1,690 research outputs found
The nonlinear states of viscous capillary jets confined in the axial direction
We report an experimental and theoretical study of the global stability and
nonlinear dynamics of vertical jets of viscous liquid confined in the axial
direction due to their impact on a bath of the same liquid. Previous works
demonstrated that in the absence of axial confinement the steady liquid thread
becomes unstable due to an axisymmetric global mode for values of the flow
rate, , below a certain critical value, , giving rise to oscillations
of increasing amplitude that finally lead to a dripping regime (Sauter &
Buggisch, J. Fluid Mech., 2005; Rubio-Rubio et al., J. Fluid Mech., 2013). Here
we focus on the effect of the jet length, , on the transitions that take
place for decreasing values of . The linear stability analysis shows good
agreement with our experiments, revealing that increases monotonically
with , reaching the semi-infinite jet asymptote for large values of .
Moreover, as decreases a quasi-static limit is reached, whereby
and the neutral conditions are given by a critical length determined by
hydrostatics. Our experiments have also revealed the existence of a new regime
intermediate between steady jetting and dripping, in which the thread reaches a
limit-cycle state without breakup. We thus show that there exist three possible
states depending on the values of the control parameters, namely steady
jetting, oscillatory jetting and dripping. For two different combinations of
liquid viscosity, and injector radius, , the boundaries separating these
regimes have been determined in the parameter plane, showing that steady
jetting exists for small enough values of or large enough values of ,
dripping prevails for small enough values of or sufficiently large values
of , and oscillatory jetting takes place in an intermediate region whose
size increases with the liquid viscosity and decreases with
ASSET PRICING AND SYSTEMATIC LIQUIDITY RISK: AN EMPIRICAL INVESTIGATION OF THE SPANISH STOCK MARKET
It seems reasonable to expect systematic liquidity shocks to affect the optimal behavior of agents in financial markets. Indeed, fluctuations in various measures of liquidity are significantly correlated across common stocks(Chordia, Roll and Subrahmanyam (2000)). Thus, this paper empirically analyzes whether Spanish expected returns during the nineties are associated cross-sectionally to betas estimated relative to two competing liquidity risk factors. On one hand, we propose a new market-wide liquidity factor which is defined as the difference between returns of stocks highly sensitive to changes in the relative bid-ask spread less returns from stocks with low sensitivities to those changes. We argue that stocks with positive covariability between returns and this factor are assets whose returns tend to go down when aggregate liquidity is low, and hence do not hedge a potential liquidity crisis. Consequently, investors will require a premium to hold these assets. Similarly, note that in the case of assets that covary negatively with the liquidity factor, investors may be willing to pay a premium rather than to require an additional compensation. On the other hand, Pastor and Stambaugh (2002) suggest that a reasonable liquidity risk factor should be associated with the strength of volume-related return reversals since order flow induces greater return reversals when liquidity is lower. Our empirical results show that neither of these proxies for systematic liquidity risk carries a premium in the Spanish stock market.
Marcelleina parvispora (Ascomycota, Pezizales), a new Marcelleina species from Catalonia (Spain)
Marcelleina parvispora (Ascomycota, Pezizales), a new Marcelleina species from
Catalonia (Spain). Marcelleina parvispora sp. nov. (Pezizales, Ascomycota) is described as a new
ascomycete species from Catalonia (NE of continental Spain). This first Spanish collection belongs to
a new, apparently saprotrophic species that grows in Eucalyptus sp. plantations. A comparison is made
with related European species of MarcelleinaMarcelleina parvispora sp. nov. (Pezizales, Ascomycota), una nueva especie de
ascomicete, aparentemente saprobia en plantaciones de Eucalyptus sp., procedente de Cataluña (NE de
España continental). La descripción se acompaña de fotografías macro y microscópicas de sus
singulares caracteres y de su comparación con algunas especies europeas próximas
Aplicación de técnicas SIG en el estudio de evaluación de degradación de suelos. Mazarrón (Murcia)
El objetivo general de este estudio ha sido evaluar el estado actual y los cambios que se han producido en
algunas características edáficas del área de Mazarrón (Murcia) en un periodo de 20 años, en relación con los
procesos de degradación. Para ello se han utilizado los resultados de dos muestreos de suelos realizados en
un intervalo de veinte años (1988 y 2008). Se han analizado algunas de las características edáficas que más
relación tienen con la vulnerabilidad de los suelos a la erosión (Textura, Materia orgánica y Pedregosidad) y de
las cuales se disponía de datos en los dos muestreos.
