8,381 research outputs found
Infinite impulse response modal filtering in visible adaptive optics
Diffraction limited resolution adaptive optics (AO) correction in visible
wavelengths requires a high performance control. In this paper we investigate
infinite impulse response filters that optimize the wavefront correction: we
tested these algorithms through full numerical simulations of a
single-conjugate AO system comprising an adaptive secondary mirror with 1127
actuators and a pyramid wavefront sensor (WFS). The actual practicability of
the algorithms depends on both robustness and knowledge of the real system:
errors in the system model may even worsen the performance. In particular we
checked the robustness of the algorithms in different conditions, proving that
the proposed method can reject both disturbance and calibration errors
Flow of magnetic repelling grains in a two-dimensional silo
During a typical silo discharge, the material flow rate is determined by the
contact forces between the grains. Here, we report an original study concerning
the discharge of a two-dimensional silo filled with repelling magnetic grains.
This non-contact interaction leads to a different dynamics from the one
observed with conventional granular materials. We found that, although the flow
rate dependence on the aperture size follows roughly the power-law with an
exponent found in non-repulsive systems, the density and velocity
profiles during the discharge are totally different. New phenomena must be
taken into account. Despite the absence of contacts, clogging and intermittence
were also observed for apertures smaller than a critical size determined by the
effective radius of the repulsive grains.Comment: 6 pages, 8 figure
Quark core formation in spinning-down pulsars
Pulsars spin-down due to magnetic torque reducing its radius and increasing
the central energy density. Some pulsar which are born with central densities
close to the critical value of quark deconfinement may undergo a phase
transition and structural re-arrengement. This process may excite oscillation
modes and emmit gravitational waves. We determine the rate of quark core
formation in neutron stars using a realistic population synthesis code.Comment: Proceedings of the 2nd International Workshop on Astronomy and
Relativistic Astrophysics, to appear in IJMP
Resistance to acid attack, abrasion and leaching behavior of alkali-activated mine waste binders
This paper report results of a research
project on the development of alkali-activated binders using mine wastes. Abrasion and acid resistance of two
ordinary Portland cement (OPC) strength class concrete mixtures (C20/25 and C30/37) and several mine waste (MW) mixtures were compared. This study
indicates that MW binders possess higher acid and abrasion resistance than OPC based concrete mixtures.The leaching assessment of the MW binders shows it
can be considered an inert material which indicates that it could be used as a building material
Expected Coalescence Rate of Double Neutron Stars for Ground Based Interferometers
In this paper we present new estimates of the coalescence rate of neutron
star binaries in the local universe and we discuss its consequences for the
first generations of ground based interferometers. Our approach based on both
evolutionary and statistical methods gives a galactic merging rate of 1.7
10 yr, in the range of previous estimates 10 - 10
yr. The local rate which includes the contribution of elliptical
galaxies is two times higher, in the order of 3.4 10 yr. We
predict one detection every 148 and 125 years with initial VIRGO and LIGO, and
up to 6 events per year with their advanced configuration. Our recent detection
rate estimates from investigations on VIRGO future improvements are quoted.Comment: talk given at the GWDAW9 (Annecy, 2004) to be published in CQ
S wave velocity structure below central Mexico using high-resolution surface wave tomography
Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB
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