9,765 research outputs found
The effects of geometric uncertainties on computational modelling of knee biomechanics
The geometry of the articular components of the knee is an important factor in predicting joint mechanics in computational models. There are a number of uncertainties in the definition of the geometry of cartilage and meniscus, and evaluating the effects of these uncertainties is fundamental to understanding the level of reliability of the models. In this study, the sensitivity of knee mechanics to geometric uncertainties was investigated by comparing polynomial-based and image-based knee models and varying the size of meniscus. The results suggested that the geometric uncertainties in cartilage and meniscus resulting from the resolution of MRI and the accuracy of segmentation caused considerable effects on the predicted knee mechanics. Moreover, even if the mathematical geometric descriptors can be very close to the imaged-based articular surfaces, the detailed contact pressure distribution produced by the mathematical geometric descriptors was not the same as that of the image-based model. However, the trends predicted by the models based on mathematical geometric descriptors were similar to those of the imaged-based models
Experimental validation of a new biphasic model of the contact mechanics of the porcine hip
Hip models that incorporate the biphasic behaviour of articular cartilage can improve understanding of the joint function, pathology of joint degeneration and effect of potential interventions. The aim of this study was to develop a specimen-specific biphasic finite element model of a porcine acetabulum incorporating a biphasic representation of the articular cartilage and to validate the model predictions against direct experimental measurements of the contact area in the same specimen. Additionally, the effect of using a different tension-compression behaviour for the solid phase of the articular cartilage was investigated. The model represented different radial clearances and load magnitudes. The comparison of the finite element predictions and the experimental measurement showed good agreement in the location, size and shape of the contact area, and a similar trend in the relationship between contact area and load was observed. There was, however, a deviation of over 30% in the magnitude of the contact area, which might be due to experimental limitations or to simplifications in the material constitutive relationships used. In comparison with the isotropic solid phase model, the tension-compression solid phase model had better agreement with the experimental observations. The findings provide some confidence that the new biphasic methodology for modelling the cartilage is able to predict the contact mechanics of the hip joint. The validation provides a foundation for future subject-specific studies of the human hip using a biphasic cartilage model
Dipolar origin of the gas-liquid coexistence of the hard-core 1:1 electrolyte model
We present a systematic study of the effect of the ion pairing on the
gas-liquid phase transition of hard-core 1:1 electrolyte models. We study a
class of dipolar dimer models that depend on a parameter R_c, the maximum
separation between the ions that compose the dimer. This parameter can vary
from sigma_{+/-} that corresponds to the tightly tethered dipolar dimer model,
to R_c --> infinity, that corresponds to the Stillinger-Lovett description of
the free ion system. The coexistence curve and critical point parameters are
obtained as a function of R_c by grand canonical Monte Carlo techniques. Our
results show that this dependence is smooth but non-monotonic and converges
asymptotically towards the free ion case for relatively small values of R_c.
This fact allows us to describe the gas-liquid transition in the free ion model
as a transition between two dimerized fluid phases. The role of the unpaired
ions can be considered as a perturbation of this picture.Comment: 16 pages, 13 figures, submitted to Physical Review
Asymmetric Primitive-Model Electrolytes: Debye-Huckel Theory, Criticality and Energy Bounds
Debye-Huckel (DH) theory is extended to treat two-component size- and
charge-asymmetric primitive models, focussing primarily on the 1:1 additive
hard-sphere electrolyte with, say, negative ion diameters, a--, larger than the
positive ion diameters, a++. The treatment highlights the crucial importance of
the charge-unbalanced ``border zones'' around each ion into which other ions of
only one species may penetrate. Extensions of the DH approach which describe
the border zones in a physically reasonable way are exact at high and low
density, , and, furthermore, are also in substantial agreement with
recent simulation predictions for \emph{trends} in the critical parameters,
and , with increasing size asymmetry. Conversely, the simplest
linear asymmetric DH description, which fails to account for physically
expected behavior in the border zones at low , can violate a new lower bound
on the energy (which applies generally to models asymmetric in both charge and
size). Other recent theories, including those based on the mean spherical
approximation, have predicted trends in the critical parameters quite opposite
to those established by the simulations.Comment: to appear in Physical Review
Mitigation of Pulsed Interference to Redshifted HI and OH Observations between 960 and 1215 MHz
The neutral hydrogen 21-cm spectral line (1420.4 MHz) and the four 18-cm
lines of the hydroxyl molecule (1612-1720 MHz) are observable at redshifts
which put their measured line frequencies well below their protected frequency
bands. Part of the redshift ranges (z = 0.171-0.477 for HI and z = 0.37-0.73
for OH) fall in the 960 to 1215 MHz band that is allocated to aircraft
navigation. Most of the signals in this band are pulsed emissions of low duty
cycle so much of the time between pulses is interference free. This paper
outlines the structure and measured properties of signals in this band and
demonstrates a signal processing strategy that is effective at removing the
pulsed signals from spectra at sensitivities produced by several hours of
integration.Comment: Astronomical Journal, May 2005, in pres
Political participation: the vocational motivations of Labour party employees
Party employees are an under-researched group in political science. This article begins to address this oversight by examining Labour Party employees using new quantitative and qualitative data. It argues that party employment should be regarded as a form of political participation and as a consequence, existing models of political participation can be utilised to help explain why people work for political parties. After testing these propositions, the article concludes that existing models are indeed helpful in explaining the motivations for party employment
Universality class of criticality in the restricted primitive model electrolyte
The 1:1 equisized hard-sphere electrolyte or restricted primitive model has
been simulated via grand-canonical fine-discretization Monte Carlo. Newly
devised unbiased finite-size extrapolation methods using temperature-density,
(T, rho), loci of inflections, Q = ^2/ maxima, canonical and C_V
criticality, yield estimates of (T_c, rho_c) to +- (0.04, 3)%. Extrapolated
exponents and Q-ratio are (gamma, nu, Q_c) = [1.24(3), 0.63(3); 0.624(2)] which
support Ising (n = 1) behavior with (1.23_9, 0.630_3; 0.623_6), but exclude
classical, XY (n = 2), SAW (n = 0), and n = 1 criticality with potentials
phi(r)>Phi/r^{4.9} when r \to \infty
Abstract composition rule for relativistic kinetic energy in the thermodynamical limit
We demonstrate by simple mathematical considerations that a power-law tailed
distribution in the kinetic energy of relativistic particles can be a limiting
distribution seen in relativistic heavy ion experiments. We prove that the
infinite repetition of an arbitrary composition rule on an infinitesimal amount
leads to a rule with a formal logarithm. As a consequence the stationary
distribution of energy in the thermodynamical limit follows the composed
function of the Boltzmann-Gibbs exponential with this formal logarithm. In
particular, interactions described as solely functions of the relative
four-momentum squared lead to kinetic energy distributions of the
Tsallis-Pareto (cut power-law) form in the high energy limit.Comment: Submitted to Europhysics Letters. LaTeX, 3 eps figure
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