Para la creación y análisis de la información espacial se han empleado dos Sistemas de Información
Geográfica de software libre. Uno de ellos ha sido GvSIG, desarrollado por la Generalitat Valenciana y el
programa GRASS. La metodología SIG empleada ha sido una herramienta fundamental para obtener la serie
de capas de variables edáficas para los años 1988 y 2008.
Los resultados muestran como el área de estudio es bastante vulnerable a los procesos erosivos, sobre
todo por erosión hidrica y antrópica, debido a las características de sus suelos. Riesgo que se incrementaría si
en futuro se abandonaran los cultivos.The main objetive of these study is to evaluate the current situation and the changes ocurred in some
edaphic characteristics of the soils in the Mazarrón area (Murcia) in a twenty years period, in relation with the
degradation processes. For these propose we have used the results of two soils samples carried out in a
twenty years intervale (1988 and 2008). The caracteristics more related with the vulnerability of the soil
erosion (texture, organic matter and stoniness) of wich we had data of the two samples were evaluated.
Two Geografical Information System of free software have been used for the creation and analisis of the
espacial information. One of these was GvSIG, developed by Generalitat Valenciana and GRASS programme.
The GIS methodology carried out was a fundamental tool to obtain edaphic variables layers for the years 1988
and 2008.
The results show how the study area is very sensible to the erosive processes, specialy by hidric and
antropic erosion, because soil caracteristics. This hazard would increase in a future if the land was
abandoned
Study of single-lobed circular polarization profiles in the quiet Sun
The existence of asymmetries in the circular polarization (Stokes V) profiles
emerging from the solar photosphere is known since the 1970s. These profiles
require the presence of a velocity gradient along the line of sight, possibly
associated with gradients of magnetic field strength, inclination and/or
azimuth. We have focused our study on the Stokes V profiles showing extreme
asymmetry in the from of only one lobe. Using Hinode spectropolarimetric
measurements we have performed a statistical study of the properties of these
profiles in the quiet sun. We show their spatial distribution, their main
physical properties, how they are related with several physical observables and
their behavior with respect to their position on the solar disk. The single
lobed Stokes V profiles occupy roughly 2% of the solar surface. For the first
time, we have observed their temporal evolution and have retrieved the physical
conditions of the atmospheres from which they emerged using an inversion code
implementing discontinuities of the atmospheric parameters along the line of
sight. In addition, we use synthetic Stokes profiles from 3D magnetoconvection
simulations to complement the results of the inversion. The main features of
the synthetic single-lobed profiles are in general agreement with the observed
ones, lending support to the magnetic and dynamic topologies inferred from the
inversion. The combination of all these different analysis suggests that most
of the single-lobed Stokes V profiles are signals coming from magnetic flux
emergence and/or submergence processes taking place in small patches in the
photospheric of the quiet sun.Comment: 21 pages, 26 figures, 1 tabl
The history of a quiet-Sun magnetic element revealed by IMaX/SUNRISE
Isolated flux tubes are considered to be fundamental magnetic building blocks
of the solar photosphere. Their formation is usually attributed to the
concentration of magnetic field to kG strengths by the convective collapse
mechanism. However, the small size of the magnetic elements in quiet-Sun areas
has prevented this scenario from being studied in fully resolved structures.
Here we report on the formation and subsequent evolution of one such
photospheric magnetic flux tube, observed in the quiet Sun with unprecedented
spatial resolution (0\farcs 15 - 0\farcs 18) and high temporal cadence (33 s).
The observations were acquired by the Imaging Magnetograph Experiment (IMaX)
aboard the \textsc{Sunrise} balloon-borne solar observatory. The equipartition
field strength magnetic element is the result of the merging of several same
polarity magnetic flux patches, including a footpoint of a previously emerged
loop. The magnetic structure is then further intensified to kG field strengths
by convective collapse. The fine structure found within the flux concentration
reveals that the scenario is more complex than can be described by a thin flux
tube model with bright points and downflow plumes being established near the
edges of the kG magnetic feature. We also observe a daisy-like alignment of
surrounding granules and a long-lived inflow towards the magnetic feature.
