598 research outputs found
Numerical Simulation of Steady Supersonic and Hypersonic Flows over Simple Bodies of Revolution. Aero Report 9902
This paper reports on numerical results for supersonic
and hypersonic steady flows over axisymmetric blunt
bodies. Two-dimensional compressible Navier- Stokes
equations are solved using a high- resolution upwind
Roeâs scheme. A modification to the boundary conditions
and the implementation of Hartenâs entropy fix is
proposed to improve the robustness of the code, which
is then tested on an axisymmetric spike, cone and cylinder
at freestream Mach numbers of 2.21, 6.00 and
30.00
Numerical Simulation of High-Speed Unsteady Flows over Axisymmetric Spiked Bodies. Aero Report 9903
The present paper reports some preliminary numerical
results for the supersonic/hypersonic unsteady flow
over a family of spiked blunt bodies. Axisymmetric
compressible Navier-Stokes equations are solved using
a high- resolution unfactored implicit upwind Roeâs
scheme and a time-accurate pseudo-time method is employed
for advancing in time. Unsteady flow arising
at Mach 2.21 and Mach 6.00 around the spiked 50°,
70° cone and cylinder are simulated and the computational
results are compared with measurements. The
study on these simple shapes for which experimental
results are available provides a demonstration that such
unsteady flows can be predicted reasonably well and
provides confidence that more complex flows such as
intake ââbuzzâ and high Reynoldsâ number ablation can
be tackled
Magneto-optical study of thermally annealed InAs-InGaAs-GaAs self-assembled quantum dots
We report a magneto-optical study of InAs-InGaAs-GaAs self-assembled quantum dots (QDs) subjected to post-growth thermal annealing at different temperatures. At low temperatures annealing strongly affects the bimodal distribution of QDs; at higher temperatures a strong blueshift of the emission occurs. Magnetophotoluminescence reveals that the annealing increases the QD size, with a larger effect occurring along the growth axis, and decreases the carrier effective masses. The main contribution to the blueshift is deduced to be an increase in the average Ga composition of the QDs. The inadvertent annealing which occurs during the growth of the upper AlGaAs cladding layer in laser structures is also studied
Evaluation of growth methods for the heteroepitaxy of non-polar (1120) GAN on sapphire by MOVPE
Non-polar a-plane gallium nitride (GaN) lms have been grown on r-plane (1102) sapphire by metal organic vapour
phase epitaxy (MOVPE). A total of ve in-situ defect reduction techniques for a-plane GaN are compared, including
two variants with a low temperature GaN nucleation layer (LTNL) and three variants without LTNL, in which the high-
temperature growth of GaN is performed directly on the sapphire using various crystallite sizes. The material quality is
investigated by photoluminescence (PL), x-ray di raction, cathodoluminescence, atomic force and optical microscopy. It
is found that all layers are anisotropically strained with threading dislocation densities over 109 cm2. The PL spectrum
is typically dominated by emission from basal plane stacking faults. Overall, growth techniques without LTNL do not
yield any particular improvement and even result in the creation of new defects, ie. inversion domains, which are seldom
observed if a low temperature GaN nucleation layer is used. The best growth method uses a LTNL combined with a
single silicon nitride interlayer.This work is supported by the Engineering and Physical Sciences Research Council (United Kingdom) under EP/J003603/1 and EP/H0495331. The European Research Council has also provided nancial support under the European Community's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no 279361 (MACONS).This is the final published version, also available from Elsevier at: http://dx.doi.org/10.1016/j.jcrysgro.2014.09.00
The consequences of high injected carrier densities on carrier localization and efficiency droop in InGaN/GaN quantum well structures
There is a great deal of interest in the underlying causes of efficiency droop in
InGaN/GaN quantum well light emitting diodes, with several physical mechanisms
being put forward to explain the phenomenon. In this paper we report on the
observation of a reduction in the localisation induced S-shape temperature
dependence of the peak photoluminescence energy with increasing excitation power
density. This S-shape dependence is a key fingerprint of carrier localisation. Over
the range of excitation power density where the depth of the S shape is reduced we
also observe a reduction in the integrated photoluminescence intensity per unit
excitation power, i.e. efficiency droop. Hence the onset of efficiency droop occurs at
the same carrier density as the onset of carrier delocalisation. We correlate these
experimental results with the predictions of a theoretical model of the effects of
carrier localisation due to local variations in the concentration of the randomly
distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the
basis of this comparison of theory with experiment we attribute the reduction in the Sshape
temperature dependence to the saturation of the available localised states. We
propose that this saturation of the localised states is a contributory factor to efficiency
droop whereby non localised carriers recombine non-radiatively
Failures of Cognitive Control or Attention? The Case of Stop-Signal Deficits in Schizophrenia
We used Bayesian cognitive modelling to identify the underlying causes of apparent inhibitory deficits in the stop-signal paradigm. The analysis was applied to stop-signal data reported by Badcock et al. (Psychological Medicine 32: 87-297, 2002) and Hughes et al. (Biological Psychology 89: 220-231, 2012), where schizophrenia patients and control participants made rapid choice responses, but on some trials were signalled to stop their ongoing response. Previous research has assumed an inhibitory deficit in schizophrenia, because estimates of the mean time taken to react to the stop signal are longer in patients than controls. We showed that these longer estimates are partly due to failing to react to the stop signal (âtrigger failuresâ) and partly due to a slower initiation of inhibition, implicating a failure of attention rather than a deficit in the inhibitory process itself. Correlations between the probability of trigger failures and event-related potentials reported by Hughes et al. are interpreted as supporting the attentional account of inhibitory deficits. Our results, and those of Matzke et al. (2016), who report that controls also display a substantial although lower trigger-failure rate, indicate that attentional factors need to be taken into account when interpreting results from the stop-signal paradigm
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