3,170 research outputs found
Drag reduction on a blunt body by self-adaption of rear flexibly hinged flaps
We study the aerodynamics of a blunt-based body with rear flexibly-hinged rigid flaps,
subject to a turbulent flow of Reynolds number Re = 12000, under aligned and cross
flow conditions with yaw angle β = 0◦ and β = 4◦. To that aim, different values of
the equivalent torsional stiffness are considered, to cover the range of reduced velocity
U∗ = (0, 3.48] in water tank experiments. The effect of the angular deflection of
plates on the drag and near wake flow is analyzed, experimentally and numerically.
The results show that, in the range of U∗ herein considered, the plates undergo an
inwards quasi-static, self-adaptive deflection, which is symmetric for yaw angles β = 0◦
and asymmetric for β = 4◦. In particular, the plates feature small mean deformation
angles for values of U∗ < 1, whereas a sharp and monotonic increase of such deflection
occurs for U∗ > 1, i.e. for lower values of the hinge’s stiffness, with an asymptotic
trend towards the larger values of U∗. A critical value of reduced velocity of U∗ ≃ 0.96
is obtained as the instability threshold above which plates depart from their initial
equilibrium position. The progressive streamlining of the trailing edge translates into
significant reductions of the associated mean drag coefficients. Thus, reductions close
to 19% with respect to reference static plates configurations are obtained for the most
flexible case of U∗ = 3.48 for both β = 0◦ and β = 4◦. A close inspection of the
near wake reveals that the inwards progressive mean displacement of the plates yields
a reduction in the recirculation bubble size. A symmetric evolution of the recirculating
bubble is observed for β = 0◦, whereas the bubble becomes asymmetric for β = 4◦,
with a larger leeward clockwise vortex. In both cases, the drag coefficient is shown
to vary linearly with the global aspect ratio of the recirculating bubble. The analysis
of the numerical results shows that the reduced extension of the recirculating bubble
significantly alters the formation length and intensity of the eddies size and associated
pressure. It is observed that despite the local pressure decrease in the vortices shed from
the trailing edges, the plates self adaption reduces their size and prevents the eddies
from entering the cavity, thus, creating a dead flow region with a consequent pressure
increase at the body base.Junta de Andalucia FEDER-UJA 1262764Universidad de JaenEuropean CommissionSpanish MCIN/AEI PDC2021-121288-I00European Union Next Generation EU/PRT
Electron- versus neutrino-nucleus scattering
We illustrate the connection between electron and neutrino scattering off
nuclei and show how the former process can be used to constrain the description
of the latter. After reviewing some of the nuclear models commonly used to
study lepton-nucleus reactions, we describe in detail the SuSAv2 model and show
how its predictions compare with the available electron- and
neutrino-scattering data over the kinematical range going from the
quasi-elastic peak to pion-production and highly inelastic scattering.Comment: Shortened version, 71 pages, review article, 52 figure
Testing the chemical tagging technique with open clusters
Context. Stars are born together from giant molecular clouds and, if we
assume that the priors were chemically homogeneous and well-mixed, we expect
them to share the same chemical composition. Most of the stellar aggregates are
disrupted while orbiting the Galaxy and most of the dynamic information is
lost, thus the only possibility of reconstructing the stellar formation history
is to analyze the chemical abundances that we observe today.
Aims. The chemical tagging technique aims to recover disrupted stellar
clusters based merely on their chemical composition. We evaluate the viability
of this technique to recover co-natal stars that are no longer gravitationally
bound.
Methods. Open clusters are co-natal aggregates that have managed to survive
together. We compiled stellar spectra from 31 old and intermediate-age open
clusters, homogeneously derived atmospheric parameters, and 17 abundance
species, and applied machine learning algorithms to group the stars based on
their chemical composition. This approach allows us to evaluate the viability
and efficiency of the chemical tagging technique.
