1,491 research outputs found
A whole farm model for quantifying total greenhouse gas emissions on South African dairy farms
This paper presents a model to quantify total greenhouse gas (GHG) emissions from dairy farms. The model, which is based on a whole farm management approach, accounts for the variability that occurs in GHG emissions among farm production and management practices. The variation is accommodated in six dairy farm management systems (FMS), which broadly include typical dairy production systems in South Africa. These are pasture-based with high or low stocking rates, total mixed ration with high or low stocking rates, and partial mixed ration with high or low stocking rates. Three variations of functional units that were used to evaluate the environmental impacts of various FMS are defined as per animal unit = kg CO2-eq head-1 yr-1; per unit of farm area = kg CO2-eq ha-1 yr-1, and per unit of product = kg CO2-eq kg FPCM-1, where FPCM is fat and protein corrected milk. The results show a range of GHG emissions in CO2-eq among the FMS with various methodological approaches because of the large impact from different emission factors, which vary between accounting methods. The more detailed equations were recommended to effectively improve environmental impacts. These more detailed non-linear equations tended to predict more biologically realistic emissions when compared with the linear equations in which over or under-predictions of GHG were observed. The most prominent drivers for GHG emissions across all FMS were from enteric methane (CH4) and nitrous oxide (N2O) from soil management. Rankings among FMS varied according to output methodology and functional units. GHG emissions expressed per animal or per unit area differ greatly from those expressed from a given level of product. In conclusion, the accounting methodologies that are described in this paper to predict GHG emissions of animal-related origin performed sufficiently across all FMS, and could be applied to quantify the carbon footprint of dairy production systems in South Africa.Keywords: Carbon dioxide equivalents, dairy production, methane, nitrous oxid
Direct Numerical Simulations of Type Ia Supernovae Flames II: The Rayleigh-Taylor Instability
A Type Ia supernova explosion likely begins as a nuclear runaway near the
center of a carbon-oxygen white dwarf. The outward propagating flame is
unstable to the Landau-Darrieus, Rayleigh-Taylor, and Kelvin-Helmholtz
instabilities, which serve to accelerate it to a large fraction of the speed of
sound. We investigate the Rayleigh-Taylor unstable flame at the transition from
the flamelet regime to the distributed-burning regime, around densities of
g/cc, through detailed, fully resolved simulations. A low Mach number,
adaptive mesh hydrodynamics code is used to achieve the necessary resolution
and long time scales. As the density is varied, we see a fundamental change in
the character of the burning--at the low end of the density range the
Rayleigh-Taylor instability dominates the burning, whereas at the high end the
burning suppresses the instability. In all cases, significant acceleration of
the flame is observed, limited only by the size of the domain we are able to
study. We discuss the implications of these results on the potential for a
deflagration to detonation transition.Comment: submitted to ApJ, some figures degraded due to size constraint
Indirect coupling between spins in semiconductor quantum dots
The optically induced indirect exchange interaction between spins in two
quantum dots is investigated theoretically. We present a microscopic
formulation of the interaction between the localized spin and the itinerant
carriers including the effects of correlation, using a set of canonical
transformations. Correlation effects are found to be of comparable magnitude as
the direct exchange. We give quantitative results for realistic quantum dot
geometries and find the largest couplings for one dimensional systems.Comment: 4 pages, 3 figure
Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario
We develop an improved method for tracking the nuclear flame during the
deflagration phase of a Type Ia supernova, and apply it to study the variation
in outcomes expected from the gravitationally confined detonation (GCD)
paradigm. A simplified 3-stage burning model and a non-static ash state are
integrated with an artificially thickened advection-diffusion-reaction (ADR)
flame front in order to provide an accurate but highly efficient representation
of the energy release and electron capture in and after the unresolvable flame.
