19,186 research outputs found
Dynamics from diffraction
A model-independent approach for the extraction of detailed
lattice dynamical information from neutron powder diffraction data is described. The technique is based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement.
Phonon dispersion curves extracted in this way are shown to
reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. The extent to which diffraction data are sensitive to lattice dynamics is explored in a
range of materials. The prospect that such detailed dynamical information might be accessible using comparatively facile experiments such as neutron
powder diffraction is incredibly valuable when studying systems for which established spectroscopic methods are prohibitive or
inappropriate
Technical innovation changes standard radiographic protocols in veterinary medicine: is it necessary to obtain two dorsoproximal-palmarodistal oblique views of the equine foot when using computerised radiography systems?
Since the 1950s, veterinary practitioners have included two separate dorsoproximal–palmarodistal oblique (DPr–PaDiO) radiographs as part of a standard series of the equine foot. One image is obtained to visualise the distal phalanx and the other to visualise the navicular bone. However, rapid development of computed radiography and digital radiography and their post-processing capabilities could mean that this practice is no longer required. The aim of this study was to determine differences in perceived image quality between DPr–PaDiO radiographs that were acquired with a computerised radiography system with exposures, centring and collimation recommended for the navicular bone versus images acquired for the distal phalanx but were subsequently manipulated post-acquisition to highlight the navicular bone. Thirty images were presented to four clinicians for quality assessment and graded using a 1–3 scale (1=textbook quality, 2=diagnostic quality, 3=non-diagnostic image). No significant difference in diagnostic quality was found between the original navicular bone images and the manipulated distal phalanx images. This finding suggests that a single DPr–PaDiO image of the distal phalanx is sufficient for an equine foot radiographic series, with appropriate post-processing and manipulation. This change in protocol will result in reduced radiographic study time and decreased patient/personnel radiation exposure
The Electrodynamics of Inhomogeneous Rotating Media and the Abraham and Minkowski Tensors II: Applications
Applications of the covariant theory of drive-forms are considered for a
class of perfectly insulating media. The distinction between the notions of
"classical photons" in homogeneous bounded and unbounded stationary media and
in stationary unbounded magneto-electric media is pointed out in the context of
the Abraham, Minkowski and symmetrized Minkowski electromagnetic
stress-energy-momentum tensors. Such notions have led to intense debate about
the role of these (and other) tensors in describing electromagnetic
interactions in moving media. In order to address some of these issues for
material subject to the Minkowski constitutive relations, the propagation of
harmonic waves through homogeneous and inhomogeneous, isotropic plane-faced
slabs at rest is first considered. To motivate the subsequent analysis on
accelerating media two classes of electromagnetic modes that solve Maxwell's
equations for uniformly rotating homogeneous polarizable media are enumerated.
Finally it is shown that, under the influence of an incident monochromatic,
circularly polarized, plane electromagnetic wave, the Abraham and symmetrized
Minkowski tensors induce different time-averaged torques on a uniformly
rotating materially inhomogeneous dielectric cylinder. We suggest that this
observation may offer new avenues to explore experimentally the covariant
electrodynamics of more general accelerating media.Comment: 29 pages, 4 figures. Accepted for publication in Proc. Roy. Soc.
Developmental design, fabrication, and test of acoustic suppressors for fans of high bypass turbofan engines
An analysis procedure was developed for design of acoustically treated nacelles for high bypass turbofan engines. The plan was applied to the conceptual design of a nacelle for the quiet engine typical of a 707/DC-8 airplane installation. The resultant design was modified to a test nacelle design for the NASA Lewis quiet fan. The acoustic design goal was a 10 db reduction in effective perceived fan noise levels during takoff and approach. Detailed nacelle designs were subsequently developed for both the quiet engine and the quiet fan. The acoustic design goal for each nacelle was 15 db reductions in perceived fan noise levels from the inlet and fan duct. Acoustically treated nacelles were fabricated for the quiet engine and quiet fan for testing. Performance of selected inlet and fan duct lining configurations was experimentally evaluated in a flow duct. Results of the tests show that the linings perform as designed
Does the galaxy correlation length increase with the sample depth?
