948 research outputs found
Automatic Fastening Large Structures: a New Approach
The external tank (ET) intertank structure for the space shuttle, a 27.5 ft diameter 22.5 ft long externally stiffened mechanically fastened skin-stringer-frame structure, was a labor intensitive manual structure built on a modified Saturn tooling position. A new approach was developed based on half-section subassemblies. The heart of this manufacturing approach will be 33 ft high vertical automatic riveting system with a 28 ft rotary positioner coming on-line in mid 1985. The Automatic Riveting System incorporates many of the latest automatic riveting technologies. Key features include: vertical columns with two sets of independently operating CNC drill-riveting heads; capability of drill, insert and upset any one piece fastener up to 3/8 inch diameter including slugs without displacing the workpiece offset bucking ram with programmable rotation and deep retraction; vision system for automatic parts program re-synchronization and part edge margin control; and an automatic rivet selection/handling system
A Streamwise Constant Model of Turbulence in Plane Couette Flow
Streamwise and quasi-streamwise elongated structures have been shown to play
a significant role in turbulent shear flows. We model the mean behavior of
fully turbulent plane Couette flow using a streamwise constant projection of
the Navier Stokes equations. This results in a two-dimensional, three velocity
component () model. We first use a steady state version of the model to
demonstrate that its nonlinear coupling provides the mathematical mechanism
that shapes the turbulent velocity profile. Simulations of the model
under small amplitude Gaussian forcing of the cross-stream components are
compared to DNS data. The results indicate that a streamwise constant
projection of the Navier Stokes equations captures salient features of fully
turbulent plane Couette flow at low Reynolds numbers. A system theoretic
approach is used to demonstrate the presence of large input-output
amplification through the forced model. It is this amplification
coupled with the appropriate nonlinearity that enables the model to
generate turbulent behaviour under the small amplitude forcing employed in this
study.Comment: Journal of Fluid Mechanics 2010, in pres
Dynamics of threads and polymers in turbulence: power-law distributions and synchronization
We study the behavior of threads and polymers in a turbulent flow. These
objects have finite spatial extension, so the flow along them differs slightly.
The corresponding drag forces produce a finite average stretching and the
thread is stretched most of the time. Nevertheless, the probability of
shrinking fluctuations is significant and is known to decay only as a
power-law. We show that the exponent of the power law is a universal number
independent of the statistics of the flow. For polymers the coil-stretch
transition exists: the flow must have a sufficiently large Lyapunov exponent to
overcome the elastic resistance and stretch the polymer from the coiled state
it takes otherwise. The probability of shrinking from the stretched state above
the transition again obeys a power law but with a non-universal exponent. We
show that well above the transition the exponent becomes universal and derive
the corresponding expression. Furthermore, we demonstrate synchronization: the
end-to-end distances of threads or polymers above the transition are
synchronized by the flow and become identical. Thus, the transition from
Newtonian to non-Newtonian behavior in dilute polymer solutions can be seen as
an ordering transition.Comment: 13 pages, version accepted to Journal of Statistical Mechanic
The sintering, sintered microstructure and mechanical properties of Ti-Fe-Si Alloys
A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (≤1 pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (TiSi) being dispersed in both the α and β phases. Conversely, additions of >1 pct Si produce coarse and/or networked TiSi silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by TiSi. Increasing the Fe content in the Ti-xFe-0.5/ 1.0Si alloys above 3 pct markedly increases the average grain size and changes the morphology of the α-phase phase to much thinner and more acicular laths. Consequently, the ductility drops t
Behavior of QQ-Plots and Genomic Control in Studies of Gene-Environment Interaction
Genome-wide association studies of gene-environment interaction (GxE GWAS) are becoming popular. As with main effects GWAS, quantile-quantile plots (QQ-plots) and Genomic Control are being used to assess and correct for population substructure. However, in GE work these approaches can be seriously misleading, as we illustrate; QQ-plots may give strong indications of substructure when absolutely none is present. Using simulation and theory, we show how and why spurious QQ-plot inflation occurs in GE GWAS, and how this differs from main-effects analyses. We also explain how simple adjustments to standard regression-based methods used in GE GWAS can alleviate this problem
