782,475 research outputs found
Mud peeling and horizontal crack formation in drying clays
Mud peeling is a common phenomenon whereby horizontal cracks propagate parallel to the surface of a drying clay. Differential stresses then cause the layer of clay above the crack to curl up to form a mud peel. By treating the clay as a poroelastic solid, we analyse the peeling phenomenon and show that it is caused by the gradient in tensile stress at the surface of the clay, analogously to the spalling of thermoelastic materials. For a constant water evaporation rate at the clay surface we derive equations for the depth of peeling and the time of peeling as functions of the evaporation rate. Our model predicts a simple relationship between the radius of curvature of a mud peel and the depth of peeling. The model predictions are in agreement with the available experimental data available
Task oriented nonlinear control laws for telerobotic assembly operations
The goal of this research is to achieve very intelligent telerobotic controllers which are capable of receiving high-level commands from the human operator and implementing them in an adaptive manner in the object/task/manipulator workspace. Initiatives by the authors at Integrated Systems, Inc. to identify and develop the key technologies necessary to create such a flexible, highly programmable, telerobotic controller are presented. The focus of the discussion is on the modeling of insertion tasks in three dimensions and nonlinear implicit force feedback control laws which incorporate tool/workspace constraints. Preliminary experiments with dual arm beam assembly in 2-D are presented
High Rate Discharge Studies of LI/SO2 Batteries
A battery composed of twelve lithium/sulfur dioxide D size cells in series is forced discharged at 21 amperes. This current is established by the proposed use of the battery and represented a discharge condition which might produce venting. Discharge of the battery into voltage reversal results not only in cells venting but also in the violent rupture of at least one cell
Localization properties of a tight-binding electronic model on the Apollonian network
An investigation on the properties of electronic states of a tight-binding
Hamiltonian on the Apollonian network is presented. This structure, which is
defined based on the Apollonian packing problem, has been explored both as a
complex network, and as a substrate, on the top of which physical models can
defined. The Schrodinger equation of the model, which includes only nearest
neighbor interactions, is written in a matrix formulation. In the uniform case,
the resulting Hamiltonian is proportional to the adjacency matrix of the
Apollonian network. The characterization of the electronic eigenstates is based
on the properties of the spectrum, which is characterized by a very large
degeneracy. The rotation symmetry of the network and large number of
equivalent sites are reflected in all eigenstates, which are classified
according to their parity. Extended and localized states are identified by
evaluating the participation rate. Results for other two non-uniform models on
the Apollonian network are also presented. In one case, interaction is
considered to be dependent of the node degree, while in the other one, random
on-site energies are considered.Comment: 7pages, 7 figure
Scaling Limit and Critical Exponents for Two-Dimensional Bootstrap Percolation
Consider a cellular automaton with state space
where the initial configuration is chosen according to a Bernoulli
product measure, 1's are stable, and 0's become 1's if they are surrounded by
at least three neighboring 1's. In this paper we show that the configuration
at time n converges exponentially fast to a final configuration
, and that the limiting measure corresponding to is in
the universality class of Bernoulli (independent) percolation.
More precisely, assuming the existence of the critical exponents ,
, and , and of the continuum scaling limit of crossing
probabilities for independent site percolation on the close-packed version of
(i.e., for independent -percolation on ), we
prove that the bootstrapped percolation model has the same scaling limit and
critical exponents.
This type of bootstrap percolation can be seen as a paradigm for a class of
cellular automata whose evolution is given, at each time step, by a monotonic
and nonessential enhancement.Comment: 15 page
Deep 10 and 18 micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles & Tentative Evidence for a Brightness Asymmetry
We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that
was recently discovered to have a spectacular, nearly edge-on, circumstellar
disk prominent at ~20 microns (Jayawardhana et al. 1998; Koerner et al. 1998).
These new images, obtained with OSCIR at Keck II, show that the disk's size at
10 microns is comparable to its size at 18 microns. Therefore, the 18
micron-emitting dust may also emit some, or all, of the 10 micron radiation.
Using these multi-wavelength images, we determine a "characteristic" diameter
of 2-3 microns for the mid-infrared-emitting dust particles if they are
spherical and composed of astronomical silicates. Particles this small are
expected to be blown out of the system by radiation pressure in a few hundred
years, and therefore these particles are unlikely to be primordial. Dynamical
modeling of the disk (Wyatt et al. 2000) indicates that the disk surface
density is relatively sharply peaked near 70 AU, which agrees with the mean
annular radius deduced by Schneider et al. (1999) from their NICMOS images. We
present evidence (~1.8 sigma significance) for a brightness asymmetry that may
result from the presence of the hole and the gravitational perturbation of the
disk particle orbits by the low-mass stellar companion or a planet. This
"pericenter glow," which must still be confirmed, results from a very small (a
few AU) shift of the disk's center of symmetry relative to the central star HR
4796A; one side of the inner boundary of the annulus is shifted towards HR
4796A, thereby becoming warmer and more infrared-emitting. The possible
detection of pericenter glow implies that the detection of even complex
dynamical effects of planets on disks is within reach.Comment: 18 pages. 9 GIF images. Total size ~800 kB. High resolution images
available upon request. Accepted for publication in the Astrophysical Journal
(scheduled for January 10, 2000
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