668 research outputs found
The dynamic characterisation of disk geometry particle dampers
Particle dampers (PDs) have the advantages of being simple in geometry, small in volume and applicable in extreme temperature environments. Experimental studies have shown that PDs can offer considerable potential for suppressing structural resonant conditions over a wide frequency range. In this paper, the nonlinear characteristics of PDs are studied experimentally in a series of response-level-controlled tests. The effect of the geometry is studied and a method is developed to model the nonlinear damping of PDs as equivalent viscous dampers that can be applied directly to engineering structures at the design stage
Extending the D'Alembert Solution to Space-Time Modified Riemann-Liouville Fractional Wave Equations
In the realm of complexity, it is argued that adequate modeling of
TeV-physics demands an approach based on fractal operators and fractional
calculus (FC). Non-local theories and memory effects are connected to
complexity and the FC. The non-differentiable nature of the microscopic
dynamics may be connected with time scales. Based on the Modified
Riemann-Liouville definition of fractional derivatives, we have worked out
explicit solutions to a fractional wave equation with suitable initial
conditions to carefully understand the time evolution of classical fields with
a fractional dynamics. First, by considering space-time partial fractional
derivatives of the same order in time and space, a generalized fractional
D'Alembertian is introduced and by means of a transformation of variables to
light-cone coordinates, an explicit analytical solution is obtained. To address
the situation of different orders in the time and space derivatives, we adopt
different approaches, as it will become clear throughout the paper. Aspects
connected to Lorentz symmetry are analyzed in both approaches.Comment: 8 page
Proton-proton bremsstrahlung below and above pion-threshold: the influence of the -isobar
The proton-proton bremsstrahlung is investigated within a coupled-channel
model with the degree of freedom. The model is consistent with the
scattering up to 1 GeV and the vertex determined in the
study of pion photoproduction reactions. It is found that the
excitation can significantly improve the agreements with the at MeV. Predictions at and MeV are
presented for future experimental tests.Comment: 26 pages Revtex, 12 figures are available from the authors upon
request ([email protected]
Entropy Identity and Material-Independent Equilibrium Conditions in Relativistic Thermodynamics
On the basis of the balance equations for energy-momentum, spin, particle and
entropy density, an approach is considered which represents a comparatively
general framework for special- and general-relativistic continuum
thermodynamics. In the first part of the paper, a general entropy density
4-vector, containing particle, energy-momentum, and spin density contributions,
is introduced which makes it possible, firstly, to judge special assumptions
for the entropy density 4-vector made by other authors with respect to their
generality and validity and, secondly, to determine entropy supply and entropy
production. Using this entropy density 4-vector, in the second part,
material-independent equilibrium conditions are discussed. While in literature,
at least if one works in the theory of irreversible thermodynamics assuming a
Riemann space-time structure, generally thermodynamic equilibrium is determined
by introducing a variety of conditions by hand, the present approach proceeds
as follows: For a comparatively wide class of space-time geometries the
necessary equilibrium conditions of vanishing entropy supply and entropy
production are exploited and, afterwards, supplementary conditions are assumed
which are motivated by the requirement that thermodynamic equilibrium
quantities have to be determined uniquely.Comment: Research Paper, 30 page
Chiral Symmetry and light resonances in hot and dense matter
We present a study of the scattering amplitude in the and
channels at finite temperature and nuclear density within a chiral
unitary framework. Meson resonances are dynamically generated in our approach,
which allows us to analyze the behavior of their associated scattering poles
when the system is driven towards chiral symmetry restoration. Medium effects
are incorporated in three ways: (a) by thermal corrections of the unitarized
scattering amplitudes, (b) by finite nuclear density effects associated to a
renormalization of the pion decay constant, and complementarily (c) by
extending our calculation of the scalar-isoscalar channel to account for finite
nuclear density and temperature effects in a microscopic many-body
implementation of pion dynamics. Our results are discussed in connection with
several phenomenological aspects relevant for nuclear matter and Heavy-Ion
Collision experiments, such as mass scaling vs broadening from dilepton
spectra and chiral restoration signals in the channel. We also
elaborate on the molecular nature of resonances.Comment: 14 pages, 14 figures. Contribution to Hard Probes 2008, Illa de A
Toxa, Spain, June 8th-14th 200
Density waves in dry granular media falling through a vertical pipe
We report experimental measurements of density waves in granular materials
flowing down in a capillary tube. The density wave regime occurs at
intermediate flow rates between a low density free fall regime and a high
compactness slower flow.Comment: LaTeX file, 17 pages, 6 EPS figures, Phys.Rev.E (Feb.1996
Depression and anxiety in adolescents and adults with cystic fibrosis in the UK: A cross-sectional study
Background
The International Depression/anxiety Epidemiological Study (TIDES) in the UK aimed: (i) to establish the prevalence of anxiety and depression amongst people with CF compared to a normative sample; (ii) to establish the association between mood, demographic and clinical variables; and (iii) to provide guidance for specialist-referral decision-making.
