9,459 research outputs found
Creep-rupture reliability analysis
A probabilistic approach to the correlation and extrapolation of creep-rupture data is presented. Time temperature parameters (TTP) are used to correlate the data, and an analytical expression for the master curve is developed. The expression provides a simple model for the statistical distribution of strength and fits neatly into a probabilistic design format. The analysis focuses on the Larson-Miller and on the Manson-Haferd parameters, but it can be applied to any of the TTP's. A method is developed for evaluating material dependent constants for TTP's. It is shown that optimized constants can provide a significant improvement in the correlation of the data, thereby reducing modelling error. Attempts were made to quantify the performance of the proposed method in predicting long term behavior. Uncertainty in predicting long term behavior from short term tests was derived for several sets of data. Examples are presented which illustrate the theory and demonstrate the application of state of the art reliability methods to the design of components under creep
Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations
We study magneto-elastic oscillations of highly magnetized neutron stars
(magnetars) which have been proposed as an explanation for the quasi-periodic
oscillations (QPOs) appearing in the decaying tail of the giant flares of soft
gamma-ray repeaters (SGRs). We extend previous studies by investigating various
magnetic field configurations, computing the Alfv\'en spectrum in each case and
performing magneto-elastic simulations for a selected number of models. By
identifying the observed frequencies of 28 Hz (SGR 1900+14) and 30 Hz (SGR
1806-20) with the fundamental Alfv\'en QPOs, we estimate the required surface
magnetic field strength. For the magnetic field configurations investigated
(dipole-like poloidal, mixed toroidal-poloidal with a dipole-like poloidal
component and a toroidal field confined to the region of field lines closing
inside the star, and for poloidal fields with an additional quadrupole-like
component) the estimated dipole spin-down magnetic fields are between 8x10^14 G
and 4x10^15 G, in broad agreement with spin-down estimates for the SGR sources
producing giant flares. A number of these models exhibit a rich Alfv\'en
continuum revealing new turning points which can produce QPOs. This allows one
to explain most of the observed QPO frequencies as associated with
magneto-elastic QPOs. In particular, we construct a possible configuration with
two turning points in the spectrum which can explain all observed QPOs of SGR
1900+14. Finally, we find that magnetic field configurations which are entirely
confined in the crust (if the core is assumed to be a type I superconductor)
are not favoured, due to difficulties in explaining the lowest observed QPO
frequencies (f<30 Hz).Comment: 21 pages, 16 figures, 6 tables, matched to version accepted by MNRAS
with extended comparison/discussion to previous wor
Size segregation and convection
The size segregation of granular materials in a vibrating container is
investigated using Molecular Dynamics. We find that the rising of larger
particles is accompanied by the existence of convection cells even in the case
of the lowest possible frequencies. The convection can, however, also be
triggered by the larger particle itself. The possibility of rising through this
mechanism strongly depends on the depth of the larger particle.Comment: 7 pages, 4 figure
Geometry dependence of the clogging transition in tilted hoppers
We report the effect of system geometry on the clogging of granular material
flowing out of flat-bottomed hoppers with variable aperture size D. For such
systems, there exists a critical aperture size Dc at which there is a
divergence in the time for a flow to clog. To better understand the origins of
Dc, we perturb the system by tilting the hopper an angle Q and mapping out a
clogging phase diagram as a function of Q and D. The clogging transition
demarcates the boundary between the freely-flowing (large D, small Q) and
clogging (small D, large Q) regimes. We investigate how the system geometry
affects Dc by mapping out this phase diagram for hoppers with either a circular
hole or a rectangular narrow slit. Additionally, we vary the grain shape,
investigating smooth spheres (glass beads), compact angular grains (beach
sand), disk-like grains (lentils), and rod-like grains (rice). We find that the
value of Dc grows with increasing Q, diverging at pi-Qr where Qr is the angle
of repose. For circular apertures, the shape of the clogging transition is the
same for all grain types. However, this is not the case for the narrow slit
apertures, where the rate of growth of the critical hole size with tilt angle
depends on the material
Speckle visibility spectroscopy and variable granular fluidization
We introduce a dynamic light scattering technique capable of resolving motion
that changes systematically, and rapidly, with time. It is based on the
visibility of a speckle pattern for a given exposure duration. Applying this to
a vibrated layer of glass beads, we measure the granular temperature and its
variation with phase in the oscillation cycle. We observe several transitions
involving jammed states, where the grains are at rest during some portion of
the cycle. We also observe a two-step decay of the temperature on approach to
jamming.Comment: 4 pages, 4 figures, experimen
Measured Laboratory and In-Use Fuel Economy Observed over Targeted Drive Cycles for Comparable Hybrid and Conventional Package Delivery Vehicles
In-use and laboratory-derived fuel economies were analyzed for a medium-duty hybrid electric drivetrain with 'engine off at idle' capability and a conventional drivetrain in a typical commercial package delivery application. Vehicles studied included eleven 2010 Freightliner P100H hybrids in service at a United Parcel Service facility in Minneapolis during the first half of 2010. The hybrids were evaluated for 18 months against eleven 2010 Freightliner P100D diesels at the same facility. Both vehicle groups use the same 2009 Cummins ISB 200-HP engine. In-use fuel economy was evaluated using UPS's fueling and mileage records, periodic ECM image downloads, and J1939 CAN bus recordings during the periods of duty cycle study. Analysis of the in-use fuel economy showed 13%-29% hybrid advantage depending on measurement method, and a delivery route assignment analysis showed 13%-26% hybrid advantage on the less kinetically intense original diesel route assignments and 20%-33% hybrid advantage on the more kinetically intense original hybrid route assignments. Three standardized laboratory drive cycles were selected that encompassed the range of real-world in-use data. The hybrid vehicle demonstrated improvements in ton-mi./gal fuel economy of 39%, 45%, and 21% on the NYC Comp, HTUF Class 4, and CARB HHDDT test cycles, respectively
Metallochaperones Are Needed for Mycobacterium tuberculosis and Escherichia coli Nicotinamidase-Pyrazinamidase Activity.
