SEARCH · Engineering Papers

Results for “observation geometry”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17

All sources NASA NTRS DOE OSTI

NASA NTRS1990

Low power arcjet performance

An experimental investigation was performed to evaluate arc jet operation at low power. A standard, 1 kW, constricted arc jet was run using nozzles with three different constrictor diameters. Each nozzle was run over a range of current and mass flow rates to explore stability and performance in the low power engine. A standard pulse-width modulated power processor was modified to accommodate the high operating voltages required under certain conditions. Stable, reliable operation at power levels below 0.5 kW was obtained at efficiencies between 30 and 40 percent. The operating range was found to be somewhat dependent on constrictor geometry at low mass flow rates. Quasi-periodic voltage fluctuations were observed at the low power end of the operating envelope, The nozzle insert geometry was found to have little effect on the performance of the device. The observed performance levels show that specific impulse levels above 350 seconds can be obtained at the 0.5 kW power level.

Curran, Francis M.↗

NASA NTRS1990

Low power arcjet performance

An experimental investigation was performed to evaluate arcjet operation at low power. A standard, 1 kW, constricted arcjet was run using nozzles with three different constrictor diameters. Each nozzle was run over a range of current and mass flow rates to explore stability and performance in the low power regime. A standard pulse-width modulated power processor was modified to accommodate the high operating voltages required under certain conditions. Stable, reliable operation at power levels below 0.5 kW was obtained at efficiencies between 30 and 40 percent. The operating range was found to be somewhat dependent on constrictor geometry at low mass flow rates. Quasi-periodic voltage fluctuations were observed at the low power end of the operating envelope. The nozzle insert geometry was found to have little effect on the performance of the device. The observed performance levels show that specific impulse levels above 350 seconds can be obtained at the 0.5 kW power level.

Curran, Francis M.↗

NASA NTRS2012

Constraining Viewing Geometries of Pulsars with Single-Peaked Gamma-ray Profiles Using a Multiwavelength Approach

Since the launch of the Large Area Telescope (LAT) on board the Fermi spacecraft in June 2008, the number of observed gamma-ray pulsars has increased dramatically. A large number of these are also observed at radio frequencies. Constraints on the viewing geometries of 5 of 6 gamma-ray pulsars exhibiting single-peaked gamma-ray profiles were derived using high-quality radio polarization data [1]. We obtain independent constraints on the viewing geometries of 6 by using a geometric emission code to model the Fermi LAT and radio light curves (LCs). We find fits for the magnetic inclination and observer angles by searching the solution space by eye. Our results are generally consistent with those previously obtained [1], although we do find small differences in some cases. We will indicate how the gamma-ray and radio pulse shapes as well as their relative phase lags lead to constraints in the solution space. Values for the flux correction factor (f(omega)) corresponding to the fits are also derived (with errors).

Seyffert, A. S.↗

NASA NTRS1994

A qualitative and quantitative laser-based computer-aided flow visualization method

The experimental approach presented here offers a nonintrusive, qualitative and quantitative evaluation of full field flow patterns applicable in various geometries in a variety of fluids. This Full Flow Field Tracking (FFFT) Particle Image Velocimetry (PIV) technique, by means of particle tracers illuminated by a laser light sheet, offers an alternative to Laser Doppler Velocimetry (LDV), and intrusive systems such as Hot Wire/Film Anemometry. The method makes obtainable the flow patterns, and allows quantitative determination of the velocities, accelerations, and mass flows of an entire flow field. The method uses a computer based digitizing system attached through an imaging board to a low luminosity camera. A customized optical train allows the system to become a long distance microscope (LDM), allowing magnifications of areas of interest ranging up to 100 times. Presented in addition to the method itself, are studies in which the flow patterns and velocities were observed and evaluated in three distinct geometries, with three different working fluids. The first study involved pressure and flow analysis of a brush seal in oil. The next application involved studying the velocity and flow patterns in a cowl lip cooling passage of an air breathing aircraft engine using water as the working fluid. Finally, the method was extended to a study in air to examine the flows in a staggered pin arrangement located on one side of a branched duct.

Canacci, Victor A.↗

NASA NTRS1984

Comparison of the X-ray observations of the AM Herculis objects to theory

The X-ray observations of the original AM Her objects (AM Her, VV Pup, EF Eri, and AN UMa are reexamined. The work differs from that carried out previously in that the observed X-ray spectral properties are compared to theoretical models calculated using a dipolar accretion geometry rather than a spherically symmetric one, and the observed X-ray light curves are compared to theoretical light curves calculated assuming that the X-ray sources are cataclysmic variable-like binaries which contain a strongly magnetic white dwarf. The added constraints offered by the light curve properties restrict the possible kinds of X-ray emission scenarios in some cases. For instance, it is found that the X-ray observations of AM Her can be self-consistently explained by accretion powered radiative shock models. Further, the observations cannot be self-consistently explained by conduction dominated models as has been suggested by previous workers.

