225 research outputs found
Gas-dynamic shock heating of post-flare loops due to retraction following localized, impulsive reconnection
We present a novel model in which shortening of a magnetic flux tube
following localized, three-dimensional reconnection generates strong
gas-dynamic shocks around its apex. The shortening releases magnetic energy by
progressing away from the reconnection site at the Alfven speed. This launches
inward flows along the field lines whose collision creates a pair of
gas-dynamic shocks. The shocks raise both the mass density and temperature
inside the newly shortened flux tube. Reconnecting field lines whose initial
directions differ by more that 100 degrees can produce a concentrated knot of
plasma hotter that 20 MK, consistent with observations. In spite of these high
temperatures, the shocks convert less than 10% of the liberated magnetic energy
into heat - the rest remains as kinetic energy of bulk motion. These
gas-dynamic shocks arise only when the reconnection is impulsive and localized
in all three dimensions; they are distinct from the slow magnetosonic shocks of
the Petschek steady-state reconnection model
Observation of two-wave structure in strongly nonlinear dissipative granular chains
In a strongly nonlinear viscous granular chain under conditions of loading
that exclude stationary waves (e.g., impact by a single grain) we observe a
pulse that consists of two interconnected but distinct parts. One is a leading
narrow "primary pulse" with properties similar to a solitary wave in a "sonic
vacuum." It arises from strong nonlinearity and discreteness in the absence of
dissipation, but now decays due to viscosity. The other is a broad, much more
persistent shock-like "secondary pulse" trailing the primary pulse and caused
by viscous dissipation. The medium behind the primary pulse is transformed from
a "sonic vacuum" to a medium with finite sound speed. When the rapidly decaying
primary pulse dies, the secondary pulse continues to propagate in the "sonic
vacuum," with an oscillatory front if the viscosity is relatively small, until
its eventual (but very slow) disintegration. Beyond a critical viscosity there
is no separation of the two pulses, and the dissipation and nonlinearity
dominate the shock-like attenuating pulse which now exhibits a nonoscillatory
front
Experimental evidence of solitary wave interaction in Hertzian chains
We study experimentally the interaction between two solitary waves that
approach one to another in a linear chain of spheres interacting via the Hertz
potential. When these counter propagating waves collide, they cross each other
and a phase shift respect to the noninteracting waves is introduced, as a
result of the nonlinear interaction potential. This observation is well
reproduced by our numerical simulations and it is shown to be independent of
viscoelastic dissipation at the beads contact. In addition, when the collision
of equal amplitude and synchronized counter propagating waves takes place, we
observe that two secondary solitary waves emerge from the interacting region.
The amplitude of secondary solitary waves is proportional to the amplitude of
incident waves. However, secondary solitary waves are stronger when the
collision occurs at the middle contact in chains with even number of beads.
Although numerical simulations correctly predict the existence of these waves,
experiments show that their respective amplitude are significantly larger than
predicted. We attribute this discrepancy to the rolling friction at the beads
contacts during solitary wave propagation
Simulation of Particle Size Effect on Dynamic Properties and Fracture of PTFE-W-Al Composites
Recent investigations of the dynamic compressive strength of cold
isostatically pressed composites of polytetrafluoroethylene (PTFE), tungsten
(W) and aluminum (Al) powders show significant differences depending on the
size of metallic particles. The addition of W increases the density and overall
strength of the sample. To investigate relatively large deformations
multi-material Eulerian and arbitrary Lagrangian-Eulerian methods, which have
the ability to efficiently handle the formation of free surfaces, were used.
The calculations indicate that the increased strength of the sample with fine
metallic particles is due to the formation of force chains under dynamic
loading. This phenomenon occurs even at larger porosity of the PTFE matrix in
comparison with samples with larger particle size of W and higher density of
the PTFE matrix.Comment: 5 pages, 6 figure
Anomalous wave reflection from the interface of two strongly nonlinear granular media
Granular materials exhibit a strongly nonlinear behaviour affecting the
propagation of information in the medium. Dynamically self-organized strongly
nonlinear solitary waves are the main information carriers in granular chains.
