2,959 research outputs found
Lattice Black Holes
We study the Hawking process on lattices falling into static black holes. The
motivation is to understand how the outgoing modes and Hawking radiation can
arise in a setting with a strict short distance cutoff in the free-fall frame.
We employ two-dimensional free scalar field theory. For a falling lattice with
a discrete time-translation symmetry we use analytical methods to establish
that, for Killing frequency and surface gravity satisfying
in lattice units, the continuum Hawking spectrum
is recovered. The low frequency outgoing modes arise from exotic ingoing modes
with large proper wavevectors that "refract" off the horizon. In this model
with time translation symmetry the proper lattice spacing goes to zero at
spatial infinity. We also consider instead falling lattices whose proper
lattice spacing is constant at infinity and therefore grows with time at any
finite radius. This violation of time translation symmetry is visible only at
wavelengths comparable to the lattice spacing, and it is responsible for
transmuting ingoing high Killing frequency modes into low frequency outgoing
modes.Comment: 26 pages, LaTeX, 2 figures included with psfig. Several improvements
in the presentation. One figure added. Final version to appear in Phys.Rev.
Controlling charge injection in organic field-effect transistors using self-assembled monolayers
We have studied charge injection across the metal/organic semiconductor
interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect
transistors, with Au source and drain electrodes modified by self-assembled
monolayers (SAMs) prior to active polymer deposition. By using the SAM to
engineer the effective Au work function, we markedly affect the charge
injection process. We systematically examine the contact resistivity and
intrinsic channel mobility, and show that chemically increasing the injecting
electrode work function significantly improves hole injection relative to
untreated Au electrodes.Comment: 5 pages, 2 figures. Supplementary information available upon reques
Gravity on Noncommutative D-Branes
The effective action for the low energy scattering of two gravitons with a
D-brane in the presence of a constant antisytmetric field in bosonic string
theory is calculated and the modification to the standard D-brane action to
first order in is obtained.Comment: 18 pages, Latex file, accepted in Int. J. Mod. Phys.
Probing the Enhancon via Calculations in Supersymmetric Gauge Theory
We consider the N=2 gauge theory on N D7-branes wrapping K3, with D3-brane
probes. In the large N limit, the D7-branes blow up to form an enhancon shell.
We probe the region inside and outside the enhancon shell using the D3-branes,
and compute the probe metric using the Seiberg-Witten formalism. Supergravity
arguments suggest a flat interior up to 1/N corrections, and indeed our results
for the D3-brane probes are consistent with that. By including the dynamics of
the branes, these results, together with those of hep-th/0204050, demonstrate
the robustness of the enhancon mechanism beyond patching together of
supergravity solutions with D-brane source junction conditions.Comment: 20 pages, 2 figures, minor correction
Wind turbine gearbox condition monitoring : experimental validation of a thermal network model
Condition monitoring is an important part of predictive maintenance. Early detection of incipient faults prevents major component failures and allows for the implementation of predictive repair strategies. Gearboxes are responsible for up to one third of all lost wind turbine availability1 , each failure downtime of around 600 hours2 . Deterioration of the drivetrain components will often be reflected in an increase in losses, as a result of increased friction or a reduced efficiency of energy transfer in the cooling mechanism, resulting in elevated temperatures3 . Monitoring temperature changes are useful to understand how the thermal behaviour of a gearbox can change as a result of a fault. In the author’s previous research, a thermal network model of a ”healthy” gearbox was produced. The model was partially validated by prelimary experimental data using an 11kW wind turbine gearbox test rig located at University of Strathclyde. Figure 1 shows the how this research links together. The confidence in the preliminary experiment is limited due to the simplistic nature of the data aquisition (DAQ) equipment. The DAQ system itself plays a significant role, as temperature measurement method influences diagnostic capabilities. A number of studies in literature used experimental methods to use temperature to detect faults4 , in most cases the input variables and measurement uncertainty weren’t quantified. This research focusses on improving the certainty of the experimental data by redeveloping the DAQ system and experiment strategy to improve accuracy. Figure 2 shows all aspects of experiment that have been taken into consideration to ensure validity, reliability and reproducibility. The experiment was carried out on a ”healthy” gearbox, with 8 point sensors and a thermal camera. The rotational speed was fixed and the torque was varied to reflect the wind turbine operation. The results show the way in which heat propagates through the gearbox, and are compared to the thermal model to identify normal operational losses. Faults were then simulated on the test rig with the addition of heat at component locations. The magnitude and location of the applied heat were varied to imitate failure modes. The results show how the additional heat propagates to other parts of the gearbox and indicate the extent to which a fault can be detected. These results can also be used to validate the simulated faults in the thermal model. The outcome of this research can determine if temperature measurements can be used to detect and locate faults, to make condition monitoring more accurate
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