527 research outputs found
Comportamiento de estudiantes de maestro al medir el volumen
Las magnitudes y su medida, por su uso cotidiano, son un contenido de las matemáticas escolares que los estudiantes para maestro de primaria deben dominar. Estos estudiantes presentan lagunas cuando se les proponen tareas para el desarrollo de la competencia de comparación y medida de la capacidad y del volumen. En el presente trabajo se aportan tareas de este tipo y se identifican las estrategias, los errores y dificultades que cometen los estudiantes, al resolverlas. Además, se describe una actuación realizada en el aula para que los estudiantes detecten y superen las estrategias erróneas
Modelling with a CFD code the near-range dispersion of particles unexpectedly released from a nuclear power plant
An event in November 2007 in Ascó-1 nuclear power plant (Spain) originated the release of a significant amount of hot metallic particles through the discharge stack. Particles were dispersed and deposited in roofs and neighbouring areas within the NPP controlled area. However, the event was not detected until March 2008. More than 1,300 hot points with radioactive particles were found, 94% located inside the double fenced controlled area and 6% within the exclusion area; 5 particles were out of the exclusion area, across the river. To provide additional insights on the potential consequences of the release, a computational fluid dynamics (CFD) code, Ansys-CFX-11, has been used to simulate the near-range atmospheric dispersion and deposition of the particles. The purpose of the analysis was to assess the distance travelled by particles of different sizes. A very detailed model of the site was built, taking into account the buildings and the terrain features including the river valley and the surrounding hills. The modelled domain was 3.2 x 5.2 km, with the atmospheric layer up to 4 km height. The atmospheric conditions recorded during different periods of time were classified into 37 representative categories. In general, the distribution of the particles found was adequately reproduced. Particles larger than 100 microns could not travel beyond the double fence. Particles between 50 and 100 microns could have been deposited mainly within the exclusion area, with a small probability of travelling farther. Smaller particles could have travelled beyond, but also should have been deposited in the nearby area, while the majority of particles found are larger, thus indicating that the size of the released particles should be above 50 microns. The detailed CFD simulation allowed answering relevant questions concerning the possibility of having an impacted region larger than the exclusion area
Infalling clouds onto super-massive black hole binaries - I. Formation of discs, accretion and gas dynamics
There is compelling evidence that most -if not all- galaxies harbour a
super-massive black hole (SMBH) at their nucleus, hence binaries of these
massive objects are an inevitable product of the hierarchical evolution of
structures in the universe, and represent an important but thus-far elusive
phase of galaxy evolution. Gas accretion via a circumbinary disc is thought to
be important for the dynamical evolution of SMBH binaries, as well as in
producing luminous emission that can be used to infer their properties. One
plausible source of the gaseous fuel is clumps of gas formed due to turbulence
and gravitational instabilities in the interstellar medium, that later fall
toward and interact with the binary. In this context, we model numerically the
evolution of turbulent clouds in near-radial infall onto equal-mass SMBH
binaries, using a modified version of the SPH code GADGET-3. We present a total
of 12 simulations that explore different possible pericentre distances and
relative inclinations, and show that the formation of circumbinary discs and
discs around each SMBH ('mini-discs') depend on those parameters. We also study
the dynamics of the formed discs, and the variability of the feeding rate onto
the SMBHs in the different configurations.Comment: Accepted for publication in MNRAS, 17 pages, 13 figures. Animations
available at http://www.astro.puc.cl/~fgarrido/animation
A Persistent High-Energy Flux from the Heart of the Milky Way : Integral's view of the Galactic Center
The Ibis/Isgri imager on Integral detected for the first time a hard X-ray
source, IGR J17456-2901, located within 1' of Sgr A* over the energy range
20-100 keV. Here we present the results of a detailed analysis of ~7 Ms of
Integral observations of the GC. With an effective exposure of 4.7 Ms we have
obtained more stringent positional constraints on this HE source and
constructed its spectrum in the range 20-400 keV. Furthermore, by combining the
Isgri spectrum with the total X-ray spectrum corresponding to the same physical
region around SgrA* from XMM data, and collected during part of the Integral
observations, we constructed and present the first accurate wide band HE
spectrum for the central arcmins of the Galaxy. Our complete analysis of the
emission properties of IGR shows that it is faint but persistent with no
variability above 3 sigma contrary to what was alluded to in our first paper.
