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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

An analysis of the wear behavior of SiC whisker reinforced alumina from 25 to 1200 C

A model is described for predicting the wear behavior of whisker reinforced ceramics. The model was successfully applied to a silicon carbide whisker reinforced alumina ceramic composite subjected to sliding contact. The model compares the friction forces on the whiskers due to sliding, which act to pull or push them out of the matrix, to the clamping or compressive forces on the whiskers due to the matrix, which act to hold the whiskers in the composite. At low temperatures, the whiskers are held strongly in the matrix and are fractured into pieces during the wear process along with the matrix. At elevated temperatures differential thermal expansion between the whiskers and matrix can cause loosening of the whiskers and lead to pullout during the wear process and to higher wear. The model, which represents the combination of elastic stress analysis and a friction heating analysis, predicts a transition temperature at which the strength of the whiskers equals the clamping force holding them in the matrix. Above the transition the whiskers are pulled out of the matrix during sliding, and below the transition the whiskers are simply fractured. The existence of the transition gives rise to a dual wear mode or mechanism behavior for this material which was observed in laboratory experiments. The results from this model correlate well with experimentally observed behavior indicating that the model may be useful in obtaining a better understanding of material behavior and in making material improvements.

Dellacorte, Christopher↗
NASA NTRS1991

Experimental modelling of lightning interaction phenomena with a free potential conducting objects

Laboratory experiments were conducted to investigate the physical processes of the development of air discharge and its interaction with free potential conducting objects. The space-time development of lightning in gaps was recorded by a motion picture camera and an optoelectronic transducer. The electric field at different points in the gap was measured using a Pockels device both in the leader stage and in the stage of the return stroke. Experimental results of the streamer zone length measurements in the gaps with lengths up to 65 meters are presented. The physical processes occurring during the interaction of positive and negative long sparks with isolated objects were investigated. The striking probability of isolated conducting spheres with different diameters and the dependence of the strike on the location of the gap are investigated.

Chernov, E. N.↗
NASA NTRS1991

Response of vegetation indices to changes in three measures of leaf water stress

The responses of vegetation indices to changes in water stress were evaluated in two separate laboratory experiments. In one experiment the normalized difference vegetation index (NDVI), the near-IR to red ratio (near-IR/red), the Infrared Index (II), and the Moisture Stress Index (MSI) were more highly correlated to leaf water potential in lodgepole pine branches than were the Leaf Water Content Index (LWCI), the mid-IR ratio (Mid-IR), or any of the single Thematic Mapper (TM) bands. In the other experiment, these six indices and the TM Tasseled Cap brightness, greenness, and wetness indices responded to changes in leaf relative water content (RWC) differently than they responded to changes in leaf water content (WC) of three plant species, and the responses were dependent on how experimental replicates were pooled. With no pooling, the LWCI was the most highly correlated index to both RWC and WC among replications, followed by the II, MSI, and wetness. Only the LWCI was highly correlated to RWC and WC when replications were pooled within species. With among species pooling the LWCI was the only index highly correlated with RWC, while the II, MSI, Mid-IR, and wetness were most highly correlated with WC.

Cohen, Warren B.↗
NASA NTRS1989

Linear estimation and control studies of vibration suppression of SCOLE flexible mast

A Kalman filter and a controller are presented for vibration suppression of the Spacecraft Control Laboratory Experiment flexible mast mounted in the cantilevered configuration. Mode shapes and frequencies of the structure obtained from a finite element analysis are used to compute the controller and filter gains. The paper presents results and discussion from simulation and experimental studies. Comparison of experimental results with those obtained by simulation show close agreement.

Ghosh, D.↗
NASA NTRS1990

Ultra-low-frequency magnetic pulsations in the earth's magnetosphere

Spacecraft observations have shown that geomagnetic pulsations originating in magnetospheric processes, in spite of their small amplitude on the ground, have amplitudes in space relative to the local magnetic field of 5-10 percent and occasionally up to about 50 percent. It is noted that by studying geomagnetic pulsations, a detailed comparison can be made between plasma physics theory and observations that are not possible in laboratory experiments. Also geomagnetic pulsations play a role in magnetospheric dynamics and energy transport, and their study forms an integral part of enhancing the knowledge of the magnetosphere. The importance of spacecraft observations are discussed and attention is given to such topics as waves in the magnetosphere, field-line resonances, the quantitative analysis of a dipole field, plasma instabilities, and energy flow.

Anderson, Brian J.↗
NASA NTRS1991

A new class of absorption feature in Io's near-infrared spectrum

A relatively weak IR absorption feature detected at 1200 resolving power in Io at 2.1253 microns does not correspond to any gas- or solid-phase absorption expected on the basis of previously identified Io surface constituents. The source material of the feature appears to be stable and more uniformly distributed in longitude than Io's hot spots. These characteristics imply the feature's participation in a class different from those of other Io absorption spectrum features, thereby potentially serving as a major indicator of Io's atmosphere-surface composition and interactions. Results of laboratory experiments with plausible surface ices are compared with these observations.

