11,896 research outputs found
Modeling Helical Structures in Relativistic Jets
Many jets exhibit twisted helical structures. Where superluminal motions are
detected, jet orientation and pattern/flow speed are considerably constrained.
In this case modeling efforts can place strong limits on conditions in the jet
and in the external environment. This can be done by modeling the spatial
development of helical structures which are sensitively dependent on these
conditions. Along an expanding jet this sensitivity manifests itself in
predictable changes in pattern speed and observed wavelength. In general,
twists of low frequency relative to the local resonant frequency are advected
along the expanding jet into a region in which the twist frequency is high
relative to the local resonant frequency. The wave speed can be very different
in these two frequency regimes. Potential effects include helical twists with a
nearly constant apparent wavelength, an apparent wavelength scaling
approximately with the jet radius for up to two orders of magnitude of jet
expansion, or multiple twist wavelengths with vastly different intrinsic scale
and vastly different wave speeds that give rise to similar observed twist
wavelengths but with very different observed motion. In this paper I illustrate
the basic intrinsic and observed behavior of these structures and show how to
place constraints on jet conditions in superluminal jets using the apparent
structures and motions in the inner 3C 120 jet.Comment: 18 pages, 7 figure
Imaging high-speed friction at the nanometer scale
Friction is a complicated phenomenon involving nonlinear dynamics at
different length and time scales[1, 2]. The microscopic origin of friction is
poorly understood, due in part to a lack of methods for measuring the force on
a nanometer-scale asperity sliding at velocity of the order of cm/s.[3, 4]
Despite enormous advance in experimental techniques[5], this combination of
small length scale and high velocity remained illusive. Here we present a
technique for rapidly measuring the frictional forces on a single asperity (an
AFM tip) over a velocity range from zero to several cm/s. At each image pixel
we obtain the velocity dependence of both conservative and dissipative forces,
revealing the transition from stick-slip to a smooth sliding friction[1, 6]. We
explain measurements on graphite using a modified Prandtl-Tomlinson model that
takes into account the damped elastic deformation of the asperity. With its
greatly improved force sensitivity and very small sliding amplitude, our method
enables rapid and detailed surface mapping of the full velocity-dependence of
frictional forces with less than 10~nm spatial resolution.Comment: 7 pages, 4 figure
Macroscopic Characterization of Mechanical Properties in Electric Current Treated Dry Drawn High Strength Wires
The present paper investigates the use of electric current treatment in improving the drawability of plain carbon steel wire for high strength steel applications. The mechanical properties for wires of composition
0.80C-0.65Mn-0.27Si wt.% of diameters 4.09 and 3.00 mm dry drawn from 10.00 mm rods are characterised. The total number of passes for 4.09 and 3.00 mm diameter wires are 7 and 10 respectively resulting in true strains of 1.79 and 2.41. Samples are treated with electric currents in-between the two drawing stages of 4.09 and 3.00 mm, and tested at both stages in tension, torsion and reverse bending along with control samples for comparison. The applied currents are pulsed at a frequency of 100 Hz with each pulse being approximated by a square wave of loading width 80μs and modest current densities of 7.96 Amm-2. Thus the infuence of electric current on the drawability of plain carbon steel wire is assessed between stages of reduction
Symmetric Interconnection Networks from Cubic Crystal Lattices
Torus networks of moderate degree have been widely used in the supercomputer
industry. Tori are superb when used for executing applications that require
near-neighbor communications. Nevertheless, they are not so good when dealing
with global communications. Hence, typical 3D implementations have evolved to
5D networks, among other reasons, to reduce network distances. Most of these
big systems are mixed-radix tori which are not the best option for minimizing
distances and efficiently using network resources. This paper is focused on
improving the topological properties of these networks.
By using integral matrices to deal with Cayley graphs over Abelian groups, we
have been able to propose and analyze a family of high-dimensional grid-based
interconnection networks. As they are built over -dimensional grids that
induce a regular tiling of the space, these topologies have been denoted
\textsl{lattice graphs}. We will focus on cubic crystal lattices for modeling
symmetric 3D networks. Other higher dimensional networks can be composed over
these graphs, as illustrated in this research. Easy network partitioning can
also take advantage of this network composition operation. Minimal routing
algorithms are also provided for these new topologies. Finally, some practical
issues such as implementability and preliminary performance evaluations have
been addressed
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