Results. We found that stars at different evolutionary stages have distinct
chemical patterns that may be due to NLTE effects, atomic diffusion, mixing,
and biases. When separating stars into dwarfs and giants, we observed that a
few open clusters show distinct chemical signatures while the majority show a
high degree of overlap. This limits the recovery of co-natal aggregates by
applying the chemical tagging technique. Nevertheless, there is room for
improvement if more elements are included and models are improved.Comment: accepted for publication in Astronomy and Astrophysics. Corrected
typo
The effect of substrate on high-temperature annealing of GaN epilayers: Si versus sapphire
We have studied the effects of rapid thermal annealing at 1300¿°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display visible alterations with craterlike morphology scattered over the surface. The annealed GaN/Si layers were characterized by a range of experimental techniques: scanning electron microscopy, optical confocal imaging, energy dispersive x-ray microanalysis, Raman scattering, and cathodoluminescence. A substantial Si migration to the GaN epilayer was observed in the crater regions, where decomposition of GaN and formation of Si3N4 crystallites as well as metallic Ga droplets and Si nanocrystals have occurred. The average diameter of the Si nanocrystals was estimated from Raman scattering to be around 3¿nm. Such annealing effects, which are not observed in GaN grown on sapphire, are a significant issue for applications of GaN grown on Si(111) substrates when subsequent high-temperature processing is required
The central structure of Broad Absorption Line QSOs: observational characteristics in the cm-mm wavelength domain
Accounting for ~20% of the total QSO population, Broad Absorption Line QSOs
are still an unsolved problem in the AGN context. They present wide troughs in
the UV spectrum, due to material with velocities up to 0.2 c toward the
observer. The two models proposed in literature try to explain them as a
particular phase of the evolution of QSOs or as normal QSOs, but seen from a
particular line of sight.
We built a statistically complete sample of Radio-Loud BAL QSOs, and carried
out an observing campaign to piece together the whole spectrum in the cm
wavelength domain, and highlight all the possible differences with respect to a
comparison sample of Radio-Loud non-BAL QSOs. VLBI observations at high angular
resolution have been performed, to study the pc-scale morphology of these
objects. Finally, we tried to detect a possible dust component with
observations at mm-wavelengths.
Results do not seem to indicate a young age for all BAL QSOs. Instead a
variety of orientations and morphologies have been found, constraining the
outflows foreseen by the orientation model to have different possible angles
with respect to the jet axis
In vitro characterization of solute transport in the spinal canal
This paper presents results of an experimental investigation of solute transport in a simplified model of the spinal canal. The work aims to provide increased understanding of the mechanisms responsible for drug dispersion in intrathecal drug delivery (ITDD) procedures. The model consists of an annular channel bounded externally by a rigid transparent tube of circular section, representing the dura mater, and internally by an eccentric cylindrical compliant insert, representing the spinal cord. The tube, closed at one end, is connected to a rigid acrylic reservoir, representing the cranial cavity. The system is filled with water, whose properties are almost identical to those of the cerebrospinal fluid. A programmable peristaltic pump is employed to generate oscillatory motion at frequencies that are representative of those induced by the cardiac and respiratory cycles. Laser induced fluorescence is used to characterize the dispersion of fluorescent dye along the canal and into the cranial cavity for different values of the relevant Womersley number and different eccentricities of the annular section. The present work corroborates experimentally, for the first time, the existence of a steady bulk flow, associated with the mean Lagrangian motion, which plays a key role in the transport of the solute along the spinal canal. The measurements of solute dispersion are found to be in excellent agreement with theoretical predictions obtained using a simplified transport equation derived earlier on the basis of a two-timescale asymptotic analysis. The experimental results underscore the importance of the eccentricity and its variations along the canal and identifies changes in the flow topology associated with differences in the Womersley number, with potential implications in guiding future designs of ITDD protocols.This work was supported by the coordinated project, PID2020-115961RB-C31, PID2020-115961RB-C32, and PID2020-115961RA-C33, financed by MCIN/AEI/10.13039/501100011033, and by the Junta de Andalucia and European Funds, Project No. P18-FR-4619. F. Moral-Pulido wants to thank the Spanish Ministry of Universities for the financial support provided by the Fellowship FPU18/05694. The work of A. L. Sánchez was supported by the U.S. National Science Foundation through Grant No. 1853954
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