5,845 research outputs found
Blueprint for Iteratively Hardening Power Grids Employing Unified Power Flow Controllers
A stable electricity supply is vital for modern society. However, many parts of our power transmission grid are operating near their operational limits. Such stressed systems are vulnerable to cascading failures, where a few small faults can induce a cascade of failures potentially leading to a major blackout The unified power flow controller (UPFC), the most powerful highspeed, semi-conductor based power flow device, can be used as a theoretical model to study how these devices can be used to improve power grid resilience. The blueprint presented here can be used to iteratively identify critical weaknesses in power grids and to recommend a means of fixing these weaknesses via the installation of UPFCs. This approach to hardening the power transmission grid will make it less prone to blackouts and better able to forestall or reduce the severity of unavoidable blackouts
Thermal And Mechanical Response Of Particulate Composite Plates Under Inertial Excitation
The thermal and mechanical, near-resonant responses of particulate composite plates formed from hydroxyl-terminated polybutadiene (HTPB) binder and varying volume ratios of ammonium chloride (NH4Cl) particles (50, 65, 75%) are investigated. Each test specimen is clamped and forced with three levels of band-limited, white noise inertial excitation (10–1000 Hz at 1.00, 1.86 and 2.44 g RMS). The mechanical response of each plate is recorded via scanning laser Doppler vibrometry. The plates are then excited at a single resonant frequency and the thermal response is recorded via infrared thermography. Comparisons are made between the mechanical operational deflection shapes of each plate and spatial temperature distributions, with correlation seen between the observed level of strain, as visualized by strain energy density, and heat generation. The effect of particle/binder ratio on both the thermal and mechanical responses is discussed. Acquired results are also compared to an analytical model of the system. The observed thermomechanical effects render an improved understanding of the thermomechanics of plastic-bonded composites, an essential step in support of the development of new technologies for the vapor-based detection of hidden explosives
Chandra observations of the bursting X-ray transient SAX J1747.0-2853 during low-level accretion activity
We present Chandra/ACIS observations of the bursting X-ray transient SAX
J1747.0-2853 performed on 18 July 2001. We detected a bright source at the
position of R.A = 17^h 47^m 02.60^s and Dec. = -28 52' 58.9'' (J2000.0; with a
1 sigma error of ~0.7 arcseconds), consistent with the BeppoSAX and ASCA
positions of SAX J1747.0-2853 and with the Ariel V position of the transient GX
+0.2,-0.2, which was active during the 1970's. The 0.5-10 keV luminosity of the
source during our observations was ~3 x 10^{35} erg/s (assuming a distance of 9
kpc) demonstrating that the source was in a low-level accretion state. We also
report on the long-term light curve of the source as observed with the all sky
monitor aboard the Rossi X-ray Timing Explorer. After the initial 1998
outburst, two more outbursts (in 2000 and 2001) were detected with peak
luminosities about two orders of magnitude larger than our Chandra luminosity.
Our Chandra observation falls in-between those two outbursts, making the
outburst history for SAX J1747.0-2853 complex. Those bright 2000 and 2001
outbursts combined with the likely extended period of low level activity
in-between those outbursts strongly suggest that the classification of SAX
J1747.0-2853 as a faint X-ray transient was premature. It might be possible
that the other faint X-ray transients also can exhibit bright, extended
outbursts which would eliminate the need for a separate sub-class of X-ray
transients. We discuss our results also in the context of the behavior of X-ray
binaries accreting at low levels with luminosities around 10^{35} erg/s, a
poorly studied accretion rate regime.Comment: Accepte for publication in ApJ, 11 July 200
Exploring the joint association of road traffic noise and air quality with hypertension using QGIS
Improving the validity of activity of daily living dependency risk assessment
OBJECTIVES: Efforts to prevent activity of daily living (ADL) dependency may be improved through models that assess older adults' dependency risk. We evaluated whether cognition and gait speed measures improve the predictive validity of interview-based models.
METHOD: Participants were 8,095 self-respondents in the 2006 Health and Retirement Survey who were aged 65 years or over and independent in five ADLs. Incident ADL dependency was determined from the 2008 interview. Models were developed using random 2/3rd cohorts and validated in the remaining 1/3rd.
RESULTS: Compared to a c-statistic of 0.79 in the best interview model, the model including cognitive measures had c-statistics of 0.82 and 0.80 while the best fitting gait speed model had c-statistics of 0.83 and 0.79 in the development and validation cohorts, respectively.
CONCLUSION: Two relatively brief models, one that requires an in-person assessment and one that does not, had excellent validity for predicting incident ADL dependency but did not significantly improve the predictive validity of the best fitting interview-based models
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