8,570 research outputs found
Fast computation of effective diffusivities using a semi-analytical solution of the homogenization boundary value problem for block locally-isotropic heterogeneous media
Direct numerical simulation of diffusion through heterogeneous media can be
difficult due to the computational cost of resolving fine-scale
heterogeneities. One method to overcome this difficulty is to homogenize the
model by replacing the spatially-varying fine-scale diffusivity with an
effective diffusivity calculated from the solution of an appropriate boundary
value problem. In this paper, we present a new semi-analytical method for
solving this boundary value problem and computing the effective diffusivity for
pixellated, locally-isotropic, heterogeneous media. We compare our new solution
method to a standard finite volume method and show that equivalent accuracy can
be achieved in less computational time for several standard test cases. We also
demonstrate how the new solution method can be applied to complex heterogeneous
geometries represented by a grid of blocks. These results indicate that our new
semi-analytical method has the potential to significantly speed up simulations
of diffusion in heterogeneous media.Comment: 29 pages, 4 figures, 5 table
On the controversy concerning the definition of quark and gluon angular momentum
A major controversy has arisen in QCD as to how to split the total angular
momentum into separate quark and gluon contributions, and as to whether the
gluon angular momentum can itself be split, in a gauge invariant way, into a
spin and orbital part. Several authors have proposed various answers to these
questions and offered a variety of different expressions for the relevant
operators. I argue that none of these is acceptable and suggest that the
canonical expression for the momentum and angular momentum operators is the
correct and physically meaningful one. It is then an inescapable fact that the
gluon angular momentum operator cannot, in general, be split in a gauge
invariant way into a spin and orbital part. However, the projection of the
gluon spin onto its direction of motion i.e. its helicity is gauge invariant
and is measured in deep inelastic scattering on nucleons. The Ji sum rule,
relating the quark angular momentum to generalized parton distributions, though
not based on the canonical operators, is shown to be correct, if interpreted
with due care. I also draw attention to several interesting aspects of QED and
QCD, which, to the best of my knowledge, are not commented upon in the standard
textbooks on Field Theory.Comment: 41 pages; Some incorrect statements have been rectified and a
detailed discussion has been added concerning the momentum carried by quarks
and the Ji sum rule for the angular momentu
Double Charge Exchange And Configuration Mixing
The energy dependence of forward pion double charge exchange reactions on
light nuclei is studied for both the Ground State transition and the
Double-Isobaric-Analog-State transitions. A common characteristic of these
double reactions is a resonance-like peak around 50 MeV pion lab energy. This
peak arises naturally in a two-step process in the conventional pion-nucleon
system with proper handling of nuclear structure and pion distortion. A
comparison among the results of different nuclear structure models demonstrates
the effects of configuration mixing. The angular distribution is used to fix
the single particle wave function.Comment: Added 1 figure (now 8) corrected references and various other change
Achievement goals, self-handicapping, and performance: A 2 × 2 achievement goal perspective
Elliot and colleagues (2006) examined the effects of experimentally induced achievement goals, proposed by the
trichotomous model, on self-handicapping and performance in physical education. Our study replicated and extended the
work of Elliot et al. by experimentally promoting all four goals proposed by the 262 model (Elliot & McGregor, 2001),
measuring the participants’ own situational achievement goals, using a relatively novel task, and testing the participants in a group setting. We used a randomized experimental design with four conditions that aimed to induce one of the four goals advanced by the 262 model. The participants (n¼138) were undergraduates who engaged in a dart-throwing task. The results pertaining to self-handicapping partly replicated Elliot and colleagues’ findings by showing that experimentally promoted performance-avoidance goals resulted in less practice. In contrast, the promotion of mastery-avoidance goals did
not result in less practice compared with either of the approach goals. Dart-throwing performance did not differ among the four goal conditions. Personal achievement goals did not moderate the effects of experimentally induced goals on selfhandicapping and performance. The extent to which mastery-avoidance goals are maladaptive is discussed, as well as the interplay between personal and experimentally induced goals
Buoyancy waves in Pluto's high atmosphere: Implications for stellar occultations
We apply scintillation theory to stellar signal fluctuations in the
high-resolution, high signal/noise, dual-wavelength data from the MMT
observation of the 2007 March 18 occultation of P445.3 by Pluto. A well-defined
high wavenumber cutoff in the fluctuations is consistent with viscous-thermal
dissipation of buoyancy waves (internal gravity waves) in Pluto's high
atmosphere, and provides strong evidence that the underlying density
fluctuations are governed by the gravity-wave dispersion relation.Comment: Accepted 18 June 2009 for publication in Icaru
Commensurate and modulated magnetic phases in orthorhombic A1C60
Competing magnetically ordered structures in polymerized orthorhombic A1C60
are studied. A mean-field theory for the equilibrium phases is developed using
an Ising model and a classical Heisenberg model to describe the competition
between inter- and intra-chain magnetic order in the solid. In the Ising model,
the limiting commensurate one-dimensional and three-dimensional phases are
separated by a commensurate three-sublattice state and by two sectors
containing higher-order commensurate phases. For the Heisenberg model the
quasi-1D phase is never the equilibrium state; instead the 3D commensurate
phases exhibits a transition to a continuum of coplanar spiral magnetic phases.Comment: 11 pages REVTeX 3.0 plus 4 figures appende
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A bulky glycocalyx fosters metastasis formation by promoting G1 cell cycle progression.
Metastasis depends upon cancer cell growth and survival within the metastatic niche. Tumors which remodel their glycocalyces, by overexpressing bulky glycoproteins like mucins, exhibit a higher predisposition to metastasize, but the role of mucins in oncogenesis remains poorly understood. Here we report that a bulky glycocalyx promotes the expansion of disseminated tumor cells in vivo by fostering integrin adhesion assembly to permit G1 cell cycle progression. We engineered tumor cells to display glycocalyces of various thicknesses by coating them with synthetic mucin-mimetic glycopolymers. Cells adorned with longer glycopolymers showed increased metastatic potential, enhanced cell cycle progression, and greater levels of integrin-FAK mechanosignaling and Akt signaling in a syngeneic mouse model of metastasis. These effects were mirrored by expression of the ectodomain of cancer-associated mucin MUC1. These findings functionally link mucinous proteins with tumor aggression, and offer a new view of the cancer glycocalyx as a major driver of disease progression
Pluto-Charon Stellar Occultation Candidates: 1990-1995
We have carried out a search to identify stars that might be occulted by Pluto or Charon during the period 1990-1995 and part of 1996. This search was made with an unfiltered CCD camera operated in the strip scanning mode, and it reaches an R magnitude of approximately 17.5-about 1.5 mag fainter than previous searches. Circumstances for each of the 162 potential occultations are given, including an approximate R magnitude of the star, which allows estimation of the signal-to-noise ratio (S/N) for observation of each occultation. The faintest stars in our list would yield an S/N of about 20 for a 1 S integration when observed with a CCD detector on an 8 m telescope under a dark sky. Our astrometric precision (+/- 0.2 arcsec, with larger systematic errors possible for individual cases) is insufficient to serve as a final prediction for these potential occultations, but is sufficient to identify stars deserving of further, more accurate, astrometric observations. Statistically, we expect about 32 of these events to be observable somewhere on Earth. The number of events actually observed will be substantially smaller because of clouds and the sparse distribution of large telescopes. Finder charts for each of the 91 stars involved are presented
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