715 research outputs found
A planar reacting shear layer system for the study of fluid dynamics-combustion interaction
A versatile planar reacting shear layer facility is constructed at NASA-Lewis. The research objectives, as well as design, instrumentations and the operational procedures developed for the system are described. The fundamental governing equations and the type of quantitative information that are needed from experiments are described. Additionally, a review of earlier work is presented. Whenever appropriate, comparisons are made with similar systems in other facilities and the main differences are described. Finally, the nonintrusive measurement techniques (PLIF, PMS, LDV, and Schlieren photography) and the type of experiments that are planned are described
Vapor growth of GeTe single crystals in micro-gravity
The positive effects of micro-gravity on crystal growth and fundamental properties of the vapor transport reaction were established by analyzing the results of GeSe and GeTe vapor transport experiments performed on board Skylab. The analysis was based on a direct comparison of GeSe and GeTe crystals and of mass transport rate data obtained on earth and in space. For this purpose, a total of six transport experiments employing different concentrations of transport agent (GeI4) and two temperature gradients were performed during the Skylab 3 and 4 missions. Extensive ground-based studies demonstrated that the crystal morphology and the mass transport rates of the above systems are affected by the transport conditions, in particular by gravity-driven convection. The results demonstrate unambiguously a considerable improvement of the space crystals in terms of surface perfection, crystalline homogeneity and defect density. The observation of greater mass transport rates than expected in micro-gravity environment is of basic scientific and technological significance. This indicates that conventional transport models are incomplete and demonstrates that crystals of improved quality can be grown at reasonable rates by this technique in space. Results are of practical importance for the modification of crystal growth techniques on earth
Nanosecond electro-optical switching with a repetition rate above 20MHz
We describe an electro-optical switch based on a commercial electro-optic
modulator (modified for high-speed operation) and a 340V pulser having a rise
time of 2.2ns (at 250V). It can produce arbitrary pulse patterns with an
average repetition rate beyond 20MHz. It uses a grounded-grid triode driven by
transmitting power transistors. We discuss variations that enable analog
operation, use the step-recovery effect in bipolar transistors, or offer other
combinations of output voltage, size, and cost.Comment: 3 pages, 3 figures. Minor change
Effects of Increased Drought in Amazon Forests Under Climate Change: Separating the Roles of Canopy Responses and Soil Moisture
The Amazon forests are one of the largest ecosystem carbon pools on Earth. Although more frequent and prolonged future droughts have been predicted, the impacts have remained largely uncertain, as most land surface models (LSMs) fail to capture the vegetation drought responses. In this study, the ability of the LSM JSBACH to simulate the drought responses of leaf area index (LAI) and leaf litter production in the Amazon forests is evaluated against artificial drought experiments. Based on the evaluation, improvements are implemented, including a dependency of leaf growth on leaf carbon allocation and a better representation of drought-dependent leaf shedding. The modified JSBACH is shown to capture the drought responses at two sites and across different regions of the basin. It is then coupled with an atmospheric model to simulate the carbon and biogeophysical feedbacks of drought under future climate. We separate the drought impacts into (a) the direct effect, resulting from drier soil and stomatal closure, which does not involve a change in canopy structure, and (b) the LAI effect, resulting from leaf shedding and involving canopy response. We show that the latter accounts for 35% of reduced land carbon uptake (9 ± 10 vs. 26 ± 7 g/m2/yr; mean ± 1 sd) and 12% of surface warming (0.09 ± 0.03 vs. 0.7 ± 0.07 K) during the late 21st century. A north-south dipole of precipitation change is found, which is largely attributable to the direct effect. The results highlight the importance of incorporating drought deciduousness of tropical rainforests in LSMs to better simulate land-atmosphere interactions in the future
