16,474 research outputs found
Zinc abundances of planetary nebulae
Zinc is a useful surrogate element for measuring Fe/H as, unlike iron, it is
not depleted in the gas phase media. Zn/H and O/Zn ratios have been derived
using the [Zn IV] emission line at 3.625um for a sample of nine Galactic
planetary nebulae, seven of which are based upon new observations using the
VLT. Based on photoionization models, O/O++ is the most reliable ionisation
correction factor for zinc that can readily be determined from optical emission
lines, with an estimated accuracy of 10% or better for all targets in our
sample. The majority of the sample is found to be sub-solar in [Zn/H]. [O/Zn]
in half of the sample is found to be consistent with Solar within
uncertainties, whereas the remaining half are enhanced in [O/Zn]. [Zn/H] and
[O/Zn] as functions of Galactocentric distance have been investigated and there
is little evidence to support a trend in either case.Comment: Accepted MNRAS, 11 pages, 8 figure
Experimental evaluation of heat transfer on a 1030:1 area ratio rocket nozzle
A 1030:1 carbon steel, heat-sink nozzle was tested. The test conditions included a nominal chamber pressure of 2413 kN/sq m and a mixture ratio range of 2.78 to 5.49. The propellants were gaseous oxygen and gaseous hydrogen. Outer wall temperature measurements were used to calculate the inner wall temperature and the heat flux and heat rate to the nozzle at specified axial locations. The experimental heat fluxes were compared to those predicted by the Two-Dimensional Kinetics (TDK) computer model analysis program. When laminar boundary layer flow was assumed in the analysis, the predicted values were within 15% of the experimental values for the area ratios of 20 to 975. However, when turbulent boundary layer conditions were assumed, the predicted values were approximately 120% higher than the experimental values. A study was performed to determine if the conditions within the nozzle could sustain a laminar boundary layer. Using the flow properties predicted by TDK, the momentum-thickness Reynolds number was calculated, and the point of transition to turbulent flow was predicted. The predicted transition point was within 0.5 inches of the nozzle throat. Calculations of the acceleration parameter were then made to determine if the flow conditions could produce relaminarization of the boundary layer. It was determined that if the boundary layer flow was inclined to transition to turbulent, the acceleration conditions within the nozzle would tend to suppress turbulence and keep the flow laminar-like
Comparison of theoretical and experimental thrust performance of a 1030:1 area ratio rocket nozzle at a chamber pressure of 2413 kN/m2 (350 psia)
The joint Army. Navy, NASA. Air Force (JANNAF) rocket engine peformnace prediction procedure is based on the use of various reference computer programs. One of the reference programs for nozzle analysis is the Two-Dimensional Kinetics (TDK) Program. The purpose of this report is to calibrate the JANNAF procedure incorporated into the December l984 version of the TDK program for the high-area-ratio rocket engine regime. The calibration was accomplished by modeling the performance of a 1030:1 rocket nozzle tested at NASA Lewis Research Center. A detailed description of the experimental test conditions and TDK input parameters is given. The results show that the computer code predicts delivered vacuum specific impulse to within 0.12 to 1.9 percent of the experimental data. Vacuum thrust coefficient predictions were within + or - 1.3 percent of experimental results. Predictions of wall static pressure were within approximately + or - 5 percent of the measured values. An experimental value for inviscid thrust was obtained for the nozzle extension between area ratios of 427.5 and 1030 by using an integration of the measured wall static pressures. Subtracting the measured thrust gain produced by the nozzle between area ratios of 427.5 and 1030 from the inviscid thrust gain yielded experimental drag decrements of 10.85 and 27.00 N (2.44 and 6.07 lb) for mixture ratios of 3.04 and 4.29, respectively. These values correspond to 0.45 and 1.11 percent of the total vacuum thrust. At a mixture ratio of 4.29, the TDK predicted drag decrement was 16.59 N (3.73 lb), or 0.71 percent of the predicted total vacuum thrust
3D Micron-scale Imaging of the Cortical Bone Canal Network in Human Osteogenesis Imperfecta (OI)
Osteogenesis imperfecta (OI) is a genetic disorder leading to increased bone fragility. Recent work has shown that the hierarchical structure of bone plays an important role in determining its mechanical properties and resistance to fracture. The current study represents one of the first attempts to characterize the 3D structure and composition of cortical bone in OI at the micron-scale. A total of 26 pediatric bone fragments from 18 individuals were collected during autopsy (Nc=5) or routing orthopaedic procedures (NOI=13) and imaged by microtomography with a synchrotron light source (SRµCT) for several microstructural parameters including cortical porosity (Ca.V/TV), canal surface to tissue volume (Ca.S/TV), canal diameter (Ca.Dm), canal separation (Ca.Sp), canal connectivity density (Ca.ConnD), and volumetric tissue mineral density (TMD). Results indicated significant differences in all imaging parameters between pediatric controls and OI tissue, with OI bone showing drastically increased cortical porosity, canal diameter, and connectivity. Preliminary mechanical testing revealed a possible link between cortical porosity and strength. Together these results suggest that the pore network in OI contributes greatly to its reduced mechanical properties
Model-Free or Not?
