27,175 research outputs found
A search for biogenic trace gases in the atmosphere of Mars
The detection of certain trace gases in the atmosphere of Mars may serve as a possible indicator of microbial life on the surface of Mars. Candidate biogenic gases include methane CH4, ammonia NH3, nitrous oxide N2O, and several reduced sulfur species. Chemical thermodynamic equilibrium and photochemical calculations preclude the presence of these gases in any measurable concentrations in the atmosphere of Mars in the absence of biogenic production. A search for these gases utilizing either high resolution (spectral and spatial) spectroscopy from a Mars orbiter, such as the Observer, and or in situ measurements from a Mars lander or rover, is proposed
Performance of coated columbium and tantalum alloys in plasma arc reentry simulation tests
The evaluation of coated refractory metals screened in stagnation model plasma arc tests is reported. Columbium alloys FS-85, C-129Y, and Cb-752 coated with Si-20Cr-20Fe (R512E) were tested at 1390 C. Three silicide coatings on Ta-10W were tested at 1470 C. Half-hour cycles and a 6500 N/sqm stagnation pressure were used. The best R512E coated columbium alloy was FS-85 with first local coating breakdowns occurring in 12 to 50 cycles. At coating defects, low metal recession rates (0.005 mm/min) were generally observed on coated columbium alloys while high rates (0.15 mm/min) were observed on coated Ta-10W. Coated columbium suffered large emittance losses (to below 0.7) due to surface refractory metal pentoxide formation
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Information, VARs and DSGE Models
How informative is a time series representation of a given vector of observables about the structural shocks and impulse response functions in a DSGE model? In this paper we refer to this econometrician’s problem as “E-invertibility” and consider the corresponding information problem of the agents in the assumed DGP, the DSGE model, which we refer to as “A-invertibility” We consider how the general nature of the agents’ signal extraction problem under imperfect information impacts on the econometrician’s problem of attempting to infer the nature of structural shocks and associated impulse responses from the data. We also examine a weaker condition of recoverability. A general conclusion is that validating a DSGE model by comparing its impulse response functions with those of a data VAR is more problematic when we drop the common assumption in the literature that agents have perfect information as an endowment. We develop measures of approximate fundamentalness for both perfect and imperfect information cases and illustrate our results using analytical and numerical examples
Corrosion resistant thermal barrier coating
A thermal barrier coating system for protecting metal surfaces at high temperature in normally corrosive environments is described. The thermal barrier coating system includes a metal alloy bond coating, the alloy containing nickel, cobalt, iron, or a combination of these metals. The system further includes a corrosion resistant thermal barrier oxide coating containing at least one alkaline earth silicate. The preferred oxides are calcium silicate, barium silicate, magnesium silicate, or combinations of these silicates
A status review of Lewis Research Center supported protection system development
Protective coatings for heat resistant materials for aircraft gas turbine engines, and refractory metals for reentry vehicle
Dovetail: Stronger Anonymity in Next-Generation Internet Routing
Current low-latency anonymity systems use complex overlay networks to conceal
a user's IP address, introducing significant latency and network efficiency
penalties compared to normal Internet usage. Rather than obfuscating network
identity through higher level protocols, we propose a more direct solution: a
routing protocol that allows communication without exposing network identity,
providing a strong foundation for Internet privacy, while allowing identity to
be defined in those higher level protocols where it adds value.
Given current research initiatives advocating "clean slate" Internet designs,
an opportunity exists to design an internetwork layer routing protocol that
decouples identity from network location and thereby simplifies the anonymity
problem. Recently, Hsiao et al. proposed such a protocol (LAP), but it does not
protect the user against a local eavesdropper or an untrusted ISP, which will
not be acceptable for many users. Thus, we propose Dovetail, a next-generation
Internet routing protocol that provides anonymity against an active attacker
located at any single point within the network, including the user's ISP. A
major design challenge is to provide this protection without including an
application-layer proxy in data transmission. We address this challenge in path
construction by using a matchmaker node (an end host) to overlap two path
segments at a dovetail node (a router). The dovetail then trims away part of
the path so that data transmission bypasses the matchmaker. Additional design
features include the choice of many different paths through the network and the
joining of path segments without requiring a trusted third party. We develop a
systematic mechanism to measure the topological anonymity of our designs, and
we demonstrate the privacy and efficiency of our proposal by simulation, using
a model of the complete Internet at the AS-level
Overlapping resonances in the control of intramolecular vibrational redistribution
Coherent control of bound state processes via the interfering overlapping
resonances scenario [Christopher et al., J. Chem. Phys. 123, 064313 (2006)] is
developed to control intramolecular vibrational redistribution (IVR). The
approach is applied to the flow of population between bonds in a model of
chaotic OCS vibrational dynamics, showing the ability to significantly alter
the extent and rate of IVR by varying quantum interference contributions.Comment: 10 pages, 7 figure
Discovery of orbital decay in SMC X-1
The results are reported of three observations of the binary X ray pulsar SMC X-1 with the Ginga satellite. Timing analyses of the 0.71 s X ray pulsations yield Doppler delay curves which, in turn, provide the most accurate determination of the SMC X-1 orbital parameters available to date. The orbital phase of the 3.9 day orbit is determined in May 1987, Aug. 1988, and Aug. 1988 with accuracies of 11, 1, and 3.5 s, respectively. These phases are combined with two previous determinations of the orbital phase to yield the rate of change in the orbital period: P sub orb/P sub orb = (-3.34 + or - 0.023) x 10(exp -6)/yr. An interpretation of this measurement and the known decay rate for the orbit of Cen X-3 is made in the context of tidal evolution. Finally, a discussion is presented of the relation among the stellar evolution, orbital decay, and neutron star spinup time scales for the SMC X-1 system
Rheology and Contact Lifetime Distribution in Dense Granular Flows
We study the rheology and distribution of interparticle contact lifetimes for
gravity-driven, dense granular flows of non-cohesive particles down an inclined
plane using large-scale, three dimensional, granular dynamics simulations.
Rather than observing a large number of long-lived contacts as might be
expected for dense flows, brief binary collisions predominate. In the hard
particle limit, the rheology conforms to Bagnold scaling, where the shear
stress is quadratic in the strain rate. As the particles are made softer,
however, we find significant deviations from Bagnold rheology; the material
flows more like a viscous fluid. We attribute this change in the collective
rheology of the material to subtle changes in the contact lifetime distribution
involving the increasing lifetime and number of the long-lived contacts in the
softer particle systems.Comment: 4 page
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