Rural vs. Non-rural Differences and Longitudinal Bone Changes by DXA and pQCT in Men Aged 20-66 Years: A Population-Based Study
The purpose of this research was to determine whether there were differences in estimated means and rates of change in BMC, bone area, BMD and measures of bone geometry among men (n=544) from three distinct populations (Hutterite [rural], rural non-Hutterite, non-rural), and whether activity levels or calcium intake explain these population differences. Men were enrolled in the South Dakota Rural Bone Health Study and followed for 7.5 years to estimate means and rates of change in bone mass, density, size and geometry. Femoral neck (FN) and spine measurements were obtained every 18 months by DXA and distal radius (4% and 20%) measurements by pQCT. Activity measurements and calcium intake were obtained quarterly for the first 3 years and at 54, 72, and 90 months. Rural men had greater percent time in moderate plus vigorous activity (mean ± SD: 22 ± 10 vs. 15 ± 8%,
Higher BMC and areal BMD in Children and Grandchildren of Individuals with Hip or Knee Replacement
The relationship between aBMD and osteoarthritis (OA) remains unclear. We compared aBMD, BMC and bone size among children and grandchildren of Hutterites with hip or knee replacement (n=23 each) to children and grandchildren of age- and sex-matched controls (178 children and 267 grandchildren). There were no differences in anthropometric measures or activity levels between case and control probands, but femoral neck (FN) and spine (LS) aBMD and Z-scores were greater in cases than controls (0.89 vs. 0.80 g/cm2; 1.15 vs. 1.03 g/cm2; 1.5 vs. 0.8; 2.4 vs. 1.2: all por =2 years post-menarcheal or males\u3e or =18 years): 33 were not classified. Post-menarcheal, but not premenarcheal, granddaughters of cases had greater hip, FN and LS aBMD Z-scores (0.7 vs. -0.1; 0.6 vs. -0.1; 0.8 vs. -0.3); greater hip and spine aBMD (1.03 vs. 0.95, 1.10 vs. 0.98 g/cm2); greater femoral neck and spine BMC (4.77 vs. 4.21, 66.7 vs. 55.4 g); and greater spine bone area (60.7 vs. 56.6 cm2) compared to granddaughters of controls (all,
Engine Gaseous, Aerosol Precursor and Particulated Flight Altitude Conditions
The overall objective of the NASA Atmospheric Effects of Aviation Project (AEAP) is to develop scientific bases for assessing atmospheric impacts of the exhaust emissions by both current and future fleets of subsonic and supersonic aircraft. Among the six primary elements of the AEAP is Emissions Characterization. The objective of the Emission Characterization effort is to determine the exhaust emission constituents and concentrations at the engine exit plane. The specific objective of this engine test is to obtain a database of gaseous and particulate emissions as a function of fuel sulfur and engine operating conditions. The database of the particulate emission properties is to be used as a comparative baseline with subsequent flight measurement. The engine used in this test was a Pratt & Whitney F1OO-200E turbofan engine. Aviation fuel (Jet A) with a range of fuel sulfur was used. Low and high sulfur values are limited by commercially available fuels and by fuel specification limits of O.3% by weight. Test matrix was set by parametrically varying the combustor inlet temperature (T(sub 3) between idle and maximum power setting at simulated SLS and up to five other altitudes for each fuel. Four diagnostic systems, extractive and non-intrusive, were assembled for the gaseous and particulate emissions characterization measurements study. NASA extractive system includes smoke meter and analyzers for measurement of CO, CO2, NO, NOx, O2, total unburnt hydrocarbons (THC), and SO2. Particulate emissions were characterized by University of Missouri-Rolla Mobile Aerosol Sampling System. A chemical ionization mass spectrometer from the Air Force Research Laboratory at Hanscom AFB was used to measure SO2 and HNO3. Aerodyne Research. Inc. used infrared tunable diode laser absorption to measure SO2, SO3, NO, H2O and CO2
State-Level Immunization Information Systems: Potential for Childhood Immunization Data Linkages.
