18,422 research outputs found
The shape and mechanics of curved fold origami structures
We develop recursion equations to describe the three-dimensional shape of a
sheet upon which a series of concentric curved folds have been inscribed. In
the case of no stretching outside the fold, the three-dimensional shape of a
single fold prescribes the shape of the entire origami structure. To better
explore these structures, we derive continuum equations, valid in the limit of
vanishing spacing between folds, to describe the smooth surface intersecting
all the mountain folds. We find that this surface has negative Gaussian
curvature with magnitude equal to the square of the fold's torsion. A series of
open folds with constant fold angle generate a helicoid
Models for the 3-D axisymmetric gravitational potential of the Milky Way Galaxy - A detailed modelling of the Galactic disk
Aims. Galaxy mass models based on simple and analytical functions for the
density and potential pairs have been widely proposed in the literature. Disk
models constrained by kinematic data alone give information on the global disk
structure only very near the Galactic plane. We attempt to circumvent this
issue by constructing disk mass models whose three-dimensional structures are
constrained by a recent Galactic star counts model in the near-infrared and
also by observations of the hydrogen distribution in the disk. Our main aim is
to provide models for the gravitational potential of the Galaxy that are fully
analytical but also with a more realistic description of the density
distribution in the disk component. Methods. From the disk model directly based
on the observations (here divided into the thin and thick stellar disks and the
HI and H disks subcomponents), we produce fitted mass models by combining
three Miyamoto-Nagai disk profiles of any "model order" (1, 2, or 3) for each
disk subcomponent. The Miyamoto-Nagai disks are combined with models for the
bulge and "dark halo" components and the total set of parameters is adjusted by
observational kinematic constraints. A model which includes a ring density
structure in the disk, beyond the solar Galactic radius, is also investigated.
Results. The Galactic mass models return very good matches to the imposed
observational constraints. In particular, the model with the ring density
structure provides a greater contribution of the disk to the rotational support
inside the solar circle. The gravitational potential models and their
associated force-fields are described in analytically closed forms, and in
addition, they are also compatible with our best knowledge of the stellar and
gas distributions in the disk component. The gravitational potential models are
suited for investigations of orbits in the Galactic disk.Comment: 22 pages, 13 figures, 11 tables, accepted for publication in A&
Enhancement of the Benjamin-Feir instability with dissipation
It is shown that there is an overlooked mechanism whereby some kinds of
dissipation can enhance the Benjamin-Feir instability of water waves. This
observation is new, and although it is counterintuitive, it is due to the fact
that the Benjamin-Feir instability involves the collision of modes with
opposite energy sign (relative to the carrier wave), and it is the negative
energy perturbations which are enhanced.Comment: 15 pages, 2 figures To download more papers, go to
http://www.cmla.ens-cachan.fr/~dias. Physics of Fluids (2007) to appea
Non-perturbative fixed points and renormalization group improved effective potential
The stability conditions of a renormalization group improved effective
potential have been discussed in the case of scalar QED and QCD with a
colorless scalar. We calculate the same potential in these models assuming the
existence of non-perturbative fixed points associated to a conformal phase. In
the case of scalar QED the barrier of instability found previously is barely
displaced as we approach the fixed point, and in the case of QCD with a
colorless scalar not only the barrier is changed but the local minimum of the
potential is also changed.Comment: 6 pages, 8 figures, References added. Matching the journal versio
Monte Carlo Simulations of Ultrathin Magnetic Dots
In this work we study the thermodynamic properties of ultrathin ferromagnetic
dots using Monte Carlo simulations. We investigate the vortex density as a
function of the temperature and the vortex structure in monolayer dots with
perpendicular anisotropy and long-range dipole interaction. The interplay
between these two terms in the hamiltonian leads to an interesting behavior of
the thermodynamic quantities as well as the vortex density.Comment: 10 figure
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