Relaxation in nuclear magnetic resonance is a powerful method for obtaining spatially
resolved, timescale-specific dynamics information about molecular systems. However,
dynamics in biomolecular systems are generally too complex to be fully characterized
based on NMR data alone. This is a familiar problem, addressed by the Lipari-Szabo
model-free analysis, a method that captures the full information content of NMR relaxation
data in case all internal motion of a molecule in solution is sufficiently fast. We investigate
model-free analysis, as well as several other approaches, and find that model-free, spectral
density mapping, LeMaster’s approach, and our detector analysis form a class of analysis
methods, for which behavior of the fitted parameters has a well-defined relationship to the
distribution of correlation times of motion, independent of the specific form of that
distribution. In a sense, they are all “model-free.” Of these methods, only detectors are
generally applicable to solid-state NMR relaxation data. We further discuss how detectors
may be used for comparison of experimental data to data extracted from molecular
dynamics simulation, and how simulation may be used to extract details of the dynamics
that are not accessible via NMR, where detector analysis can be used to connect those
details to experiments. We expect that combined methodology can eventually provide
enough insight into complex dynamics to provide highly accurate models of motion, thus
lending deeper insight into the nature of biomolecular dynamics
Finite Element Assessment of Pediatric Femoral Response to Loading During Ambulation: Normal vs. Osteogenesis Imperfecta (OI) Bone
A disk inside the bipolar planetary nebula M2-9
Bipolarity in proto-planetary and planetary nebulae is associated with events
occurring in or around their cores. Past infrared observations have revealed
the presence of dusty structures around the cores, many in the form of disks.
Characterising those dusty disks provides invaluable constraints on the
physical processes that govern the final mass expulsion of intermediate-mass
stars. We focus this study on the famous M2-9 bipolar nebula, where the moving
lighthouse beam pattern indicates the presence of a wide binary. The compact
and dense dusty core in the center of the nebula can be studied by means of
optical interferometry. M2-9 was observed with VLTI/MIDI at 39-47 m baselines
with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are
interpreted using a dust radiative transfer Monte Carlo code. A disk-like
structure is detected perpendicular to the lobes and a good fit is found with a
stratified disk model composed of amorphous silicates. The disk is compact,
2535 mas at 8, and 3746 mas at 13. For
the adopted distance of 1.2 kpc, the inner rim of the disk is 15 AU. The
mass represents a few percent of the mass found in the lobes. The compactness
of the disk puts strong constraints on the binary content of the system, given
an estimated orbital period 90-120yr. We derive masses of the binary components
between 0.6--1.0M_{\sun} for a white dwarf and 0.6--1.4M_{\sun} for an
evolved star. We present different scenarios on the geometric structure of the
disk accounting for the interactions of the binary system, which includes an
accretion disk as well.Comment: 9 figures, A&A accepte
Some conjectures about integral means of ∂f and ∂¯f
The final version of the paper appears in: "Complex Analysis and Differential Equations" 64 (1999): 91-109. Print.We discuss some conjectural inequalities concerning a problem from the calculus of variations, namely that rank 1 convex functions are quasi-convex. An affirmative answer would also give the best constants for the Beurling-Ahlfors operator that appears in the theory of quasi-conformal mappings on the plane
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