rectangular RC columns confined with fiber-reinforced polymer (FRP) wrapping. Experimental tests are conducted to obtain the stress-strain response and ultimate load for three field-size columns having different aspect ratios and/or corner radii. Effective transverse FRP failure strain and the effect of increasing confining action on the stress-strain behavior are examined. Existing strength models, the majority of which were developed for small-scale specimens, are applied to predict the structural response. Since some of them fail to adequately characterize the test data and others are complex and require significant calculation, a simple design-oriented model is developed. The new model is based on the confinement effectiveness coefficient, an aspect ratio coefficient, and a corner radius coefficient. It accurately predicts the axial ultimate strength of the large-scale columns at hand and, when applied to the small-scale columns studied by other investigators, produces reasonable results
Properties of Planetary Caustics in Gravitational Microlensing
Although some of the properties of the caustics in planetary microlensing
have been known, our understanding of them is mostly from scattered information
based on numerical approaches. In this paper, we conduct a comprehensive and
analytic analysis of the properties of the planetary caustics, which are one of
the two sets of caustics in planetary microlensing, those located away from the
central star. Under the perturbative approximation, we derive analytic
expressions for the location, size, and shape of the planetary caustic as a
function of the star-planet separation and the planet/star mass ratio. Based on
these expressions combined with those for the central caustic, which is the
other set of caustics located close to the central star, we compare the
similarities and differences between the planetary and central caustics. We
also present the expressions for the size ratio between the two types of
caustics and for the condition of the merging of the two types of caustics.
These analytic expressions will be useful in understanding the dependence of
the planetary lensing behavior on the planet parameters and thus in
interpreting the planetary lensing signalsComment: total 6 pages, including 6 figures, ApJ, submitte
Analysis of Microlensing Light Curves Induced by Multiple-Planet Systems
To maximize the number of planet detections by increasing efficiency, current
microlensing follow-up observation experiments are focusing on
high-magnification events to search for planet-induced perturbations near the
peak of lensing light curves. It was known that by monitoring
high-magnification events, it is possible to detect multiplicity signatures of
planetary systems. However, it was believed that the interpretation of the
signals and the characterization of the detected multiple-planet systems would
be difficult due to the complexity of the magnification pattern in the central
region combined with the large number of lensing parameters required to model
multiple-planet systems. In this paper, we demonstrate that in many cases the
central planetary perturbations induced by multiple planets can be well
approximated by the superposition of the single planetary perturbations where
the individual planet-primary pairs act as independent binary lens systems
(binary superposition). The validity of the binary-superposition approximation
implies that the analysis of perturbations induced by multiple planets can be
greatly simplified because the anomalies produced by the individual planet
components can be investigated separately by using relatively much simpler
single-planetary analysis, and thus enables better characterization of these
systems.Comment: Manuscript with high-resolution figures are available at
http://astroph.chungbuk.ac.kr/~cheongho/preprint.htm
Supersolid and charge density-wave states from anisotropic interaction in an optical lattice
We show anisotropy of the dipole interaction between magnetic atoms or polar
molecules can stabilize new quantum phases in an optical lattice. Using a well
controlled numerical method based on the tensor network algorithm, we calculate
phase diagram of the resultant effective Hamiltonian in a two-dimensional
square lattice - an anisotropic Hubbard model of hard-core bosons with
attractive interaction in one direction and repulsive interaction in the other
direction. Besides the conventional superfluid and the Mott insulator states,
we find the striped and the checkerboard charge density wave states and the
supersolid phase that interconnect the superfluid and the striped solid states.
The transition to the supersolid phase has a mechanism different from the case
of the soft-core Bose Hubbard model.Comment: 5 pages, 5 figures
Perfect Output Feedback in the Two-User Decentralized Interference Channel
In this paper, the -Nash equilibrium (-NE) region of the two-user
Gaussian interference channel (IC) with perfect output feedback is approximated
to within bit/s/Hz and arbitrarily close to bit/s/Hz. The
relevance of the -NE region is that it provides the set of rate-pairs
that are achievable and stable in the IC when both transmitter-receiver pairs
autonomously tune their own transmit-receive configurations seeking an
-optimal individual transmission rate. Therefore, any rate tuple outside
the -NE region is not stable as there always exists one link able to
increase by at least bits/s/Hz its own transmission rate by updating its
own transmit-receive configuration. The main insights that arise from this work
are: The -NE region achieved with feedback is larger than or equal
to the -NE region without feedback. More importantly, for each rate pair
achievable at an -NE without feedback, there exists at least one rate
pair achievable at an -NE with feedback that is weakly Pareto superior.
There always exists an -NE transmit-receive configuration that
achieves a rate pair that is at most bit/s/Hz per user away from the outer
bound of the capacity region.Comment: Revised version (Aug. 2015
Local Spin Susceptibility of the S=1/2 Kagome Lattice in ZnCu3(OD)6Cl2
We report single-crystal 2-D NMR investigation of the nearly ideal spin S=1/2
kagome lattice ZnCu3(OD)6Cl2. We successfully identify 2-D NMR signals
originating from the nearest-neighbors of Cu2+ defects occupying Zn sites. From
the 2-D Knight shift measurements, we demonstrate that weakly interacting Cu2+
spins at these defects cause the large Curie-Weiss enhancement toward T=0
commonly observed in the bulk susceptibility data. We estimate the intrinsic
spin susceptibility of the kagome planes by subtracting defect contributions,
and explore several scenarios.Comment: 4 figures; published in PR-B Rapid Communication
On the origin of the Fermi arc phenomena in the underdoped cuprates: signature of KT-type superconducting transition
We study the effect of thermal phase fluctuation on the electron spectral
function in a d-wave superconductor with Monte Carlo simulation.
The phase degree of freedom is modeled by a XY-type model with build-in d-wave
character. We find a ridge-like structure emerges abruptly on the underlying
Fermi surface in above the KT-transition temperature of the XY
model. Such a ridge-like structure, which shares the same characters with the
Fermi arc observed in the pseudogap phase of the underdoped cuprates, is found
to be caused by the vortex-like phase fluctuation of the XY model.Comment: 5 page
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