We demonstrate that both our ADR and energy release methods do not generate
significant acoustic noise, as has been a problem with previous ADR-based
schemes. We proceed to model aspects of the deflagration, particularly the role
of buoyancy of the hot ash, and find that our methods are reasonably
well-behaved with respect to numerical resolution. We show that if a detonation
occurs in material swept up by the material ejected by the first rising bubble
but gravitationally confined to the white dwarf (WD) surface (the GCD
paradigm), the density structure of the WD at detonation is systematically
correlated with the distance of the deflagration ignition point from the center
of the star. Coupled to a suitably stochastic ignition process, this
correlation may provide a plausible explanation for the variety of nickel
masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa
Model Flames in the Boussinesq Limit: The Effects of Feedback
We have studied the fully nonlinear behavior of pre-mixed flames in a
gravitationally stratified medium, subject to the Boussinesq approximation. Key
results include the establishment of criterion for when such flames propagate
as simple planar flames; elucidation of scaling laws for the effective flame
speed; and a study of the stability properties of these flames. The simplicity
of some of our scalings results suggests that analytical work may further
advance our understandings of buoyant flames.Comment: 11 pages, 14 figures, RevTex, gzipped tar fil
The mixed problem for the Lam\'e system in two dimensions
We consider the mixed problem for the Lam\'e system of elasticity in a
bounded Lipschitz domain . We suppose that the
boundary is written as the union of two disjoint sets, . We take traction data from the space and Dirichlet data from a
Sobolev space and look for a solution of with the
given boundary conditions. We give a scale invariant condition on and find
an exponent so that for , we have a unique solution of this
boundary value problem with the non-tangential maximal function of the gradient
of the solution in . We also establish the existence of a
unique solution when the data is taken from Hardy spaces and Hardy-Sobolev
spaces with in for some
Book Reviews
Book Review 1Book Title: Early Life Histories of Fishes: New Developmental, Ecological and Evolutionary PerspectivesBook Author: Edited by E.K. BalonDr W. Junk, Dordrecht. 280 pp.Book Review 2Book Title: Comparative Aspects of Extracellular Acid-base BalanceBook Author: J.P. TruchotSpringer, 1987. 248 pp. 51 figures.Book Review 3Book Title: Insect Flight: Dispersal and MigrationBook Author: Edited by W. DanthanarayanaSpringer-Verlag, Berlin, 1986. 289 pp.Book Review 4Book Title: The Mammalian Herbivore Stomach. Comparative Anatomy, Function and EvolutionBook Author: Peter LangerGustav Fischer, Stuttgart, 1988. 557 pages, 246 figures and 72 tablesBook Review 5Book Title: Biology of the Integument. Vol. 2: VertebratesBook Authors: Edited by J. Bereiter-Hahn, A.G. Matoltsy & K.S. RichardsSpringer, Berlin, 1986. 855 pp.Book Review 6Book Title: Advances in the Biology of Turbellarians and related PlatyhelminthesBook Author: Edited by Seth TylerDr. W. Junk Publishers, 1986. 357 pages; 253 figuresBook Review 7Book Title: Evolutionary Physiological EcologyBook Author: Edited by P. CalowCambridge University Press, Cambridge. 239 pp.Book Review 8Book Title: DragonfliesBook Author: Peter L. MillerCambridge University Press, Cambridge, New York and Melbourne, 1987. 84 pp
Effective emotion regulation strategies improve fMRI and ECG markers of psychopathology in panic disorder: Implications for psychological treatment action
© 2015 Macmillan Publishers Limited. All rights reserved. Impairments in emotion regulation are thought to have a key role in the pathogenesis of anxiety disorders, but the neurobiological underpinnings contributing to vulnerability remain poorly understood. It has been a long-held view that exaggerated fear is linked to hyperresponsivity of limbic brain areas and impaired recruitment of prefrontal control. However, increasing evidence suggests that prefrontal-cortical networks are hyperactive during threat processing in anxiety disorders. This study directly explored limbic-prefrontal neural response, connectivity and heart-rate variability (HRV) in patients with a severe anxiety disorder during incidental versus intentional emotion regulation. During 3 Tesla functional magnetic resonance imaging, 18 participants with panic disorder and 18 healthy controls performed an emotion regulation task. They either viewed negative images naturally (Maintain), or they were instructed to intentionally downregulate negative affect using previously taught strategies of cognitive reappraisal (Reappraisal). Electrocardiograms were recorded throughout to provide a functional measure of regulation and emotional processing. Compared with controls, patients showed increased neural activation in limbic-prefrontal areas and reduced HRV during incidental emotion regulation (Maintain). During intentional regulation (Reappraisal), group differences were significantly attenuated. These findings emphasize patients' ability to regulate negative affect if provided with adaptive strategies. They also bring prefrontal hyperactivation forward as a potential mechanism of psychopathology in anxiety disorders. Although these results challenge models proposing impaired allocation of prefrontal resources as a key characteristic of anxiety disorders, they are in line with more recent neurobiological frameworks suggesting that prefrontal hyperactivation might reflect increased utilisation of maladaptive regulation strategies quintessential for anxiety disorders
Spectroscopic variability of two Oe stars
The Oe stars HD45314 and HD60848 have recently been found to exhibit very
different X-ray properties: whilst HD60848 has an X-ray spectrum and emission
level typical of most OB stars, HD45314 features a much harder and brighter
X-ray emission, making it a so-called gamma Cas analogue. Monitoring the
optical spectra could provide hints towards the origin of these very different
behaviours. We analyse a large set of spectroscopic observations of HD45314 and
HD60848, extending over 20 years. We further attempt to fit the H-alpha line
profiles of both stars with a simple model of emission line formation in a
Keplerian disk. Strong variations in the strengths of the H-alpha, H-beta, and
He I 5876 emission lines are observed for both stars. In the case of HD60848,
we find a time lag between the variations in the equivalent widths of these
lines. The emission lines are double peaked with nearly identical strengths of
the violet and red peaks. The H-alpha profile of this star can be successfully
reproduced by our model of a disk seen under an inclination of 30 degrees. In
the case of HD45314, the emission lines are highly asymmetric and display
strong line profile variations. We find a major change in behaviour between the
2002 outburst and the one observed in 2013. This concerns both the relationship
between the equivalent widths of the various lines and their morphologies at
maximum strength (double-peaked in 2002 versus single-peaked in 2013). Our
simple disk model fails to reproduce the observed H-alpha line profiles of
HD45314. Our results further support the interpretation that Oe stars do have
decretion disks similar to those of Be stars. Whilst the emission lines of
HD60848 are explained by a disk with a Keplerian velocity field, the disk of
HD45314 seems to have a significantly more complex velocity field that could be
related to the phenomenon that produces its peculiar X-ray emission.Comment: Accepted for Publication in A&
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