2,079 research outputs found
Correction of diffraction effects in confocal raman microspectroscopy
A mathematical approach developed to correct depth profiles of
wet-chemically modified polymer films obtained by confocal Raman
microscopy is presented which takes into account scattered contributions originated from a diffraction-limited laser focal volume. It is demonstrated that the problem can be described using a linear Fredholm integral equation of the first kind which correlates apparent and true Raman intensities with the depth resolution curve of the instrument.
The calculations of the corrected depth profiles show that considerable differences between apparent and corrected depth profiles exist at the surface, especially when profiles with strong concentration gradients are dealt with or an instrument with poor depth resolution is used. Degrees of modification at the surface obtained by calculation of the corrected depth profiles are compared with those measured by FTIR-ATR and show an excellent concordance.</p
A new model for deflagration fronts in reactive fluids
We present a new way of modeling deflagration fronts in reactive fluids, the
main emphasis being on turbulent thermonuclear deflagration fronts in white
dwarfs undergoing a Type Ia supernova explosion. Our approach is based on a
level set method which treats the front as a mathematical discontinuity and
allows full coupling between the front geometry and the flow field. With only
minor modifications, this method can also be applied to describe contact
discontinuities. Two different implementations are described and their
physically correct behaviour for simple testcases is shown. First results of
the method applied to the concrete problems of Type Ia supernovae and chemical
hydrogen combustion are briefly discussed; a more extensive analysis of our
astrophysical simulations is given in (Reinecke et al. 1998, MPA Green Report
1122b).Comment: 11 pages, 13 figures, accepted by A&A, corrected and extended
according to referee's comment
Carbon Ignition in Type Ia Supernovae: An Analytic Model
The observable properties of a Type Ia supernova are sensitive to how the
nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point
at its center, off-center, or at multiple points and times. We present a simple
analytic model for the runaway based upon a combination of stellar
mixing-length theory and recent advances in understanding Rayleigh-Benard
convection. The convective flow just prior to runaway is likely to have a
strong dipolar component, though higher multipoles may contribute appreciably
at the very high Rayleigh number (10) appropriate to the white dwarf
core. A likely outcome is multi-point ignition with an exponentially increasing
number of ignition points during the few tenths of a second that it takes the
runaway to develop. The first sparks ignite approximately 150 - 200 km off
center, followed by ignition at smaller radii. Rotation may be important to
break the dipole asymmetry of the ignition and give a healthy explosion.Comment: 14 pages, 0 figures, submitted to ApJ, corrected typo in first
author's nam
A Conceptual Framework for B2B Electronic Contracting
Electronic contracting aims at improving existing business relationship paradigms and at enabling new forms of contractual relationships. To successfully realize these objectives, an integral understanding of the contracting field must be established. In this paper, we propose a conceptual framework for business-to-business contracting support. The framework provides a complete view over the contracting field. It allows positioning research efforts in the domain, analysing them, placing their goals into perspective, and overseeing future research topics and issues. It is the basis for drawing conclusions about basic requirements to contracting systems
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
Can Deflagration-Detonation-Transitions occur in Type Ia Supernovae?
The mechanism for deflagration-detonation-transition (DDT) by turbulent
preconditioning, suggested to explain the possible occurrence of delayed
detonations in Type Ia supernova explosions, is argued to be conceptually
inconsistent. It relies crucially on diffusive heat losses of the burned
material on macroscopic scales. Regardless of the amplitude of turbulent
velocity fluctuations, the typical gradient scale for temperature fluctuations
is shown to be the laminar flame width or smaller, rather than the factor of
thousand more required for a DDT. Furthermore, thermonuclear flames cannot be
fully quenched in regions much larger than the laminar flame width as a
consequence of their simple ``chemistry''. Possible alternative explosion
scenarios are briefly discussed.Comment: 8 pages, uses aastex; added references. Accepted by ApJ Letter
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
Aerothermodynamic radiation studies
We have built and made operational a 6 in. electric arc driven shock tube which alloys us to study the non-equilibrium radiation and kinetics of low pressure (0.1 to 1 torr) gases processed by 6 to 12 km/s shock waves. The diagnostic system allows simultaneous monitoring of shock radiation temporal histories by a bank of up to six radiometers, and spectral histories with two optical multi-channel analyzers. A data set of eight shots was assembled, comprising shocks in N2 and air at pressures between 0.1 and 1 torr and velocities of 6 to 12 km/s. Spectrally resolved data was taken in both the non-equilibrium and equilibrium shock regions on all shots. The present data appear to be the first spectrally resolved shock radiation measurements in N2 performed at 12 km/s. The data base was partially analyzed with salient features identified
Nucleosynthesis in thermonuclear supernovae with tracers: convergence and variable mass particles
Nucleosynthetic yield predictions for multi-dimensional simulations of
thermonuclear supernovae generally rely on the tracer particle method to obtain
isotopic information of the ejected material for a given supernova simulation.
We investigate how many tracer particles are required to determine converged
integrated total nucleosynthetic yields. For this purpose, we conduct a
resolution study in the number of tracer particles for different hydrodynamical
explosion models at fixed spatial resolution. We perform hydrodynamic
simulations on a co-expanding Eulerian grid in two dimensions assuming
rotational symmetry for both pure deflagration and delayed detonation Type Ia
supernova explosions. Within a given explosion model, we vary the number of
tracer particles to determine the minimum needed for the method to give a
robust prediction of the integrated yields of the most abundant nuclides. For
the first time, we relax the usual assumption of constant tracer particle mass
and introduce a radially vary- ing distribution of tracer particle masses. We
find that the nucleosynthetic yields of the most abundant species (mass
fraction > 10E-5) are reasonably well predicted for a tracer number as small as
32 per axis and direction - more or less independent of the explosion model. We
conclude that the number of tracer particles that were used in extant published
works appear to have been sufficient as far as integrated yields are concerned
for the most copiously produced nuclides. Additionally we find that a suitably
chosen tracer mass distribution can improve convergence for nuclei produced in
the outer layer of the supernova where the constant tracer mass prescription
suffers from poor spatial resolution.Comment: 9 pages, 5 figures, accepted for publication in MNRA
- …