We have analyzed the behavior of the correlation length, , as a function
of the sample depth by extracting from the CfA2 redshift survey volume--limited
samples out to increasing distances. For a fractal distribution, the value of
would increase with the volume occupied by the sample. We find no linear
increase for the CfA2 samples of the sort that would be expected if the
Universe preserved its small scale fractal character out to the distances
considered (60--100\hmpc). The results instead show a roughly constant value
for as a function of the size of the sample, with small fluctuations due
to local inhomogeneities and luminosity segregation. Thus the fractal picture
can safely be discarded.Comment: Accepted for publication in ApJ
A combinatorial investigation of sputtered Ta-Al-C thin films
We describe a combinatorial experiment investigating the Ta–Al–C material system, conducted with the aim of determining why the tantalum-containing Mn + 1AXn phases have so far proved to be not amenable to thin-film synthesis. Samples were deposited onto (0001) Al2O3 wafers at 850 °C and characterized by X-ray diffraction wafer maps, scanning electron microscopy, and surface optical scattering. Elemental Ta, the binary phases TaC, Ta2C, and TaAl3, and the ternary phases Ta3Al2C and Ta5Al3C were identified. The morphology, phase composition and preferred orientation of the films deposited were found to be highly sensitive to the Ta fraction of the incident flux during deposition. No MAX phase material was observed, indicating that the Ta-containing MAX phases do not form under the deposition conditions investigated. Explanations associated with inadequate coverage of stochiometries, preferential sputtering, and thermodynamic instability have been ruled out. An explanation based on reduced surface diffusion of Ta during growth is proposed. A substantially higher substrate temperature during deposition is likely to be required to synthesize Ta-containing MAX phases
The supercluster--void network III. The correlation function as a geometrical statistic
We investigate properties of the correlation function of clusters of galaxies
using geometrical models. On small scales the correlation function depends on
the shape and the size of superclusters. On large scales it describes the
geometry of the distribution of superclusters. If superclusters are distributed
randomly then the correlation function on large scales is featureless. If
superclusters and voids have a tendency to form a regular lattice then the
correlation function on large scales has quasi-regularly spaced maxima and
minima of decaying amplitude; i.e., it is oscillating. The period of
oscillations is equal to the step size of the grid of the lattice.
We calculate the power spectrum for our models and compare the geometrical
information of the correlation function with other statistics. We find that
geometric properties (the regularity of the distribution of clusters on large
scales) are better quantified by the correlation function. We also analyse
errors in the correlation function and the power spectrum by generating random
realizations of models and finding the scatter of these realizations.Comment: MNRAS LaTex style, 12 pages, 7 PostScript figures embedded, accepted
by MNRA
Dynamic Effect of Cold-Air Bypass Valve for Compressor Surge Recovery and Prevention in Fuel Cell Gas Turbine Hybrid Systems
A large volume between compressor and turbine is present in fuel cell gas turbine hybrid systems. The substantially larger compressor plenum volume modifies the dynamic behaviour of these systems, increasing the risk of compressor surge during transients and subsequent destruction of both turbomachinery and fuel cell components. Diverting part of the compressor inlet flow directly to the turbine inlet through a cold-air bypass valve, bypassing the fuel cell stack, has been proven to be an effective method to increase the surge margin during normal operation and also to recover the machine from fully developed surge. This study investigates the dynamic effect of different cold-air bypass valve opening/closing procedures, especially steps and ramps changing the valve fractional opening. This analysis was carried out with reference to the Hybrid Performance (Hyper) facility: a hybrid system emulated using hardware and a cyber-physical fuel cell system at the National Energy Technology Laboratory (NETL), U.S. Department of Energy (DOE). Simulations performed on a Matlab\uae-Simulink\uae dynamic model of the system based on Greitzer's theory showed a different behaviour varying the valve fractional opening with steps or ramps. Many experimental tests were performed on the Hyper facility to confirm the trends obtained from the simulations results. From the outcomes of this study, it has been possible to determine how to maximize the surge recovery effect of the cold-air bypass valve opening and to minimize surge related risks during the valve closure
Physical Bias of Galaxies From Large-Scale Hydrodynamic Simulations
We analyze a new large-scale (Mpc) numerical hydrodynamic
simulation of the popular CDM cosmological model, including in our
treatment dark matter, gas and star-formation, on the basis of standard
physical processes. The method, applied with a numerical resolution of
kpc (which is still quite coarse for following individual galaxies,
especially in dense regions), attempts to estimate where and when galaxies
form. We then compare the smoothed galaxy distribution with the smoothed mass
distribution to determine the "bias" defined as on scales large compared with the code
numerical resolution (on the basis of resolution tests given in the appendix of
this paper). We find that (holding all variables constant except the quoted
one) bias increases with decreasing scale, with increasing galactic age or
metallicity and with increasing redshift of observations. At the Mpc
fiducial comoving scale bias (for bright regions) is 1.35 at reaching to
3.6 at , both numbers being consistent with extant observations. We also
find that Mpc voids in the distribution of luminous objects are
as observed (i.e., observed voids are not an argument against CDM-like models)
and finally that the younger systems should show a colder Hubble flow than do
the early type galaxies (a testable proposition). Surprisingly, little
evolution is found in the amplitude of the smoothed galaxy-galaxy correlation
function (as a function of {\it comoving} separation). Testing this prediction
vs observations will allow a comparison between this work and that of Kauffmann
et al which is based on a different physical modelingmethod.Comment: in press, ApJ, 26 latex pages plus 7 fig
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