18,554 research outputs found
State-space model identification and feedback control of unsteady aerodynamic forces
Unsteady aerodynamic models are necessary to accurately simulate forces and
develop feedback controllers for wings in agile motion; however, these models
are often high dimensional or incompatible with modern control techniques.
Recently, reduced-order unsteady aerodynamic models have been developed for a
pitching and plunging airfoil by linearizing the discretized Navier-Stokes
equation with lift-force output. In this work, we extend these reduced-order
models to include multiple inputs (pitch, plunge, and surge) and explicit
parameterization by the pitch-axis location, inspired by Theodorsen's model.
Next, we investigate the na\"{\i}ve application of system identification
techniques to input--output data and the resulting pitfalls, such as unstable
or inaccurate models. Finally, robust feedback controllers are constructed
based on these low-dimensional state-space models for simulations of a rigid
flat plate at Reynolds number 100. Various controllers are implemented for
models linearized at base angles of attack , and . The resulting control laws are
able to track an aggressive reference lift trajectory while attenuating sensor
noise and compensating for strong nonlinearities.Comment: 20 pages, 13 figure
Different forms of the bovine PrP gene have five or six copies of a short, G-C-rich element within the protein-coding exon
Current models of the virus-like agents of scrapie and bovine spongiform encephalopathy (BSE) have to take into account that structural changes in a host-encoded protein (PrP protein) exhibit an effect on the time course of these diseases and the survival time of any man or animal exposed to these pathogens. We report here the sequence of different forms of the bovine PrP gene which contain either five or six copies of a short, G-C-rich element which encodes the octapeptide Pro-His-Gly-Gly-Gly-Trp-Gly-Gln or its longer variants Pro-Gln/His-Gly-Gly-Gly-Gly-Trp-Gly-Gln. Out of 12 cattle, we found eight animals homozygous for genes with six copies of the Gly-rich peptide (6:6), while four were heterozygous (6:5). Two confirmed cases of BSE occurred in (6: 6) homozygous animals. Bovine spongiform encephalopathy (BSE) is a transmissible disease (Fraser et al., 1988; Dawson et al., 1990; Barlow & Middleton, 1990) which produces neuropathological lesions in cattle similar to those seen in ovine scrapie (Wells et al., 1987) and the rare human dementias Creutzfeldt-Jakob disease (CJD) and Gerstmann-Str/iussler syndrome (GSS) (Beck & Daniel, 1987). A cellular membrane protein (PrP) has a key role in the transmission and development of these diseases. This protein accumulates in the brain and other tissues during the protracted time course of these diseases and, in a disease-specific, protease-resistant isoform (SAF-PrP), has been purified by subcellular fractionation of scrapie
Mechanism of enhanced light output in InGaN-based microlight emitting diodes
Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output
Sensitivity of arrest in mode-coupling glasses to low-q structure
We quantify, within mode coupling theory, how changes in the liquid structure
affect that of the glass. Apart from the known sensitivity to the structure
factor at wavevectors around the first sharp diffraction peak , we
find a strong (and inverted) response to structure at wavevectors \emph{below}
this peak: an increase in {\em lowers} the degree of arrest over a
wide -range. This strong sensitivity to `caged cage' packing effects, on
length scales of order 2d, is much weaker in attractive glasses where
short-range bonding dominates the steric caging effect.Comment: 4 pages, 5 figures. v2: 3 figures replaced; text rewritte
Characterizing and correcting for the effect of sensor noise in the dynamic mode decomposition
Dynamic mode decomposition (DMD) provides a practical means of extracting
insightful dynamical information from fluids datasets. Like any data processing
technique, DMD's usefulness is limited by its ability to extract real and
accurate dynamical features from noise-corrupted data. Here we show
analytically that DMD is biased to sensor noise, and quantify how this bias
depends on the size and noise level of the data. We present three modifications
to DMD that can be used to remove this bias: (i) a direct correction of the
identified bias using known noise properties, (ii) combining the results of
performing DMD forwards and backwards in time, and (iii) a total
least-squares-inspired algorithm. We discuss the relative merits of each
algorithm, and demonstrate the performance of these modifications on a range of
synthetic, numerical, and experimental datasets. We further compare our
modified DMD algorithms with other variants proposed in recent literature
Estimates of production rates of SUSY particles in ultra-relativistic heavy-ion collisions
We estimate the production rates of supersymmetric particles in central
heavy-ion collisions at LHC. The parton cascade model is used to seek for
possible collective phenomena which enlarge the production probability of very
heavy particles. Even if there is some indication of such cooperative effects,
higher energy and higher luminosity of proton beams at LHC disfavor heavy-ion
reactions in the search for supersymmetric particles.Comment: 19 pages including 10 EPS figure
InGaN nano-ring structures for high-efficiency light emitting diodes
A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated
Discovery of the supernova remnant G351.0-5.4
Context. While searching the NRAO VLA Sky Survey (NVSS) for diffuse radio
emission, we have serendipitously discovered extended radio emission close to
the Galactic plane. The radio morphology suggests the presence of a previously
unknown Galactic supernova remnant. An unclassified {\gamma}-ray source
detected by EGRET (3EG J1744-3934) is present in the same location and may stem
from the interaction between high-speed particles escaping the remnant and the
surrounding interstellar medium.
Aims. Our aim is to confirm the presence of a previously unknown supernova
remnant and to determine a possible association with the {\gamma}-ray emission
3EG J1744-3934.
Methods. We have conducted optical and radio follow-ups of the target using
the Dark Energy Camera (DECam) on the Blanco telescope at Cerro Tololo
Inter-American Observatory (CTIO) and the Giant Meterwave Radio Telescope
(GMRT). We then combined these data with archival radio and {\gamma}-ray
observations.
Results. While we detected the extended emission in four different radio
bands (325, 1400, 2417, and 4850 MHz), no optical counterpart has been
identified. Given its morphology and brightness, it is likely that the radio
emission is caused by an old supernova remnant no longer visible in the optical
band. Although an unclassified EGRET source is co-located with the supernova
remnant, Fermi-LAT data do not show a significant {\gamma}-ray excess that is
correlated with the radio emission. However, in the radial distribution of the
{\gamma}-ray events, a spatially extended feature is related with SNR at a
confidence level {\sigma}.
Conclusions. We classify the newly discovered extended emission in the radio
band as the old remnant of a previously unknown Galactic supernova: SNR
G351.0-5.4.Comment: 6 pages, 6 figures, accepted A&
- …