1,324 research outputs found
A method for the measurement of hydrodynamic oil films using ultrasonic reflection
The measurement of the thickness of an oil film in a lubricated component is essential information for performance monitoring and control. In this work, a new method for oil film thickness measurement, based on the reflection of ultrasound, is evaluated for use in fluid film journal bearing applications. An ultrasonic wave will be partially reflected when it strikes a thin layer between two solid media. The proportion of the wave reflected depends on the thickness of the layer and its acoustic properties. A simple quasi-static spring model shows how the reflection depends on the stiffness of the layer alone. This method has been first evaluated using flat plates separated by a film of oil, and then used in the measurement of oil films in a hydrodynamic journal bearing. A transducer is mounted on the outside of the journal and a pulse propagated through the shell. The pulse is reflected back at the oil film and received by the same transducer. The amplitude of the reflected wave is processed in the frequency domain. The spring model is then used to determine the oil film stiffness that can be readily converted to film thickness. Whilst the reflected amplitude of the wave is dependent on the frequency component, the measured film thickness is not; this indicates that the quasi-static assumption holds. Measurements of the lubricant film generated in a simple journal bearing have been taken over a range of loads and speeds. The results are compared with predictions from classical hydrodynamic lubrication theory. The technique has also been used to measure oil film thickness during transient loading events. The response time is rapid and film thickness variation due to step changes in load and oil feed pressure can be clearly observed
Oil film measurement in polytetrafluoroethylene-faced thrust pad bearings for hydrogenerator applications
There is a growing trend in the replacement of the babbit facing in thrust pad bearings with a composite polytetrafluoroethylene (PTFE) surface layer. The PTFE-faced bearings have been shown to allow a greater specific pressure, reduce thermal crowning, and, in some cases, negate the need for an oil-lift (jacking) system. These designs of bearing require new methods for the measurement of oil film thickness both to assist in their development and for plant condition monitoring. In this work, an ultrasonic method of oil film measurement is evaluated for this purpose. An ultrasonic transducer is mounted on the back face of the thrust pad. Pulses are generated and transmitted through the pad material, bonding interlayer, and PTFE surface layer. The proportion of the wave that reflects back from the oil film layer is determined. This is then related to the oil film thickness using a series of calibration experiments and a spring stiffness model. In practice, the reflected signal is difficult to distinguish, in the time domain, from other internal reflections from the pad. Signals are compared with reflections when no oil film is present and processing is carried out in the frequency domain. Experiments have been performed on a full size PTFE-faced thrust pad destined for a hydroelectric power station turbine. The instrumented pad was installed in a test facility and subjected to a range of loading conditions both with and without oil lift. Although there were some problems with the robustness of the experimental procedure, oil films were successfully measured and used to study the effect of the oil-lift system on film formation. © IMechE 2006
Quasar-galaxy associations
There is controversy about the measurement of statistical associations
between bright quasars and faint, presumably foreground galaxies. We look at
the distribution of galaxies around an unbiased sample of 63 bright, moderate
redshift quasars using a new statistic based on the separation of the quasar
and its nearest neighbour galaxy. We find a significant excess of close
neighbours at separations less than about 10 arcsec which we attribute to the
magnification by gravitational lensing of quasars which would otherwise be too
faint to be included in our sample. About one quarter to one third of the
quasars are so affected although the allowed error in this fraction is large.Comment: uuencoded Postscript file (including figures and tables), SUSSEX-AST
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Total focussing method for volumetric imaging in immersion non destructive evaluation
This paper describes the use of a 550 (25x22) element 2MHz 2D piezoelectric composite array in immersion mode to image an aluminum test block containing a collection of artificial defects. The defects included a 1mm diameter side-drilled hole, a collection of 1mm slot defects with varying degrees of skew to the normal and a flat bottomed hole. The data collection was carried out using the full matrix capture; a scanning procedure was developed to allow the operation of the large element count array through a conventional 64-channel phased array controller. A 3D TFM algorithm capable of imaging in a dual media environment was implemented in MATLAB for the offline processing the raw scan data. This algorithm facilitates the creation of 3D images of defects while accounting for refraction effects at material boundaries. In each of the test samples interrogated the defects, and their spatial position, are readily identified using TFM. Defect directional information has been characterized using VTFM for defect exhibiting angles up to and including 45o of skew
Formation of ultra-compact dwarf galaxies: tests of the galaxy threshing scenario in Fornax
This paper investigates the possibility that UCD galaxies in the Fornax
cluster are formed by the threshing of nucleated, early-type dwarf galaxies
(hereafter dwarf galaxies).
Similar to the results of Cote et al. (2006) for the Virgo cluster, we show
that the Fornax Cluster observations are consistent with a single population in
which all dwarfs are nucleated, with a ratio of nuclear to total magnitude that
varies slowly with magnitude. Importantly, the magnitude distribution of the
UCD population is similar to that of the dwarf nuclei in the Fornax cluster.
