2,238 research outputs found
Slow flows of yield stress fluids: complex spatio-temporal behaviour within a simple elasto-plastic model
A minimal athermal model for the flow of dense disordered materials is
proposed, based on two generic ingredients: local plastic events occuring above
a microscopic yield stress, and the non-local elastic release of the stress
these events induce in the material. A complex spatio-temporal rheological
behaviour results, with features in line with recent experimental observations.
At low shear rates, macroscopic flow actually originates from collective
correlated bursts of plastic events, taking place in dynamically generated
fragile zones. The related correlation length diverges algebraically at small
shear rates. In confined geometries bursts occur preferentially close to the
walls yielding an intermittent form of flow localization.Comment: 4 pages, 4 figure
On the effects of roughness on the nonlinear dynamics of a bolted joint: a multiscale analysis
Accurate prediction of the vibration response of friction joints is of great importance when estimating both the performance and the life of build-up structures. The contact conditions at the joint interface, including local normal load distribution and contact stiffness, play a critical role in the nonlinear dynamic response. These parameters strongly depend on the mating surfaces, where the surface roughness is well known to have a significant impact on the contact conditions in the static case. In contrast, its effects on the global and local nonlinear dynamic response of a build-up structure is not as well understood due to the complexity of the involved mechanisms. To obtain a better understanding of the dependence of the nonlinear dynamic response on surface roughness, a newly proposed multiscale approach has been developed. It links the surface roughness to the contact pressure and contact stiffness, and in combination with a multiharmonic balance solver, allows to compute the nonlinear dynamic response for different interface roughness. An application of the technique to a single bolted lap joint highlighted a strong impact of larger roughness values on the pressure distribution and local contact stiffness and in turn on the nonlinear dynamic response
The ground state of binary systems with a periodic modulation of the linear coupling
We consider a quasi-one-dimensional two-component systm, described by a pair
of Nonlinear Schr\"{o}dinger/Gross-Pitaevskii Equations (NLSEs/GPEs), which are
coupled by the linear mixing, with local strength , and by the
nonlinear incoherent interaction. We assume the self-repulsive nonlinearity in
both components, and include effects of a harmonic trapping potential. The
model may be realized in terms of periodically modulated slab waveguides in
nonlinear optics, and in Bose-Einstein condensates too. Depending on the
strengths of the linear and nonlinear couplings between the components, the
ground states (GSs) in such binary systems may be symmetric or asymmetric. In
this work, we introduce a periodic spatial modulation of the linear coupling,
making an odd, or even function of the coordinate. The sign flips of
strongly modify the structure of the GS in the binary system, as
the relative sign of its components tends to lock to the local sign of . Using a systematic numerical analysis, and an analytical approximation, we
demonstrate that the GS of the trapped system contains one or several kinks
(dark solitons) in one component, while the other component does not change its
sign. Final results are presented in the form of maps showing the number of
kinks in the GS as a function of the system's parameters, with the odd/even
modulation function giving rise to the odd/even number of the kinks. The
modulation of also produces a strong effect on the transition
between states with nearly equal and strongly unequal amplitudes of the two
components.Comment: 8 pages, 3 figure
Classical Loop Actions of Gauge Theories
Since the first attempts to quantize Gauge Theories and Gravity in the loop
representation, the problem of the determination of the corresponding classical
actions has been raised. Here we propose a general procedure to determine these
actions and we explicitly apply it in the case of electromagnetism. Going to
the lattice we show that the electromagnetic action in terms of loops is
equivalent to the Wilson action, allowing to do Montecarlo calculations in a
gauge invariant way. In the continuum these actions need to be regularized and
they are the natural candidates to describe the theory in a ``confining
phase''.Comment: LaTeX 14 page
Fluctuations of Fluctuation-Induced "Casimir" Forces
The force experienced by objects embedded in a correlated medium undergoing
thermal fluctuations--the so-called fluctuation--induced force--is actually
itself a fluctuating quantity. We compute the corresponding probability
distribution and show that it is a Gaussian centered on the well-known Casimir
force, with a non-universal standard deviation that can be typically as large
as the mean force itself. The relevance of these results to the experimental
measurement of fluctuation-induced forces is discussed, as well as the
influence of the finite temporal resolution of the measuring apparatus.Comment: 4 pages, 2 figure