Comparison of some Reduced Representation Approximations
In the field of numerical approximation, specialists considering highly
complex problems have recently proposed various ways to simplify their
underlying problems. In this field, depending on the problem they were tackling
and the community that are at work, different approaches have been developed
with some success and have even gained some maturity, the applications can now
be applied to information analysis or for numerical simulation of PDE's. At
this point, a crossed analysis and effort for understanding the similarities
and the differences between these approaches that found their starting points
in different backgrounds is of interest. It is the purpose of this paper to
contribute to this effort by comparing some constructive reduced
representations of complex functions. We present here in full details the
Adaptive Cross Approximation (ACA) and the Empirical Interpolation Method (EIM)
together with other approaches that enter in the same category
Socio-Economic Status and Pregnancy Outcome: An Australian Study
A prospective cohort of 8556 pregnant women attending the Mater Misericordiae Mothers' Hospital in Brisbane was examined to consider the impact of socio-economic status on pregnancy outcome. The indicators of socio-economic status selected were family income, maternal education and paternal occupational status. Pregnancy outcomes considered were preterm delivery, low birthweight, low birthweight for gestational age, and perinatal death. Subsidiary analyses were also undertaken for Apgar scores, time to establish respiration, need for mechanical respiration and admission to intensive care. Before adjustment, the main consistent association was between the occupational status of the father and three measures of perinatal morbidity. Initial adjustment for the mother's socio-demographic background and weight/height ratio reduced the strength and statistical significance of the above associations, while further adjustment for lifestyle variations between the three status groups further reduced the above associations to marginal statistical significance. The findings suggest that observed class differences in pregnancy outcome are attributable to the mother's personal characteristics (height/weight, parity) and her lifestyle
Particles and fields in fluid turbulence
The understanding of fluid turbulence has considerably progressed in recent
years. The application of the methods of statistical mechanics to the
description of the motion of fluid particles, i.e. to the Lagrangian dynamics,
has led to a new quantitative theory of intermittency in turbulent transport.
The first analytical description of anomalous scaling laws in turbulence has
been obtained. The underlying physical mechanism reveals the role of
statistical integrals of motion in non-equilibrium systems. For turbulent
transport, the statistical conservation laws are hidden in the evolution of
groups of fluid particles and arise from the competition between the expansion
of a group and the change of its geometry. By breaking the scale-invariance
symmetry, the statistically conserved quantities lead to the observed anomalous
scaling of transported fields. Lagrangian methods also shed new light on some
practical issues, such as mixing and turbulent magnetic dynamo.Comment: 165 pages, review article for Rev. Mod. Phy
A failed RCT to determine if antibiotics prevent mastitis: Cracked nipples colonized with Staphylococcus aureus: A randomized treatment trial [ISRCTN65289389]
BACKGROUND: A small, non-blinded, RCT (randomised controlled trial) had reported that oral antibiotics reduced the incidence of mastitis in lactating women with Staphylococcus aureus (S. aureus)- colonized cracked nipples. We aimed to replicate the study with a more rigorous design and adequate sample size. METHODS: Our intention was to conduct a double-blind placebo-controlled trial to determine if an antibiotic (flucloxacillin) could prevent mastitis in lactating women with S. aureus-colonized cracked nipples. We planned to recruit two groups of 133 women with S. aureus-colonized cracked nipples. RESULTS: We spent over twelve months submitting applications to five hospital ethics committees and seven funding bodies, before commencing the trial. Recruitment to the trial was very slow and only ten women were randomized to the trial after twelve months, and therefore the trial was stopped early. CONCLUSIONS: In retrospect we should have conducted a feasibility study, which would have revealed the low number of women in these Melbourne hospitals (maternity wards and breastfeeding clinics) with damaged nipples. The appropriate use of antibiotics for breastfeeding women with cracked nipples still needs to be tested
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