Trafton, L. M.↗
NASA NTRS1989

Advanced turboprop aircraft flyover noise annoyance

Results from recent laboratory experiments in which human subjects were exposed to synthetic sounds simulating the flyover noise emitted by advanced turbofan aircraft engines are briefly summarized. The Aircraft Noise Synthesis System described by McCurdy et al. (1987) is used to simulate the noise from (1) a conventional turboprop engine, (2) a jet engine, (3) a single-rotating turbofan engine, and counterrotating turbofans with (4) equal or (5) unequal numbers of blades on the two rotors. The measured annoyance levels are compared with effective perceived noise levels in a graph. For a given noise level, the annoyance levels for (3) are shown to be slightly lower than those for all the other engine types, especially if the tone/broadband noise ratio is relatively high.

Mccurdy, David A.↗
NASA NTRS1991

Supercritical transition in plane channel flow with spatially periodic perturbations

Laboratory experiments and numerical simulations have been conducted for plane channel flow with a streamwise-periodic array of cylinders. The primary transition in this open flow occurs as a convective rather than absolute instability and leads to traveling-wave packets, which advect out of the system. The ordered secondary state is characteristic of closed flows, in contrast with other open flows where the primary transition often leads directly to turbulence.

Schatz, Michael F.↗
NASA NTRS1991

The states of carbon and nitrogen atoms after photodissociation of CN, CH, CH(+), C2, C3, and CO in comets

The photodissociation of carbon compounds by solar UV radiation at a heliocentric distance of 1 AU is examined, comparing published observational data with the predictions of theoretical models and results from laboratory experiments. It is shown that species other than CO, including CN, CH, CH(+), C2, and C3, can contribute to the observed brightness of the VUV lines of C I (156.1, 165.7, and 193.1 nm) and C II (133.5 nm) in comet comae. CN photodissociation is also found to produce metastable 2D0 and 2P0 N I atoms, possibly leading (at heliocentric distances less than 0.25 AU) to 143.9-nm emission via resonance fluorescence.

Singh, P. D.↗
NASA NTRS1991

Tests of stratospheric models - The reactions of atomic chlorine with O3 and CH4 at low temperature

The rate-constant ratio of the photochemical reactions of atomic chlorine with O3 and CH4 was determined using data from laboratory experiments on competitive chlorination of O3/CH4 mixtures at stratospheric temperatures (197-217 K). Two experimental approaches were used: (1) measuring the k1/k2 ratio for the reactions of atomic chlorine with ozone and methane and (2) testing for some of the ClO/CH3O2 chemistry. The chlorine and ozone concentrations were monitored by UV-Vis spectroscopy, and the CH3Cl concentration was measured by FTIR. The results on the k1/k2 ratio are in excellent agreement with the current NASA recommendation (DeMore et al., 1990), being only 12 percent higher. On the other hand, results on the ClO + CH3O2 reaction do not support the rate constant suggested by Simon et al. (1989).

Demore, W. B.↗
NASA NTRS1991

Deep-focus earthquakes and recycling of water into the earth's mantle

For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of the planet.

Meade, Charles↗
NASA NTRS1991

Heterogeneous reactions with NaCl in the El Chichon volcanic aerosols

Previous investigations of the effects of the 1982 eruption of the El Chichon volcano could not explain all the observations of changes in O3, HCl, NO and NO2 simultaneously without proposing unproven chemical reactions. Since reactions between solid NaCl and gaseous ClNO3 and N2O5 rapidly produce photochemically active chlorine species and solid NaNO3 in laboratory experiments, it is suggested that these reactions could have occurred with the NaCl observed to be present in the El Chichon sulfuric acid areosols. As a consequence, it is predicted that HCl should increase substantially, while NO(x) should decrease, in agreement with the measurements after the eruption. Ozone should be only slightly affected by these reactions. Reactions between solid NaCl and the acids H2SO4 and HNO3 might prove to be important, but sufficient evidence regarding their efficiency and the pressure of HNO3 in the aerosols is lacking.

Michelangeli, Diane V.↗
NASA NTRS1991

Detection of solid methanol toward W33A

A recently detected absorption feature at 3.53 microns in the spectrum of W33A has been assigned to methanol (CH3OH). Its optical depth implies that methanol is the second most abundant molecule (7 percent relative to H2O) in the grain mantles in the line of sight toward W33A observed to date. Laboratory experiments have shown that the implied abundance is difficult to explain by UV irradiation of the dust grains alone. Grain surface reactions or condensation directly out of the gas phase must also play roles, but the relative contributions of the various processes are difficult to estimate. The optical depth of the 3.53-micron feature constrains the contribution of methanol to the 6.8-micron feature in W33A to be 10 percent or less, requiring the contribution of at least one other compound to this feature, while the estimated contribution to the 4.9-micron absorption band is even smaller. Only a small contribution of formaldehyde (H2CO) is consistent with the observed 3.53-micron band profile. In contrast to methanol, formaldehyde can be produced readily by photochemical reactions within the ice mantle.

