156 research outputs found
Impact of sodium aluminate on the hydration process of alkali-activated ground granulated blast furnace slag
In the present investigation, the effect of the presence of highly soluble alumina source (sodium aluminate, NaAlO2) on Ground Granulated Blast furnace Slag (GGBS) hydration was considered. The GGBS was alkali-activated with sodium hydroxide (NaOH) or sodium carbonate (Na2CO3). The impact on the evolution on the setting time, hydration kinetics and strength development was first considered. In the case of NaOH-activated mixes, the presence of the alumina source led to the delay in setting and an extension of the induction period. The alumina additive led then to an extension of the workable time of the binder. For instance the initial setting time was extended from 30 min to more than 6 hours when adding 1% by weight of NaAlO2. The strength development was also significantly improved with addition of NaAlO2, both at early age and long term. In contrast, addition of NaAlO2 to the Na2CO3-activated mixes led to almost complete cessation of hydration (no measurable strength) within the whole dosage rate interval considered (0.1% to 2% of NaAlO2). Different techniques were used to investigate the impact of NaAlO2 on the hydration products development, including XRD, SEM image analysis and 27Al/29Si/23Na high-resolution NMR spectroscopy. It was found that the presence of the highly soluble alumina source led to rapid precipitation of Afm phases. It is suspected that, due to the low pH level in the case of the Na2CO3-activated mixes, the calcium ions are mainly distributed near the slag grain surfaces. The Afm phases may then mainly precipitate on the slag grains leading hydration blockage. Actually only Afm phases and Gaylussite were detected in the case of NaAlO2 blended Na2CO3-blended mixes
Structural styles and Neogene petroleum system around the Yusuf-Habibas Ridge (Alboran Basin, Mediterranean Sea)
International audienceThe Algerian offshore is part of the southern margin of the western Mediterranean Sea. The western part of this offshore area represents the transitional margin between the South Algero-Balearic Basin and the Alboran Basin. The study area includes the southern and eastern parts of the Alboran Basin and the northwestern part of the Algerian margin and is in the western part of the plate boundary between Eurasia and Africa (Figure 1). The Yusuf-Habibas Ridge is a major EW-striking structure of this complex plate boundary, separating the eastern and southern parts of the Alboran Basin from the South Algero-Balearic Basin (Martinez-Garcia et al., 2011, and references therein). The ridge played an important role during the Neogene Alboran westward block migration between the Africa and Iberia plates, while the Kabylies blocks migrated southward and accreted to Africa. Furthermore, the ongoing NW-SE convergence between Africa and Iberia has induced a new stress field, since 7 Ma ago, replacing an earlier stress field (Fernandez-Ibañez et al., 2007) and leading to reactivation and polyphased deformation on the main structures in the basin, including the Yusuf-Habibas Ridge
Simulation de l'écoulement des pâtes cimentaires par un modèle diphasique
La modélisation de l'écoulement des pâtes cimentaires est un problème difficile car le matériau a non seulement un comportement rhéologique complexe mais il peut également présenter des hétérogénéités induites par l'écoulement. L'apparition de ces hétérogénéités résulte de la filtration de phases fluides au travers de phases solides et peut conduire au blocage de l'écoulement. Pour modéliser l'écoulement d'un tel matériau il est nécessaire de prendre en compte la présence d'au moins deux phases. Dans notre modélisation nous considérons que les deux phases sont continues et admettent un comportement rhéologique en loi de puissance. Le couplage entre les deux phases est pris en compte au moyen d'une loi de Darcy généralisée à un fluide en loi de puissance. Le modèle est résolu par la méthode des éléments finis et validé dans le cas d'un test d'écrasement. Nous montrons que ces simulations permettent d'établir des diagrammes d'ouvrabilité des pâtes cimentaires
Comparison of the thin flux tube approximation with 3D MHD simulations
The structure and dynamics of small vertical photospheric magnetic flux
concentrations has been often treated in the framework of an approximation
based upon a low-order truncation of the Taylor expansions of all quantities in
the horizontal direction, together with the assumption of instantaneous total
pressure balance at the boundary to the non-magnetic external medium. Formally,
such an approximation is justified if the diameter of the structure (a flux
tube or a flux sheet) is small compared to all other relevant length scales
(scale height, radius of curvature, wavelength, etc.). The advent of realistic
3D radiative MHD simulations opens the possibility of checking the consistency
of the approximation with the properties of the flux concentrations that form
in the course of a simulation.