This result, in conjunction with the spectral characteristics of the X-ray
emission from this region, suggests that the source is most likely not
point-like but, rather, that it is a compact, yet diffuse, non-thermal emission
region. The centroid of IGR is estimated to be R.A.=17h45m42.5,
decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with a
positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35
erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also located
within 1' of Sgr A*. We present arguments in favor of an interpretation
according to which the photons detected by Integral and Hess arise from the
same compact region of diffuse emission near the central BH and that the
supernova remnant Sgr A East could play an important role as a contributor of
very HE g-rays to the overall spectrum from this region.Comment: 14 pages, 11 figures, Accepted for publication in Ap
Evolution of binary black holes in self gravitating discs: dissecting the torques
We study the interplay between gas accretion and gravity torques in changing a binary elements and its total angular momentum (L) budget. Especially, we analyse the physical origin of the gravity torques (T_g) and their location within the disc. We analyse 3D SPH simulations of the evolution of initially quasi-circular massive black hole binaries (BHBs) residing in the central hollow of massive self-gravitating circumbinary discs. We use different thermodynamics within the cavity and for the numerical size of the black holes to show that (i) the BHB eccentricity growth found previously is a general result, independent of the accretion and the adopted thermodynamics; (ii) the semi-major axis decay depends both on the T_g and on the interplay with the disc-binary L-transfer due to accretion; (iii) the spectral structure of the T_g is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc and, in general, does not reflect directly the period of the binary; (iv) the net T_g changes sign across the BHB corotation radius. We quantify the relative importance of the two, which appear to depend on the thermodynamical properties of the instreaming gas, and which is crucial in assessing the disc-binary L-transfer; (v) the net torque manifests as a purely kinematic (non-resonant) effect as it stems from the cavity, where the material flows in and out in highly eccentric orbits. Both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the BHB. Moreover, the total torque exerted by the disc affects L(BHB) by changing all the elements (mass, mass ratio, eccentricity, semimajor axis) of the BHB. Common prescriptions equating tidal torque to semi-major axis shrinking might therefore be poor approximations for real astrophysical systems
Dissecting X-ray-emitting Gas around the Center of our Galaxy
Most supermassive black holes (SMBHs) are accreting at very low levels and
are difficult to distinguish from the galaxy centers where they reside. Our own
Galaxy's SMBH provides a uniquely instructive exception, and we present a
close-up view of its quiescent X-ray emission based on 3 mega-second of Chandra
observations. Although the X-ray emission is elongated and aligns well with a
surrounding disk of massive stars, we can rule out a concentration of low-mass
coronally active stars as the origin of the emission based on the lack of
predicted Fe Kalpha emission. The extremely weak H-like Fe Kalpha line further
suggests the presence of an outflow from the accretion flow onto the SMBH.
These results provide important constraints for models of the prevalent
radiatively inefficient accretion state.Comment: 18 pages, 5 PDF figures, pdflatex format; Final version, published in
Scienc
Point cloud voxel classification of aerial urban LiDAR using voxel attributes and random forest approach
The opportunities now afforded by increasingly available, dense, aerial urban LiDAR point clouds (greater than100 pts/m2) are arguably stymied by their sheer size, which precludes the effective use of many tools designed for point cloud data mining and classification. This paper introduces the point cloud voxel classification (PCVC) method, an automated, two-step solution for classifying terabytes of data without overwhelming the computational infrastructure. First, the point cloud is voxelized to reduce the number of points needed to be processed sequentially. Next, descriptive voxel attributes are assigned to aid in further classification. These attributes describe the point distribution within each voxel and the voxel's geo-location. These include 5 point-descriptors (density, standard deviation, clustered points, fitted plane, and plane's angle) and 2 voxel position attributes (elevation and neighbors). A random forest algorithm is then used for final classification of the object within each voxel using four categories: ground, roof, wall, and vegetation. The proposed approach was evaluated using a 297,126,417 point dataset from a 1 km2 area in Dublin, Ireland and 50% denser dataset of New York City of 13,912,692 points (150 m2). PCVC's main advantage is scalability achieved through a 99 % reduction in the number of points that needed to be sequentially categorized. Additionally, PCVC demonstrated strong classification results (precision of 0.92, recall of 0.91, and F1-score of 0.92) compared to previous work on the same data set (precision of 0.82-0.91, recall 0.86-0.89, and F1-score of 0.85-0.90).This work was funded by the National Science Foundation award 1940145
Structure of the Beariz granite (Ourense, Galicia)
The geological map, cross-sections and structural analysis of the Beariz granite are presented. The Geometric
and structural characteristics of this intrusive body are in good agreement with a sinkynematic emplacement
within a strike-slip shear zon
Low angular momentum flow model of Sgr A* activity
Sgr A* is the closest massive black hole and can be observed with the highest
angular resolution. Nevertheless, our current understanding of the accretion
process in this source is very poor. The inflow is almost certainly of low
radiative efficiency and it is accompanied by a strong outflow and the flow is
strongly variable but the details of the dynamics are unknown. Even the amount
of angular momentum in the flow is an open question. Here we argue that low
angular momentum scenario is better suited to explain the flow variability. We
present a new hybrid model which describes such a flow and consists of an outer
spherically symmetric Bondi flow and an inner axially symmetric flow described
through MHD simulations. The assumed angular momentum of the matter is low,
i.e. the corresponding circularization radius in the equatorial plane of the
flow is just above the innermost stable circular orbit in pseudo-Newtonian
potential. We compare the radiation spectrum from such a flow to the broad band
observational data for Sgr A*.Comment: Proceedings of the AHAR 2008 Conference: The Universe under the
Microscope; Astrophysics at High Angular Resolution, Bad Honef
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