Here we report the first experimental observation of the dramatic change of
reflectivity from the interface of two granular media triggered by a noncontact
magnetically induced initial precompression. It may be appropriate to name this
phenomenon the "acoustic diode" effect. Based on numerical simulations, we
explain this effect by the high gradient of particle velocity near the
interface.Comment: 14 pages, 3 figure
Short Pulse Dynamics in Strongly Nonlinear Dissipative Granular Chains
We study the energy decay properties of a pulse propagating in a strongly
nonlinear granular chain with damping proportional to the relative velocity of
the grains. We observe a wave disturbance that at low viscosities consists of
two parts exhibiting two entirely different time scales of dissipation. One
part is an attenuating solitary wave, is dominated by discreteness and
nonlinearity effects as in a dissipationless chain, and has the shorter
lifetime. The other is a purely dissipative shocklike structure with a much
longer lifetime and exists only in the presence of dissipation. The range of
viscosities and initial configurations that lead to this complex wave
disturbance are explored
Particle size effect on strength, failure and shock behavior in Polytetrafluoroethylene-Al-W granular composites
The variation of metallic particle size and sample porosity significantly
alters the dynamic mechanical properties of high density granular composites
processed using a cold isostatically pressed mixture of polytetrafluoroethylene
(PTFE), aluminum (Al) and tungsten (W) powders. Quasi-static and dynamic
experiments are performed with identical constituent mass fractions with
variations in the size of the W particles and pressing conditions. The
relatively weak polymer matrix allows the strength and fracture modes of this
material to be governed by the granular type behavior of agglomerated metal
particles. A higher ultimate compressive strength was observed in relatively
high porosity samples with small W particles compared to those with coarse W
particles in all experiments. Mesoscale granular force chains comprised of the
metallic particles explain this unusual phenomenon as observed in a hydrocode
simulation of a drop-weight test. Macrocracks forming below the critical
failure strain for the matrix and unusual behavior due to a competition between
densification and fracture in dynamic tests of porous samples were also
observed. Shock loading of this granular composite resulted in higher fraction
of total internal energy deposition in the soft PTFE matrix, specifically
thermal energy, which can be tailored by the W particle size distribution.Comment: 35 pages, 13 figure
A cardinal role for cathepsin D in co-ordinating the host-mediated apoptosis of macrophages and killing of pneumococci
The bactericidal function of macrophages against pneumococci is enhanced by their apoptotic demise, which is controlled by the anti-apoptotic protein Mcl-1. Here, we show that lysosomal membrane permeabilization (LMP) and cytosolic translocation of activated cathepsin D occur prior to activation of a mitochondrial pathway of macrophage apoptosis. Pharmacological inhibition or knockout of cathepsin D during pneumococcal infection blocked macrophage apoptosis. As a result of cathepsin D activation, Mcl-1 interacted with its ubiquitin ligase Mule and expression declined. Inhibition of cathepsin D had no effect on early bacterial killing but inhibited the late phase of apoptosis-associated killing of pneumococci in vitro. Mice bearing a cathepsin D-/- hematopoietic system demonstrated reduced macrophage apoptosis in vivo, with decreased clearance of pneumococci and enhanced recruitment of neutrophils to control pulmonary infection. These findings establish an unexpected role for a cathepsin D-mediated lysosomal pathway of apoptosis in pulmonary host defense and underscore the importance of apoptosis-associated microbial killing to macrophage function
Historical Analysis: Tracking, Problematizing, and Reterritorializing Achievement and the Achievement Gap
For more than a century, state and federal governments and organizations have used different measures to determine if students and groups of students have achieved in a particular subject or grade level. While the construct of achievement is applied irrespective of student differences, this equal application turns out to be anything but equitable. In this chapter, we work to understand the way achievement plays out for Black students by deconstructing how the word achievement works. In doing so, we track the history of education, testing, and curriculum as it has been applied to Black youth and youth of color
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