21,404 research outputs found
Propulsion system study for Small Transport Aircraft Technology (STAT)
Propulsion system technologies applicable to the generation of commuter airline aircraft expected to enter service in the 1990's are identified and evaluated in terms of their impact on aircraft operating economics and fuel consumption. The most promising technologies in the areas of engine, propeller, gearbox, and nacelle design are recommended for future research. Each item under consideration is evaluated relative to a modern baseline engine, the General Electric CT7-5, in a current technology aircraft flying a fixed range and payload. The analysis is presented for two aircraft sizes (30 and 50 passenger), over a range of mission lengths (100 to 1100 km) and fuel costs (396 per cu m)
Deformation-induced accelerated dynamics in polymer glasses
Molecular dynamics simulations are used to investigate the effects of
deformation on the segmental dynamics in an aging polymer glass. Individual
particle trajectories are decomposed into a series of discontinuous hops, from
which we obtain the full distribution of relaxation times and displacements
under three deformation protocols: step stress (creep), step strain, and
constant strain rate deformation. As in experiments, the dynamics can be
accelerated by several orders of magnitude during deformation, and the history
dependence is entirely erased during yield (mechanical rejuvenation). Aging can
be explained as a result of the long tails in the relaxation time distribution
of the glass, and similarly, mechanical rejuvenation is understood through the
observed narrowing of this distribution during yield. Although the relaxation
time distributions under deformation are highly protocol specific, in each case
they may be described by a universal acceleration factor that depends only on
the strain.Comment: 15 pages, 15 figure
The cognitive organization of music knowledge: a clinical analysis
Despite much recent interest in the clinical neuroscience of music processing, the cognitive organization of music as a domain of non-verbal knowledge has been little studied. Here we addressed this issue systematically in two expert musicians with clinical diagnoses of semantic dementia and Alzheimer’s disease, in comparison with a control group of healthy expert musicians. In a series of neuropsychological experiments, we investigated associative knowledge of musical compositions (musical objects), musical emotions, musical instruments (musical sources) and music notation (musical symbols). These aspects of music knowledge were assessed in relation to musical perceptual abilities and extra-musical neuropsychological functions. The patient with semantic dementia showed relatively preserved recognition of musical compositions and musical symbols despite severely impaired recognition of musical emotions and musical instruments from sound. In contrast, the patient with Alzheimer’s disease showed impaired recognition of compositions, with somewhat better recognition of composer and musical era, and impaired comprehension of musical symbols, but normal recognition of musical emotions and musical instruments from sound. The findings suggest that music knowledge is fractionated, and superordinate musical knowledge is relatively more robust than knowledge of particular music. We propose that music constitutes a distinct domain of non-verbal knowledge but shares certain cognitive organizational features with other brain knowledge systems. Within the domain of music knowledge, dissociable cognitive mechanisms process knowledge derived from physical sources and the knowledge of abstract musical entities
Simulations of aging and plastic deformation in polymer glasses
We study the effect of physical aging on the mechanical properties of a model
polymer glass using molecular dynamics simulations. The creep compliance is
determined simultaneously with the structural relaxation under a constant
uniaxial load below yield at constant temperature. The model successfully
captures universal features found experimentally in polymer glasses, including
signatures of mechanical rejuvenation. We analyze microscopic relaxation
timescales and show that they exhibit the same aging characteristics as the
macroscopic creep compliance. In addition, our model indicates that the entire
distribution of relaxation times scales identically with age. Despite large
changes in mobility, we observe comparatively little structural change except
for a weak logarithmic increase in the degree of short-range order that may be
correlated to an observed decrease in aging with increasing load.Comment: 9 pages, 12 figure
Investigation of Noise Field and Velocity Profiles of an Afterburning Engine
Sound pressure levels, frequency spectrum, and jet velocity profiles are presented for an engine-afterburner combination at various values of afterburner fuel - air ratio. At the high fuel-air ratios, severe low-frequency resonance was encountered which represented more than half the total energy in the sound spectrum. At similar thrust conditions, lower sound pressure levels were obtained from a current fighter air craft with a different afterburner configuration. The lower sound pressure levels are attributed to resonance-free afterburner operation and thereby indicate the importance of acoustic considerations in afterburner design
Precision measurement of the 5 2S1/2 - 4 2D5/2 quadrupole transition isotope shift between 88Sr+ and 86Sr+
We have measured the isotope shift of the narrow quadrupole-allowed 5 2S1/2 -
4 2D5/2 transition in 86Sr+ relative to the most abundant isotope 88Sr+. This
was accomplished using high-resolution laser spectroscopy of individual trapped
ions, and the measured shift is Delta-nu_meas^(88,86) = 570.281(4) MHz. We have
also tested a recently developed and successful method for ab-initio
calculation of isotope shifts in alkali-like atomic systems against this
measurement, and our initial result of Delta-nu_calc^(88,86) = 457(28) MHz is
also presented. To our knowledge, this is the first high precision measurement
and calculation of that isotope shift. While the measurement and the
calculation are in broad agreement, there is a clear discrepancy between them,
and we believe that the specific mass shift was underestimated in our
calculation. Our measurement provides a stringent test for further refinements
of theoretical isotope shift calculation methods for atomic systems with a
single valence electron
Neural precursor cells derived from induced pluripotent stem cells exhibit reduced susceptibility to infection with a neurotropic coronavirus.
The present study examines the susceptibility of mouse induced pluripotent stem cell-derived neural precursor cells (iPSC-NPCs) to infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV). Similar to NPCs derived from striatum of day 1 postnatal GFP-transgenic mice (GFP-NPCs), iPSC-derived NPCs (iPSC-NPCs) are able to differentiate into terminal neural cell types and express MHC class I and II in response to IFN-γ treatment. However, in contrast to postnatally-derived NPCs, iPSC-NPCs express low levels of carcinoembryonic antigen-cell adhesion molecule 1a (CEACAM1a), the surface receptor for JHMV, and are less susceptible to infection and virus-induced cytopathic effects. The relevance of this in terms of therapeutic application of NPCs resistant to viral infection is discussed
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