The joint population of UCDs and the dwarfs from which they may originate is
modelled and shown to be consistent with an NFW profile with a characteristic
radius of 5 kpc. Furthermore, a steady-state dynamical model reproduces the
known mass profile of Fornax. However, there are a number of peculiarities in
the velocity dispersion data that remain unexplained.
The simplest possible threshing model is tested, in which dwarf galaxies move
on orbits in a static cluster potential and are threshed if they pass within a
radius at which the tidal force from the cluster exceeds the internal gravity
at the core of their dark matter halo. This fails to reproduce the observed
fraction of UCDs at radii greater than 30 kpc from the core of Fornax.Comment: 18 pages, accepted for publication by MNRAS Changes in response to
referee's comments: Amended Figure 6 to allow for missing UCDs at large radii
Modified discussio
Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups
We present the results of a search for ultra-compact dwarf galaxies (UCDs) in
six different galaxy groups: Dorado, NGC1400, NGC0681, NGC4038, NGC4697 and
NGC5084. We searched in the apparent magnitude range 17.5 < b_j < 20.5 (except
NGC5084: 19.2 < b_j < 21.0). We found 1 definite plus 2 possible UCD candidates
in the Dorado group and 2 possible UCD candidates in the NGC1400 group. No UCDs
were found in the other groups. We compared these results with predicted
luminosities of UCDs in the groups according to the hypothesis that UCDs are
globular clusters formed in galaxies. The theoretical predictions broadly agree
with the observational results, but deeper surveys are needed to fully test the
predictions.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Interactive manipulation of microparticles in an octagonal sonotweezer
An ultrasonic device for micro-patterning and precision manipulation of micrometre-scale particles is demonstrated. The device is formed using eight piezoelectric transducers shaped into an octagonal cavity. By exciting combinations of transducers simultaneously, with a controlled phase delay between them, different acoustic landscapes can be created, patterning micro-particles into lines, squares, and more complex shapes. When operated with all eight transducers the device can, with appropriate phase control, manipulate the two dimensional acoustic pressure gradient; it thus has the ability to position and translate a single tweezing zone to different locations on a surface in a precise and programmable manner
Compact Stellar Systems around NGC 1399
We have obtained spectroscopic redshifts of colour-selected point sources in
four wide area VLT-FLAMES fields around the Fornax Cluster giant elliptical
galaxy NGC 1399, identifying as cluster members 30 previously unknown faint
(-10.5<M_g'<-8.8) compact stellar systems (CSS), and improving redshift
accuracy for 23 previously catalogued CSS.
By amalgamating our results with CSS from previous 2dF observations and
excluding CSS dynamically associated with prominent (non-dwarf) galaxies
surrounding NGC 1399, we have isolated 80 `unbound' systems that are either
part of NGC 1399's globular cluster (GC) system or intracluster GCs. For these
unbound systems, we find (i) they are mostly located off the main stellar locus
in colour-colour space; (ii) their projected distribution about NGC 1399 is
anisotropic, following the Fornax Cluster galaxy distribution, and there is
weak evidence for group rotation about NGC 1399; (iii) their
completeness-adjusted radial surface density profile has a slope similar to
that of NGC 1399's inner GC system; (iv) their mean heliocentric recessional
velocity is between that of NGC 1399's inner GCs and that of the surrounding
dwarf galaxies, but their velocity dispersion is significantly lower; (v)
bright CSS (M_V<-11) are slightly redder than the fainter systems, suggesting
they have higher metallicity; (vi) CSS show no significant trend in
colour index with radial distance from NGC 1399.Comment: 13 pages (including supplementary table), 13 figures, 5 tables.
Accepted for publication in MNRA
Red Parkes-Quasars: Evidence for Soft X-ray Absorption
The Parkes Half-Jansky Flat Spectrum Sample contains a large number of
sources with unusually red optical-to-near-infrared continua. If this is to be
interpreted as extinction by dust in the line-of-sight, then associated
material might also give rise to absorption in the soft X-ray regime. This
hypothesis is tested using broadband (0.1-2.4 keV) data from the {\it ROSAT}
All-Sky Survey provided by Siebert et al. (1998). Significant (
confidence level) correlations between optical (and near-infrared)--to--soft
X-ray continuum slope and optical extinction are found in the data, consistent
with absorption by material with metallicity and a range in gas-to-dust ratio
as observed in the local ISM. Under this simple model, the soft X-rays are
absorbed at a level consistent with the range of extinctions (
magnitudes) implied by the observed optical reddening. Excess X-ray absorption
by warm (ionised) gas, (ie. a `warm absorber') is not required.Comment: 23 pages of text, 3 figures, to appear in Jan 10 (1999) issue of The
Astrophysical Journa
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