3,056 research outputs found
A Reanalysis of the Hydrodynamic Theory of Fluid, Polar-Ordered Flocks
I reanalyze the hydrodynamic theory of fluid, polar ordered flocks. I find
new linear terms in the hydrodynamic equations which slightly modify the
anisotropy, but not the scaling, of the damping of sound modes. I also find
that the nonlinearities allowed {\it in equilibrium} do not stabilize long
ranged order in spatial dimensions ; in accord with the Mermin-Wagner
theorem. Nonequilibrium nonlinearities {\it do} stabilize long ranged order in
, as argued by earlier work. Some of these were missed by earlier work; it
is unclear whether or not they change the scaling exponents in .Comment: 6 pages, no figures. arXiv admin note: text overlap with
arXiv:0909.195
Integral throat entrance development, qualification and production for the Antares 3 nozzle
Although design analyses of a G-90 graphite integral throat entrance for the Antares 3 solid rocket motor nozzle indicated acceptable margins of safety, the nozzle throat insert suffered a thermostructural failure during the first development firing. Subsequent re-analysis using properties measured on material from the same billet as the nozzle throat insert showed negative margins. Carbon-carbon was investigated and found to result in large positive margins of safety. The G-90 graphite was replaced by SAI fast processed 4-D material which uses Hercules HM 10000 fiber as the reinforcement. Its construction allows powder filling of the interstices after preform fabrication which accelerates the densification process. Allied 15V coal tar pitch is then used to complete densification. The properties were extensively characterized on this material and six nozzles were subjected to demonstration, development and qualification firings
Simulations of the nEDM@SNS light collection system efficiency
A system for collecting the scintillation light produced by the capture
process of ultra-cold neutrons (UCN) on polarized He is discussed and
results from simulations of its performance are presented. This system will be
implemented in nEDM@SNS, the experiment searching for the neutron electric
dipole moment (nEDM) at the Spallation Neutron Source (SNS) at Oak Ridge
National Laboratory. Simulation results show that the light collection system
collects on average 17 photoelectrons per UCN-He capture event
(sufficient to generate a robust signal), reconstructs the event location in
the beam direction to approximately 3 cm accuracy, detects capture events with
a high and spatially uniform efficiency (0.95 with 1% variation), and rejects
greater than 50% of beta decay background events.Comment: 14 pages, 13 figures. Submitted to Journal of Instrumentatio
The benefits of using a walking interface to navigate virtual environments
Navigation is the most common interactive task performed in three-dimensional virtual environments (VEs), but it is also a task that users often find difficult. We investigated how body-based information about the translational and rotational components of movement helped participants to perform a navigational search task (finding targets hidden inside boxes in a room-sized space). When participants physically walked around the VE while viewing it on a head-mounted display (HMD), they then performed 90% of trials perfectly, comparable to participants who had performed an equivalent task in the real world during a previous study. By contrast, participants performed less than 50% of trials perfectly if they used a tethered HMD (move by physically turning but pressing a button to translate) or a desktop display (no body-based information). This is the most complex navigational task in which a real-world level of performance has been achieved in a VE. Behavioral data indicates that both translational and rotational body-based information are required to accurately update one's position during navigation, and participants who walked tended to avoid obstacles, even though collision detection was not implemented and feedback not provided. A walking interface would bring immediate benefits to a number of VE applications
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The design of a distributed computing system
This paper describes a design for a computing system structure whose implementation should provide reliable, faiI-soft service at relativeIy Iow cost. The design reIies heavily on modularity and dynamic reconfigurability (in combination referred to as distribution) to achieve reliability. Low cost is maintainted by using standard hardware and software components, each of which handles a portion of the workload. When a component fails, its workload is assumed by the remaining components. The evolution of a particular network architecture for this computing system, i.e., a network utilizing a communication loop with hardware components connected to and distributed about the ring, is described
Increased HCO production in the outer disk around HD 163296
Three formaldehyde lines were observed (HCO 3--2, HCO
3--2, and HCO 3--2) in the protoplanetary disk
around the Herbig Ae star HD 163296 with ALMA at 0.5 arcsecond (60 AU) spatial
resolution. HCO 3--2 was readily detected via imaging, while
the weaker HCO 3--2 and HCO 3--2 lines
required matched filter analysis to detect. HCO is present throughout most
