15,621 research outputs found
Prediction of unsteady aerodynamic loadings caused by leading edge and trailing edge control surface motions in subsonic compressible flow: Computer program description
A digital computer program has been developed to calculate unsteady loadings caused by motions of lifting surfaces with leading edge or trailing edge controls based on the subsonic kernel function approach. The pressure singularities at hinge line and side edges have been extracted analytically as a preliminary step to solving the integral equation by collocation. The program calculates generalized aerodynamic forces for user supplied deflection modes. Optional intermediate output includes pressure at an array of points, and sectional generalized forces. From one to six controls on the half span can be accommodated
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The Influence of Water on Dielectric Behavior of Silica-filled Epoxy Nano-composites and Percolation Phenomenon
The dielectric properties of epoxy resin were studied as a function of hydration by dielectric spectroscopy. The dielectric spectroscopy measurements show different conduction and quasi-DC behaviors at very low frequencies (<10-2 Hz) with activation energies dependent on the hydration. These observations lead to the development of a model in which a “water shell” is formed around the nano-particles. The multiple shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 10 monolayers as the first layer. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 5 nm thick at 100% RH. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. PEA results show that the wetted epoxy specimens have a higher threshold field of space charge accumulation than such dry specimens since water enhances charge decay
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Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites
Work on dielectric spectroscopy of epoxy resin filled with nano-SiO2 at different relative humidities and temperatures is reported. Above the glass-transition temperature (Tg), dc-like imperfect charge transport (QDC or LFD) dominates the low frequency dielectric spectrum. Another mid-frequency relaxation process was found in the non-dried composites. Water also induces glass-transition temperature decreases, which can be measured both by dielectric spectroscopy and DSC. Both theory and experiment demonstrated that a higher water content could exist in nanocomposites than unfilled epoxy suggesting a bigger free volume when nanostructured. In our system, the hydrophilic surface of silica is likely to cause water to surround and lead to delamination of the epoxy from SiO2. This is a potential mechanical and dielectric weakness in the nanocomposites, which may lead to an ageing phenomenon. Hydrophobic surface group may reduce the water adsorption in nanocomposites
Allozyme and mitochondrial DNA variability within the New Zealand damselfly genera Xanthocnemis, Austrolestes, and Ischnura (Odonata)
We collected larval damselflies from 17 sites in the North, South and Chatham Islands, and tested the hypotheses that: (1) genetic markers (e.g., allozymes, mtDNA) would successfully ¬discriminate taxa; and (2) the dispersal capabilities of adult damselflies would limit differentiation among locations. Four species from three genera were identified based on available taxonomic keys. Using 11 allozyme loci and the mitochondrial cytochrome c-oxidase subunit I (COI) gene, we confirmed that all taxa were clearly discernible. We found evidence for low to moderate differentiation among locations based on allozyme (mean FST = 0.09) and sequence (COI) divergence (<0.034). No obvious patterns with respect to geographic location were detected, although slight differences were found between New Zealand’s main islands (North Island, South Island) and the Chatham Islands for A. colensonis (sequence divergence 0.030–0.034). We also found limited intraspecific genetic variability based on allozyme data (Hexp < 0.06 in all cases). We conclude that levels of gene flow/dispersal on the main islands may have been sufficient to maintain the observed homogeneous population structure, and that genetic techniques, particularly the COI gene locus, will be a useful aid in future identifications
Experimental Determination of the Lorenz Number in Cu0.01Bi2Te2.7Se0.3 and Bi0.88Sb0.12
Nanostructuring has been shown to be an effective approach to reduce the
lattice thermal conductivity and improve the thermoelectric figure of merit.
