Low Mach number effect in simulation of high Mach number flow

In this note, we relate the two well-known difficulties of Godunov schemes: the carbuncle phenomena in simulating high Mach number flow, and the inaccurate pressure profile in simulating low Mach number flow. We introduced two simple low-Mach-number modifications for the classical Roe flux to decrease the difference between the acoustic and advection contributions of the numerical dissipation. While the first modification increases the local numerical dissipation, the second decreases it. The numerical tests on the double-Mach reflection problem show that both modifications eliminate the kinked Mach stem suffered by the original flux. These results suggest that, other than insufficient numerical dissipation near the shock front, the carbuncle phenomena is strongly relevant to the non-comparable acoustic and advection contributions of the numerical dissipation produced by Godunov schemes due to the low Mach number effect.Comment: 9 pages, 1 figur

Analysing conversational data with computer-aided content analysis: the importance of data partitioning

This article highlights the distinctive outcomes generated by different approaches to computer-aided content analysis, and discusses how partition decisions reveal or conceal possible data interpretations. Drawing on data collected from focus groups set up during a European research study, we demonstrate how the chosen encoding technique leads to different views of the same texts, regardless of the software chosen. This analysis produces a user’s guide for researchers who need to analyze conversations and concludes with a discussion of the implications for management and organization research

The accumulation of barium by marine phytoplankton grown in culture

Marine phytoplankton have been implicated as potentially important vectors for the vertical transport of barium in the oceans. To better assess the extent to which phytoplankton can influence the geochemical cycling of barium, its bioconcentration was studied in 21 clones of 19 species of marine phytoplankters belonging to 9 algal classes. Barium levels in the ash ranged from less than 2 μg g–1 for the coccolithophore Emiliania huxleyi and the red alga Porphyridium cruenturn to 589 μg g–1 for the flagellate Tetraselmis levis. Concentrations ≥4000 μg g–1, previously reported for certain samples of diatom ash were not encountered in this study. Concentration factors on a volume basis (VCF) ranged from 0 to 3.2 × 104; the geometric mean VCF for all species was 225. Diatoms and coccolithophores generally had lower VCFs (geometric means of 90 and 12, respectively) than did other species; dinoflagellates had a geometric mean VCF of 490. Experiments with the diatom Thalassiosira pseudonana indicated that Ba cell–1 increased linearly with ambient Ba concentration. Experiments to localize the site of Ba deposition in diatom cells indicated that most of the Ba was associated with the frustules rather than with the organic fraction. Dinoflagellates and several other algae not only concentrated Ba to relatively high levels, but also accumulated Si when grown in Si-enriched medium, although they grew at least as well without added Si as with it. Ba and Si accumulation were generally negatively correlated

Powerful H2_2 Line-cooling in Stephan's Quintet : I - Mapping the Significant Cooling Pathways in Group-wide Shocks

We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km/s collision has produced a group-wide shock and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 $\times10^{8}$ M$_{\odot}$ of warm H$_2$ spread over $\sim$ 480 kpc$^2$ and additionally report the discovery of a second major shock-excited H$_2$ feature. This brings the total H$_2$ line luminosity of the group in excess of 10$^42$ erg/s. In the main shock, the H$_2$ line luminosity exceeds, by a factor of three, the X-ray luminosity from the hot shocked gas, confirming that the H$_2$-cooling pathway dominates over the X-ray. [Si II]34.82$\mu$m emission, detected at a luminosity of 1/10th of that of the H$_2$, appears to trace the group-wide shock closely and in addition, we detect weak [FeII]25.99$\mu$m emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 < $V_{s}$ < 300 km/s) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, PAH and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H$_{2}$ power is driven by turbulent energy transfer from motions in a post-shocked layer. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.Comment: 23 pages, 15 figures, Accepted to Ap

Optical performance of the JWST MIRI flight model: characterization of the point spread function at high-resolution