We carry out a comparative analysis between the thin flux tube/sheet models
and flux concentrations formed in a 3D radiation-MHD simulation. We compare the
distribution of the vertical and horizontal components of the magnetic field in
a 3D MHD simulation with the field distribution in the case of the thin flux
tube/sheet approximation. We also consider the total (gas plus magnetic)
pressure in the MHD simulation box. Flux concentrations with
super-equipartition fields are reasonably well reproduced by the second-order
thin flux tube/sheet approximation. The differences between approximation and
simulation are due to the asymmetry and the dynamics of the simulated
structures
Simulation of a flux emergence event and comparison with observations by Hinode
We study the observational signature of flux emergence in the photosphere
using synthetic data from a 3D MHD simulation of the emergence of a twisted
flux tube. Several stages in the emergence process are considered. At every
stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5
{\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral
resolution of Hinode's SOT/SP. Then, following observational practice, we apply
Milne-Eddington-type inversions to the synthetic spectra in order to retrieve
various atmospheric parameters and compare the results with recent Hinode
observations. During the emergence sequence, the spectral lines sample
different parts of the rising flux tube, revealing its twisted structure. The
horizontal component of the magnetic field retrieved from the simulations is
close to the observed values. The flattening of the flux tube in the
photosphere is caused by radiative cooling, which slows down the ascent of the
tube to the upper solar atmosphere. Consistent with the observations, the
rising magnetized plasma produces a blue shift of the spectral lines during a
large part of the emergence sequence.Comment: A&A Letter, 3 figure
Participatory analysis for adaptation to climate change in Mediterranean agricultural systems: possible choices in process design (versão Pre Print)
There is an increasing call for local measures to adapt to climate change, based on foresight analyses in collaboration with actors. However, such analyses involve many challenges, particularly because the actors concerned may not consider climate change to be an urgent concern. This paper examines the methodological choices made by three research teams in the design and implementation of participatory foresight analyses to explore agricultural and water management options for adaptation to climate change. Case studies were conducted in coastal areas of France, Morocco, and Portugal where the groundwater is intensively used for irrigation, the aquifers are at risk or are currently overexploited, and a serious agricultural crisis is underway. When designing the participatory processes, the researchers had to address four main issues: whether to avoid or prepare dialogue between actors whose relations may be limited or tense; how to select participants and get them involved; how to facilitate discussion of issues that the actors may not initially consider to be of great concern; and finally, how to design and use scenarios. In each case, most of the invited actors responded and met to discuss and evaluate a series of scenarios. Strategies were discussed at different levels, from farming practices to aquifer management. It was shown that such participatory analyses can be implemented in situations which may initially appear to be unfavourable. This was made possible by the flexibility in the methodological choices, in particular the possibility of framing the climate change issue in a broader agenda for discussion with the actors
Multiscale magnetic underdense regions on the solar surface: Granular and Mesogranular scales
The Sun is a non-equilibrium dissipative system subjected to an energy flow
which originates in its core. Convective overshooting motions create
temperature and velocity structures which show a temporal and spatial
evolution. As a result, photospheric structures are generally considered to be
the direct manifestation of convective plasma motions. The plasma flows on the
photosphere govern the motion of single magnetic elements. These elements are
arranged in typical patterns which are observed as a variety of multiscale
magnetic patterns. High resolution magnetograms of quiet solar surface revealed
the presence of magnetic underdense regions in the solar photosphere, commonly
called voids, which may be considered a signature of the underlying convective
structure. The analysis of such patterns paves the way for the investigation of
all turbulent convective scales from granular to global. In order to address
the question of magnetic structures driven by turbulent convection at granular
and mesogranular scales we used a "voids" detection method. The computed voids
distribution shows an exponential behavior at scales between 2 and 10 Mm and
the absence of features at 5-10 Mm mesogranular scales. The absence of
preferred scales of organization in the 2-10 Mm range supports the multiscale
nature of flows on the solar surface and the absence of a mesogranular
convective scale
Decay of a simulated mixed-polarity magnetic field in the solar surface layers
Magnetic flux is continuously being removed and replenished on the solar
surface. To understand the removal process we carried out 3D radiative MHD
simulations of the evolution of patches of photospheric magnetic field with
equal amounts of positive and negative flux. We find that the flux is removed
at a rate corresponding to an effective turbulent diffusivity, of 100--340
km^2/s, depending on the boundary conditions. For average unsigned flux
densities above about 70 Gauss, the percentage of surface magnetic energy
coming from different field strengths is almost invariant. The overall process
is then one where magnetic elements are advected by the horizontal granular
motions and occasionally come into contact with opposite-polarity elements.