Methods
Patients (≥ 12 years) completed the Hospital Anxiety and Depression Scale (HADS). CF-HADS scores, expressed as percentiles, were compared with a normative sample. Multiple-regression analysis explored associations between demographic, clinical and mood variables.
Results
Thirty-nine CF centres recruited 2065 patients. Adults with CF were similar in terms of anxiety and depression to the general population. Adolescents with CF were less anxious and depressed. For adult patients, older age, unemployment for health reasons and poor lung function were associated with disordered mood. Gender-specific CF-percentile scores were calculated.
Conclusion
Surveillance, with attention to gender and risk factors is advocated. This work provides unique benchmark scores to aid referral decision-making
Passing to the Limit in a Wasserstein Gradient Flow: From Diffusion to Reaction
We study a singular-limit problem arising in the modelling of chemical
reactions. At finite {\epsilon} > 0, the system is described by a Fokker-Planck
convection-diffusion equation with a double-well convection potential. This
potential is scaled by 1/{\epsilon}, and in the limit {\epsilon} -> 0, the
solution concentrates onto the two wells, resulting into a limiting system that
is a pair of ordinary differential equations for the density at the two wells.
This convergence has been proved in Peletier, Savar\'e, and Veneroni, SIAM
Journal on Mathematical Analysis, 42(4):1805-1825, 2010, using the linear
structure of the equation. In this paper we re-prove the result by using solely
the Wasserstein gradient-flow structure of the system. In particular we make no
use of the linearity, nor of the fact that it is a second-order system. The
first key step in this approach is a reformulation of the equation as the
minimization of an action functional that captures the property of being a
curve of maximal slope in an integrated form. The second important step is a
rescaling of space. Using only the Wasserstein gradient-flow structure, we
prove that the sequence of rescaled solutions is pre-compact in an appropriate
topology. We then prove a Gamma-convergence result for the functional in this
topology, and we identify the limiting functional and the differential equation
that it represents. A consequence of these results is that solutions of the
{\epsilon}-problem converge to a solution of the limiting problem.Comment: Added two sections, corrected minor typos, updated reference
Three-body decay of the d* dibaryon
Under certain circumstances, a three-body decay width can be approximated by
an integral involving a product of two off-shell two-body decay widths. This
``angle-average'' approximation is used to calculate the decay width
of the dibaryon in a simple model for the most
important Feynman diagrams describing pion emissions with baryon-baryon recoil
and meson retardation. The decay width is found to be about 0.006 (0.07, 0.5)
MeV at the mass of 2065 (2100, 2150) MeV for input dynamics derived from
the Full Bonn potential. The smallness of this width is qualitatively
understood as the result of the three-body decay being ``third forbidden''. The
concept of forbiddenness and the threshold behavior of a three-body
decay are further studied in connection with the decay of the dibaryon
where the idea of unfavorness has to be introduced.
The implications of these results are briefly discussed.Comment: 15 pages, RevTeX, two-column journal style, six figure
Simultaneous Softening of sigma and rho Mesons associated with Chiral Restoration
Complex poles of the unitarized pi-pi scattering amplitude in nuclear matter
are studied. Partial restoration of chiral symmetry is modeled by the decrease
of in-medium pion decay constant f*_{pi}.
For large chiral restoration (f*_{pi}/f_{pi} << 1),
2nd sheet poles in the scalar (sigma) and the vector (rho) mesons are both
dictated by the Lambert W function and show universal softening as f*_{pi}
decreases.
In-medium pi-pi cross section receives substantial contribution from the soft
mode and exhibits a large enhancement in low-energy region.
Fate of this universality for small chiral restoration (f*_{pi}/f_{pi} ~ 1)
is also discussed.Comment: 5 pages, 4-eps figures, version accepted by Phys. Rev. C (R) with
minor modification
- …