Grim, R. J. A.↗
NASA NTRS1991

The SiH vibrational stretch as an indicator of the chemical state of interstellar grains

The infrared spectra of amorphous silicate grains and films formed in a variety of laboratory experiments have been measured. It is found that the frequency of the SiH fundamental stretch is extremely sensitive to the chemical environment of the grain in which the silicon is bound and varies from 2270/cm in oxidized grains to 2110/cm in a reducing environment. It is proposed that features previously observed in W33A could be due to -SiH groups in amorphous silica, partially reduced silica, silicon nitride or slightly oxidized silicon carbide. It is predicted that observations at 2270/cm will reveal a feature due to -SiH groups in highly oxidized interstelar and circumstellar grains. Such a feature might also appear in the spectra of comets.

Moore, Marla H.↗
NASA NTRS1991

Laboratory studies of the newly discovered infrared band at 4705.2/cm (2.1253 microns) in the spectrum of Io - The tentative identification of CO2

The present evaluation of results from over 120 laboratory experiments relevant to the Trafton et al. (1991) discovery of a 2.1253-micron line in the spectrum of Io demonstrates that this band is produced by neither overtones nor combinations of the fundamental bands of molecules already noted on Io. Since the band's frequency is close to the first overtone of the nu(3) asymmetric stretching mode of CO2, attention is given this molecule's behavior under Io conditions. While pure solid CO2, and CO2 intimately mixed in a matrix of solid SO2 and H2S, generate bands of similar widths, these fall at frequencies lower than the Io band. Attention is given to the possible identification of the Io band with CO2 multimers of 'clusters' on Io.

Sandford, Scott A.↗
NASA NTRS1991

Raman shifting of KrF laser radiation for tropospheric ozone measurements

The differential absorption lidar (DIAL) measurement of tropospheric ozone requires use of high average power UV lasers operating at two appropriate DIAL wavelengths. Laboratory experiments have demonstrated that a KrF excimer laser can be used to generate several wavelengths with good energy conversion efficiencies by stimulated Raman shifting using hydrogen (H2) and deuterium (D2). Computer simulations for an airborne lidar have shown that these laser emissions can be used for the less than 5 percent random error, high resolution measuremment of ozone across the troposphere using the DIAL technique. In the region of strong ozone absorption, laser wavelengths of 277.0 and 291.7 nm were generated using H2 and D2, respectively. In addition, a laser wavelength at 302.0 nm was generated using two cells in series, with the first containing D2 and the second containing H2. The energy conversion efficiency for each wavelength was between 14 and 27 percent.

Grant, William B.↗
NASA NTRS1991

Segmented Ge detector rejection of internal beta activity produced by neutron irradiation

Future Ge spectrometers flown in space to observe cosmic gamma-ray sources will incorporate segmented detectors to reduce the background from radioactivity produced by energetic particle reactions. To demonstrate the effectiveness of a segmented Ge detector in rejecting background events due to the beta decay of internal radioactivity, a laboratory experiment has been carried out in which radioactivity was produced in the detector by neutron irradiation. A Cf-252 source of neutrons was used to produce, by neutron capture on Ge-74 (36.5 percent of natural Ge) in the detector itself, Ge-75 (t sub 1/2 = 82.78 min), which decays by beta emission with a maximum electron kinetic energy of 1188 keV. By requiring that an ionizing event deposit energy in two or more of the five segments of the detector, each about 1-cm thick, the beta particles, which have a range of about 1-mm, are rejected, while most external gamma rays incident on the detector are counted. Analysis of this experiment indicates that over 85 percent of the beta events from the decay of Ge-75 are rejected, which is in good agreement with Monte Carlo calculations.

Varnell, L. S.↗
NASA NTRS1990

Segmented Ge detectors and mechanical coolers for future gamma-ray astronomy instruments

The effectiveness of a segmented Ge detector in rejecting background events due to the beta decay of internal radioactivity is demonstrated by a laboratory experiment in which radioactivity was produced in the detector by neutron irradiation. A Cf-252 source of neutrons was used to produce, by neutron capture on Ge-74 in the detector itself, Ge-75, which decays by beta emission with a maximum energy of 1188 keV. Simultaneous spectra are taken of the activity in the detector under two conditions: a free spectrum in which all events in the detector are accumulated, and a gated spectrum in which events are accumulated only if they deposit energy in two or more segments. A comparison of the spectra shows that over 85 percent of the beta events are rejected, which is in good agreement with predictions.

Varnell, Larry S.↗