108 research outputs found
Adaptive Wing/Aerofoil Design Optimisation Using MOEA Coupled to Uncertainty Design Method
The use of adaptive wing/aerofoil designs is being considered as promising techniques in aeronautic/aerospace since they can reduce aircraft emissions, improve aerodynamic performance of manned or unmanned aircraft. The paper investigates the robust design and optimisation for one type of adaptive techniques; Active Flow Control (AFC) bump at transonic flow conditions on a Natural Laminar Flow (NLF) aerofoil designed to increase aerodynamic efficiency (especially high lift to drag ratio). The concept of using Shock Control Bump (SCB) is to control supersonic flow on the suction/pressure side of NLF aerofoil: RAE 5243 that leads to delaying shock occurrence or weakening its strength. Such AFC technique reduces total drag at transonic speeds due to reduction of wave drag. The location of Boundary Layer Transition (BLT) can influence the position the supersonic shock occurrence. The BLT position is an uncertainty in aerodynamic design due to the many factors, such as surface contamination or surface erosion. The paper studies the SCB shape design optimisation using robust Evolutionary Algorithms (EAs) with uncertainty in BLT positions. The optimisation method is based on a canonical evolution strategy and incorporates the concepts of hierarchical topology, parallel computing and asynchronous evaluation. Two test cases are conducted; the first test assumes the BLT is at 45% of chord from the leading edge and the second test considers robust design optimisation for SCB at the variability of BLT positions and lift coefficient. Numerical result shows that the optimisation method coupled to uncertainty design techniques produces Pareto optimal SCB shapes which have low sensitivity and high aerodynamic performance while having significant total drag reduction
Effects of epitaxial strain on the growth mechanism of YBa2Cu3O7-x thin films in [YBa2Cu3O7-x / PrBa2Cu3O7-x] superlattices
We report on the growth mechanism of YBa2Cu3O7-x (YBCO). Our study is based
on the analysis of ultrathin, YBa2Cu3O7-x layers in c-axis oriented YBa2Cu3O7-x
/ PrBa2Cu3O7-x superlattices. We have found that the release of epitaxial
strain in very thin YBCO layers triggers a change in the dimensionality of the
growth mode. Ultrathin, epitaxially strained, YBCO layers with thickness below
3 unit cells grow in a block by block two dimensional mode coherent over large
lateral distances. Meanwhile, when thickness increases, and the strain relaxes,
layer growth turns into three dimensional, resulting in rougher layers and
interfaces.Comment: 10 pages + 9 figures, accepted in Phys. Rev.
Is there a vortex-glass transition in high-temperature superconductors?
We show that DC voltage versus current measurements of a YBCO micro-bridge in
a magnetic field can be collapsed onto scaling functions proposed by Fisher,
Fisher, and Huse, as is widely reported in the literature. We find, however,
that good data collapse is achieved for a wide range of critical exponents and
temperatures. These results strongly suggest that agreement with scaling alone
does not prove the existence of a phase transition. We propose a criterion to
determine if the data collapse is valid, and thus if a phase transition occurs.
To our knowledge, none of the data reported in the literature meet our
criterion.Comment: 4 pages, 4 figure
Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3 / YBa2Cu3O7 superlattices
We study the interplay between magnetism and superconductivity in high
quality YBa2Cu3O7 (YBCO) / La0.7Ca0.3MnO3(LCMO)superlattices. We find evidence
for the YBCO superconductivity depression in presence of the LCMO layers. We
show that due to its short coherence length superconductivity survives in the
YBCO down to much smaller thickness in presence of the magnetic layer than in
low Tc superconductors. We also find that for a fixed thickness of the
superconducting layer, superconductivity is depressed over a thickness interval
of the magnetic layer in the 100 nm range. This is a much longer length scale
than that predicted by the theory of ferromagnetic/superconducting proximity
effect.Comment: 10 pages + 5 figures, submitted to Phys. Rev.
Critical temperature depression and persistent photoconductivity in ion irradiated YBa2Cu3O7−x films and YBa2Cu3O7−x/PrBa2Cu3O7 superlattices
Muon spin rotation studies of electronic excitations and magnetism in the vortex cores of superconductors
The focus of this paper is on recent progress in muon spin rotation (muSR)
studies of the vortex cores in type-II superconductors. By comparison of muSR
measurements of the vortex core size in a variety of materials with results
from techniques that directly probe electronic states, the effect of
delocalized quasiparticles on the spatial variation of field in a lattice of
interacting vortices has been determined for both single-band and multi-band
superconductors. These studies demonstrate the remarkable accuracy of what some
still consider an exotic technique. In recent years muSR has also been used to
search for magnetism in and around the vortex cores of high-temperature
superconductors. As a local probe muSR is specially suited for detecting static
or quasistatic magnetism having short-range or random spatial correlations. As
discussed in this review, muSR experiments support a generic phase diagram of
competing superconducting and magnetic order parameters, characterized by a
quantum phase transition to a state where the competing order is spatially
nonuniform.Comment: 28 pages, 22 figure
The temperature-dependent magnetization profile across an epitaxial bilayer of ferromagnetic La2/3Ca1/3MnO3 and superconducting YBa2Cu3O7-d
Epitaxial bilayers of ferromagnetic La2/3Ca1/3MnO3 (LCMO) and superconducting
YBa2Cu3O7-d (YBCO) have been grown on single-crystalline SrTiO3 (STO)
substrates by pulsed laser deposition. The Manganese magnetization profile
across the FM layer has been determined with high spatial resolution at low
temperatures by X-ray resonant magnetic reflectivity (XRMR). It is found that
not only the adjacent superconductor but also the substrate underneath
influences the magnetization of the LCMO film at the interfaces at low
temperatures. Both effects can be investigated individually by XRMR
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