28,278 research outputs found
The Rocketdyne Multifunction Tester. Part 2: Operation of a Radial Magnetic Bearing as an Excitation Source
The operation of the magnetic bearing used as an excitation source in the Rocketdyne Multifunction Tester is described. The tester is scheduled for operation during the summer of 1990. The magnetic bearing can be used in two control modes: (1) open loop mode, in which the magnetic bearing operates as a force actuator; and (2) closed loop mode, in which the magnetic bearing provides shaft support. Either control mode can be used to excite the shaft; however, response of the shaft in the two control modes is different due to the alteration of the eigenvalues by closed loop mode operation. A rotordynamic model is developed to predict the frequency response of the tester due to excitation in either control mode. Closed loop mode excitation is shown to be similar to the excitation produced by a rotating eccentricity in a conventional bearing. Predicted frequency response of the tester in the two control modes is compared, and the maximum response is shown to be the same for the two control modes when synchronous unbalance loading is not considered. The analysis shows that the response of this tester is adequate for the extraction of rotordynamic stiffness, damping, and inertia coefficients over a wide range of test article stiffnesses
The Impact of Late-Career Health and Employment Shocks on Social Security and Other Wealth
About one-quarter of workers age 51 to 55 in 1992 developed health-related work limitations and about one-fifth were laid off from their jobs before age 62. Although late-career health and employment shocks often derail retirement savings plans, Social Security's disability insurance, spouse and survivor benefits, and progressive benefit formula provide important protections. In fact, health shocks increase Social Security's lifetime value, primarily because the system's disability insurance allows some disabled workers to collect benefits before age 62. However, if the system's disability insurance program did not exist, the onset of health-related work limitations would substantially reduce Social Security wealth
Criteria for Core-Collapse Supernova Explosions by the Neutrino Mechanism
We investigate the criteria for successful core-collapse supernova explosions
by the neutrino mechanism. We find that a
critical-luminosity/mass-accretion-rate condition distinguishes non-exploding
from exploding models in hydrodynamic one-dimensional (1D) and two-dimensional
(2D) simulations. We present 95 such simulations that parametrically explore
the dependence on neutrino luminosity, mass accretion rate, resolution, and
dimensionality. While radial oscillations mediate the transition between 1D
accretion (non-exploding) and exploding simulations, the non-radial standing
accretion shock instability characterizes 2D simulations. We find that it is
useful to compare the average dwell time of matter in the gain region with the
corresponding heating timescale, but that tracking the residence time
distribution function of tracer particles better describes the complex flows in
multi-dimensional simulations. Integral quantities such as the net heating
rate, heating efficiency, and mass in the gain region decrease with time in
non-exploding models, but for 2D exploding models, increase before, during, and
after explosion. At the onset of explosion in 2D, the heating efficiency is
2% to 5% and the mass in the gain region is 0.005 M_{\sun}
to 0.01 M_{\sun}. Importantly, we find that the critical luminosity for
explosions in 2D is 70% of the critical luminosity required in 1D. This
result is not sensitive to resolution or whether the 2D computational domain is
a quadrant or the full 180. We suggest that the relaxation of the
explosion condition in going from 1D to 2D (and to, perhaps, 3D) is of a
general character and is not limited by the parametric nature of this study.Comment: 32 pages in emulateapj, including 17 figures, accepted for
publication in ApJ, included changes suggested by the refere
A constraint on a varying proton--electron mass ratio 1.5 billion years after the Big Bang
A molecular hydrogen absorber at a lookback time of 12.4 billion years,
corresponding to 10 of the age of the universe today, is analyzed to put a
constraint on a varying proton--electron mass ratio, . A high resolution
spectrum of the J14432724 quasar, which was observed with the Very Large
Telescope, is used to create an accurate model of 89 Lyman and Werner band
transitions whose relative frequencies are sensitive to , yielding a limit
on the relative deviation from the current laboratory value of
.Comment: Accepted for publication in PRL. Includes supplemental materia
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