1,952 research outputs found
Noise predictions and economic effects of Boeing nacelle modifications
Noise level predictions and economics of Boeing nacelle modification
Dynamical heterogeneity in soft particle suspensions under shear
We present experimental measurements of dynamical heterogeneities in a dense
system of microgel spheres, sheared at different rates and at different packing
fractions in a microfluidic channel, and visualized with high speed digital
video microscopy. A four-point dynamic susceptibility is deduced from video
correlations, and is found to exhibit a peak that grows in height and shifts to
longer times as the jamming transition is approached from two different
directions. In particular, the time for particle-size root-mean square relative
displacements is found to scale as where is the strain rate and
is the distance from the random close packing volume
fraction. The typical number of particles in a dynamical heterogeneity is
deduced from the susceptibility peak height and found to scale as . Exponent uncertainties are less than ten
percent. We emphasize that the same power-law behavior is found at packing
fractions above and below . Thus, our results considerably extend a
previous observation of for granular heap flow at
fixed packing below . Furthermore, the implied result compares well with expectation from mode-coupling theory and
with prior observations for driven granular systems
Improving Internal Consistency of Standard State Thermodynamic Data for Sulfate Ion, Portlandite, Gypsum, Barite, Celestine, and Associated Ions
AbstractThermochemical measurements in the gypsum-portlandite-water system are actually better than those reflected in the thermodynamic properties of CODATA. It is argued that by careful choice of starting points and pathways and with new data, improvements in standard state properties can achieve improved consistency with solubilities of gypsum, portlandite, barite, witherite, celestine, and strontianite if better entropy estimates of these minerals are made
Conceptual design study of advanced acoustic-composite nacelles
Conceptual studies were conducted to assess the impact of incorporating advanced technologies in the nacelles of a current wide-bodied transport and an advanced technology transport. The improvement possible in the areas of fuel consumption, flyover noise levels, airplane weight, manufacturing costs, and airplane operating cost were evaluated for short and long-duct nacelles. Use of composite structures for acoustic duct linings in the fan inlet and exhaust ducts was considered as well as for other nacelle components. For the wide-bodied transport, the use of a long-duct nacelle with an internal mixer nozzle in the primary exhaust showed significant improvement in installed specific fuel consumption and airplane direct operating costs compared to the current short-duct nacelle. The long-duct mixed-flow nacelle is expected to achieve significant reductions in jet noise during takeoff and in turbo-machinery noise during landing approach. Recommendations were made of the technology development needed to achieve the potential fuel conservation and noise reduction benefits
Neural mechanisms underlying target detection in a dragonfly centrifugal neuron
© The Company of Biologists Ltd 2007Visual identification of targets is an important task for many animals searching for prey or conspecifics. Dragonflies utilize specialized optics in the dorsal acute zone, accompanied by higher-order visual neurons in the lobula complex, and descending neural pathways tuned to the motion of small targets. While recent studies describe the physiology of insect small target motion detector (STMD) neurons, little is known about the mechanisms that underlie their exquisite sensitivity to target motion. Lobula plate tangential cells (LPTCs), a group of neurons in dipteran flies selective for wide-field motion, have been shown to take input from local motion detectors consistent with the classic correlation model developed by Hassenstein and Reichardt in the 1950s. We have tested the hypothesis that similar mechanisms underlie the response of dragonfly STMDs. We show that an anatomically characterized centrifugal STMD neuron (CSTMD1) gives responses that depend strongly on target contrast, a clear prediction of the correlation model. Target stimuli are more complex in spatiotemporal terms than the sinusoidal grating patterns used to study LPTCs, so we used a correlation-based computer model to predict response tuning to velocity and width of moving targets. We show that increasing target width in the direction of travel causes a shift in response tuning to higher velocities, consistent with our model. Finally, we show how the morphology of CSTMD1 allows for impressive spatial interactions when more than one target is present in the visual field.Bart R. H. Geurten, Karin Nordström, Jordanna D. H. Sprayberry, Douglas M. Bolzon and David C. O'Carrol
