9,220 research outputs found
Modeling the buckling and delamination of thin films
I study numerically the problem of delamination of a thin film elastically
attached to a rigid substrate. A nominally flat elastic thin film is modeled
using a two-dimensional triangular mesh. Both compression and bending
rigidities are included to simulate compression and bending of the film. The
film can buckle (i.e., abandon its flat configuration) when enough compressive
strain is applied. The possible buckled configurations of a piece of film with
stripe geometry are investigated as a function of the compressive strain. It is
found that the stable configuration depends strongly on the applied strain and
the Poisson ratio of the film. Next, the film is considered to be attached to a
rigid substrate by springs that can break when the detaching force exceeds a
threshold value, producing the partial delamination of the film. Delamination
is induced by a mismatch of the relaxed configurations of film and substrate.
The morphology of the delaminated film can be followed and compared with
available experimental results as a function of model parameters.
`Telephone-cord', polygonal, and `brain-like' patterns qualitatively similar to
experimentally observed configurations are obtained in different parameter
regions. The main control parameters that select the different patterns are the
mismatch between film and substrate and the degree of in-plane relaxation
within the unbuckled regions.Comment: 8 pages, 10 figure
Electron Temperature of Ultracold Plasmas
We study the evolution of ultracold plasmas by measuring the electron
temperature. Shortly after plasma formation, competition between heating and
cooling mechanisms drives the electron temperature to a value within a narrow
range regardless of the initial energy imparted to the electrons. In agreement
with theory predictions, plasmas exhibit values of the Coulomb coupling
parameter less than 1.Comment: 4 pages, plus four figure
Coating thermal noise of a finite-size cylindrical mirror
Thermal noise of a mirror is one of the limiting noise sources in the high
precision measurement such as gravitational-wave detection, and the modeling of
thermal noise has been developed and refined over a decade. In this paper, we
present a derivation of coating thermal noise of a finite-size cylindrical
mirror based on the fluctuation-dissipation theorem. The result agrees to a
previous result with an infinite-size mirror in the limit of large thickness,
and also agrees to an independent result based on the mode expansion with a
thin-mirror approximation. Our study will play an important role not only to
accurately estimate the thermal-noise level of gravitational-wave detectors but
also to help analyzing thermal noise in quantum-measurement experiments with
lighter mirrors.Comment: 13 pages, 4 figure
Musculoskeletal modelling of an ostrich (Struthio camelus) pelvic limb: influence of limb orientation on muscular capacity during locomotion
We developed a three-dimensional, biomechanical computer model of the 36 major pelvic limb muscle groups in an ostrich (Struthio camelus) to investigate muscle function in this, the largest of extant birds and model organism for many studies of locomotor mechanics, body size, anatomy and evolution. Combined with experimental data, we use this model to test two main hypotheses. We first query whether ostriches use limb orientations (joint angles) that optimize the moment-generating capacities of their muscles during walking or running. Next, we test whether ostriches use limb orientations at mid-stance that keep their extensor muscles near maximal, and flexor muscles near minimal, moment arms. Our two hypotheses relate to the control priorities that a large bipedal animal might evolve under biomechanical constraints to achieve more effective static weight support. We find that ostriches do not use limb orientations to optimize the moment-generating capacities or moment arms of their muscles. We infer that dynamic properties of muscles or tendons might be better candidates for locomotor optimization. Regardless, general principles explaining why species choose particular joint orientations during locomotion are lacking, raising the question of whether such general principles exist or if clades evolve different patterns (e.g., weighting of muscle force–length or force–velocity properties in selecting postures). This leaves theoretical studies of muscle moment arms estimated for extinct animals at an impasse until studies of extant taxa answer these questions. Finally, we compare our model’s results against those of two prior studies of ostrich limb muscle moment arms, finding general agreement for many muscles. Some flexor and extensor muscles exhibit self-stabilization patterns (posture-dependent switches between flexor/extensor action) that ostriches may use to coordinate their locomotion. However, some conspicuous areas of disagreement in our results illustrate some cautionary principles. Importantly, tendon-travel empirical measurements of muscle moment arms must be carefully designed to preserve 3D muscle geometry lest their accuracy suffer relative to that of anatomically realistic models. The dearth of accurate experimental measurements of 3D moment arms of muscles in birds leaves uncertainty regarding the relative accuracy of different modelling or experimental datasets such as in ostriches. Our model, however, provides a comprehensive set of 3D estimates of muscle actions in ostriches for the first time, emphasizing that avian limb mechanics are highly three-dimensional and complex, and how no muscles act purely in the sagittal plane. A comparative synthesis of experiments and models such as ours could provide powerful synthesis into how anatomy, mechanics and control interact during locomotion and how these interactions evolve. Such a framework could remove obstacles impeding the analysis of muscle function in extinct taxa
V39: an unusual object in the field of IC 1613
The variable star V39 in the field of IC 1613 is discussed in the light of
the available photometric and new spectroscopic data. It has strong emission
Balmer lines, and the observed characteristics could be explained by a W Vir
pulsating star with a period of 14.341 d, located at more than 115 kpc, that is
in the very outer halo of our Galaxy. It should have an apparent companion, a
long period (1118d) red variable, belonging to IC 1613. The main uncertainty in
this interpretation is an emission feature at 668.4 nm, which we tentatively
identified as a He I line.Comment: 5 pages; accepted for publication in Astronomy & Astrophysic
Dynamics of Large-Scale Plastic Deformation and the Necking Instability in Amorphous Solids
We use the shear transformation zone (STZ) theory of dynamic plasticity to
study the necking instability in a two-dimensional strip of amorphous solid.
Our Eulerian description of large-scale deformation allows us to follow the
instability far into the nonlinear regime. We find a strong rate dependence;
the higher the applied strain rate, the further the strip extends before the
onset of instability. The material hardens outside the necking region, but the
description of plastic flow within the neck is distinctly different from that
of conventional time-independent theories of plasticity.Comment: 4 pages, 3 figures (eps), revtex4, added references, changed and
added content, resubmitted to PR
Community Health Evaluation in Normanton
The Community Health research project was based in the Normanton Neighbourhood Renewal area of Derby. It involved five key parties; the Derby City Partnership, Derby City Primary Care Trust, the Derby City Council Environmental Health and Trading Standards Department, the Centre for
Community Regeneration and the International Centre for Guidance Studies (iCeGS) at the University of Derby. The research activities were led by the iCeGS.
The project had two main objectives. The first was to map health-related service provision in the Normanton area, and then secondly, to ascertain the reach and impact of that provision. An important sub-component of the evaluation element was to enhance community capacity by recruiting and
training a group of community evaluators in research and evaluation techniques to become competent community evaluators
Excitations of a Bose-condensed gas in anisotropic traps
We investigate the zero-temperature collective excitations of a
Bose-condensed atomic gas in anisotropic parabolic traps. The condensate
density is determined by solving the Gross-Pitaevskii (GP) equation using a
spherical harmonic expansion. The GP eigenfunctions are then used to solve the
Bogoliubov equations to obtain the collective excitation frequencies and mode
densities. The frequencies of the various modes, classified by their parity and
the axial angular momentum quantum number, m, are mapped out as a function of
the axial anisotropy. Specific emphasis is placed upon the evolution of these
modes from the modes in the limit of an isotropic trap.Comment: 7 pages Revtex, 9 Postscript figure
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