Recommended from our members
Anomalous x-ray scattering studies of short-, intermediate- and extended-range order in glasses
The authors present the formalism of anomalous x-ray scattering as applied to partial structure analysis of disordered materials, and give an example of how the technique has been applied, together with that of neutron diffraction, to investigate short-, intermediate- and extended-range order in vitreous germania and rubidium germanate
Cosmic Histories of Stars, Gas, Heavy Elements, and Dust
We present a set of coupled equations that relate the stellar, gaseous,
chemical, and radiation constituents of the universe averaged over the whole
galaxy population. Using as input the available data from quasar
absorption-line surveys, optical imaging and redshift surveys, and the COBE
DIRBE and FIRAS extragalactic infrared background measurements, we obtain
solutions for the cosmic histories of stars, interstellar gas, heavy elements,
dust, and radiation from stars and dust in galaxies. Our solutions reproduce
remarkably well a wide variety of observations that were not used as input,
including the integrated background light from galaxy counts, the optical and
near-infrared emissivities from galaxy surveys, the local infrared emissivities
from the IRAS survey, the mean abundance of heavy elements from surveys of
damped Lyman-alpha systems, and the global star formation rates from H
surveys and submillimeter observations. The solutions presented here suggest
that the process of galaxy formation appears to have undergone an early period
of substantial inflow to assemble interstellar gas at , a subsequent
period of intense star formation and chemical enrichment at , and a recent period of rapid decline in the gas content, star
formation rate, optical stellar emissivity, and infrared dust emission at
. [abridged version]Comment: 29 pages, ApJ in press, 10 Sept 9
The gait and balance of patients with diabetes can be improved: a randomised controlled trial
Aims/hypothesis: Gait characteristics and balance are altered in diabetic patients. Little is known about possible treatment strategies. This study evaluates the effect of a specific training programme on gait and balance of diabetic patients. Methods: This was a randomised controlled trial (n = 71) with an intervention (n = 35) and control group (n = 36). The intervention consisted of physiotherapeutic group training including gait and balance exercises with function-orientated strengthening (twice weekly over 12weeks). Controls received no treatment. Individuals were allocated to the groups in a central office. Gait, balance, fear of falls, muscle strength and joint mobility were measured at baseline, after intervention and at 6-month follow-up. Results: The trial is closed to recruitment and follow-up. After training, the intervention group increased habitual walking speed by 0.149m/s (p < 0.001) compared with the control group. Patients in the intervention group also significantly improved their balance (time to walk over a beam, balance index recorded on Biodex balance system), their performance-oriented mobility, their degree of concern about falling, their hip and ankle plantar flexor strength, and their hip flexion mobility compared with the control group. After 6months, all these variables remained significant except for the Biodex sway index and ankle plantar flexor strength. Two patients developed pain in their Achilles tendon: the progression for two related exercises was slowed down. Conclusions/interpretation: Specific training can improve gait speed, balance, muscle strength and joint mobility in diabetic patients. Further studies are needed to explore the influence of these improvements on the number of reported falls, patients' physical activity levels and quality of life. Trial registration:: ClinicalTrials.gov NCT00637546 Funding:: This work was supported by the Swiss National Foundation (SNF): PBSKP-123446/
Effective interactions between inclusions in complex fluids driven out of equilibrium
The concept of fluctuation-induced effective interactions is extended to
systems driven out of equilibrium. We compute the forces experienced by
macroscopic objects immersed in a soft material driven by external shaking
sources. We show that, in contrast with equilibrium Casimir forces induced by
thermal fluctuations, their sign, range and amplitude depends on specifics of
the shaking and can thus be tuned. We also comment upon the dispersion of these
shaking-induced forces, and discuss their potential application to phase
ordering in soft-materials.Comment: 10 pages, 8 figures, to appear in PR
A Survey of Satisfiability Modulo Theory
Satisfiability modulo theory (SMT) consists in testing the satisfiability of
first-order formulas over linear integer or real arithmetic, or other theories.
In this survey, we explain the combination of propositional satisfiability and
decision procedures for conjunctions known as DPLL(T), and the alternative
"natural domain" approaches. We also cover quantifiers, Craig interpolants,
polynomial arithmetic, and how SMT solvers are used in automated software
analysis.Comment: Computer Algebra in Scientific Computing, Sep 2016, Bucharest,
Romania. 201
- …