18,919 research outputs found
Bisector and zero-macrospin co-rotational systems for shell elements
A principal issue in any co-rotational approach for large displacement analysis of plates and shells is associated with the specific choice of the local reference system in relation to the current deformed element configuration. Previous approaches utilised local co-rotational systems, which are invariant to nodal ordering, a characteristic that is deemed desirable on several fronts; however, the associated definitions of the local reference system suffered from a range of shortcomings, including undue complexity, dependence on the local element formulation and possibly an asymmetric tangent stiffness matrix. In this paper, new definitions of the local co-rotational system are proposed for quadrilateral and triangular shell elements, which achieve the invariance characteristic to the nodal ordering in a relatively simple manner and address the aforementioned shortcomings. The proposed definitions utilise only the nodal coordinates in the deformed configuration, where two alternative definitions, namely, bisector and zero-macrospin definitions, are presented for each of quadrilateral and triangular finite elements. In each case, the co-rotational transformations linking the local and global element entities are presented, highlighting the simplicity of the proposed approach. Several numerical examples are finally presented to demonstrate the effectiveness and relative accuracy of the alternative definitions proposed for the local co-rotational system
Quantum radiation reaction force on a one-dimensional cavity with two relativistic moving mirrors
We consider a real massless scalar field inside a cavity with two moving
mirrors in a two-dimensional spacetime, satisfying Dirichlet boundary condition
at the instantaneous position of the boundaries, for arbitrary and relativistic
laws of motion. Considering vacuum as the initial field state, we obtain
formulas for the exact value of the energy density of the field and the quantum
force acting on the boundaries, which extend results found in previous papers.
For the particular cases of a cavity with just one moving boundary,
non-relativistic velocities, or in the limit of infinity length of the cavity
(a single mirror), our results coincide with those found in the literature.Comment: 6 pages 9 figure
A dynamical model for the dusty ring in the Coalsack
Lada et al. recently presented a detailed near-infrared extinction map of
Globule G2 in the Coalsack molecular cloud complex, showing that this starless
core has a well-defined central extinction minimum. We propose a model for G2
in which a rapid increase in external pressure is driving an approximately
symmetric compression wave into the core. The rapid increase in external
pressure could arise because the core has recently been assimilated by the
Coalsack cloud complex, or because the Coalsack has recently been created by
two large-scale converging flows. The resulting compression wave has not yet
converged on the centre of the core, so there is a central rarefaction. The
compression wave has increased the density in the swept-up gas by about a
factor of ten, and accelerated it inwards to speeds of order . It is shown that even small levels of initial turbulence destroy the
ring seen in projection almost completely. In the scenario of strong external
compression that we are proposing this implies that the initial turbulent
energy in this globule is such that .
Protostar formation should occur in about .Comment: Accepted for publication in A&
Integrated design for integrated photonics: from the physical to the circuit level and back
Silicon photonics is maturing rapidly on a technology basis, but design challenges are still prevalent. We discuss these challenges and explain how design of photonic integrated circuits needs to be handled on both the circuit as on the physical level. We also present a number of tools based on the IPKISS design framework
Rugged Metropolis Sampling with Simultaneous Updating of Two Dynamical Variables
The Rugged Metropolis (RM) algorithm is a biased updating scheme, which aims
at directly hitting the most likely configurations in a rugged free energy
landscape. Details of the one-variable (RM) implementation of this
algorithm are presented. This is followed by an extension to simultaneous
updating of two dynamical variables (RM). In a test with Met-Enkephalin in
vacuum RM improves conventional Metropolis simulations by a factor of about
four. Correlations between three or more dihedral angles appear to prevent
larger improvements at low temperatures. We also investigate a multi-hit
Metropolis scheme, which spends more CPU time on variables with large
autocorrelation times.Comment: 8 pages, 5 figures. Revisions after referee reports. Additional
simulations for temperatures down to 220
Systematic identification of transcription factors associated with patient survival in cancers
<p>Abstract</p> <p>Background</p> <p>Aberrant activation or expression of transcription factors has been implicated in the tumorigenesis of various types of cancer. In spite of the prevalent application of microarray experiments for profiling gene expression in cancer samples, they provide limited information regarding the activities of transcription factors. However, the association between transcription factors and cancers is largely dependent on the transcription regulatory activities rather than mRNA expression levels.</p> <p>Results</p> <p>In this paper, we propose a computational approach that integrates microarray expression data with the transcription factor binding site information to systematically identify transcription factors associated with patient survival given a specific cancer type. This approach was applied to two gene expression data sets for breast cancer and acute myeloid leukemia. We found that two transcription factor families, the steroid nuclear receptor family and the ATF/CREB family, are significantly correlated with the survival of patients with breast cancer; and that a transcription factor named T-cell acute lymphocytic leukemia 1 is significantly correlated with acute myeloid leukemia patient survival.</p> <p>Conclusion</p> <p>Our analysis identifies transcription factors associating with patient survival and provides insight into the regulatory mechanism underlying the breast cancer and leukemia. The transcription factors identified by our method are biologically meaningful and consistent with prior knowledge. As an insightful tool, this approach can also be applied to other microarray cancer data sets to help researchers better understand the intricate relationship between transcription factors and diseases.</p
Direct observation of a sharp transition to coherence in Dense Cores
We present NH3 observations of the B5 region in Perseus obtained with the
Green Bank Telescope (GBT). The map covers a region large enough (~11'x14')
that it contains the entire dense core observed in previous dust continuum
surveys. The dense gas traced by NH3(1,1) covers a much larger area than the
dust continuum features found in bolometer observations. The velocity
dispersion in the central region of the core is small, presenting subsonic
non-thermal motions which are independent of scale. However, it is thanks to
the coverage and high sensitivity of the observations that we present the
detection, for the first time, of the transition between the coherent core and
the dense but more turbulent gas surrounding it. This transition is sharp,
increasing the velocity dispersion by a factor of 2 in less than 0.04 pc (the
31" beam size at the distance of Perseus, ~250 pc). The change in velocity
dispersion at the transition is ~3 km/s/pc. The existence of the transition
provides a natural definition of dense core: the region with nearly-constant
subsonic non-thermal velocity dispersion. From the analysis presented here we
can not confirm nor rule out a corresponding sharp density transition.Comment: Accepted for publication in ApJ Letters. 6 pages, 5 figure
Intrinsic Charm Contribution to Double Quarkonium Hadroproduction
Double production has been observed by the NA3 collaboration in and collisions with a cross section of the order of 20-30 pb. The
pairs measured in nucleus interactions at 150 and 280
GeV are observed to carry an anomalously large fraction of the projectile
momentum in the laboratory frame, at 150 GeV and
at 280 GeV. We postulate that these forward pairs
are created by the materialization of Fock states in the projectile containing
two pairs of intrinsic quarks. We calculate the overlap of the
charmonium states with the
Fock state as described by the intrinsic charm model and find that the longitudinal momentum and invariant mass distributions
are both well reproduced. We also discuss double production in
interactions and the implications for other heavy quarkonium production
channels in QCD.Comment: Revtex, APS style, 7 pages, 3 figures in uuencoded fil
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