Imamura, J. N.↗

NASA NTRS2017

Remote Sensing of Crystal Shapes in Ice Clouds

Ice crystals in clouds exist in a virtually limitless variation of geometries. The most basic shapes of ice crystals are columnar or plate-like hexagonal prisms with aspect ratios determined by relative humidity and temperature. However, crystals in ice clouds generally display more complex structures owing to aggregation, riming and growth histories through varying temperature and humidity regimes. Crystal shape is relevant for cloud evolution as it affects microphysical properties such as fall speeds and aggregation efficiency. Furthermore, the scattering properties of ice crystals are affected by their general shape, as well as by microscopic features such as surface roughness, impurities and internal structure. To improve the representation of ice clouds in climate models, increased understanding of the global variation of crystal shape and how it relates to, e.g., location, cloud temperature and atmospheric state is crucial. Here, the remote sensing of ice crystal macroscale and microscale structure from airborne and space-based lidar depolarization observations and multi-directional measurements of total and polarized reflectances is reviewed. In addition, a brief overview is given of in situ and laboratory observations of ice crystal shape as well as the optical properties of ice crystals that serve as foundations for the remote sensing approaches. Lidar depolarization is generally found to increase with increasing cloud height and to vary with latitude. Although this variation is generally linked to the variation of ice crystal shape, the interpretation of the depolarization remains largely qualitative and more research is needed before quantitative conclusions about ice shape can be deduced. The angular variation of total and polarized reflectances of ice clouds has been analyzed by numerous studies in order to infer information about ice crystal shapes from them. From these studies it is apparent that pristine crystals with smooth surfaces are generally inconsistent with the data and thus crystal impurity, distortion or surface roughness is prevalent. However, conclusions about the dominating ice shapes are often inconclusive and contradictory and are highly dependent on the limited selection of shapes included in the investigations. Since ice crystal optical properties are mostly determined by the aspect ratios of the crystal components and their microscale structure, it is advised that remote sensing applications focus on the variation of these ice shape characteristics, rather than on the macroscale shape or habit. Recent studies use databases with nearly continuous ranges of crystal component aspect ratio and-or roughness levels to infer the variation of ice crystal shape from satellite and airborne remote sensing measurements. Here, the rationale and results of varying strategies for the remote sensing of ice crystal shape are reviewed. Observed systematic variations of ice crystal geometry with location, cloud height and atmospheric state suggested by the data are discussed. Finally, a prospective is given on the future of the remote sensing of ice cloud particle shapes.

clouds↗

NASA NTRS2020

MODIS Reflective Solar Bands Lunar Calibration Update and Improvements

The MODerate resolution Imaging Spectroradiometer (MODIS) instruments on-board the Terra and Aqua spacecrafts were launched on December 18, 1999 and May 4, 2002, respectively. Each instrument has been scheduled to view the Moon through its Space View (SV) port approximately once per month in order to monitor the long-term radiometric stability of their reflective solar bands (RSB). The lunar calibration has played a key role in tracking the change in the response versus scan angle (RVS) of the instrument’s scan mirror, which is an essential part of the RSB calibration. The lunar irradiance observed by MODIS depends on the view geometry and it is crucial to remove the geometric effects in order to derive accurate RSB calibration coefficients. With a scheduled roll maneuver, the lunar phase angles are kept in a narrow range of 1° for each MODIS instrument such that the impact of these geometric effects is minimized. Nevertheless, it remains a challenge to completely remove the impact of geometric effects in the calibration coefficients derived from the lunar observations, considering the high expectations for the accuracy and quality of the results. In the current MODIS lunar calibration, the geometric effects on the lunar irradiance are corrected by the RObotic Lunar Observatory (ROLO) model. The overall relative uncertainty of the ROLO model for the MODIS calibration has been assessed to be about 1% in the selected lunar phase angle range of 1°. It could be as large as 4% beyond this small phase angle range, especially for the shortest wavelengths. This uncertainty induces noticeable oscillations in the calibration coefficients derived from the lunar observations. We developed a simple lunar model, which is based on MODIS lunar measurements, derived from scheduled lunar observations, as well as those obtained from intrusion of the Moon in the SV, referred to as unscheduled lunar observations, in the time period from 2005 to 2012. Here, the scheduled lunar observations for the entire mission are reprocessed with the new lunar model applied to correct the view geometry effects for each MODIS instrument. New calibration coefficients for the RSBs are calculated. We show that oscillations and noise in the derived lunar calibration coefficients are significantly reduced for both MODIS instruments.

