63,447 research outputs found
Valve recession: From experiment to predictive model
Increasing demands on engine performance and cost reductions have meant that advances made in materials and production technology are often outpaced This frequently results in wear problems occurring with engine components. Few models exist for predicting wear, and consequently each wear problem has to be investigated, the cause isolated and remedial action taken. The objective of this work was to carry out experimental studies to investigate valve and seat insert wear mechanisms and use the test results to develop a recession prediction tool to assess the potential for valve recession and solve problems that occur more quickly. Experimental apparatus has been developed that is capable of providing a valid simulation of the wear of diesel automotive inlet valves and seats. Test methodologies developed have isolated the effects of impact and sliding. A semi-empirical wear model for predicting valve recession has been developed based on data gathered during the bench testing. A software program, RECESS, was developed to run the model. Model predictions are compared with engine dynamometer tests and bench tests. The model can be used to give a quantitative prediction of the valve recession to be expected with a particular material pair or a qualitative assessment of how parameters need to be altered in order to reduce recession. The valve recession model can be integrated into an industrial environment in order to help reduce costs and timescales involved in solving valve/seat wear problems
A block diagonalization theorem in the energy-momentum method
We prove a geometric generalization of a block diagonalization theorem first found by the authors for
rotating elastic rods. The result here is given in the general context of simple mechanical systems with a
symmetry group acting by isometries on a configuration manifold. The result provides a choice of
variables for linearized dynamics at a relative equilibrium which block diagonalizes the second variation of
an augmented energy these variables effectively separate the rotational and internal vibrational modes. The
second variation of the effective Hamiltonian is block diagonal. separating the modes completely. while the
symplectic form has an off diagonal term which represents the dynamic interaction between these modes.
Otherwise, the symplectic form is in a type of normal form. The result sets the stage for the development
of useful criteria for bifurcation as well as the stability criteria found here. In addition, the techniques
should apply to other systems as well, such as rotating fluid masses
Uncovering CDM halo substructure with tidal streams
Models for the formation and growth of structure in a cold dark matter
dominated universe predict that galaxy halos should contain significant
substructure. Studies of the Milky Way, however, have yet to identify the
expected few hundred sub-halos with masses greater than about 10^6 Msun. Here
we propose a test for the presence of sub-halos in the halos of galaxies. We
show that the structure of the tidal tails of ancient globular clusters is very
sensitive to heating by repeated close encounters with the massive dark
sub-halos. We discuss the detection of such an effect in the context of the
next generation of astrometric missions, and conclude that it should be easily
detectable with the GAIA dataset. The finding of a single extended cold stellar
stream from a globular cluster would support alternative theories, such as
self-interacting dark matter, that give rise to smoother halos.Comment: 7 pages, 7 figures, submitted to MNRA
The Role of Financial Factors in the Trade Collapse: A Skeptic's View
This paper explores the role of financial factors in the 2008-9 collapse of U.S. imports
and exports. Using highly disaggregated international trade data, we examine whether the
cross-sectoral variation in how much imports or exports fell during this episode can be
explained by financial variables. To do this, we employ a wide variety of possible
indicators, such as standard measures of trade credit and external finance dependence,
proxies for shipping lags at the sector level, and shares of intra-firm trade in each sector.
Overall, there is very little evidence that financial factors played a role in the collapse of
U.S. trade.2008-2009 Crisis, International Trade, Financial Factors
Estimating proportions of objects from multispectral scanner data
Progress is reported in developing and testing methods of estimating, from multispectral scanner data, proportions of target classes in a scene when there are a significiant number of boundary pixels. Procedures were developed to exploit: (1) prior information concerning the number of object classes normally occurring in a pixel, and (2) spectral information extracted from signals of adjoining pixels. Two algorithms, LIMMIX and nine-point mixtures, are described along with supporting processing techniques. An important by-product of the procedures, in contrast to the previous method, is that they are often appropriate when the number of spectral bands is small. Preliminary tests on LANDSAT data sets, where target classes were (1) lakes and ponds, and (2) agricultural crops were encouraging
iCosmo: an Interactive Cosmology Package
Aims: The interactive software package iCosmo, designed to perform
cosmological calculations is described. Methods: iCosmo is a software package
to perform interactive cosmological calculations for the low redshift universe.
Computing distance measures, the matter power spectrum, and the growth factor
is supported for any values of the cosmological parameters. It also computes
derived observed quantities for several cosmological probes such as cosmic
shear, baryon acoustic oscillations and type Ia supernovae. The associated
errors for these observables can be derived for customised surveys, or for
pre-set values corresponding to current or planned instruments. The code also
allows for the calculation of cosmological forecasts with Fisher matrices which
can be manipulated to combine different surveys and cosmological probes. The
code is written in the IDL language and thus benefits from the convenient
interactive features and scientific library available in this language. iCosmo
can also be used as an engine to perform cosmological calculations in batch
mode, and forms a convenient adaptive platform for the development of further
cosmological modules. With its extensive documentation, it may also serve as a
useful resource for teaching and for newcomers in the field of cosmology.
Results: The iCosmo package is described with various examples and command
sequences. The code is freely available with documentation at
http://www.icosmo.org, along with an interactive web interface and is part of
the Initiative for Cosmology, a common archive for cosmological resources.Comment: 6 pages including 2 tables and 4 figures. Accepted and published in
Astronomy and Astrophysics. Public code and further resources available at
http://www.icosmo.or
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