These reconnect above the photosphere on a comparatively short time scale after
which the U loops produced rapidly escape through the upper surface while the
downward retraction of inverse-U loops is significantly slower, because of the
higher inertia and lower plasma beta in the deeper layers.Comment: 8 pages, 10 figures accepted in A&
Power spectrum of turbulent convection in the solar photosphere
The solar photosphere provides us with a laboratory for understanding
turbulence in a layer where the fundamental processes of transport vary rapidly
and a strongly superadiabatic region lies very closely to a subadiabatic layer.
Our tools for probing the turbulence are high-resolution spectropolarimetric
observations such as have recently been obtained with the two sunrise missions,
and numerical simulations. Our aim is to study photospheric turbulence with the
help of Fourier power spectra that we compute from observations and
simulations. We also attempt to explain some properties of the photospheric
overshooting flow with the help of its governing equations and simulations. We
find that quiet-Sun observations and smeared simulations exhibit a power-law
behavior in the subgranular range of their Doppler velocity power spectra with
an index of. The unsmeared simulations exhibit a power-law index
of. The smearing considerably reduces the extent of the
power-law-like portion of the spectra. Therefore, the limited spatial
resolution in some observations might eventually result in larger uncertainties
in the estimation of the power-law indices.
The simulated vertical velocity power spectra as a function of height show a
rapid change in the power-law index from the solar surface to ~km above
it. A scale-dependent transport of the vertical momentum occurs. At smaller
scales, the vertical momentum is more efficiently transported sideways than at
larger scales. This results in less vertical velocity power transported upward
at small scales than at larger scales and produces a progressively steeper
vertical velocity power law below km. Above this height, the gravity work
progressively gains importance at all scales, making the atmosphere
progressively more hydrostatic and resulting in a gradually less steep power
law.Comment: 10 pages, 7 figures, Accepted in A and
GCN5 modulates salicylic acid homeostasis by regulating H3K14ac levels at the 5ʹ and 3ʹ ends of its target genes
The modification of histones by acetyl groups has a key role in the regulation of chromatin structure and transcription. The Arabidopsis thaliana histone acetyltransferase GCN5 regulates histone modifications as part of the Spt-Ada-Gcn5 Acetyltransferase (SAGA) transcriptional coactivator complex. GCN5 was previously shown to acetylate lysine 14 of histone 3 (H3K14ac) in the promoter regions of its target genes even though GCN5 binding did not systematically correlate with gene activation. Here, we explored the mechanism through which GCN5 controls transcription. First, we fine-mapped its GCN5 binding sites genome-wide and then used several global methodologies (ATAC-seq, ChIP-seq and RNA-seq) to assess the effect of GCN5 loss-of-function on the expression and epigenetic regulation of its target genes. These analyses provided evidence that GCN5 has a dual role in the regulation of H3K14ac levels in their 5′ and 3′ ends of its target genes. While the gcn5 mutation led to a genome-wide decrease of H3K14ac in the 5′ end of the GCN5 down-regulated targets, it also led to an increase of H3K14ac in the 3′ ends of GCN5 up-regulated targets. Furthermore, genome-wide changes in H3K14ac levels in the gcn5 mutant correlated with changes in H3K9ac at both 5′ and 3′ ends, providing evidence for a molecular link between the depositions of these two histone modifications. To understand the biological relevance of these regulations, we showed that GCN5 participates in the responses to biotic stress by repressing salicylic acid (SA) accumulation and SA-mediated immunity, highlighting the role of this protein in the regulation of the crosstalk between diverse developmental and stress-responsive physiological programs. Hence, our results demonstrate that GCN5, through the modulation of H3K14ac levels on its targets, controls the balance between biotic and abiotic stress responses and is a master regulator of plant-environmental interactions
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