Mycobacterium tuberculosis nicotinamidase-pyrazinamidase (PZAse) is a metalloenzyme that catalyzes conversion of nicotinamide-pyrazinamide to nicotinic acid-pyrazinoic acid. This study investigated whether a metallochaperone is required for optimal PZAse activity. M. tuberculosis and Escherichia coli PZAses (PZAse-MT and PZAse-EC, respectively) were inactivated by metal depletion (giving PZAse-MT-Apo and PZAse-EC-Apo). Reactivation with the E. coli metallochaperone ZnuA or Rv2059 (the M. tuberculosis analog) was measured. This was repeated following proteolytic and thermal treatment of ZnuA and Rv2059. The CDC1551 M. tuberculosis reference strain had the Rv2059 coding gene knocked out, and PZA susceptibility and the pyrazinoic acid (POA) efflux rate were measured. ZnuA (200 μM) achieved 65% PZAse-EC-Apo reactivation. Rv2059 (1 μM) and ZnuA (1 μM) achieved 69% and 34.3% PZAse-MT-Apo reactivation, respectively. Proteolytic treatment of ZnuA and Rv2059 and application of three (but not one) thermal shocks to ZnuA significantly reduced the capacity to reactivate PZAse-MT-Apo. An M. tuberculosis Rv2059 knockout strain was Wayne positive and susceptible to PZA and did not have a significantly different POA efflux rate than the reference strain, although a trend toward a lower efflux rate was observed after knockout. The metallochaperone Rv2059 restored the activity of metal-depleted PZAse in vitro Although Rv2059 is important in vitro, it seems to have a smaller effect on PZA susceptibility in vivo. It may be important to mechanisms of action and resistance to pyrazinamide in M. tuberculosis Further studies are needed for confirmation.IMPORTANCE Tuberculosis is an infectious disease caused by the bacterium Mycobacterium tuberculosis and remains one of the major causes of disease and death worldwide. Pyrazinamide is a key drug used in the treatment of tuberculosis, yet its mechanism of action is not fully understood, and testing strains of M. tuberculosis for pyrazinamide resistance is not easy with the tools that are presently available. The significance of the present research is that a metallochaperone-like protein may be crucial to pyrazinamide's mechanisms of action and of resistance. This may support the development of improved tools to detect pyrazinamide resistance, which would have significant implications for the clinical management of patients with tuberculosis: drug regimens that are appropriately tailored to the resistance profile of a patient's individual strain lead to better clinical outcomes, reduced onward transmission of infection, and reduction of the development of resistant strains that are more challenging and expensive to treat
Nitrous oxide in fresh water systems: An estimate for the yield of atmospheric N2O associated with disposal of human waste
The N2O content of waters in the Potomac and Merrimack Rivers was measured on a number of occasions over the period April to July 1977. The concentrations of dissolved N2O exceeded those which would apply in equilibrium with air by factors ranging from about 46 in the Potomac to 1.2 in the Merrimack. Highest concentrations of dissolved N2O were associated with sewage discharges from the vicinity of Washington, D. C., and analysis indicates a relatively high yield, 1.3 to 11%, for prompt conversion of waste nitrogen to N2O. Measurements of dissolved N2O in fresh water ponds near Boston demonstrated that aquatic systems provide both strong sources and sinks for atmospheric N2O
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