2,057 research outputs found
Non-paraxial Talbot effect in one-dimensional gratings with period comparable to the wavelength
The Talbot effect is a diffractive phenomenon which was found since its inception in the Gaussian optics, but when the period of the object is comparable with the wavelength is considered entering its non-paraxial regime. There, it has attracted interest for its applications related to the design of objects and their restorative effects posing in damaged gratings on this scale. In this contribution a study of this phenomenon at level non-paraxial (metaxial) is presented by using scalar diffraction theory in plane waves representation for periodic transversal fields. To this end, a one-dimensional amplitude grating of period p is used which is illuminated by a monochromatic plane wave of wavelength λ where p ranges between 1,5 λ and 4,2 λ.Fil: Arrieta, E.. Universidad Popular del Cesar; ColombiaFil: Bolognini, Nestor Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; ArgentinaFil: Torres, C.O.. Universidad Popular del Cesar; Colombi
Mechanical characterization of disordered and anisotropic cellular monolayers
We consider a cellular monolayer, described using a vertex-based model, for
which cells form a spatially disordered array of convex polygons that tile the
plane. Equilibrium cell configurations are assumed to minimize a global energy
defined in terms of cell areas and perimeters; energy is dissipated via dynamic
area and length changes, as well as cell neighbour exchanges. The model
captures our observations of an epithelium from a Xenopus embryo showing that
uniaxial stretching induces spatial ordering, with cells under net tension
(compression) tending to align with (against) the direction of stretch, but
with the stress remaining heterogeneous at the single-cell level. We use the
vertex model to derive the linearized relation between tissue-level stress,
strain and strain-rate about a deformed base state, which can be used to
characterize the tissue's anisotropic mechanical properties; expressions for
viscoelastic tissue moduli are given as direct sums over cells. When the base
state is isotropic, the model predicts that tissue properties can be tuned to a
regime with high elastic shear resistance but low resistance to area changes,
or vice versa.Comment: 9 figure
The Method of Multiple Scales: Asymptotic Solutions and Normal Forms for Nonlinear Oscillatory Problems
AbstractThe method of multiple scales is implemented in Maple V Release 2 to generate a uniform asymptotic solutionO(ϵr) for a weakly nonlinear oscillator.In recent work, it has been shown that the method of multiple scales also transforms the differential equations into normal form, so the given algorithm can be used to simplify the equations describing the dynamics of a system near a fixed point.These results are equivalent to those obtained with the traditional method of normal forms which uses a near-identity coordinate transformation to get the system into the “simplest” form.A few Duffing type oscillators are analysed to illustrate the power of the procedure. The algorithm can be modified to take care of systems of ODEs, PDEs and other nonlinear cases
Marinas as habitats for nearshore fish assemblages: comparative analysis of underwater visual census, baited cameras and fish traps
Understanding the ecological role that artificial structures might play on nearshore fish assemblages requires the collection of accurate and reliable data through efficient sampling techniques. In this work, differences in the composition and structure of fish assemblages between the inner and outer sides of three marinas located in the temperate northern-eastern Atlantic Ocean were tested using three complementary sampling techniques: underwater visual censuses (UVC), baited cameras (BCs) and fish traps (FTs). UVCs and BCs recorded a comparable number and relative abundance of species, which in turn were much greater than those recorded by FTs. This finding supports the use of UVCs and BCs over FTs for broad ecologically studies, especially when dealing with structurally complex habitats such as artificial structures. We found differences in fish assemblage structure between the inner and outer sides of marinas, independently of the sampling method. Four small-sized species (Similiparma lurida, Thalassoma pavo, Sarpa salpa and Symphodus roissali) associated with structurally complex vegetated habitats dominated, in terms of abundance, the outer sides of marinas; Diplodus vulgaris, Diplodus sargus and Gobius niger, species with high ecological plasticity in habitat requirements, dominated the inner sides of marinas. The information provided in this study is of great interest for developing sound monitoring programmes to ascertain the effects of artificial structures on fish communities.info:eu-repo/semantics/publishedVersio
The Quasar-frame Velocity Distribution of Narrow CIV Absorbers
We report on a survey for narrow (FWHM < 600 km/s) CIV absorption lines in a
sample of bright quasars at redshifts in the Sloan Digital
Sky Survey. Our main goal is to understand the relationship of narrow CIV
absorbers to quasar outflows and, more generally, to quasar environments. We
determine velocity zero-points using the broad MgII emission line, and then
measure the absorbers' quasar-frame velocity distribution. We examine the
distribution of lines arising in quasar outflows by subtracting model fits to
the contributions from cosmologically intervening absorbers and absorption due
to the quasar host galaxy or cluster environment. We find a substantial number
( per cent) of absorbers with REW \AA in the velocity range
+750 km/s \la v \la +12000 km/s are intrinsic to the AGN outflow. This
`outflow fraction' peaks near km/s with a value of . At velocities below km/s the incidence
of outflowing systems drops, possibly due to geometric effects or to the
over-ionization of gas that is nearer the accretion disk. Furthermore, we find
that outflow-absorbers are on average broader and stronger than
cosmologically-intervening systems. Finally, we find that per cent of
the quasars in our sample exhibit narrow, outflowing CIV absorption with REW \AA, slightly larger than that for broad absorption line systems.Comment: 11 pages, 9 figures, accepted for publication in MNRA
- …