The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard the James Webb Space Telescope (JWST), providing imaging, coronagraphy and spectroscopy over the 5-28 microns band. To verify the optical performance of the instrument, extensive tests were performed at CEA on the flight model (FM) of the Mid-InfraRed IMager (MIRIM) at cryogenic temperatures and in the infrared. This paper reports on the point spread function (PSF) measurements at 5.6 microns, the shortest operating wavelength for imaging. At 5.6 microns the PSF is not Nyquist-sampled, so we use am original technique that combines a microscanning measurement strategy with a deconvolution algorithm to obtain an over-resolved MIRIM PSF. The microscanning consists in a sub-pixel scan of a point source on the focal plane. A data inversion method is used to reconstruct PSF images that are over-resolved by a factor of 7 compared to the native resolution of MIRI. We show that the FWHM of the high-resolution PSFs were 5-10% wider than that obtained with Zemax simulations. The main cause was identified as an out-of-specification tilt of the M4 mirror. After correction, two additional test campaigns were carried out, and we show that the shape of the PSF is conform to expectations. The FWHM of the PSFs are 0.18-0.20 arcsec, in agreement with simulations. 56.1-59.2% of the total encircled energy (normalized to a 5 arcsec radius) is contained within the first dark Airy ring, over the whole field of view. At longer wavelengths (7.7-25.5 microns), this percentage is 57-68%. MIRIM is thus compliant with the optical quality requirements. This characterization of the MIRIM PSF, as well as the deconvolution method presented here, are of particular importance, not only for the verification of the optical quality and the MIRI calibration, but also for scientific applications.Comment: 13 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wav

Magnetostructure of MnAs on GaAs revisited

The ferromagnetic to nonferromagnetic (&#945;-&#946;) phase transition in epitaxial MnAs layers on GaAs(100) is studied by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism photoemission electron microscopy in order to elucidate the nature of the controversial nonferromagnetic state of &#946;-MnAs. In the coexistence region of the two phases the &#946; phase shows a clear XMLD signal characteristic of antiferromagnetism. The nature and the possible causes of the elusiveness of this magnetic state are discussed

Effects of graft pretensioning in anterior cruciate ligament reconstruction

Purpose Graft pretensioning is used in anterior cruciate ligament (ACL) reconstruction to prevent secondary slackening. Its effects on collagen fibrillar ultrastructure are not known. In this study, we hypothesized that graft pretensioning in ACL reconstruction creates ultrastructural changes detectable in scanning electron microscopy (SEM). Methods A prospective comparative study was carried out on 38 ACL reconstructions using a 4-strand semitendinosus graft. Samples were harvested intra-operatively before and after pretensioning for 30 s, 2 or 5 min. The images produced in SEM were analyzed using an original semi-quantitative «CIP» score taking into account collagen cohesion, integrity, and parallelism. Intra- and inter-tester reliability for the CIP score were tested. Results The CIP scores decreased by 3.5 (1.6) points after pretensioning (P &lt; 0.05). Significant differences were found in the 5, 2 min and 30 s subgroups for the global CIP score. Relative decrease (Delta CIP) was significantly higher in the 2 and 5 min subgroups after pretensioning in comparison with the 30 s subgroups. Intra- and inter-tester reliability for the CIP score were 0.85 and 0.92 (P &lt; 0.05). Conclusion Pretensioning ACL grafts resulted in alteration of the collagen fibrillar ultrastructure, detectable using SEM. These results confirm the existence of collagen ultrastructural changes after pretensioning that may be related to its duration. Level of evidence Prospective comparative study, Level II

Combined and single effects of pesticide carbaryl and toxic Microcystis aeruginosa on the life history of Daphnia pulicaria

The combined influence of a pesticide (carbaryl) and a cyanotoxin (microcystin LR) on the life history of Daphnia pulicaria was investigated. At the beginning of the experiments animals were pulse exposed to carbaryl for 24 h and microcystins were delivered bound in Microcystis’ cells at different, sub-lethal concentrations (chronic exposure). In order to determine the actual carbaryl concentrations in the water LC–MS/MS was used. For analyses of the cyanotoxin concentration in Daphnia’s body enzyme-linked immunosorbent assay (ELISA) was used. Individual daphnids were cultured in a flow-through system under constant light (16 h of light: 8 h of dark), temperature (20°C), and food conditions (Scenedesmus obliquus, 1 mg of C l−1). The results showed that in the treatments with carbaryl egg numbers per female did not differ significantly from controls, but the mortality of newborns increased significantly. Increasing microcystin concentrations significantly delayed maturation, reduced size at first reproduction, number of eggs, and newborns. The interaction between carbaryl and Microcystis was highly significant. Animals matured later and at a smaller size than in controls. The number of eggs per female was reduced as well. Moreover, combined stressors caused frequent premature delivery of offspring with body deformations such as dented carapax or an undeveloped heart. This effect is concluded to be synergistic and could not be predicted from the effects of the single stressors.