Because the experimentally measured thermal conductivity includes contributions
from both carriers and phonons, separating out the phonon contribution has been
difficult and is mostly based on estimating the electronic contributions using
the Wiedemann-Franz law. In this paper, an experimental method to directly
measure electronic contributions to the thermal conductivity is presented and
applied to Cu0.01Bi2Te2.7Se0.3, [Cu0.01Bi2Te2.7Se0.3]0.98Ni0.02, and
Bi0.88Sb0.12. By measuring the thermal conductivity under magnetic field,
electronic contributions to thermal conductivity can be extracted, leading to
knowledge of the Lorenz number in thermoelectric materials
Genetic heterogeneity of residual variance in broiler chickens
Aims were to estimate the extent of genetic
heterogeneity in environmental variance. Data comprised 99 535 records of
35-day body weights from broiler chickens reared in a controlled
environment. Residual variance within dam families was estimated using
ASREML, after fitting fixed effects such as genetic groups and hatches, for
each of 377 genetically contemporary sires with a large number of progeny
(100 males or females each). Residual variance was computed separately
for male and female offspring, and after correction for sampling, strong
evidence for heterogeneity was found, the standard deviation between sires
in within variance amounting to 15–18% of its mean. Reanalysis using
log-transformed data gave similar results, and elimination of 2–3% of
outlier data reduced the heterogeneity but it was still over 10%. The
correlation between estimates for males and females was low, however. The
correlation between sire effects on progeny mean and residual variance for
body weight was small and negative (-0.1). Using a data set bigger than any
yet presented and on a trait measurable in both sexes, this study has shown
evidence for heterogeneity in the residual variance, which could not be
explained by segregation of major genes unless very few determined the
trait
Reversable heat flow through the carbon nanotube junctions
Microscopic mechanisms of externally controlled reversable heat flow through
the carbon nanotube junctions (NJ) are studied theoretically. Our model
suggests that the heat is transfered along the tube section by
electrons () and holes () moving ballistically in either in parallel or
in opposite directions and accelerated by the bias source-drain voltage (Peltier effect). We compute the Seebeck coefficient , electric
and thermal conductivities and find that their magnitudes
strongly depend on and . The sign reversal of
versus the sign of formerly observed experimentally is interpreted
in this work in terms of so-called chiral tunneling phenomena (Klein paradox)
Thermoelectricity in Nanowires: A Generic Model
By employing a Boltzmann transport equation and using an energy and size
dependent relaxation time () approximation (RTA), we evaluate
self-consistently the thermoelectric figure-of-merit of a quantum wire
with rectangular cross-section. The inferred shows abrupt enhancement in
comparison to its counterparts in bulk systems. Still, the estimated for
the representative BiTe nanowires and its dependence on wire parameters
deviate considerably from those predicted by the existing RTA models with a
constant . In addition, we address contribution of the higher energy
subbands to the transport phenomena, the effect of chemical potential tuning on
, and correlation of with quantum size effects (QSEs). The obtained
results are of general validity for a wide class of systems and may prove
useful in the ongoing development of the modern thermoelectric applications.Comment: 15 pages, 6 figures; Dedicated to the memory of Amirkhan Qezell
Examining 'window dressing' : the views of black police associations on recruitment and training
In a previous issue of this journal, Ellis Cashmore (2002 Cashmore, E. 2002. ‘Behind the window dressing: minority ethnic police perspectives on cultural diversity’. Journal of Ethnic and Migration Studies, 28(2): 327–41.
[Taylor & Francis Online], [CSA], [Google Scholar]
) discussed two key issues currently confronting police constabularies in England and Wales: the recruitment of minority ethnic officers and civilian staff, and the impact of diversity training now in place for all police officers. Cashmore argued that not only are these policies ineffective in enhancing cultural diversity within constabularies, but that they are harmful, presenting a false outward image of effective action. This article examines Cashmore's arguments and develops them in light of findings from recent research on Black Police Associations (BPAs) in England and Wales. Our findings firstly suggest that, because of heavy involvement with these initiatives and the close relationship BPAs have developed with senior management (in comparison to non-BPA members), they must be considered in any discussion of minority ethnic recruitment and diversity training. In addition, the majority of the officers we interviewed were supportive of current recruitment and training programmes. Secondly, we argue that BPAs are helping to change the nature of the overall police culture to a certain extent. Many minority ethnic officers no longer feel they must downplay their ethnicity as members of constabularies
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