16,269 research outputs found
A two-layer shallow water model for bedload sediment transport: convergence to Saint-Venant-Exner model
A two-layer shallow water type model is proposed to describe bedload sediment
transport. The upper layer is filled by water and the lower one by sediment.
The key point falls on the definition of the friction laws between the two
layers, which are a generalization of those introduced in Fern\'andez-Nieto et
al. (ESAIM: M2AN, 51:115-145, 2017). This definition allows to apply properly
the two-layer shallow water model for the case of intense and slow bedload
sediment transport. Moreover, we prove that the two-layer model converges to a
Saint-Venant-Exner system (SVE) including gravitational effects when the ratio
between the hydrodynamic and morphodynamic time scales is small. The SVE with
gravitational effects is a degenerated nonlinear parabolic system. This means
that its numerical approximation is very expensive from a computational point
of view, see for example T. Morales de Luna et al. (J. Sci. Comp., 48(1):
258-273, 2011). In this work, gravitational effects are introduced into the
two-layer system without such extra computational cost. Finally, we also
consider a generalization of the model that includes a non-hydrostatic pressure
correction for the fluid layer and the boundary condition at the sediment
surface. Numerical tests show that the model provides promising results and
behave well in low transport rate regimes as well as in many other situations
Formal deduction of the Saint-Venant-Exner model including arbitrarily sloping sediment beds and associated energy
In this work we present a deduction of the Saint-Venant-Exner model through
an asymptotic analysis of the Navier-Stokes equations. A multi-scale analysis
is performed in order to take into account that the velocity of the sediment
layer is smaller than the one of the fluid layer. This leads us to consider a
shallow water type system for the fluid layer and a lubrication Reynolds
equation for the sediment one. This deduction provides some improvements with
respect to the classical Saint-Venant-Exner model: (i) the deduced model has an
associated energy. Moreover, it allows us to explain why classical models do
not have an associated energy and how to modify them in order to recover a
model with this property. (ii) The model incorporates naturally a necessary
modification that must be taken into account in order to be applied to
arbitrarily sloping beds. Furthermore, we show that this modification is
different of the ones considered classically, and that it coincides with a
classical one only if the solution has a constant free surface. (iii) The
deduced solid transport discharge naturally depends on the thickness of the
moving sediment layer, what allows to ensure sediment mass conservation.
Moreover, we include a simplified version of the model for the case of
quasi-stationary regimes. Some of these simplified models correspond to the
generalization of classical ones such as Meyer-PeterM\"uller and
Ashida-Michiue models. Three numerical tests are presented to study the
evolution of a dune for several definition of the repose angle, to see the
influence of the proposed definition of the effective shear stress in
comparison with the classical one, and by comparing with experimental data.Comment: 44 pages, sumbitted to Advances in Water Resources 17 july 201
Neutron background at the Canfranc Underground Laboratory and its contribution to the IGEX-DM dark matter experiment
A quantitative study of the neutron environment in the Canfranc Underground
Laboratory has been performed. The analysis is based on a complete set of
simulations and, particularly, it is focused on the IGEX-DM dark matter
experiment. The simulations are compared to the IGEX-DM low energy data
obtained with different shielding conditions. The results of the study allow us
to conclude, with respect to the IGEX-DM background, that the main neutron
population, coming from radioactivity from the surrounding rock, is practically
eliminated after the implementation of a suitable neutron shielding. The
remaining neutron background (muon-induced neutrons in the shielding and in the
rock) is substantially below the present background level thanks to the muon
veto system. In addition, the present analysis gives us a further insight on
the effect of neutrons in other current and future experiments at the Canfranc
Underground Laboratory. The comparison of simulations with the body of data
available has allowed to set the flux of neutrons from radioactivity of the
Canfranc rock, (3.82 +- 0.44) x 10^{-6} cm^{-2} s^{-1}, as well as the flux of
muon-induced neutrons in the rock, (1.73 +- 0.22(stat) \+- 0.69(syst)) x
10^{-9} cm^{-2} s^{-1}, or the rate of neutron production by muons in the lead
shielding, (4.8 +- 0.6 (stat) +- 1.9 (syst)) x 10^{-9} cm^{-3} s^{-1}.Comment: 17 pages, 8 figures, elsart document class; final version to appear
in Astroparticle Physic
A multi-wavelength view of magnetic flaring from PMS stars
Flares from the Sun and other stars are most prominently observed in the soft
X-ray band. Most of the radiated energy, however, is released at optical/UV
wavelengths. In spite of decades of investigation, the physics of flares is not
fully understood. Even less is known about the powerful flares routinely
observed from pre-main sequence stars, which might significantly influence the
evolution of circumstellar disks. Observations of the NGC2264 star forming
region were obtained in Dec. 2011, simultaneously with three telescopes,
Chandra (X-rays), CoRoT (optical), and Spitzer (mIR), as part of the
"Coordinated Synoptic Investigation of NGC2264" (CSI-NGC2264). Shorter Chandra
and CoRoT observations were also obtained in March 2008. We analyzed the
lightcurves to detect X-ray flares with an optical and/or mIR counterpart.
Basic flare properties from the three datasets, such as emitted energies and
peak luminosities, were then compared to constrain the spectral energy
distribution of the flaring emission and the physical conditions of the
emitting regions. Flares from stars with and without circumstellar disks were
also compared to establish any difference that might be attributed to the
presence of disks. Seventy-eight X-ray flares with an optical and/or mIR
counterpart were detected. Their optical emission is found to correlate well
with, and to be significantly larger than, the X-ray emission. The slopes of
the correlations suggest that the difference becomes smaller for the most
powerful flares. The mIR flare emission seems to be strongly affected by the
presence of a circumstellar disk: flares from stars with disks have a stronger
mIR emission with respect to stars without disks. This might be attributed to
the reprocessing of the optical (and X-ray) flare emission by the inner
circumstellar disk, providing evidence for flare-induced disk heating.Comment: 16 pages (36 including appendixes), 8 figures (main text), accepted
for publication by Astronomy & Astrophysics (section 8
Using Wavelets to reject background in Dark Matter experiments
A method based on wavelet techniques has been developed and applied to
background rejection in the data of the IGEX dark matter experiment. The method
is presented and described in some detail to show how it efficiently rejects
events coming from noise and microphonism through a mathematical inspection of
their recorded pulse shape. The result of the application of the method to the
last data of IGEX is presented.Comment: 14 pages, 8 figures. Submitted to Astrop. Phy
Mercury removal in wastewater by iron oxide nanoparticles
Mercury is one of the persistent pollutants in wastewater; it is becoming a severe environmental and public health problem, this is why nowadays its removal is an obligation. Iron oxide nanoparticles are receiving much attention due to their properties, such as: great biocompatibility, ease of separation, high relation of surface-area to volume, surface modifiability, reusability, excellent magnetic properties and relative low cost. In this experiment, Fe3O4 and γ-Fe2O3 nanoparticles were synthesized using iron salts and NaOH as precipitation agents, and Aloe Vera as stabilizing agent; then these nanoparticles were characterized by three different measurements: first, using a Zetasizer Nano ZS for their size estimation, secondly UV-visible spectroscopy which showed the existence of resonance of plasmon at λmax∼360 nm, and lastly by Scanning Electron Microscopy (SEM) to determine nanoparticles form. The results of this characterization showed that the obtained Iron oxides nanoparticles have a narrow size distribution (∼100nm). Mercury removal of 70% approximately was confirmed by atomic absorption spectroscopy measurements
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