More on the Narrowing of Impact Broadened Radio Recombination Lines at High Principal Quantum Number

Recently Alexander and Gulyaev have suggested that the apparent decrease in impact broadening of radio recombination lines seen at high principal quantum number n may be a product of the data reduction process, possibly resulting from the presence of noise on the telescope spectra that is not present on the calculated comparison spectra. This is an interesting proposal. However, there are serious problems with their analysis that need to be pointed out. Perhaps the most important of these is the fact that for principal quantum numbers below n = 200, where the widths are not in question, their processed generated profile widths do not fit the widths of the processed lines obtained at the telescope. After processing, the halfwidths of the generated and telescope profiles must agree below n = 200 if we are to believe that the processed generated linewidths above n = 200 are meaningful. Theirs do not. Furthermore, we find that after applying the linewidth reduction factors found by Alexander and Gulyaev for their noise added profiles to our generated profiles to simulate their noise adding effect, the processed widths we obtain still do not come close to explaining the narrowing seen in the telescope lines for n values in the range 200 < n < 250. It is concluded that what is needed to solve this mystery is a completely new approach using a different observing technique instead of simply a further manipulation of the frequency-switched data.Comment: Six pages with 4 figures. Accepted for publication in Astrophysics and Space Scienc

A Novel Image Segmentation Approach for Microstructure Modelling

Microstructure models are used to investigate bulk properties of a material given images of it’s microstructure. Through their use the effect of microstructural features can be investigated independently. Processes can then be optimised to give the desired selection of microstructural features. Currently automatic methods of segmenting SEM images either miss cracks leading to large overestimates of properties such as thermal conductivity or use unjustifiable methods to select a threshold point which class cracks as porosity leading to over estimates of porosity. In this work a novel automatic image segmentation method is presented which produces maps for each phase in the microstructure and an additional phase of cracks. The selection of threshold points is based on the assumption that the brightness values for each phase should be normally distributed. Additional image processing is used to ensure results remain physically relevant. The image segmentation method has been compared to other available methods and shown to be as or more repeatable with changes of brightness and contrast of the input image than relevant alternatives. The resulting modelling route is able to predict density and specific heat to within experimental error, while the expected under predictions for thermal conductivity are observed

The influence of material properties on the wear of abradable materials

In aero-engines it is possible for the blades of the compressor, turbine or fan to incur into their casings. At these interfaces a lining of composite abradable material is used to limit damage to components and thereby sustain the efficiency and longevity of the engine as a whole. These composite materials must have good abradability and erosion resistance. Previously, the wear mechanisms at the contact between the blade and the coating have been characterised using stroboscopic imaging and force measurement on a scaled test-rig platform. This work is focused on the characterisation of the wear mechanism for two different hardnesses of abradable lining. The established stroboscopic imaging technique and contact force measurements are combined with sectioning of the abradable material in order to analyse the material’s response during the tests. A measure of the thermal properties and the resulting temperature of the linings during the test have also been made to further understand the effect of coating hardness. The wear mechanism, material response, contact force and thermal properties of the coating have been used to characterise the different material behaviour with different hardness. At low incursion rates, with a soft coating, the blade tip becomes worn after an initial adhesive transfer from the coating. Post-test sectioning showed blade material and significant compaction present in the coating. The harder coating produced adhesion on the blade tip with solidification observed in the coating. Thermal diffusivity measurements and modelling indicated that thermally driven wear observed was as a consequence of the increased number of boundaries between the metal and hBN phases present interrupting heat flow, leading to a concentration of surface heat. At higher incursion rates, the wear mechanism is more similar between the coatings and a cutting mechanism dominates producing negligible adhesion and blade wear

An investigation of the relationship between wear and contact force for abradable materials

Abradable linings are frequently used on the inside of aero-engine casings. During the operation of engine, the rotating blades may strike the lining of the casing. The wear mechanisms present during these incursions have been re-produced on a scaled test rig platform. Previously, characterisation of the wear has been performed using a stroboscopic imaging technique in order to identify the different wear mechanisms at the incursion conditions investigated. In the present study, a dynamometer has been included in the test arrangement allowing the measurement of the contact force. This approach has then been combined with sectioning of the abradable test samples, in order to investigate the material response to the different incursion conditions. The wear results, the cutting force and material structure post-incursion show a high degree of correlation. At low incursion rates, significant consolidation and solidification of abradable material was observed, whilst at the same time adhesive transfer to the blade was recorded along with a low tangential to normal force ratio. At high incursion rates, little solidification and consolidation was observed, together with negligible adhesion and a higher tangential force, suggesting a cutting mechanism. Transitions in material behaviour, wear mechanism, and force ratio were observed at the same incursion condition, further highlighting the link between the different experimental measurements

