158 research outputs found
{Tris[2-(imidazol-2-ylmethylimino)ethyl]methylammonium}iron(II) tris(perchlorate) dihydrate
The title complex, [Fe(C19H27N10)](ClO4)3·2H2O, is a new polymorph of an iron(II) Schiff base complex of tris(2-aminoethyl)methylammonium with imidazole-2-carboxaldehyde. The octahedral FeII atom is bound to three facial imidazole N atoms with average Fe—Nimidazole and Fe—Nimine bond distances of 1.963 (5) and 1.951 (5) Å, respectively. The central N atom of the tripodal ligand is outside the bonding distance at 3.92 Å. The crystal packing is stabilized by the hydrogen-bonding interactions between the two water molecules (acceptor) and two of the three imidazole NH groups (donor). The third imidazole NH group (donor) forms a hydrogen bond to one of the three perchlorate counter-ions (acceptor)
Wear and Reactivity Studies of Melt infiltrated Ceramic Matrix Composite
As interest grows in the use of ceramic matrix composites (CMCs) for critical gas turbine engine components, the effects of the CMCs interaction with the adjoining structure needs to be understood. A series of CMC/material couples were wear tested in a custom elevated temperature test rig and tested as diffusion couples, to identify interactions. Specifically, melt infiltrated silicon carbide/silicon carbide (MI SiC/SiC) CMC was tested in combination with a nickel-based super alloy, Waspaloy, a thermal barrier coating, Yttria Stabilized Zirconia (YSZ), and a monolithic ceramic, silicon nitride (Si3N4). To make the tests more representative of actual hardware, the surface of the CMC was kept in the as-received state (not machined) with the full surface features/roughness present. Test results include: scanning electron microscope characterization of the surfaces, micro-structural characterization, and microprobe analysis
Assessment of Neural Network Augmented Reynolds Averaged Navier Stokes Turbulence Model in Extrapolation Modes
A machine-learned (ML) model is developed to enhance the accuracy of
turbulence transport equations of Reynolds Averaged Navier Stokes (RANS) solver
and applied for periodic hill test case, which involves complex flow regimes,
such as attached boundary layer, shear-layer, and separation and reattachment.
The accuracy of the model is investigated in extrapolation modes, i.e., the
test case has much larger separation bubble and higher turbulence than the
training cases. A parametric study is also performed to understand the effect
of network hyperparameters on training and model accuracy and to quantify the
uncertainty in model accuracy due to the non-deterministic nature of the neural
network training. The study revealed that, for any network, less than optimal
mini-batch size results in overfitting, and larger than optimal batch size
reduces accuracy. Data clustering is found to be an efficient approach to
prevent the machine-learned model from over-training on more prevalent flow
regimes, and results in a model with similar accuracy using almost one-third of
the training dataset. Feature importance analysis reveals that turbulence
production is correlated with shear strain in the free-shear region, with shear
strain and wall-distance and local velocity-based Reynolds number in the
boundary layer regime, and with streamwise velocity gradient in the
accelerating flow regime. The flow direction is found to be key in identifying
flow separation and reattachment regime. Machine-learned models perform poorly
in extrapolation mode, wherein the prediction shows less than 10% correlation
with Direct Numerical Simulation (DNS). A priori tests reveal that model
predictability improves significantly as the hill dataset is partially added
during training in a partial extrapolation model, e.g., with the addition of
only 5% of the hill data increases correlation with DNS to 80%.Comment: 50 pages, 18 figure
The White Dwarf Cooling Sequence of NGC6397
We present the results of a deep Hubble Space Telescope (HST) exposure of the
nearby globular cluster NGC6397, focussing attention on the cluster's white
dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in
depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using
both artificial star tests and the detectability of background galaxies at
fainter magnitudes, that the cutoff is real and represents the truncation of
the white dwarf luminosity function in this cluster. We perform a detailed
comparison between cooling models and the observed distribution of white dwarfs
in colour and magnitude, taking into account uncertainties in distance,
extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model
uncertainties. After marginalising over these variables, we obtain values for
the cluster distance modulus and age of \mu_0 = 12.02 \pm 0.06 and T_c = 11.47
\pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf
initial-final mass relations are in good agreement with other independent
determinations, and the cluster age is consistent with, but more precise than,
prior determinations made using the main sequence turnoff method. In