of the gaseous disk, extending out to 550 AU. An apparent 50 AU inner radius of
the HCO emission is likely caused by an optically thick dust continuum. The
HCO radial intensity profile shows a peak at 100 AU and a secondary bump at
around 300 AU, suggesting increased production in the outer disk. Different
parameterizations of the HCO abundance were compared to the observed
visibilities with minimization, using either a characteristic
temperature, a characteristic radius or a radial power law index to describe
the HCO chemistry. Similar models were applied to ALMA Science Verification
data of CO. In all modeling scenarios, fits to the HCO data show an
increased abundance in the outer disk. The overall best-fit HCO model shows
a factor of two enhancement beyond a radius of 27020 AU, with an inner
abundance of . The HCO emitting region has a lower
limit on the kinetic temperature of K. The CO modeling suggests
an order of magnitude depletion in the outer disk and an abundance of in the inner disk. The increase in HCO outer disk emission
could be a result of hydrogenation of CO ices on dust grains that are then
sublimated via thermal desorption or UV photodesorption, or more efficient
gas-phase production beyond about 300 AU if CO is photodisocciated in this
region
Wide Angle Decentered Lens Beam Steering for Infrared Countermeasures Applications
A beam-steering system consisting of three cemented achromatic doublets is presented. Intended for use in IR countermeasure applications, our system is designed to operate over the 2- to 5-μm spectrum with minimum angular dispersion. We show that dispersion can be minimized by using doublet lenses fashioned from AMTIR-1 and germanium. Our system is designed to be compact and lightweight, with no internal foci, while allowing steering to ±22.5 deg. We also maintain a minimum 2-in. clear aperture for all steering angles, and a nominal divergence of 1 mrad. Plane wave and Gaussian beam analyses of our system are presented
Uniformity of V minus Near Infrared Color Evolution of Type Ia Supernovae, and Implications for Host Galaxy Extinction Determination
From an analysis of SNe 1972E, 1980N, 1981B, 1981D, 1983R, 1998bu, 1999cl,
and 1999cp we find that the intrinsic V-K colors of Type Ia SNe with
multi-color light curve shape (MLCS) parameter -0.4 < Delta < +0.2 suggest a
uniform color curve. V-K colors become bluer linearly with time from roughly
one week before B-band maximum until one week after maximum, after which they
redden linearly until four weeks after maximum. V-H colors exhibit very similar
color evolution. V-J colors exhibit slightly more complex evolution, with
greater scatter. The existence of V minus near infrared color relations allows
the construction of near infrared light curve templates that are an improvement
on those of Elias et al. (1985).
We provide optical BVRI and infrared JHK photometry of the Type Ia supernovae
1999aa, 1999cl, and 1999cp. SN 1999aa is an overluminous "slow decliner" (with
Delta = -0.47 mag). SN 1999cp is a moderately bright SN unreddened in its host.
SN 1999cl is extremely reddened in its host. The V minus near infrared colors
of SN 1999cl yield A_V = 2.01 +/- 0.11 mag. This leads to a distance for its
host galaxy (M 88) in agreement with other distance measurements for members of
the Virgo cluster.Comment: 57 pages, 13 postscript figures, to appear in the August 20, 2000,
issue of the Astrophysical Journal. Contains updated references and a number
of minor corrections dealt with when page proofs were correcte
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Vanadium alloys for structural applications in fusion systems: A review of vanadium alloy mechanical and physical properties
The current knowledge is reviewed on (1) the effects of neutron irradiation on tensile strength and ductility, ductile-brittle transition temperature, creep, fatigue, and swelling of vanadium-base alloys, (2) the compatibility of vanadium-base alloys with liquid lithium, water, and helium environments, and (3) the effects of hydrogen and helium on the physical and mechanical properties of vanadium alloys that are potential candidates for structural materials applications in fusion systems. Also, physical and mechanical properties issues are identified that have not been adequately investigated in order to qualify a vanadium-base alloy for the structural material in experimental fusion devices and/or in fusion reactors
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Vanadium alloys for structural applications in fusion systems: A review of vanadium alloy mechanical and physical properties
The current knowledge is reviewed on (1) the effects of neutron irradiation on tensile strength and ductility, ductile-brittle transition temperature, creep, fatigue, and swelling of vanadium-base alloys, (2) the compatibility of vanadium-base alloys with liquid lithium, water, and helium environments, and (3) the effects of hydrogen and helium on the physical and mechanical properties of vanadium alloys that are potential candidates for structural materials applications in fusion systems. Also, physical and mechanical properties issues are identified that have not been adequately investigated in order to qualify a vanadium-base alloy for the structural material in experimental fusion devices and/or in fusion reactors
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