4,388 research outputs found
The effect of a non-volatile dust mantle on the energy balance of cometary surface layers
It is likely that large parts of a cometary surface layer consist of porous ices, which are covered by a thin layer of non-volatile debris, whose structure is also fluffy and porous. In this paper the results of model calculations are presented. The calculations show the effect of ice and dust pore sizes and of the dust mantle thickness upon the thermal behavior of such a dust-ice system, when it is irradiated by the sun. In particular, it is found that the average pore size of the ice and the dust material has a large influence both on the dust surface temperature and on the temperature at the dust-ice interface
Stellar hydrodynamical modeling of dwarf galaxies: simulation methodology, tests, and first results
Cosmological simulations still lack numerical resolution or physical
processes to simulate dwarf galaxies in sufficient details. Accurate numerical
simulations of individual dwarf galaxies are thus still in demand. We aim at
(i) studying in detail the coupling between stars and gas in a galaxy,
exploiting the so-called stellar hydrodynamical approach, and (ii) studying the
chemo-dynamical evolution of individual galaxies starting from
self-consistently calculated initial gas distributions. We present a novel
chemo-dynamical code in which the dynamics of gas is computed using the usual
hydrodynamics equations, while the dynamics of stars is described by the
stellar hydrodynamics approach, which solves for the first three moments of the
collisionless Boltzmann equation. The feedback from stellar winds and dying
stars is followed in detail. In particular, a novel and detailed approach has
been developed to trace the aging of various stellar populations, which enables
an accurate calculation of the stellar feedback depending on the stellar age.
We build initial equilibrium models of dwarf galaxies that take gas
self-gravity into account and present different levels of rotational support.
Models with high rotational support develop prominent bipolar outflows; a
newly-born stellar population in these models is preferentially concentrated to
the galactic midplane. Models with little rotational support blow away a large
fraction of the gas and the resulting stellar distribution is extended and
diffuse. The stellar dynamics turns out to be a crucial aspect of galaxy
evolution. If we artificially suppress stellar dynamics, supernova explosions
occur in a medium heated and diluted by the previous activity of stellar winds,
thus artificially enhancing the stellar feedback (abridged).Comment: 22 pages, 19 figures, accepted for publication in Astronomy &
Astrophysic
Upper Bound on the Capacity of a Cascade of Nonlinear and Noisy Channels
An upper bound on the capacity of a cascade of nonlinear and noisy channels
is presented. The cascade mimics the split-step Fourier method for computing
waveform propagation governed by the stochastic generalized nonlinear
Schroedinger equation. It is shown that the spectral efficiency of the cascade
is at most log(1+SNR), where SNR is the receiver signal-to-noise ratio. The
results may be applied to optical fiber channels. However, the definition of
bandwidth is subtle and leaves open interpretations of the bound. Some of these
interpretations are discussed.Comment: The main change is to define the noise as bandlimited already in (8)
rather than before (15). This serves to clarify subsequent step
Made-to-Measure models of the Galactic Box/Peanut bulge: stellar and total mass in the bulge region
We construct dynamical models of the Milky Way's Box/Peanut (B/P) bulge,
using the recently measured 3D density of Red Clump Giants (RCGs) as well as
kinematic data from the BRAVA survey. We match these data using the NMAGIC
Made-to-Measure method, starting with N-body models for barred discs in
different dark matter haloes. We determine the total mass in the bulge volume
of the RCGs measurement (+-2.2 x +- 1.4 x +- 1.2 kpc) with unprecedented
accuracy and robustness to be 1.84 +- 0.07 x10^10 Msun. The stellar mass in
this volume varies between 1.25-1.6 x10^10 Msun, depending on the amount of
dark matter in the bulge. We evaluate the mass-to-light and mass-to-clump
ratios in the bulge and compare them to theoretical predictions from population
synthesis models. We find a mass-to-light ratio in the K-band in the range
0.8-1.1. The models are consistent with a Kroupa or Chabrier IMF, but a
Salpeter IMF is ruled out for stellar ages of 10 Gyr. To match predictions from
the Zoccali IMF derived from the bulge stellar luminosity function requires
about 40% or 0.7 x10^10 Msun dark matter in the bulge region. The BRAVA data
together with the RCGs 3D density imply a low pattern speed for the Galactic
