15,850 research outputs found
Non-destructive evaluation of concrete using a capacitive imaging technique : preliminary modelling and experiments
This paper describes the application of capacitive imaging to the inspection of concrete. A two-dimensional finite-element method was employed to model the electric field distribution from capacitive imaging probe, and how it interacts with concrete samples. Physical experiments with prototype capacitive imaging probes were also carried out. The proof-of-concept results indicated that the capacitive imaging technique could be used to detect cracks on the surface of concrete samples, as well as sub-surface air voids and steel reinforcement bars
The Discovery of 1000 km/s Outflows in Massive Post-starburst Galaxies at z=0.6
Numerical simulations suggest that active galactic nuclei (AGNs) play an
important role in the formation of early-type galaxies by expelling gas and
dust in powerful galactic winds and quenching star formation. However, the
existence of AGN feedback capable of halting galaxy-wide star formation has yet
to be observationally confirmed. To investigate this question, we have obtained
spectra of 14 post-starburst galaxies at z~0.6 to search for evidence of
galactic winds. In 10/14 galaxies we detect Mg II 2796,2803 absorption lines
which are blueshifted by 490 - 2020 km/s with respect to the stars. The median
blueshift is 1140 km/s. We hypothesize that the outflowing gas represents a
fossil galactic wind launched near the peak of the galaxy's activity, a few 100
Myr ago. The velocities we measure are intermediate between those of luminous
starbursts and broad absorption line quasars, which suggests that feedback from
an AGN may have played a role in expelling cool gas and shutting down star
formation.Comment: 5 pages, 2 figures, accepted to ApJ Letter
The opposites task: Using general rules to test cognitive flexibility in preschoolers
A brief narrative description of the journal article, document, or resource. Executive functions play an important role in cognitive development, and during the preschool years especially, children's performance is limited in tasks that demand flexibility in their behavior. We asked whether preschoolers would exhibit limitations when they are required to apply a general rule in the context of novel stimuli on every trial (the "opposites" task). Two types of inhibitory processing were measured: response interference (resistance to interference from a competing response) and proactive interference (resistance to interference from a previously relevant rule). Group data show 3-year-olds have difficulty inhibiting prepotent tendencies under these conditions, whereas 5-year-olds' accuracy is near ceiling in the task. (Contains 4 footnotes and 1 table.
On certain finiteness questions in the arithmetic of modular forms
We investigate certain finiteness questions that arise naturally when
studying approximations modulo prime powers of p-adic Galois representations
coming from modular forms. We link these finiteness statements with a question
by K. Buzzard concerning p-adic coefficient fields of Hecke eigenforms.
Specifically, we conjecture that for fixed N, m, and prime p with p not
dividing N, there is only a finite number of reductions modulo p^m of
normalized eigenforms on \Gamma_1(N). We consider various variants of our basic
finiteness conjecture, prove a weak version of it, and give some numerical
evidence.Comment: 25 pages; v2: one of the conjectures from v1 now proved; v3:
restructered parts of the article; v4: minor corrections and change
Influence of zonal flows on unstable drift modes in ETG turbulence
The linear instability of the electron temperature gradient (ETG) driven
modes in the presence of zonal flows is investigated. Random and deterministic
- like profiles of the zonal flow are considered. It is shown that the
presence of shearing by zonal flows can stabilize the linear instability of ETG
drift modes
Nonlinear shock acceleration beyond the Bohm limit
We suggest a physical mechanism whereby the acceleration time of cosmic rays
by shock waves can be significantly reduced. This creates the possibility of
particle acceleration beyond the knee energy at ~10^15eV. The acceleration
results from a nonlinear modification of the flow ahead of the shock supported
by particles already accelerated to the knee momentum at p ~ p_*. The particles
gain energy by bouncing off converging magnetic irregularities frozen into the
flow in the shock precursor and not so much by re-crossing the shock itself.
The acceleration rate is thus determined by the gradient of the flow velocity
and turns out to be formally independent of the particle mean free path
(m.f.p.). The velocity gradient is, in turn, set by the knee-particles at p ~
p_* as having the dominant contribution to the CR pressure. Since it is
independent of the m.f.p., the acceleration rate of particles above the knee
does not decrease with energy, unlike in the linear acceleration regime. The
reason for the knee formation at p ~ p_* is that particles with are
effectively confined to the shock precursor only while they are within limited
domains in the momentum space, while other particles fall into
``loss-islands'', similar to the ``loss-cone'' of magnetic traps. This
structure of the momentum space is due to the character of the scattering
magnetic irregularities. They are formed by a train of shock waves that
naturally emerge from unstably growing and steepening magnetosonic waves or as
a result of acoustic instability of the CR precursor. These losses steepen the
spectrum above the knee, which also prevents the shock width from increasing
with the maximum particle energy.Comment: aastex, 13 eps figure
Consumer credit information systems: A critical review of the literature. Too little attention paid by lawyers?
This paper reviews the existing literature on consumer credit reporting, the most extensively used instrument to overcome information asymmetry and adverse selection problems in credit markets. Despite the copious literature in economics and some research in regulatory policy, the legal community has paid almost no attention to the legal framework of consumer credit information systems, especially within the context of the European Union. Studies on the topic, however, seem particularly relevant in view of the establishment of a single market for consumer credit. This article ultimately calls for further legal research to address consumer protection concerns and inform future legislation
On the Structure and Scale of Cosmic Ray Modified Shocks
Strong astrophysical shocks, diffusively accelerating cosmic rays (CR) ought
to develop CR precursors. The length of such precursor is believed to
be set by the ratio of the CR mean free path to the shock speed,
i.e., , which is formally
independent of the CR pressure . However, the X-ray observations of
supernova remnant shocks suggest that the precursor scale may be significantly
shorter than which would question the above estimate unless the
magnetic field is strongly amplified and the gyroradius is strongly
reduced over a short (unresolved) spatial scale. We argue that while the CR
pressure builds up ahead of the shock, the acceleration enters into a strongly
nonlinear phase in which an acoustic instability, driven by the CR pressure
gradient, dominates other instabilities (at least in the case of low
plasma). In this regime the precursor steepens into a strongly nonlinear front
whose size scales with \emph{the CR pressure}as , where is the scale of
the developed acoustic turbulence, and is the ratio of CR to gas
pressure. Since , the precursor scale reduction may be strong
in the case of even a moderate gas heating by the CRs through the acoustic and
(possibly also) the other instabilities driven by the CRs.Comment: EPS 2010 paper, to appear in PPC
On radial geodesic forcing of zonal modes
The elementary local and global influence of geodesic field line curvature on
radial dispersion of zonal modes in magnetised plasmas is analysed with a
primitive drift wave turbulence model. A net radial geodesic forcing of zonal
flows and geodesic acoustic modes can not be expected in any closed toroidal
magnetic confinement configuration, since the flux surface average of geodesic
curvature identically vanishes. Radial motion of poloidally elongated zonal
jets may occur in the presence of geodesic acoustic mode activity.
Phenomenologically a radial propagation of zonal modes shows some
characteristics of a classical analogon to second sound in quantum condensates.Comment: Submitted to Phys. Plasmas (2011
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