Microfluidic rheology of soft colloids above and below jamming
The rheology near jamming of a suspension of soft colloidal spheres is
studied using a custom microfluidic rheometer that provides stress versus
strain rate over many decades. We find non-Newtonian behavior below the jamming
concentration and yield stress behavior above it. The data may be collapsed
onto two branches with critical scaling exponents that agree with expectations
based on Hertzian contacts and viscous drag. These results support the
conclusion that jamming is similar to a critical phase transition, but with
interaction-dependent exponents.Comment: 4 pages, experimen
Spin-dynamic field coupling in strongly THz driven semiconductors : local inversion symmetry breaking
We study theoretically the optics in undoped direct gap semiconductors which
are strongly driven in the THz regime. We calculate the optical sideband
generation due to nonlinear mixing of the THz field and the near infrared
probe. Starting with an inversion symmetric microscopic Hamiltonian we include
the THz field nonperturbatively using non-equilibrium Green function
techniques. We find that a self induced relativistic spin-THz field coupling
locally breaks the inversion symmetry, resulting in the formation of odd
sidebands which otherwise are absent.Comment: 8 pages, 6 figure
Patterns in the Outer Parts of Galactic Disks
This paper describes test particle simulations of the response of the outer
parts of Galactic disks to barring and spiral structure. Simulations are
conducted for cold Mestel disks and warm quasi-exponential disks with
completely flat rotation curves, subjected to pure quadrupoles and logarithmic
spirals. Even though the starting velocity distributions are smooth, the
end-points of the bar simulations show bimodality and multi-peaked structures
at locations near the outer Lindblad resonance (OLR), although spirality can
make this smoother. The growth of a bar may cause the disk isophotes to become
boxy at the OLR, as stars accummulate particularly along the minor axis. The
growth of a bar is also accompanied by substantial heating of the disk stars
near the OLR. For the growth of a bar, the radial
velocity dispersion is typically quadrupled for initially cold disks (initial
\sigmau \kms), and typically doubled for disks with final \sigmau
\kms. Simulations performed of the growth and dissolution of bars
give very similar results, demonstrating that the heat once given to disk stars
is very difficult to remove.Comment: 14 pages, 19 figure
First Principles Study of Zn-Sb Thermoelectrics
We report first principles LDA calculations of the electronic structure and
thermoelectric properties of -ZnSb. The material is found
to be a low carrier density metal with a complex Fermi surface topology and
non-trivial dependence of Hall concentration on band filling. The band
structure is rather covalent, consistent with experimental observations of good
carrier mobility. Calculations of the variation with band filling are used to
extract the doping level (band filling) from the experimental Hall number. At
this band filling, which actually corresponds to 0.1 electrons per 22 atom unit
cell, the calculated thermopower and its temperature dependence are in good
agreement with experiment. The high Seebeck coefficient in a metallic material
is remarkable, and arises in part from the strong energy dependence of the
Fermiology near the experimental band filling. Improved thermoelectric
performance is predicted for lower doping levels which corresponds to higher Zn
concentrations.Comment: 5 pages, 6 figure
The causal structure of dynamical charged black holes
We study the causal structure of dynamical charged black holes, with a
sufficient number of massless fields, using numerical simulations. Neglecting
Hawking radiation, the inner horizon is a null Cauchy horizon and a curvature
singularity due to mass inflation. When we include Hawking radiation, the inner
horizon becomes space-like and is separated from the Cauchy horizon, which is
parallel to the out-going null direction. Since a charged black hole must
eventually transit to a neutral black hole, we studied the neutralization of
the black hole and observed that the inner horizon evolves into a space-like
singularity, generating a Cauchy horizon which is parallel to the in-going null
direction. Since the mass function is finite around the inner horizon, the
inner horizon is regular and penetrable in a general relativistic sense.
However, since the curvature functions become trans-Planckian, we cannot
saymore about the region beyond the inner horizon, and it is natural to say
that there is a 'physical' space-like singularity. However, if we assume an
exponentially large number of massless scalar fields, our results can be
extended beyond the inner horizon. In this case, strong cosmic censorship and
black hole complementarity can be violated.Comment: 23 pages, 23 figure
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