30,376 research outputs found
A partial velocity approach to subcycling structural dynamics
Subcycling, or the use of different timesteps at different nodes, can be an effective way of improving the computational efficiency of explicit transient dynamic structural solutions. The method that has been most widely adopted uses a nodal partition. extending the central difference method, in which small timestep updates are performed interpolating on the displacement at neighbouring large timestep nodes. This approach leads to narrow bands of unstable timesteps or statistical stability. It also can be in error due to lack of momentum conservation on the timestep interface. The author has previously proposed energy conserving algorithms that avoid the first problem of statistical stability. However, these sacrifice accuracy to achieve stability. An approach to conserve momentum on an element interface by adding partial velocities is considered here. Applied to extend the central difference method. this approach is simple. and has accuracy advantages. The method can be programmed by summing impulses of internal forces, evaluated using local element timesteps, in order to predict a velocity change at a node. However, it is still only statistically stable, so an adaptive timestep size is needed to monitor accuracy and to be adjusted if necessary. By replacing the central difference method with the explicit generalized alpha method. it is possible to gain stability by dissipating the high frequency response that leads to stability problems. However. coding the algorithm is less elegant, as the response depends on previous partial accelerations. Extension to implicit integration, is shown to be impractical due to the neglect of remote effects of internal forces acting across a timestep interface. (C) 2002 Elsevier Science B.V. All rights reserved
Diagrammatic Coupled Cluster Monte Carlo
We propose a modified coupled cluster Monte Carlo algorithm that
stochastically samples connected terms within the truncated
Baker--Campbell--Hausdorff expansion of the similarity transformed Hamiltonian
by construction of coupled cluster diagrams on the fly. Our new approach --
diagCCMC -- allows propagation to be performed using only the connected
components of the similarity-transformed Hamiltonian, greatly reducing the
memory cost associated with the stochastic solution of the coupled cluster
equations. We show that for perfectly local, noninteracting systems, diagCCMC
is able to represent the coupled cluster wavefunction with a memory cost that
scales linearly with system size. The favorable memory cost is observed with
the only assumption of fixed stochastic granularity and is valid for arbitrary
levels of coupled cluster theory. Significant reduction in memory cost is also
shown to smoothly appear with dissociation of a finite chain of helium atoms.
This approach is also shown not to break down in the presence of strong
correlation through the example of a stretched nitrogen molecule. Our novel
methodology moves the theoretical basis of coupled cluster Monte Carlo closer
to deterministic approaches.Comment: 31 pages, 6 figure
A Chandra ACIS view of the Thermal Composite Supernova Remnant 3C391
We present a 60 ks Chandra ACIS-S observation of the thermal composite
supernova remnant 3C391. The southeast-northwest elongated morphology is
similar to that previously found in radio and X-ray studies. This observation
unveils a highly clumpy structure of the remnant. Detailed spatially resolved
spectral analysis for the small-scale features reveals that the interior gas is
generally of normal metal abundance and has approached or basically reached
ionization equilibrium. The hydrogen column density increases from southeast to
northwest. Three mechanisms, radiative rim, thermal conduction, and cloudlet
evaporation, may all play roles in the X-ray appearance of 3C391 as a "thermal
composite" remnant, but there are difficulties with each of them in explaining
some physical properties. Comparatively, the cloudlet evaporation model is
favored by the main characteristics such as the highly clumpy structure and the
uniform temperature and density distribution over most of the remnant. The
directly measured postshock temperature also implies a young age, about 4 kyr,
for the remnant. The postshock gas pressure derived from the NE and SW rims,
which harbor maser spots, is consistent with the estimate for the maser
regions. An unresolved X-ray source is observed on the northwest border and its
spectrum is best fitted by a power-law.Comment: aastex, 27 pages (including 4 figures), to appear in the ApJ 1 Dec.
2004, v616 issu
Implicit finite element study of non-steady effects in cold roll forming
The ability of ABAQUS Standard to obtain a non-steady implicit solution to the problem of cold roll forming a channel section is investigated. A solution can be found with careful selection of parameters, but solutions are unacceptably slow for commercial use. The implicit solutions show buckling on the first pass that does not develop into an edge wave, in contrast to a published explicit solution. Faster solutions to steady rolling can be obtained using ALE models that permit convection of stress in the direction of rolling
U. S. labor supply and demand in the long run
In this paper we model U.S. labor supply and demand in considerable detail in order to capture the enormous heterogeneity of the labor force and its evolution over the next 25 years. We represent labor supplies for a large number of demographic groups as responses to prices of leisure and consumption goods and services. The price of leisure is an after-tax wage rate, while the final prices of goods and services reflect the supply prices of the industries that produce them. By including demographic characteristics among the determinants of household preferences, we incorporate the expected demographic transition into our long-run projections of the U.S. labor market.Labor supply ; Labor market
When is S=A/4?
