22,231 research outputs found
Design and numerical modeling of a pressurized airframe bulkhead joint
The structural loading on a conceptual lap joint in the empennage of a civil aircraft has been investigated. The lap joint interfaces the end-pressure part-hemispherical bulkhead to the cylindrical fuselage. The pressure bulkhead is made of carbon fiber reinforced plastic materials. The aim of the study is to present numerical results of the induced structural loading from the fuselage positive internal pressure differential and the localized high stress intensity field at the lap joint location. A methodology for the appropriate numerical approach to analyze the domed pressure bulkhead is presented. The results of the numerical investigation showed that the laminate loading levels calculated by the use of either initial sizing analytical formulas for pressurized domes or by the use of equilibrium nodal loading from finite element models of low fidelity compared to refined finite element analysis can be significantly underestimated. Some of the implications on carbon fiber reinforced plastic structural sizing at the specified location are developed
Evolution of Threats in the Global Risk Network
With a steadily growing population and rapid advancements in technology, the
global economy is increasing in size and complexity. This growth exacerbates
global vulnerabilities and may lead to unforeseen consequences such as global
pandemics fueled by air travel, cyberspace attacks, and cascading failures
caused by the weakest link in a supply chain. Hence, a quantitative
understanding of the mechanisms driving global network vulnerabilities is
urgently needed. Developing methods for efficiently monitoring evolution of the
global economy is essential to such understanding. Each year the World Economic
Forum publishes an authoritative report on the state of the global economy and
identifies risks that are likely to be active, impactful or contagious. Using a
Cascading Alternating Renewal Process approach to model the dynamics of the
global risk network, we are able to answer critical questions regarding the
evolution of this network. To fully trace the evolution of the network we
analyze the asymptotic state of risks (risk levels which would be reached in
the long term if the risks were left unabated) given a snapshot in time, this
elucidates the various challenges faced by the world community at each point in
time. We also investigate the influence exerted by each risk on others. Results
presented here are obtained through either quantitative analysis or
computational simulations.Comment: 27 pages, 15 figure
On the width of cold fronts in clusters of galaxies due to conduction
We consider the impact of thermal conduction in clusters of galaxies on the
(unmagnetized) interface between a cold gaseous cloud and a hotter gas flowing
over the cloud (the so-called cold front). We argue that near the stagnation
point of the flow conduction creates a spatially extended layer of constant
thickness , where is of order , and is
the curvature radius of the cloud, is the velocity of the flow at infinity,
and is the conductivity of the gas. For typical parameters of the observed
fronts, one finds . The formation time of such a layer is . Once the layer is formed, its thickness only slowly varies with time and
the quasi-steady layer may persist for many characteristic time scales. Based
on these simple arguments one can use the observed width of the cold fronts in
galaxy clusters to constrain the effective thermal conductivity of the
intra-cluster medium.Comment: Accepted for MNRAS. 9 pages; 6 b&w figures; 2 colour figure
"Black Star" or Astrophysical Black Hole?
Recently wide publicity has been given to a claim by T. Vachaspati that
"black holes do not exist", that the objects known as black holes in
astrophysics should rather be called "black stars" and they not only do not
have event horizons but actually can be the source of spectacular gamma ray
bursts. In this short essay (no flimsier than the original preprint where these
extravagant claims appeared) I demonstrate that these ill-considered claims are
clearly wrong. Yet they present a good occasion to reflect on some well known
but little discussed conceptual difficulties which arise when applying
relativistic terminology in an astrophysical context.Comment: Poster presented at "Compact Objects" meeting in Hunagshan, China,
2-7 July 2007. To be published in the AIP Conference Proceeding serie
Vertical axis non-linearities in wavelength scanning interferometry
The uncertainty of measurements made on an areal surface topography instrument is directly influenced by its metrological characteristics. In this work, the vertical axis deviation from linearity of a wavelength scanning interferometer is evaluated. The vertical axis non-linearities are caused by the spectral leakage resulting from the Fourier transform algorithm for phase slope estimation. These non-linearities are simulated and the results are compared with experimental measurements. In order to reduce the observed non-linearities, a
modification of the algorithm is proposed. The application of a Hamming window and the exclusion of edge points in the extracted phase are shown to increase the accuracy over the whole instrument range
RG flows, cycles, and c-theorem folklore
Monotonic renormalization group flows of the "c" and "a" functions are often
cited as reasons why cyclic or chaotic coupling trajectories cannot occur. It
is argued here, based on simple examples, that this is not necessarily true.
Simultaneous monotonic and cyclic flows can be compatible if the flow-function
is multi-valued in the couplings.Comment: 3 pages, 5 figure
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