332,791 research outputs found
Measurement of thermal conductivity and heat pipe effect in hydrophilic and hydrophobic carbon papers
In this paper, we present an experimental study on measurement of the thermal conductivity and heat pipe effect in both hydrophilic and hydrophobic (Toray TGP-H60) carbon papers (around 200 μm thickness) with/out liquid water. An experimental setup is developed for measuring thermal conductance at different liquid water contents and temperatures without dissembling the testing device for water addition. Theoretical analysis is also performed to evaluate the apparent conductance of heat pipe effect. We found that liquid water presence inside these materials increases the overall thermal conductivity. At high temperature around 80 °C, the heat pipe effect is evident for the hydrophilic paper; while for the hydrophobic one, the heat pipe effect is found to be smaller. The distinction is likely due to the different patterns of the capillary liquid flow in the two media. For the hydrophobic paper, liquid water flows back to the evaporation side when the breakthrough pressure is reached and flow is through preferred routes of small flow resistance. As a result, heat pipe effect is active only in part of the medium, therefore smaller than that in the hydrophilic one. The results are important for understanding the heat transfer phenomena occurring in porous media and effects of material surface property. © 2012 Elsevier Ltd. All rights reserved
The Magnetic Rayleigh-Taylor Instability in Three Dimensions
We study the magnetic Rayleigh-Taylor instability in three dimensions, with
focus on the nonlinear structure and evolution that results from different
initial field configurations. We study strong fields in the sense that the
critical wavelength l_c at which perturbations along the field are stable is a
large fraction of the size of the computational domain. We consider magnetic
fields which are initially parallel to the interface, but have a variety of
configurations, including uniform everywhere, uniform in the light fluid only,
and fields which change direction at the interface. Strong magnetic fields do
not suppress instability, in fact by inhibiting secondary shear instabilities,
they reduce mixing between the heavy and light fluid, and cause the rate of
growth of bubbles and fingers to increase in comparison to hydrodynamics.
Fields parallel to, but whose direction changes at, the interface produce long,
isolated fingers separated by the critical wavelength l_c, which may be
relevant to the morphology of the optical filaments in the Crab nebula.Comment: 14 pages, 9 pages, accepted by Ap
Downward continuation of the free-air gravity anomalies to the ellipsoid using the gradient solution and terrain correction: An attempt of global numerical computations
The formulas for the determination of the coefficients of the spherical harmonic expansion of the disturbing potential of the earth are defined for data given on a sphere. In order to determine the spherical harmonic coefficients, the gravity anomalies have to be analytically downward continued from the earth's surface to a sphere-at least to the ellipsoid. The goal is to continue the gravity anomalies from the earth's surface downward to the ellipsoid using recent elevation models. The basic method for the downward continuation is the gradient solution (the g sub 1 term). The terrain correction was also computed because of the role it can play as a correction term when calculating harmonic coefficients from surface gravity data. The fast Fourier transformation was applied to the computations
A More Precise Extraction of |V_{cb}| in HQEFT of QCD
The more precise extraction for the CKM matrix element |V_{cb}| in the heavy
quark effective field theory (HQEFT) of QCD is studied from both exclusive and
inclusive semileptonic B decays. The values of relevant nonperturbative
parameters up to order 1/m^2_Q are estimated consistently in HQEFT of QCD.
Using the most recent experimental data for B decay rates, |V_{cb}| is updated
to be |V_{cb}| = 0.0395 \pm 0.0011_{exp} \pm 0.0019_{th} from B\to D^{\ast} l
\nu decay and |V_{cb}| = 0.0434 \pm 0.0041_{exp} \pm 0.0020_{th} from B\to D l
\nu decay as well as |V_{cb}| = 0.0394 \pm 0.0010_{exp} \pm 0.0014_{th} from
inclusive B\to X_c l \nu decay.Comment: 7 pages, revtex, 4 figure
Large Component QCD and Theoretical Framework of Heavy Quark Effective Field Theory
Based on a large component QCD derived directly from full QCD by integrating
over the small components of quark fields with , an
alternative quantization procedure is adopted to establish a basic theoretical
framework of heavy quark effective field theory (HQEFT) in the sense of
effective quantum field theory. The procedure concerns quantum generators of
Poincare group, Hilbert and Fock space, anticommutations and velocity
super-selection rule, propagator and Feynman rules, finite mass corrections,
trivialization of gluon couplings and renormalization of Wilson loop. The
Lorentz invariance and discrete symmetries in HQEFT are explicitly illustrated.
Some new symmetries in the infinite mass limit are discussed. Weak transition
matrix elements and masses of hadrons in HQEFT are well defined to display a
manifest spin-flavor symmetry and corrections. A simple trace
formulation approach is explicitly demonstrated by using LSZ reduction formula
in HQEFT, and shown to be very useful for parameterizing the transition form
factors via expansion. As the heavy quark and antiquark fields in HQEFT
are treated on the same footing in a fully symmetric way, the quark-antiquark
coupling terms naturally appear and play important roles for simplifying the
structure of transition matrix elements, and for understanding the concept of
`dressed heavy quark' - hadron duality. In the case that the `longitudinal' and
`transverse' residual momenta of heavy quark are at the same order of power
counting, HQEFT provides a consistent approach for systematically analyzing
heavy quark expansion in terms of . Some interesting features in
applications of HQEFT to heavy hadron systems are briefly outlined.Comment: 59 pages, RevTex, no figures, published versio
Electron spin relaxation in cubic GaN quantum dots
The spin relaxation time in zinc blende GaN quantum dot is
investigated for different magnetic field, well width and quantum dot diameter.
The spin relaxation caused by the two most important spin relaxation mechanisms
in zinc blende semiconductor quantum dots, {i.e.} the electron-phonon
scattering in conjunction with the Dresselhaus spin-orbit coupling and the
second-order process of the hyperfine interaction combined with the
electron-phonon scattering, are systematically studied. The relative importance
of the two mechanisms are compared in detail under different conditions. It is
found that due to the small spin orbit coupling in GaN, the spin relaxation
caused by the second-order process of the hyperfine interaction combined with
the electron-phonon scattering plays much more important role than it does in
the quantum dot with narrower band gap and larger spin-orbit coupling, such as
GaAs and InAs.Comment: 8 pages, 5 figures, PRB 79, 2009, in pres
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