7,837 research outputs found
Pushing the Limits of 3D Color Printing: Error Diffusion with Translucent Materials
Accurate color reproduction is important in many applications of 3D printing,
from design prototypes to 3D color copies or portraits. Although full color is
available via other technologies, multi-jet printers have greater potential for
graphical 3D printing, in terms of reproducing complex appearance properties.
However, to date these printers cannot produce full color, and doing so poses
substantial technical challenges, from the shear amount of data to the
translucency of the available color materials. In this paper, we propose an
error diffusion halftoning approach to achieve full color with multi-jet
printers, which operates on multiple isosurfaces or layers within the object.
We propose a novel traversal algorithm for voxel surfaces, which allows the
transfer of existing error diffusion algorithms from 2D printing. The resulting
prints faithfully reproduce colors, color gradients and fine-scale details.Comment: 15 pages, 14 figures; includes supplemental figure
Magnetic flux captured by an accretion disk
We suggest a possible mechanism of efficient transport of the large-scale
external magnetic flux inward through a turbulent accretion disk. The outward
drift by turbulent diffusion which limits the effectiveness of this process is
reduced if the external flux passes through the disk in concentrated patches.
Angular momentum loss by a magnetocentrifugal wind from these patches of strong
field adds to the inward drift. We propose that magnetic flux accumulating in
this way at the center of the disk provides the `second parameter' determining
X-ray spectral states of black hole binaries, and the presence of relativistic
outflows
On the transition to turbulence of wall-bounded flows in general, and plane Couette flow in particular
The main part of this contribution to the special issue of EJM-B/Fluids
dedicated to Patrick Huerre outlines the problem of the subcritical transition
to turbulence in wall-bounded flows in its historical perspective with emphasis
on plane Couette flow, the flow generated between counter-translating parallel
planes. Subcritical here means discontinuous and direct, with strong
hysteresis. This is due to the existence of nontrivial flow regimes between the
global stability threshold Re_g, the upper bound for unconditional return to
the base flow, and the linear instability threshold Re_c characterized by
unconditional departure from the base flow. The transitional range around Re_g
is first discussed from an empirical viewpoint ({\S}1). The recent
determination of Re_g for pipe flow by Avila et al. (2011) is recalled. Plane
Couette flow is next examined. In laboratory conditions, its transitional range
displays an oblique pattern made of alternately laminar and turbulent bands, up
to a third threshold Re_t beyond which turbulence is uniform. Our current
theoretical understanding of the problem is next reviewed ({\S}2): linear
theory and non-normal amplification of perturbations; nonlinear approaches and
dynamical systems, basin boundaries and chaotic transients in minimal flow
units; spatiotemporal chaos in extended systems and the use of concepts from
statistical physics, spatiotemporal intermittency and directed percolation,
large deviations and extreme values. Two appendices present some recent
personal results obtained in plane Couette flow about patterning from numerical
simulations and modeling attempts.Comment: 35 pages, 7 figures, to appear in Eur. J. Mech B/Fluid
Effect of Noise on Patterns Formed by Growing Sandpiles
We consider patterns generated by adding large number of sand grains at a
single site in an abelian sandpile model with a periodic initial configuration,
and relaxing. The patterns show proportionate growth. We study the robustness
of these patterns against different types of noise, \textit{viz.}, randomness
in the point of addition, disorder in the initial periodic configuration, and
disorder in the connectivity of the underlying lattice. We find that the
patterns show a varying degree of robustness to addition of a small amount of
noise in each case. However, introducing stochasticity in the toppling rules
seems to destroy the asymptotic patterns completely, even for a weak noise. We
also discuss a variational formulation of the pattern selection problem in
growing abelian sandpiles.Comment: 15 pages,16 figure
Oceanic three-dimensional Lagrangian Coherent Structures: A study of a mesoscale eddy in the Benguela ocean region
We study three dimensional oceanic Lagrangian Coherent Structures (LCSs) in
the Benguela region, as obtained from an output of the ROMS model. To do that
we first compute Finite-Size Lyapunov exponent (FSLE) fields in the region
volume, characterizing mesoscale stirring and mixing. Average FSLE values show
a general decreasing trend with depth, but there is a local maximum at about
100 m depth. LCSs are extracted as ridges of the calculated FSLE fields. They
present a "curtain-like" geometry in which the strongest attracting and
repelling structures appear as quasivertical surfaces. LCSs around a particular
cyclonic eddy, pinched off from the upwelling front are also calculated. The
LCSs are confirmed to provide pathways and barriers to transport in and out of
the eddy
Transition from connected to fragmented vegetation across an environmental gradient: scaling laws in ecotone geometry
A change in the environmental conditions across space—for example, altitude or latitude—can cause significant changes in the density of a vegetation type and, consequently, in spatial connectivity. We use spatially explicit simulations to study the transition from connected to fragmented vegetation. A static (gradient percolation) model is compared to dynamic (gradient contact process) models. Connectivity is characterized from the perspective of various species that use this vegetation type for habitat and differ in dispersal or migration range, that is, “step length” across the landscape. The boundary of connected vegetation delineated by a particular step length is termed the “ hull edge.” We found that for every step length and for every gradient, the hull edge is a fractal with dimension 7/4. The result is the same for different spatial models, suggesting that there are universal laws in ecotone geometry. To demonstrate that the model is applicable to real data, a hull edge of fractal dimension 7/4 is shown on a satellite image of a piñon‐juniper woodland on a hillside. We propose to use the hull edge to define the boundary of a vegetation type unambiguously. This offers a new tool for detecting a shift of the boundary due to a climate change
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