After a subsequent weakening process, the field is again intensified to kG
strengths. The area of the magnetic feature is seen to change in anti-phase
with the field strength, while the brightness of the bright points and the
speed of the downflows varies in phase. We also find a relation between the
brightness of the bright point and the presence of upflows within it.Comment: 13 pages. Accepted in ApJ. Animation 1 can be viewed and downloaded
from: http://spg.iaa.es/downloads.as
On the extraction of cellulose nanowhiskers from food by-products and their comparative reinforcing effect on a polyhydroxybutyrate-co-valerate polymer
The present work reports on the characterization of cellulose nanowhiskers (CNW) extracted from three different food by-products, i.e., wheat straw (WSCNW), Brewers spent grains (BGCNW) and olive pomace (OPCNW), by using an optimized hydrolysis method similar to that developed to extract bacterial cellulose nanowhiskers (BCNW). WSCNW and BGCNW were seen to present optimal properties, with aspect ratio, crystallinity and thermal stability values comparable to those of BCNW. Additionally, the optimized hydrolysis treatment led to extraction yields higher than those previously reported for food by-products. The CNW were subsequently incorporated into a commercial polyhydroxybutyrate-co-valerate polymer (PHBV) by solution casting, and the produced nanocomposites were characterized. Although the addition of BGCNW and WSCNW was advantageous in terms of mechanical performance in comparison with OPCNW, no significant enhancement of the pure PHBV mechanical properties was reported because of the low nanofiller loadings used and the inherent difficulty of achieving a high degree of dispersion by the casting method. Interestingly, BGCNW and WSCNW presented reduced moisture sensitivity as compared with BCNW, leading to greater barrier performance and resulting in oxygen permeability reductions up to 26 % with WSCNW and 44 % with BGCNW.Noelle Peutat, on leave from the University of Grenoble in France, is acknowledged for her great dedication and support in the experimental work. M. Martinez-Sanz would like to thank the Spanish Ministry of Education for FPU Grant 1484. The authors acknowledge financial support from the EU FP7 ECOBIOCAP Project. The Electronic Microscopy Department in the SCIE from the University of Valencia is acknowledged for the support with SEM and TEM analyses. The Portuguese authors also acknowledge support from the FCT (Portuguese Foundation for Science and Technology) through strategic project PEst-OE/EQB/LA0023/2013
NifB and NifEN protein levels are regulated by ClpX2 under nitrogen fixation conditions in Azotobacter vinelandii
The major part of biological nitrogen fixation is catalysed by the molybdenum nitrogenase that carries at its active site the iron and molybdenum cofactor (FeMo-co). The nitrogen fixation (nif) genes required for the biosynthesis of FeMo-co are derepressed in the absence of a source of fixed nitrogen. The nifB gene product is remarkable because it assembles NifB-co, a complex cluster proposed to comprise a [6Fe-9S-X] cluster, from simpler [Fe-S] clusters common to other metabolic pathways. NifB-co is a common intermediate of the biosyntheses of the cofactors present in the molybdenum, vanadium and iron nitrogenases. In this work, the expression of the Azotobacter vinelandii nifB gene was uncoupled from its natural nif regulation to show that NifB protein levels are lower in cells growing diazotrophically than in cells growing at the expense of ammonium. A. vinelandii carries a duplicated copy of the ATPase component of the ubiquitous ClpXP protease (ClpX2), which is induced under nitrogen fixing conditions. Inactivation of clpX2 resulted in the accumulation of NifB and NifEN and a defect in diazotrophic growth, especially when iron was in short supply. Mutations in nifE, nifN and nifX or in nifA also affected NifB accumulation, suggesting that NifB susceptibility to degradation might vary during its catalytic cycle.Fil: Martínez Noël, Giselle María Astrid. Centro de Biotecnología y Genómica de Plantas; EspañaFil: Curatti, Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones En Biodiversidad y Biotecnología; ArgentinaFil: Hernandez, Jose A.. Midwestern University; Estados UnidosFil: Rubio, Luis M.. Centro de Biotecnología y Genómica de Plantas; Españ
Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries
Full Stokes filter-polarimeters are key instruments for investigating the
rapid evolution of magnetic structures on the solar surface. To this end, the
image quality is routinely improved using a-posteriori image reconstruction
methods. We analyze the robustness of circular polarization asymmetries to
phase-diversity image reconstruction techniques. We use snapshots of
magneto-hydrodynamical simulations carried out with different initial
conditions to synthesize spectra of the magnetically sensitive Fe I line at
5250.2 A. We degrade the synthetic profiles spatially and spectrally to
simulate observations with the IMaX full Stokes filter-polarimeter. We also
simulate the focused/defocused pairs of images used by the phase-diversity
algorithm for reconstruction and the polarimetric modulation scheme. We assume
that standard optimization methods are able to infer the projection of the
wavefront on the Zernike polynomials with 10% precision. We also consider the
less favorable case of 25% precision. We obtain reconstructed monochromatic
modulated images that are later demodulated and compared with the original
maps. Although asymmetries are often difficult to define in the quiet Sun due
to the complexity of the Stokes V profiles, we show how asymmetries are
degraded with spatial and spectral smearing. The results indicate that,
although image reconstruction techniques reduce the spatial smearing, they can
modify the asymmetries of the profiles, mainly caused by the appearance of
spatially-correlated noise.Comment: 10 pages, accepted for publication in A&
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