Terra↗

NASA NTRS1994

The effects of correlated noise in phased-array observations of radio sources

Arrays of radio telescopes are now routinely used to provide increased signal-to-noise when observing faint point sources. However, calculation of the achievable sensitivity is complicated if there are sources in the field of view other than the target source. These additional sources not only increase the system temperatures of the individual antennas, but may also contribute significant 'correlated noise' to the effective system temperature of the array. This problem has been of particular interest in the context of tracking spacecraft in the vicinity of radio-bright planets (e.g., Galileo at Jupiter), but it has broader astronomical relevance as well. This paper presents a general formulation of the problem, for the case of a point-like target source in the presence of an additional radio source of arbitrary brightness distribution. We re-derive the well known result that, in the absence of any background sources, a phased array of N indentical antennas is a factor of N more sensitive than a single antenna. We also show that an unphased array of N identical antennas is, on average, no more sensitive than a single antenna if the signals from the individual antennas are combined prior to detection. In the case where a background source is present we show that the effects of correlated noise are highly geometry dependent, and for some astronomical observations may cause significant fluctuations in the array's effective system temperature.

Dewey, Rachel J.↗

NASA NTRS1972

A shock surface geometry: The 15-16 February 1967 event

The flare-associated interplanetary shock observed by Explorer 33 and Pioneer 7 is analyzed to yield an estimation of the ecliptic plane geometry of the shock surface near 1 AU. These spacecraft were separated by 23 deg in the heliocentric longitude and Pioneer 7 was at a distance of 1.12 AU from the sun. Although a data gap occurred at the apparent time of passage of the disturbance at Pioneer 6, which was 85 deg in heliocentric longitude from Pioneer 7 and at 0.83 AU, the recovered data did suggest such a passage. A consistent picture of the shock propagation is given to explain the difference in arrival times at Pioneers 6, 7, and Explorer 33 and the difference of the shock normals observed by Pioneer 7 and Explorer 33. The average shock speed from the sun to each spacecraft and the local speed at Explorer 33 and their relations to the position of the initiating solar flare are obtained and discussed. In the region of space between the earth and Pioneer 7 the shock surface radius of curvature in the ecliptic plane was 0.4 AU or less.

Lepping, R. P.↗

NASA NTRS1977

Simulation model of erosion and deposition on a barchan dune

Erosion and deposition over a barchan dune near the Salton Sea, California, are modeled by bookkeeping the quantity of sand in saltation following streamlines of transport. Field observations of near surface wind velocity and direction plus supplemental measurements of the velocity distribution over a scale model of the dune are combined as input to Bagnold type sand transport formulas corrected for slope effects. A unidirectional wind is assumed. The resulting patterns of erosion and deposition compare closely with those observed in the field and those predicted by the assumption of equilibrium (downwind translation of the dune without change in size or geometry). Discrepancies between the simulated results and the observed or predicted erosional patterns appear to be largely due to natural fluctuations in the wind direction. The shape of barchan dunes is a function of grain size, velocity, degree of saturation of the oncoming flow, and the variability in the direction of the oncoming wind. The size of the barchans may be controlled by natural atmospheric scales, by the age of the dunes, or by the upwind roughness. The upwind roughness can be controlled by fixed elements or by sand in the saltation. In the latter case, dune scale is determined by grain size and wind velocity.

Howard, A. D.↗

NASA NTRS1990

Thermo-oxidative stability studies of PMR-15 polymer matrix composites reinforced with various continuous fibers

An experimental study was conducted to measure the thermooxidative stability of PMR-15 composites reinforced with various fibers and to observe differences in the way they degrade in air. The fibers studied include graphite and the thermally stable Nicalon and Nextel ceramic fibers. Weight-loss rates for the different composites were assessed as a function of mechanical properties, specimen geometry, fiber sizing, and interfacial bond strength. Differences were observed in rates of weight loss, matrix cracking, geometry dependency, and fiber sizing effects. It was shown that Celion 6000 fiber-reinforced composites do not exhibit a straight-line Arrhenius relationship at temperatures above 316 C.

Bowles, Kenneth J.↗

NASA NTRS1990

Thermo-oxidative stability studies of PMR-15 polymer matrix composites reinforced with various fibers

An experimental study was conducted to measure the thermo-oxidative stability of PMR-15 polymer matrix composites reinforced with various fibers and to observe differences in the way they degrade in air. The fibers that were studied included graphite and the thermally stable Nicalon and Nextel ceramic fibers. Weight loss rates for the different composites were assessed as a function of mechanical properties, specimen geometry, fiber sizing, and interfacial bond strength. Differences were observed in rates of weight loss, matrix cracking, geometry dependency, and fiber-sizing effects. It was shown that Celion 6000 fiber-reinforced composites do not exhibit a straight-line Arrhenius relationship at temperatures above 316 C.

