2,873 research outputs found
Time-resolved fuel injector flow characterisation based on 3D laser Doppler vibrometry
In order to enable investigations of the fuel flow inside unmodified
injectors, we have developed a new experimental approach to measure
time-resolved vibration spectra of diesel nozzles using a three dimensional
laser vibrometer. The technique we propose is based on the triangulation of the
vibrometer and fuel pressure transducer signals, and enables the quantitative
characterisation of quasi-cyclic internal flows without requiring modifications
to the injector, the working fluid, or limiting the fuel injection pressure.
The vibrometer, which uses the Doppler effect to measure the velocity of a
vibrating object, was used to scan injector nozzle tips during the injection
event. The data were processed using a discrete Fourier transform to provide
time-resolved spectra for valve-closed-orifice, minisac and microsac nozzle
geometries, and injection pressures ranging from 60 to 160MPa, hence offering
unprecedented insight into cyclic cavitation and internal mechanical dynamic
processes. A peak was consistently found in the spectrograms between 6 and
7.5kHz for all nozzles and injection pressures. Further evidence of a similar
spectral peak was obtained from the fuel pressure transducer and a needle lift
sensor mounted into the injector body. Evidence of propagation of the nozzle
oscillations to the liquid sprays was obtained by recording high-speed videos
of the near-nozzle diesel jet, and computing the fast Fourier transform for a
number of pixel locations at the interface of the jets. This 6-7.5kHz frequency
peak is proposed to be the natural frequency for the injector's main internal
fuel line. Other spectral peaks were found between 35 and 45kHz for certain
nozzle geometries, suggesting that these particular frequencies may be linked
to nozzle dependent cavitation phenomena.Comment: 12 pages, 10 figure
Test-retest reliability of structural brain networks from diffusion MRI
Structural brain networks constructed from diffusion MRI (dMRI) and tractography have been demonstrated in healthy volunteers and more recently in various disorders affecting brain connectivity. However, few studies have addressed the reproducibility of the resulting networks. We measured the test–retest properties of such networks by varying several factors affecting network construction using ten healthy volunteers who underwent a dMRI protocol at 1.5 T on two separate occasions. Each T1-weighted brain was parcellated into 84 regions-of-interest and network connections were identified using dMRI and two alternative tractography algorithms, two alternative seeding strategies, a white matter waypoint constraint and three alternative network weightings. In each case, four common graph-theoretic measures were obtained. Network properties were assessed both node-wise and per network in terms of the intraclass correlation coefficient (ICC) and by comparing within- and between-subject differences. Our findings suggest that test–retest performance was improved when: 1) seeding from white matter, rather than grey; and 2) using probabilistic tractography with a two-fibre model and sufficient streamlines, rather than deterministic tensor tractography. In terms of network weighting, a measure of streamline density produced better test–retest performance than tract-averaged diffusion anisotropy, although it remains unclear which is a more accurate representation of the underlying connectivity. For the best performing configuration, the global within-subject differences were between 3.2% and 11.9% with ICCs between 0.62 and 0.76. The mean nodal within-subject differences were between 5.2% and 24.2% with mean ICCs between 0.46 and 0.62. For 83.3% (70/84) of nodes, the within-subject differences were smaller than between-subject differences. Overall, these findings suggest that whilst current techniques produce networks capable of characterising the genuine between-subject differences in connectivity, future work must be undertaken to improve network reliability
Flux-limited solutions for quasi-convex Hamilton-Jacobi equations on networks
We study Hamilton-Jacobi equations on networks in the case where Hamiltonians
are quasi-convex with respect to the gradient variable and can be discontinuous
with respect to the space variable at vertices. First, we prove that imposing a
general vertex condition is equivalent to imposing a specific one which only
depends on Hamiltonians and an additional free parameter, the flux limiter.
Second, a general method for proving comparison principles is introduced. This
method consists in constructing a vertex test function to be used in the
doubling variable approach. With such a theory and such a method in hand, we
present various applications, among which a very general existence and
uniqueness result for quasi-convex Hamilton-Jacobi equations on networks.Comment: 104 pages. Version final
Gauge/gravity duality and the interplay of various fractional branes
We consider different types of fractional branes on a Z_2 orbifold of the
conifold and analyze in detail the corresponding gauge/gravity duality. The
gauge theory possesses a rich and varied dynamics, both in the UV and in the
IR. We find the dual supergravity solution which contains both untwisted and
twisted 3-form fluxes, related to what are known as deformation and N=2
fractional branes respectively. We analyze the resulting RG flow from the
supergravity perspective, by developing an algorithm to easily extract it. We
find hints of a generalization of the familiar cascade of Seiberg dualities due
to a non-trivial interplay between the different types of fractional branes. We
finally consider the IR behavior in several limits, where the dominant
effective dynamics is either confining, in a Coulomb phase or runaway, and
discuss the resolution of singularities in the dual geometric background.Comment: 38 pages + appendices, 15 figures; v2: refs added and typos correcte
Le mestre et son « cours ». Figure et institution de la transmission patrimoniale du chant dans le sud du Portugal
Le cante alentejano, une forme de chant polyphonique du sud du Portugal, offre le cadre d’une interrogation multiple et sensible sur la rencontre – et l’affrontement – entre les représentations dominantes du patrimoine et les conceptions portées par des groupes locaux engagés dans des processus de mise en patrimoine. L’arène patrimoniale locale devient le lieu où se joue la construction de solutions de compromis entre des conceptions hétérogènes, voire antagonistes, des valeurs patrimoniales. On étudie le cas d'un "cours" ouvert, où se transmet una manière de chanter et un répertoire
- …