26,645 research outputs found
Hypersonic airframe structures: Technology needs and flight test requirements
Hypersonic vehicles, that may be produced by the year 2000, were identified. Candidate thermal/structural concepts that merit consideration for these vehicles were described. The current status of analytical methods, materials, manufacturing techniques, and conceptual developments pertaining to these concepts were reviewed. Guidelines establishing meaningful technology goals were defined and twenty-eight specific technology needs were identified. The extent to which these technology needs can be satisfied, using existing capabilities and facilities without the benefit of a hypersonic research aircraft, was assessed. The role that a research aircraft can fill in advancing this technology was discussed and a flight test program was outlined. Research aircraft thermal/structural design philosophy was also discussed. Programs, integrating technology advancements with the projected vehicle needs, were presented. Program options were provided to reflect various scheduling and cost possibilities
On the temperature dependence of correlation functions in the space like direction in hot QCD
We study the temperature dependence of quark antiquark correlations in the
space like direction. In particular, we predict the temperature dependence of
space like Bethe-Salpeter amplitudes using recent Lattice gauge data for the
space like string potential. We also investigate the effect of the space like
string potential on the screening mass and discuss possible corrections which
may arise when working with point sources.Comment: 15 pages 8 figures (not included, will be sent on request),
(SUNY-NTG-94-3
Electron-phonon interaction and antiferromagnetic correlations
We study effects of the Coulomb repulsion on the electron-phonon interaction
(EPI) in a model of cuprates at zero and finite doping. We find that
antiferromagnetic correlations strongly enhance EPI effects on the electron
Green's function with respect to the paramagnetic correlated system, but the
net effect of the Coulomb interaction is a moderate suppression of the EPI.
Doping leads to additional suppression, due to reduced antiferromagnetic
correlations. In contrast, the Coulomb interaction strongly suppresses EPI
effects on phonons, but the suppression weakens with doping.Comment: 4 pages and 5 figure
General approach for studying first-order phase transitions at low temperatures
By combining different ideas, a general and efficient protocol to deal with
discontinuous phase transitions at low temperatures is proposed. For small
's, it is possible to derive a generic analytic expression for appropriate
order parameters, whose coefficients are obtained from simple simulations. Once
in such regimes simulations by standard algorithms are not reliable, an
enhanced tempering method, the parallel tempering -- accurate for small and
intermediate system sizes with rather low computational cost -- is used.
Finally, from finite size analysis, one can obtain the thermodynamic limit. The
procedure is illustrated for four distinct models, demonstrating its power,
e.g., to locate coexistence lines and the phases density at the coexistence.Comment: 5 page
Observations on the Overwintering Potential of the Striped Cucumber Beetle (Coleoptera: Chrysomelidae) in Southern Minnesota
The striped cucumber beetle, Acalymma vittatum (Fabricius) (Coleoptera: Chrysomelidae), is an important pest of cucurbit crops. However, the overwinter- ing capacity of this pest in temperate regions is poorly understood. In this study, the in-field survival of A. vittatum was examined during three consecutive winters. In addition, the supercooling points of A. vittatum were determined as an index of cold hardiness for adults. During each winter, the survival of adults decreased significantly through time, with no individuals surviving until spring. By comparing the supercooling points and in-field survival of adults to soil temperatures, it appears that winter temperatures in Minnesota are cold enough to induce freezing of the beetles. Moreover, a considerable amount of mortality occurred before minimum monthly soil temperatures dropped below the supercooling point of overwintering individuals, suggesting the occurrence of prefreeze mortality. An improved understanding of the response of A. vittatum to winter temperatures in temperate regions may aid in early season management of this pest
PANIC: A Near-infrared Camera for the Magellan Telescopes
PANIC (Persson's Auxiliary Nasmyth Infrared Camera) is a near-infrared camera
designed to operate at any one of the f/11 folded ports of the 6.5m Magellan
telescopes at Las Campanas Observatory, Chile. The instrument is built around a
simple, all-refractive design that reimages the Magellan focal plane to a plate
scale of 0.125'' pixel^{-1} onto a Rockwell 1024x1024 HgCdTe detector. The
design goals for PANIC included excellent image quality to sample the superb
seeing measured with the Magellan telescopes, high throughput, a relatively
short construction time, and low cost. PANIC has now been in regular operation
for over one year and has proved to be highly reliable and produce excellent
images. The best recorded image quality has been ~0.2'' FWHM.Comment: 8 pages, 5 figures. To appear in "Astronomical Telescopes and
Instrumentation," Proc SPIE (Glasgow), June 2004. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/panic.pd
Influence of firing mechanisms on gain modulation
We studied the impact of a dynamical threshold on the f-I curve-the
relationship between the input and the firing rate of a neuron-in the presence
of background synaptic inputs. First, we found that, while the leaky
integrate-and-fire model cannot reproduce the f-I curve of a cortical neuron,
the leaky integrate-and-fire model with dynamical threshold can reproduce it
very well. Second, we found that the dynamical threshold modulates the onset
and the asymptotic behavior of the f-I curve. These results suggest that a
cortical neuron has an adaptation mechanism and that the dynamical threshold
has some significance for the computational properties of a neuron.Comment: 7 pages, 4 figures, conference proceeding
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Polarization control at the microscopic and electronic structure observatory
The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%
Energy efficient engine: Preliminary design and integration studies
Parametric design and mission evaluations of advanced turbofan configurations were conducted for future transport aircraft application. Economics, environmental suitability and fuel efficiency were investigated and compared with goals set by NASA. Of the candidate engines which included mixed- and separate-flow, direct-drive and geared configurations, an advanced mixed-flow direct-drive configuration was selected for further design and evaluation. All goals were judged to have been met except the acoustic goal. Also conducted was a performance risk analysis and a preliminary aerodynamic design of the 10 stage 23:1 pressure ratio compressor used in the study engines
The chemical equilibration volume: measuring the degree of thermalization
We address the issue of the degree of equilibrium achieved in a high energy
heavy-ion collision. Specifically, we explore the consequences of incomplete
strangeness chemical equilibrium. This is achieved over a volume V of the order
of the strangeness correlation length and is assumed to be smaller than the
freeze-out volume. Probability distributions of strange hadrons emanating from
the system are computed for varying sizes of V and simple experimental
observables based on these are proposed. Measurements of such observables may
be used to estimate V and as a result the degree of strangeness chemical
equilibration achieved. This sets a lower bound on the degree of kinetic
equilibrium. We also point out that a determination of two-body correlations or
second moments of the distributions are not sufficient for this estimation.Comment: 16 pages, 15 figures, revtex
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