Bowles, Kenneth J.↗

NASA NTRS1972

An investigation to improve selenodetic control on the lunar limb utilizing Apollo 15 trans-earth photography

A study is made using actual metric photography taken by Apollo 15 after the trans earth injection when the spacecraft left the lunar orbit. After measurements on 12 frames were reduced for 11 known control and unknown points on the eastern limb, a least squares adjustment program provided the simultaneous solution for the selected points. The results indicate an improvement in selenodetic control may be achieved over a limited portion of the lunar limb; however, the solution could be further improved by strengthening the geometry of intersecting rays through additional observations of the same area using film from the succeeding Apollo missions.

Riotte, W.↗

NASA NTRS1975

Data correlation and analysis of arc tunnel and wind tunnel tests of RSI joints and gaps, phase 2. Volume 1: Technical report

Heat transfer data measured in gaps representative of those being employed for joints in the space shuttle reusable surface insulation (RSI) thermal protection systems (TPS) were assimilated, analyzed, and correlated. Several types of gap were investigated with emphasis on simple butt joints. Gap widths ranged from 0.0 to 0.76 cm and depths ranged from 1 to 6 cm. Laminar, transitional, and turbulent boundary layer flows over the gap opening were investigated. The angle between gap axis and external flow was varied between 0 and pi/2 radians. The contoured cross section gap performed significantly better than all other wide gaps and slightly better than all other narrow gap geometries. Three dimensional heating variations were observed within gaps in the absence of external flow pressure gradients. Interactions between heating within gaps and heating of adjacent top tile surfaces were observed. Gaps aligned with the flow were observed to promote boundary layer transition. Heat transfer correlation equations were obtained for many of the tests. The TPS thickness requirements with and without gaps were computed for a current shuttle entry trajectory. Experimental data employed in the study are summarized. A description of each test facility, run schedule and test conditions, model descriptive information, and heat flux data are included.

Cristensen, H. E.↗

NASA NTRS1980

Plasma collection by high voltage spacecraft at low earth orbit

A computer model of the three-dimensional sheath formation and plasma current collection by high voltage spacecraft has been developed. By using new space charge density and plasma collection algorithms, it is practical to perform calculations for large, complex spacecraft. The model uses NASCAP compatible objects and geometries. Results indicate that ion focusing observed in the laboratory during high voltage collection experiments is probably due to voltage gradients on the collecting surfaces.

Katz, I.↗

NASA NTRS1982

The galaxy as the origin of gamma-ray bursts. II - The effect of an intrinsic burst luminosity distribution on log N/greater than S/ versus log S

The effect of intrinsic burst luminosity distributions on log N-log S has been investigated for halo and disk galactic sources, using a power-law burst luminosity distribution characterized by a shape exponent and a luminosity range. Properly chosen burst luminosity distributions are shown to produce the log N-log S relations sufficiently different from those obtained with monoluminosity bursts to yield halo and thick disk geometries which are compatible with the observations. The soft shape of luminosity distributions capable of altering log N-log S has such observable consequences as (1) gamma-ray burst expected repetitions which first become apparent at lower fluences, and (2) the occurrence of most phenomena at luminosities on the order of about 10 to the 39th ergs, and correspondingly lower fluences.

Jennings, M. C.↗

NASA NTRS1985

Pitch angle distributions of 35-1000 keV protons at quasi-perpendicular interplanetary shocks

The characteristic features of the scatter-free acceleration process near perpendicular shocks are examined in the upstream and downstream pitch angle distributions of 35 to 1000 keV protons. Reasonable quantitative agreement is found between theoretical predictions and observations. The role played by bottle geometries, leading to enhanced acceleration, is highlighted.

Balogh, A.↗

NASA NTRS1986

Gyrating and intermediate ion distributions upstream from the earth's bow shock

Study of ISEE 2 fast plasma data reveals that many suprathermal ion events that in previous studies would have been identified as intermediate ion events are actually gyrating ion events. Of 190 apparently intermediate ion events selected on the basis of their signature in energy-time spectrograms, half are actually found to be gyrating ion events. Using a model bow shock, the shock geometries and spacecraft locations for the observed gyrating and intermediate ion events are compared and found to be quite similar. Both gyrating and intermediate ion events are found to be associated with upstream MHD-like wave activity. Most gyrating ion events are associated with large-amplitude, monochromatic, weakly compressive waves, whereas many intermediate ion events are associated with smaller-amplitude, nonmonochromatic, weakly compressive waves. Some intermediate ion events are found to have no associated waves. Detailed study of the gyrating ion distributions reveals that the distributions are typically gyrotropic within about 4 earth radii of the bow shock, whereas at larger distances (up to about 10 earth radii) upstream from the shock the distributions are frequently 'gyrophase bunched'. In light of these observational characteristics, it is concluded that gyrating ion distributions are most likely generated both through coherent wave disruption of field-aligned beams and through reflection of solar wind ions off the shock.

Fuselier, S. A.↗