Chiral dynamics of p-wave in K^- p and coupled states

We perform an evaluation of the p-wave amplitudes of meson-baryon scattering in the strangeness S=-1 sector starting from the lowest order chiral Lagrangians and introducing explicitly the Sigma^* field with couplings to the meson-baryon states obtained using SU(6) symmetry. The N/D method of unitarization is used, equivalent, in practice, to the use of the Bethe-Salpeter equation with a cut-off. The procedure leaves no freedom for the p-waves once the s-waves are fixed and thus one obtains genuine predictions for the p-wave scattering amplitudes, which are in good agreement with experimental results for differential cross sections, as well as for the width and partial decay widths of the Sigma^*(1385).Comment: LaTeX, 18 pages, 6 figure

The composition and role of convergent technological repertoires in audiovisual media consumption

This mixed-method research focuses on the growing appropriation of multiple screen devices for audiovisual media consumption. Based on survey measures, we distinguish three patterns: (a) maintaining the status quo, by mainly drawing upon television, (b) broadening up the repertoire, by extending television with computers and mobile devices, or (c) even replacing television by a computer. Next, we draw upon insights from niche theory, rationalising media choices in terms of competing gratifications. This perspective is however too one-sided, as our results indicate that habit is a much stronger explanatory variable, especially when a broad range of devices are appropriated. In a follow-up qualitative study, based on Q-methodology, we found that the orientations towards what people seek in audiovisual technologies are only mildly contingent with specific technology appropriation. This problematises the very substance of niches in the audiovisual: as technologies are capable of the same benefits, their discriminating power is declining. Hence, in future applications of niche theory, gratifications and habits of communication modes (what people do with media technologies) should be taken into account, rather than media as tied to a specific technology. Niche theory's core remains, but its applications should be updated to theoretical insights matching the evolving media environment

Masses of ground and excited-state hadrons

We present the first Dyson-Schwinger equation calculation of the light hadron spectrum that simultaneously correlates the masses of meson and baryon ground- and excited-states within a single framework. At the core of our analysis is a symmetry-preserving treatment of a vector-vector contact interaction. In comparison with relevant quantities the root-mean-square-relative-error/degree-of freedom is 13%. Notable amongst our results is agreement between the computed baryon masses and the bare masses employed in modern dynamical coupled-channels models of pion-nucleon reactions. Our analysis provides insight into numerous aspects of baryon structure; e.g., relationships between the nucleon and Delta masses and those of the dressed-quark and diquark correlations they contain.Comment: 25 pages, 7 figures, 4 table

The Infrared Behaviour of the Pure Yang-Mills Green Functions

We review the infrared properties of the pure Yang-Mills correlators and discuss recent results concerning the two classes of low-momentum solutions for them reported in literature; i.e. decoupling and scaling solutions. We will mainly focuss on the Landau gauge and pay special attention to the results inferred from the analysis of the Dyson-Schwinger equations of the theory and from "{\it quenched}" lattice QCD. The results obtained from properly interplaying both approaches are strongly emphasized.Comment: Final version to be published in FBS (54 pgs., 11 figs., 4 tabs

Unconventional Cosmology

I review two cosmological paradigms which are alternative to the current inflationary scenario. The first alternative is the "matter bounce", a non-singular bouncing cosmology with a matter-dominated phase of contraction. The second is an "emergent" scenario, which can be implemented in the context of "string gas cosmology". I will compare these scenarios with the inflationary one and demonstrate that all three lead to an approximately scale-invariant spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept. 12 - 17 2012, to be publ. in the proceedings; these lecture notes form an updated version of arXiv:1003.1745 and arXiv:1103.227

Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3

Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers. Interdimer superexchange interactions give a three-dimensional magnon dispersion and a spin gap significantly smaller than the dimer coupling. This gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a magnetic field of 5.6T, offering a unique opportunity to explore the both types of quantum phase transition and their associated critical phenomena. We use a bond-operator formulation to obtain a continuous description of all disordered and ordered phases, and thus of the transitions separating these. Both pressure- and field-induced transitions may be considered as the Bose-Einstein condensation of triplet magnon excitations, and the respective phases of staggered magnetic order as linear combinations of dimer singlet and triplet modes. We focus on the evolution with applied pressure and field of the magnetic excitations in each phase, and in particular on the gapless (Goldstone) modes in the ordered regimes which correspond to phase fluctuations of the ordered moment. The bond-operator description yields a good account of the magnetization curves and of magnon dispersion relations observed by inelastic neutron scattering under applied fields, and a variety of experimental predictions for pressure-dependent measurements.Comment: 20 pages, 17 figure