Two- and three-point Green's functions in two-dimensional Landau-gauge Yang-Mills theory

The ghost and gluon propagator and the ghost-gluon and three-gluon vertex of two-dimensional SU(2) Yang-Mills theory in (minimal) Landau gauge are studied using lattice gauge theory. It is found that the results are qualitatively similar to the ones in three and four dimensions. The propagators and the Faddeev-Popov operator behave as expected from the Gribov-Zwanziger scenario. In addition, finite volume effects affecting these Green's functions are investigated systematically. The critical infrared exponents of the propagators, as proposed in calculations using stochastic quantization and Dyson-Schwinger equations, are confirmed quantitatively. For this purpose lattices of volume up to (42.7 fm)^2 have been used.Comment: 14 pages, 14 figures, 4 tables, references adde

A MODULAR GEOMETRIC MODEL FOR UNDERWATER PHOTOGRAMMETRY

Underwater applications of photogrammetric measurement techniques usually need to deal with multimedia photogrammetry aspects, which are characterized by the necessity of handling optical rays that are broken at interfaces between optical media with different refrative indices according to Snell’s Law. This so-called multimedia geometry has to be incorporated into geometric models in order to achieve correct measurement results. The paper shows a flexible yet strict geometric model for the handling of refraction effects on the optical path, which can be implemented as a module into photogrammetric standard tools such as spatial resection, spatial intersection, bundle adjustment or epipolar line computation. The module is especially well suited for applications, where an object in water is observed by cameras in air through one or more plane parallel glass interfaces, as it allows for some simplifications here

Excitation of inertial modes in a closed grid turbulence experiment under rotation

We report an experimental study of the decay of grid-generated turbulence in a confined geometry submitted to a global rotation. Turbulence is generated by rapidly towing a grid in a parallelepipedic water tank. The velocity fields of a large number of independent decays are measured in a vertical plane parallel to the rotation axis using a corotating Particle Image Velocimetry system. We first show that, when a "simple" grid is used, a significant amount of the kinetic energy (typically 50%) is stored in a reproducible flow composed of resonant inertial modes. The spatial structure of those inertial modes, extracted by band-pass filtering, is found compatible with the numerical results of Maas [Fluid Dyn. Res. 33, 373 (2003)]. The possible coupling between these modes and turbulence suggests that turbulence cannot be considered as freely decaying in this configuration. Finally, we demonstrate that these inertial modes may be significantly reduced (down to 15% of the total energy) by adding a set of inner tanks attached to the grid. This suggests that it is possible to produce an effectively freely decaying rotating turbulence in a confined geometry

Constant force muscle stretching induces greater acute deformations and changes in passive mechanical properties compared to constant length stretching

Stretching is applied to lengthen shortened muscles in pathological conditions such as joint contractures. We investigated (i) the acute effects of different types of stretching, i.e. constant length (CL) and constant force (CF) stretching, on acute deformations and changes in passive mechanical properties of medial gastrocnemius muscle (MG) and (ii) the association of acute muscle–tendon deformations or changes in mechanical properties with the impulse or maximal strain of stretching. Forty-eight hindlimbs from 13 male and 12 female Wistar rats (13 weeks old, respectively 424.6 ± 35.5 and 261.8 ± 15.6 g) were divided into six groups (n = 8 each). The MG was initially stretched to a length at which the force was 75%, 95%, or 115% of the force corresponding to estimated maximal dorsiflexion and held at either CF or CL for 30 min. Before and after the stretching protocol, the MG peak force and peak stiffness were assessed by lengthening the passive muscle to the length corresponding to maximal ankle dorsiflexion. Also, the muscle belly length and tendon length were measured. CF stretching affected peak force, peak stiffness, muscle belly length, and tendon length more than CL stretching (p &lt; 0.01). Impulse was associated only with the decrease in peak force, while maximal strain was associated with the decrease in peak force, peak stiffness, and the increase in muscle belly length. We conclude that CF stretching results in greater acute deformations and changes in mechanical properties than CL stretching, which appears to be dependent predominantly on the differences in imposed maximal strain.</p

"Wet-to-Dry" Conformational Transition of Polymer Layers Grafted to Nanoparticles in Nanocomposite

The present communication reports the first direct measurement of the conformation of a polymer corona grafted around silica nano-particles dispersed inside a nanocomposite, a matrix of the same polymer. This measurement constitutes an experimental breakthrough based on a refined combination of chemical synthesis, which permits to match the contribution of the neutron silica signal inside the composite, and the use of complementary scattering methods SANS and SAXS to extract the grafted polymer layer form factor from the inter-particles silica structure factor. The modelization of the signal of the grafted polymer on nanoparticles inside the matrix and the direct comparison with the form factor of the same particles in solution show a clear-cut change of the polymer conformation from bulk to the nanocomposite: a transition from a stretched and swollen form in solution to a Gaussian conformation in the matrix followed with a compression of a factor two of the grafted corona. In the probed range, increasing the interactions between the grafted particles (by increasing the particle volume fraction) or between the grafted and the free matrix chains (decreasing the grafted-free chain length ratio) does not influence the amplitude of the grafted brush compression. This is the first direct observation of the wet-to-dry conformational transition theoretically expected to minimize the free energy of swelling of grafted chains in interaction with free matrix chains, illustrating the competition between the mixing entropy of grafted and free chains, and the elastic deformation of the grafted chains. In addition to the experimental validation of the theoretical prediction, this result constitutes a new insight for the nderstanding of the general problem of dispersion of nanoparticles inside a polymer matrix for the design of new nanocomposites materials

Tensor Algebra: A Combinatorial Approach to the Projective Geometry of Figures

This paper explores the combinatorial aspects of symmetric and antisymmetric forms represented in tensor algebra. The development of geometric perspective gained from tensor algebra has resulted in the discovery of a novel projection operator for the Chow form of a curve in P3 with applications to computer vision

Large-Eddy Simulation of inhomogeneous canopy flows using high resolution terrestrial laser scanning data

The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation (LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 190m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30m) along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing, where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure. Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing

Observations of fronts in the North Sea

The persistent presence of a cold bottom layer and associated bottom fronts was observed in the stratified central North Sea during an observational program in 1981 and 1982. Moored instruments, capturing a snap-shot of such a front while it was advected past these moorings, revealed the simultaneous presence of a well-defined frontal jet with velocities up to 15 cm s−1. The Coriolis force acting on this jet appeared to be in geostrophic balance with the locally intense pressure gradient forces. Hydrographic surveys revealed the presence of both small-scale and large-scale baroclinic waves on this front, the latter reaching wavelengths of 5–10 internal Ross by radii. Some evidence for a weak secondary circulation in the cross-frontal plane was obtained from the observed deformation of isolines near the front

Measurement of Strange Quark Contributions to the Nucleon's Form Factors at Q^2=0.230 (GeV/c)^2

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a $Q^2$ of 0.230 (GeV/c)^2 and a scattering angle of \theta_e = 30^o - 40^o. Using a large acceptance fast PbF_2 calorimeter with a solid angle of \Delta\Omega = 0.62 sr the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A_{phys} =(-5.44 +- 0.54_{stat} +- 0.27_{\rm sys}) 10^{-6}. The Standard Model expectation assuming no strangeness contributions to the vector form factors is $A_0=(-6.30 +- 0.43) 10^{-6}$. The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G^s_E + 0.225 G^s_M = 0.039 +- 0.034 or F^s_1 + 0.130 F^s_2 = 0.032 +- 0.028.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Letters on Dec 11, 200