510 research outputs found
Active Oxidation of a UHTC-Based CMC
The active oxidation of ceramic matrix composites (CMC) is a severe problem that must be avoided for multi-use hypersonic vehicles. Much work has been performed studying the active oxidation of silicon-based CMCs such as C/SiC and SiC-coated carbon/carbon (C/C). Ultra high temperature ceramics (UTHC) have been proposed as a possible material solution for high-temperature applications on hypersonic vehicles. However, little work has been performed studying the active oxidation of UHTCs. The intent of this paper is to present test data indicating an active oxidation process for a UHTC-based CMC similar to the active oxidation observed with Si-based CMCs. A UHTC-based CMC was tested in the HyMETS arc-jet facility (or plasma wind tunnel, PWT) at NASA Langley Research Center, Hampton, VA. The coupon was tested at a nominal surface temperature of 3000 F (1650 C), with a stagnation pressure of 0.026 atm. A sudden and large increase in surface temperature was noticed with negligible increase in the heat flux, indicative of the onset of active oxidation. It is shown that the surface conditions, both temperature and pressure, fall within the region for a passive to active transition (PAT) of the oxidation
SOL PROBE AND METHOD OF OBTAINING MOISTURE,TEMPERATURE AND ROOT DISTRIBUTION OF A SOIL PROFILE
To provide a profile of simultaneous moisture, temperature and root density characteristics at different depths of soil, a soil probe is inserted in the soil with a viewing and camera section extending above the soil. Similarly, the probe provides a profile of simultaneous moisture, temperature and presence of molds, insects or other foreign matter in grain or similar media including liquid as well as solid granulated material. The portion of the probe that is in the soil or other media includes a source of light or light conveyance and windows which transmit an image of the soil at various depths along the length of the probe through light conductors for further transmission to the viewing and camera section. Liquid crystals sense the temperature of the soil at different locations and light conductors transmit color changes of the liquid crystals to the surface so that a profile may be compiled of moisture, temperature and root patterns by observation of the transmitted images in the viewing section. Viewed and photographic images can be com pared with known color calibration standards to ascertain in situ moisture and temperature conditions of the soil profile
Computational Simulations of the NASA Langley HyMETS Arc-Jet Facility
The Hypersonic Materials Environmental Test System (HyMETS) arc-jet facility located at the NASA Langley Research Center in Hampton, Virginia, is primarily used for the research, development, and evaluation of high-temperature thermal protection systems for hypersonic vehicles and reentry systems. In order to improve testing capabilities and knowledge of the test article environment, an effort is underway to computationally simulate the flow-field using computational fluid dynamics (CFD). A detailed three-dimensional model of the arc-jet nozzle and free-jet portion of the flow-field has been developed and compared to calibration probe Pitot pressure and stagnation-point heat flux for three test conditions at low, medium, and high enthalpy. The CFD model takes into account uniform pressure and non-uniform enthalpy profiles at the nozzle inlet as well as catalytic recombination efficiency effects at the probe surface. Comparing the CFD results and test data indicates an effectively fully-catalytic copper surface on the heat flux probe of about 10% efficiency and a 2-3 kpa pressure drop from the arc heater bore, where the pressure is measured, to the plenum section, prior to the nozzle. With these assumptions, the CFD results are well within the uncertainty of the stagnation pressure and heat flux measurements. The conditions at the nozzle exit were also compared with radial and axial velocimetry. This simulation capability will be used to evaluate various three-dimensional models that are tested in the HyMETS facility. An end-to-end aerothermal and thermal simulation of HyMETS test articles will follow this work to provide a better understanding of the test environment, test results, and to aid in test planning. Additional flow-field diagnostic measurements will also be considered to improve the modeling capability
Fundamental Discreteness Limitations of Cosmological N-Body Clustering Simulations
We explore some of the effects that discreteness and two-body scattering may
have on N-body simulations with ``realistic'' cosmological initial conditions.
We use an identical subset of particles from the initial conditions for a
Particle-Mesh (PM) calculation as the initial conditions for a variety
PM and Tree code runs. We investigate the effect of mass resolution (the
mean interparticle separation) since most ``high resolution'' codes only have
high resolution in gravitational force. The phase-insensitive two--point
statistics, such as the power spectrum (autocorrelation) are somewhat affected
by these variations, but phase-sensitive statistics show greater differences.
