18,576 research outputs found
Geometric Morphology of Granular Materials
We present a new method to transform the spectral pixel information of a
micrograph into an affine geometric description, which allows us to analyze the
morphology of granular materials. We use spectral and pulse-coupled neural
network based segmentation techniques to generate blobs, and a newly developed
algorithm to extract dilated contours. A constrained Delaunay tesselation of
the contour points results in a triangular mesh. This mesh is the basic
ingredient of the Chodal Axis Transform, which provides a morphological
decomposition of shapes. Such decomposition allows for grain separation and the
efficient computation of the statistical features of granular materials.Comment: 6 pages, 9 figures. For more information visit
http://www.nis.lanl.gov/~bschlei/labvis/index.htm
The late-time development of the Richtmyer–Meshkov instability
Measurements have been made of the growth by the Richtmyer–Meshkov instability of nominally single-scale perturbations on an air/sulfur hexafluoride (SF6) interface in a large shock tube. An approximately sinusoidal shape is given to the interface by a wire mesh which supports a polymeric membrane separating the air from the SF6. A single shock wave incident on the interface induces motion by the baroclinic mechanism of vorticity generation. The visual thickness delta of the interface is measured from schlieren photographs obtained singly in each run and in high-speed motion pictures. Data are presented for delta at times considerably larger than previously reported, and they are tested for self-similarity including independence of initial conditions. Four different initial amplitude/wavelength combinations at one incident shock strength are used to determine the scaling of the data. It is found that the growth rate decreases rapidly with time, ddelta/dt[proportional]t–p (i.e., delta[proportional]t1–p), where 0.67<~p<~0.74 and that a small dependence on the initial wavelength lambda0 persists to large time. The larger value of the power law exponent agrees with the result of the late-time-decay similarity law of Huang and Leonard [Phys. Fluids 6, 3765–3775 (1994)]. The influence of the wire mesh and membrane on the mixing process is assessed
Raman spectra of GexAsySe1−x−y glasses
Various Ge–As–Se glasses spanning a mean coordination number (MCN) from 2.2 to 2.94 have been investigated using differential scanning calorimetry and Raman spectroscopy. The glass transition temperature Tg was found to increase with increasing MCN, except for those glasses located within the nanoscale phase-separated region of the phase diagram. The evolution of Raman features at wavenumbers from 150 to 350 cm⁻¹ exhibits two transitionlike features. Merging of the 225 and 250 cm⁻¹ modes at MCN=2.5 is a symbol of the extinction of Se–Se bonds. Additionally, the appearance of two modes at 280–290 and 170 cm⁻¹ at MCN>2.7 come from the defect modes of ethanelike Ge₂Se₆/₂. The increase in the scattering from these defects is an important factor leading to enhanced optical loss in the glasses with high MCN.This research was partly supported by the Australian
Research Council through its Centres of Excellence and Federation
Fellow Programs
Instability of Rotationally Tuned Dipolar Bose-Einstein Condensates
The possibility of effectively inverting the sign of the dipole-dipole
interaction, by fast rotation of the dipole polarization, is examined within a
harmonically trapped dipolar Bose-Einstein condensate. Our analysis is based on
the stationary states in the Thomas-Fermi limit, in the corotating frame, as
well as direct numerical simulations in the Thomas-Fermi regime, explicitly
accounting for the rotating polarization. The condensate is found to be
inherently unstable due to the dynamical instability of collective modes. This
ultimately prevents the realization of robust and long-lived rotationally tuned
states. Our findings have major implications for experimentally accessing this
regime.Comment: 9 pages with 5 figure
Weakly commensurable arithmetic groups, lengths of closed geodesics and isospectral locally symmetric spaces
We introduce the notion of weak commensurabilty of arithmetic subgroups and
relate it to the length equivalence and isospectrality of locally symmetric
spaces. We prove many strong consequences of weak commensurabilty and derive
from these many interesting results about isolength and isospectral locally
symmetric spaces.Comment: 62 page
Behaviour of Magnetic Tubes in Neutron Star's Interior
It is found from Maxwell's equations that the magnetic field lines are good
analogues of relativistic strings. It is shown that the super-conducting
current in the neutron star's interior causes local rotation of magnetic flux
tubes carrying quantized flux.Comment: 6 pages, no figure
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