B/P bulge of 25-30 km.s-1.kpc-1. This would place the Galaxy among the slow
rotators (R >= 1.5). Finally, we show that the Milky Way's B/P bulge has an
off-centred X structure, and that the stellar mass involved in the peanut shape
accounts for at least 20% of the stellar mass of the bulge, significantly
larger than previously thought.Comment: Accepted for publication in MNRA
Scaling-up experiments of smouldering combustion as a remediation technology for contaminated soil
Self-sustaining Treatment for Active Remediation (STAR) is a novel, patent-pending process that uses smouldering combustion as a remediation technology for land contaminated with hazardous organic liquids. Compounds such as chlorinated solvents, coal tar and petroleum products, called Non-Aqueous Phase Liquids (NAPLs) for their low miscibility with water, have a long history of use in the industrialised world and are among the most ubiquitous of contaminants worldwide. These contaminants are toxic and many are suspected or known carcinogens. Existing remediation technologies are expensive and ineffective at reducing NAPL source zones sufficiently to restore affected water resources to appropriate quality levels. STAR introduces a self-sustaining smouldering reaction within the NAPL pool in the subsurface and allows that reaction to provide all of the post-ignition energy required by the reaction to completely remediate the NAPL source zone in the soil. Results from laboratory and field experiments have been very promising. Laboratory experiments have demonstrated STAR across a wide range of NAPL fuels and focused on coal tar to identify key parameters for successful remediation. Modelling has suggested that STAR efficiency will improve with scale as effects such as heat losses from boundaries become less significant. Observations from field experiments support the modelling theory - significantly lower relative air flow in a smouldering field experiment (330L) led to faster smouldering front propagation than observed in laboratory experiments (1L and 3L). Preliminary emissions monitoring by Fourier Transform Infrared (FTIR) spectroscopy has suggested that STAR emissions might be low enough to meet regulatory requirements, but further study is necessary. As emissions are expected to vary with each contaminant, activated carbon filters are being developed and tested in case emissions filtration is necessary. Experiments at all scales have demonstrated that STAR is controllable and self-terminating. Pilot-scale (2500L) field trials are underway to demonstrate STAR on excavated contaminated soil. The materials that will be studied in these trials are manufactured coal tar in coarse sand (which is the same material as used in the laboratory and field experiments) as well as two soils obtained from coal tar contaminated sites. This poster focuses on the scale-up to these field trials, including small scale characterisation, large scale performance, emissions monitoring and post-treatment soil analysis
Experimental studies of self-sustaining thermal aquifer remediation (STAR) for non-aqueous phase liquid (NAPL) sources
Self-sustaining Thermal Aquifer Remediation (STAR) is a novel technology that employs smouldering combustion for the remediation of subsurface contamination by non-aqueous phase liquids (NAPLs). Smouldering is a form of combustion that is slower and less energetic than flaming combustion. Familiar examples of smouldering involve solid fuels that are destroyed by the reaction (e.g., a smouldering cigarette or peat smouldering after a wildfire). In STAR, the NAPL serves as the fuel within an inert, porous soil medium. Results from experiments across a range of scales are very promising. Detailed characterisation has focused on coal tar, a common denser-than-water NAPL (DNAPL) contaminant. Complete remediation is demonstrated across this range of scales. Visual observations are supported bychemical extraction results. Further experiments suggest that STAR can be self-sustaining, meaning that once ignited the process can supply its own energy to propagate. Costly energy input is reduced significantly. Comparison of large scale to small scale laboratory experiments, a volume increase by a factor of 100, suggests that STAR process efficiency increases with scale. This increase in efficiency results from reduced heat losses at larger scales while maximum the temperature achieved by STAR is unaffected. The research also demonstrates the controllability of STAR, where the termination of airflow to the reaction terminates the STAR process. The scale-up process provides important guidance to the development of full scale STAR for ex situ remediation of NAPL-contaminated soil
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