particular, within the context of the currently accepted \Lambda CDM
cosmological model, this age places the formation of NGC6397 at a redshift z=3,
at a time when the cosmological star formation rate was approaching its peak.Comment: 56 pages, 30 figure
Tris(ethane-1,2-diamine-κ2 N,N′)nickel(II) diiodide
The title compound, [Ni(C2H8N2)3]I2, crystallizes with an [Ni(en)3
2+] cation (en is ethane-1,2-diamine) and two iodide ions in the asymmetric unit. Two of the en ligands surrrounding the Ni2+ ion have disordered C atoms, while the third exhibits extensive weak N—H⋯I interactions with the two iodide ions that extend throughout the crystalline lattice, producing an infinite network along (011)
Tris(ethane-1,2-diamine-κ2 N,N′)cobalt(III) carbonate iodide tetrahydrate
The title compound, [Co(C2H8N2)3](CO3)I·4H2O, crystallizes with a [Co(en)3]3+ cation (en is ethane-1,2-diamine), CO3
2− and I− anions and four water molecules in the asymmetric unit. In the cation, the three rings formed by the ethylenediamine units and the CoIII metal ion are in slightly distorted twist conformations. Numerous O—H⋯O, N—H⋯O, N—H⋯I and O—H⋯I intermolecular hydrogen bonds between the cation and two anions in concert with the four water molecules dominate the crystal packing and create a supramolecular infinite three-dimensional framework
The White Dwarf Cooling Sequence of the Globular Cluster Messier 4
We present the white dwarf sequence of the globular cluster M4, based on a
123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I =
28. The white dwarf luminosity function rises sharply for I >25.5, consistent
with the behaviour expected for a burst population. The white dwarfs of M4
extend to approximately 2.5 magnitudes fainter than the peak of the local
Galactic disk white dwarf luminosity function. This demonstrates a clear and
significant age difference between the Galactic disk and the halo globular
cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit
each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7
+/- 0.7 Gyr for M4 (2-sigma statistical errors).Comment: 14 pages, 4 diagrams. Accepted for publication in the Astrophysical
Journal Letter
Understanding travel behaviour change during mega-events: Lessons from the London 2012 Games
This paper presents results from a longitudinal study of the travel behaviour change associated with the London 2012 Olympic and Paralympic Games (the ‘Games’). The research examines commuter travel behaviour through a panel approach enabling an understanding of individual behaviour across three waves (before, during and after), with the study utilising unique access to a Transport for London panel study (n=1132). The findings indicate that a substantial amount of change occurred during the Games (54% made at least one change), with reducing or re-timing journeys being the most likely adaptations made. A key objective of this work was to advance the discussion about the theoretical constructs that are most applicable in the study of behaviour change associated with disruptive events, which was done through the application and critical evaluation of the Transtheoretical Model. The insights from the stages of change element of the model were relatively limited but the analysis shows significant differences in the underlying factors explaining change according to the type of change made (reduce, re-time, re-mode and re-route). Whilst the long-term behavioural impacts of events like the Games appear small, the study has uncovered a need to consider these behavioural choices as distinct rather than under the collective term of “travel behaviour change”, as is current practice
HST Observations of the White Dwarf Cooling Sequence of M4
We investigate in detail the white dwarf cooling sequence of the globular
cluster Messier 4. In particular we study the influence of various systematic
uncertainties, both observational and theoretical, on the determination of the
cluster age from the white dwarf cooling sequence. These include uncertainties
in the distance to the cluster and the extinction along the line of sight, as
well as the white dwarf mass, envelope and core compositions and the white
dwarf --main sequence mass relation. We find that fitting to the full
two-dimensional colour-magnitude diagram offers a more robust method for age
determination than the traditional method of fitting the one-dimensional white
dwarf luminosity function.
After taking into account the various uncertainties, we find a best fit age
of 12.1 Gyr, with a 95% lower limit of 10.3 Gyr. We also perform fits using two
other sets of cooling models from the literature. The models of Chabrier et al
(2000) yield an encouragingly similar result, although the models of Salaris et
al (2000) do not provide as good a fit. Our results support our previous
determination of a delay between the formation of the Galactic halo and the
onset of star formation in the Galactic disk.Comment: many pages, 43 postscript figures, submitted to Ap
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