Objectives Sources of immunization data include state registries or immunization information systems (IIS), medical records, and surveys. Little is known about the quality of these data sources or the feasibility of using IIS data for research. We assessed the feasibility of collecting immunization information for a national children\u27s health study by accessing existing IIS data and comparing the completeness of these data against medical record abstractions (MRA) and parent report. Staff time needed to obtain IIS and MRA data was assessed. Methods We administered a questionnaire to state-level IIS representatives to ascertain availability and completeness of their data for research and gather information about data formats. We evaluated quality of data from IIS, medical records, and reports from parents of 119 National Children\u27s Study participants at three locations. Results IIS data were comparable to MRA data and both were more complete than parental report. Agreement between IIS and MRA data was greater than between parental report and MRA, suggesting IIS and MRA are better sources than parental report. Obtaining IIS data took less staff time than chart review, making IIS data linkage for research a preferred choice. Conclusions IIS survey results indicate data can be obtained by researchers using data linkages. IIS are an accessible and feasible child immunization information source and these registries reduce reliance on parental report or medical record abstraction. Researchers seeking to link IIS data with large multi-site studies should consider acquiring IIS data, but may need strategies to overcome barriers to data completeness and linkage
Reductions in aircraft particulate emissions due to the use of Fischer–Tropsch fuels
The use of alternative fuels for aviation is likely to increase due to
concerns over fuel security, price stability, and the sustainability of fuel
sources. Concurrent reductions in particulate emissions from these
alternative fuels are expected because of changes in fuel composition
including reduced sulfur and aromatic content. The NASA Alternative Aviation
Fuel Experiment (AAFEX) was conducted in January–February 2009 to
investigate the effects of synthetic fuels on gas-phase and particulate
emissions. Standard petroleum JP-8 fuel, pure synthetic fuels produced from
natural gas and coal feedstocks using the Fischer–Tropsch (FT) process, and
50% blends of both fuels were tested in the CFM-56 engines on a DC-8
aircraft. To examine plume chemistry and particle evolution with time,
samples were drawn from inlet probes positioned 1, 30, and 145 m downstream
of the aircraft engines. No significant alteration to engine performance was
measured when burning the alternative fuels. However, leaks in the aircraft
fuel system were detected when operated with the pure FT fuels as a result
of the absence of aromatic compounds in the fuel.
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Dramatic reductions in soot emissions were measured for both the pure FT
fuels (reductions in mass of 86% averaged over all powers) and blended
fuels (66%) relative to the JP-8 baseline with the largest reductions at
idle conditions. At 7% power, this corresponds to a reduction from 7.6 mg kg<sup>−1</sup>
for JP-8 to 1.2 mg kg<sup>−1</sup> for the natural gas FT fuel. At full
power, soot emissions were reduced from 103 to 24 mg kg<sup>−1</sup>
(JP-8 and natural gas FT, respectively). The alternative fuels also produced
smaller soot (e.g., at 85% power, volume mean diameters were reduced from
78 nm for JP-8 to 51 nm for the natural gas FT fuel), which may reduce their
ability to act as cloud condensation nuclei (CCN). The reductions in
particulate emissions are expected for all alternative fuels with similar
reductions in fuel sulfur and aromatic content regardless of the feedstock.
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As the plume cools downwind of the engine, nucleation-mode aerosols form.
For the pure FT fuels, reductions (94% averaged over all powers) in
downwind particle number emissions were similar to those measured at the
exhaust plane (84%). However, the blended fuels had less of a reduction
(reductions of 30–44%) than initially measured (64%). The likely
explanation is that the reduced soot emissions in the blended fuel exhaust
plume results in promotion of new particle formation microphysics, rather
than coating on pre-existing soot particles, which is dominant in the JP-8
exhaust plume. Downwind particle volume emissions were reduced for both the
pure (79 and 86% reductions) and blended FT fuels (36 and 46%) due to
the large reductions in soot emissions. In addition, the alternative fuels
had reduced particulate sulfate production (near zero for FT fuels) due to
decreased fuel sulfur content.
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To study the formation of volatile aerosols (defined as any aerosol formed as
the plume ages) in more detail, tests were performed at varying ambient
temperatures (−4 to 20 °C). At idle, particle number and volume
emissions were reduced linearly with increasing ambient temperature, with
best fit slopes corresponding to −8 × 10<sup>14</sup> particles
(kg fuel)<sup>−1</sup> °C<sup>−1</sup> for particle number emissions and
−10 mm<sup>3</sup> (kg fuel)<sup>−1</sup> °C<sup>−1</sup> for particle volume
emissions. The temperature dependency of aerosol formation can have large
effects on local air quality surrounding airports in cold regions.
Aircraft-produced aerosols in these regions will be much larger than levels
expected based solely on measurements made directly at the engine exit plane.
The majority (90% at idle) of the volatile aerosol mass formed as
nucleation-mode aerosols, with a smaller fraction as a soot coating.
Conversion efficiencies of up to 2.8% were measured for the partitioning
of gas-phase precursors (unburned hydrocarbons and SO<sub>2</sub>) to form volatile
aerosols. Highest conversion efficiencies were measured at 45% power
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