Results converge at the mean interparticle separation scale of the lowest
mass-resolution code. As more particles are added, but the force resolution is
held constant, the PM and the Tree runs agree more and more strongly with
each other and with the PM run which had the same initial conditions. This
shows high particle density is necessary for correct time evolution, since many
different results cannot all be correct. However, they do not so converge to a
PM run which continued the fluctuations to small scales. Our results show that
ignoring them is a major source of error on comoving scales of the missing
wavelengths. This can be resolved by putting in a high particle density. Since
the codes never agree well on scales below the mean comoving interparticle
separation, we find little justification for quantitative predictions on this
scale. Some measures vary by 50%, but others can be off by a factor of three or
more. Our results suggest possible problems with the density of galaxy halos,
formation of early generation objects such as QSO absorber clouds, etc.Comment: Revised version to be published in Astrophysical Journal. One figure
changed; expanded discussion, more information on code parameters. Latex, 44
pages, including 19 figures. Higher resolution versions of Figures 10-15
available at: ftp://kusmos.phsx.ukans.edu/preprints/nbod
Demonstrating Discreteness and Collision Error in Cosmological N-body Simulations of Dark Matter Gravitational Clustering
Two-body scattering and other discreteness effects are unimportant in
cosmological gravitational clustering in most scenarios, since the dark matter
has a small particle mass. The collective field should determine evolution:
Two-body scattering in simulations violates the Poisson-Vlasov equations. We
test this in PM, PM, Tree, and NGPM codes, noting that a collisionless code
will preserve the one-dimensional character of plane wave collapse. We find
collisionality vanishing as the softening parameter approaches the mean
interparticle separation. Solutions for the problem are suggested, involving
greater computer power, PM-based nested grid codes, and a more conservative
approach to resolution claims.Comment: Final version accepted for ApJ Letters. Minor revisions, including
due to bug fix in tree code. Uses aasms4.sty. 15 pages. Higher resolution
figures available at ftp://kusmos.phsx.ukans.edu/preprints/discret
A Statewide Screening For Acid Rainfall In Iowa
From April through October 1980, a statewide screening for acid rainfall was conducted in Iowa. Ninety-seven National Weather Service observers were provided with pH color-indicating strips and measured and recorded the pH of every precipitation sample. Results of the screening indicate pH values of rainfall ranged from a low of 4.0 to a high of 7.0. The pH values 5.7 and 5.9 were observed most often and represented 66% of the 4,197 values measured. Approximately 80% of the rainfall pH values fell in the 5.7 to 7.0 range and 20% in the acid rainfall range (5.6 or less). Median pH values calculated for the 97 sampling locations ranged from 5.1 to 6.2, with medians of 5.7 and 5.9 occurring most frequently. With the exception of one 3-county area, most of the 11 sampling locations demonstrating acid rainfall (median values 5.6 or less) were widely separated and probably represented localized problems. Three adjacent counties located in southeast Iowa had low median pH values and this area is recommended for future study
An adaptive hierarchical particle-mesh code with isolated boundary conditions
This article describes a new, fully adaptive Particle-Multiple-Mesh numerical
simulation code developed primarily for simulations of small regions (such as a
group of galaxies) in a cosmological context. It integrates the equations of
motion of a set of particles subject to their mutual gravitational interaction
and to an arbitrary external field. The interactions are computed using a
hierarchy of nested grids constructed anew at each integration step to enhance
the spatial resolution in high-density regions of interest. Significant effort
has gone into supporting isolated boundary conditions at the top grid level.
This makes our method also applicable to non-cosmological problems, at the cost
of some complications which we discuss. We point out the implications of some
differences between our approach and those of other authors of similar codes,
in particular with respect to the handling of the interface between regions of
different spatial resolution. We present a selection of tests performed to
verify the correctness and performance of our implementation. The conclusion
suggests possible further improvements in the areas of independent time steps
and particle softening lengths.Comment: 35 pages, LaTeX, uses aaspp4.sty, 8 figures included ApJ, in press.
Only modest changes from earlier posted versio
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