Black hole entropy and its relation to the horizon area are considered. More
precisely, the conditions and specifications that are expected to be required
for the assignment of entropy, and the consequences that these expectations
have when applied to a black hole are explored. In particular, the following
questions are addressed: When do we expect to assign an entropy?; when are
entropy and area proportional? and, what is the nature of the horizon? It is
concluded that our present understanding of black hole entropy is somewhat
incomplete, and some of the relevant issues that should be addressed in
pursuing these questions are pointed out.Comment: 14 pages, no figures. Revtex file. Manuscript edited and discussion
expanded. References added, conclusions unchanged. Version to be published in
MPL
A discrete time-dependent method for metastable atoms in intense fields
The full-dimensional time-dependent Schrodinger equation for the electronic
dynamics of single-electron systems in intense external fields is solved
directly using a discrete method.
Our approach combines the finite-difference and Lagrange mesh methods. The
method is applied to calculate the quasienergies and ionization probabilities
of atomic and molecular systems in intense static and dynamic electric fields.
The gauge invariance and accuracy of the method is established. Applications to
multiphoton ionization of positronium and hydrogen atoms and molecules are
presented. At very high intensity above saturation threshold, we extend the
method using a scaling technique to estimate the quasienergies of metastable
states of the hydrogen molecular ion. The results are in good agreement with
recent experiments.Comment: 10 pages, 9 figure, 4 table
Behavior of nozzles and acoustic liners in three dimensional acoustic fields
Theoretical values of the admittances of various nozzles were computed and compared with the corresponding experimental values. The existing data reduction scheme was corrected and all available experimental data has been rechecked and corrected whenever necessary; the updated experimental admittance values are presented. An analysis associated with the frequency sensitivity of experimental admittance values was initiated and the analog-to-digital Data Reduction Program which has become operational is discussed. Fourteen nozzle tests were conducted during this report period
Is strong CP invariance due to a massless up quark?
A standing mystery in the Standard Model is the unnatural smallness of the
strong CP violating phase. A massless up quark has long been proposed as one
potential solution. A lattice calculation of the constants of the chiral
Lagrangian essential for the determination of the up quark mass, 2 alpha_8 -
alpha_5, is presented. We find 2 alpha_8 - alpha_5 = 0.29 +/- 0.18, which
corresponds to m_u / m_d = 0.410 +/- 0.036. This is the first such calculation
using a physical number of dynamical light quarks, N_f = 3.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett., corrected small
normalization error in f_pi (conclusions were unaffected), improved lattice
spacing analysis, improved finite volume analysi
Predicting spatial spread of rabies in skunk populations using surveillance data reported by the public
Background:
Prevention and control of wildlife disease invasions relies on the ability to predict spatio-temporal dynamics and understand the role of factors driving spread rates, such as seasonality and transmission distance. Passive disease surveillance (i.e., case reports by public) is a common method of monitoring emergence of wildlife diseases, but can be challenging to interpret due to spatial biases and limitations in data quantity and quality.
Methodology/Principal findings:
We obtained passive rabies surveillance data from dead striped skunks (Mephitis mephitis) in an epizootic in northern Colorado, USA. We developed a dynamic patch-occupancy model which predicts spatio-temporal spreading while accounting for heterogeneous sampling. We estimated the distance travelled per transmission event, direction of invasion, rate of spatial spread, and effects of infection density and season. We also estimated mean transmission distance and rates of spatial spread using a phylogeographic approach on a subsample of viral sequences from the same epizootic. Both the occupancy and phylogeographic approaches predicted similar rates of spatio-temporal spread. Estimated mean transmission distances were 2.3 km (95% Highest Posterior Density (HPD95): 0.02, 11.9; phylogeographic) and 3.9 km (95% credible intervals (CI95): 1.4, 11.3; occupancy). Estimated rates of spatial spread in km/year were: 29.8 (HPD95: 20.8, 39.8; phylogeographic, branch velocity, homogenous model), 22.6 (HPD95: 15.3, 29.7; phylogeographic, diffusion rate, homogenous model) and 21.1 (CI95: 16.7, 25.5; occupancy). Initial colonization probability was twice as high in spring relative to fall.
Conclusions/Significance:
Skunk-to-skunk transmission was primarily local (< 4 km) suggesting that if interventions were needed, they could be applied at the wave front. Slower viral invasions of skunk rabies in western USA compared to a similar epizootic in raccoons in the eastern USA implies host species or landscape factors underlie the dynamics of rabies invasions. Our framework provides a straightforward method for estimating rates of spatial spread of wildlife diseases
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