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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.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Numerical analysis of natural convection in liquid droplets by phase change

A numerical analysis is performed on thermocapillary buoyancy convection induced by phase change in a liquid droplet. A finite-difference code is developed using an alternating-direction implicit (ADI) scheme. The intercoupling relation between thermocapillary force, buoyancy force, fluid property, heat transfer, and phase change, along with their effects on the induced flow patterns, are disclosed. The flow is classified into three types: thermocapillary, buoyancy, and combined convection. Among the three mechanisms, the combined convection simulates the experimental observations quite well, and the basic mechanism of the observed convection inside evaporating sessile drops is thus identified. It is disclosed that evaporation initiates unstable convection, while condensation always brings about a stable density distribution which eventually damps out all fluid disturbances. Another numerical model is presented to study the effect of boundary recession due to evaporation, and the 'peeling-off' effect (the removal of the surface layer of fluid by evaporation) is shown to be relevant.

Duh, J. C.↗
NASA NTRS1991

Active control of sound radiation from a vibrating rectangular panel by sound sources and vibration inputs - An experimental comparison

Active control of sound radiation from a rectangular panel by two different methods has been experimentally studied and compared. In the first method a single control force applied directly to the structure is used with a single error microphone located in the radiated acoustic field. Global attenuation of radiated sound was observed to occur by two main mechanisms. For 'on-resonance' excitation, the control force had the effect of increasing the total panel input impedance presented to the nosie source, thus reducing all radiated sound. For 'off-resonance' excitation, the control force tends not significantly to modify the panel total response amplitude but rather to restructure the relative phases of the modes leading to a more complex vibration pattern and a decrease in radiation efficiency. For acoustic control, the second method, the number of acoustic sources required for global reduction was seen to increase with panel modal order. The mechanism in this case was that the acoustic sources tended to create an inverse pressure distribution at the panel surface and thus 'unload' the panel by reducing the panel radiation impedance. In general, control by structural inputs appears more effective than control by acoustic sources for structurally radiated noise.

Fuller, C. R.↗
NASA NTRS1992

Transmission Of Power Via Combined Laser Beams

Laser Diode Array (LDA) appears to be most efficient means of transferring power from Earth to satellites and between satellites, in terms of mass and size, of various laser configurations. To form large-scale-array amplifier (LSAA), element LDA's must generate well-defined diffraction-limited beams. Coherent matching of phases among LDA's enables system to generate good beam pattern in far field over thousands of kilometers. By passing beam from master laser through number of LDA amplifiers simultaneously, one realizes coherence among amplified output beams. LSAA used for transmission of power with efficiency of approximately 80 percent into receiver of moderate size at 5,000 km. Also transmits data at high rates by line-of-sight rather than fiber optics.

Kwon, Jin H.↗
NASA NTRS1992

Propagating confined states in phase dynamics

Theoretical treatment is given to the possibility of the existence of propagating confined states in the nonlinear phase equation by generalizing stationary confined states. The nonlinear phase equation is set forth for the case of propagating patterns with long wavelengths and low-frequency modulation. A large range of parameter values is shown to exist for propagating confined states which have spatially localized regions which travel on a background with unique wavelengths. The theoretical phenomena are shown to correspond to such physical systems as spirals in Taylor instabilities, traveling waves in convective systems, and slot-convection phenomena for binary fluid mixtures.

Brand, Helmut R.↗
NASA NTRS1989

Response of a mouse hybridoma cell line to heat shock, agitation, and sparging

A mouse hybridoma cell line is used as a model system for studying the effect of environmental stress on attachment-independent mammalian cells. The full time course of recovery for a mouse hybridoma cell line from both a mild and intermediate heat shock is examined. The pattern of intracellular synthesis is compared for actively growing, log phase cells and nondividing, stationary phase cells.

Passini, Cheryl A.↗
NASA NTRS2002

Karyotypes of Human Lymphocytes Exposed to High-Energy Iron Ions

Chromosomal aberrations were analyzed using multicolor fluorescence in situ hybridization (mFISH) in human peripheral blood lymphocytes after in vitro exposure to gamma rays or accelerated (56)Fe ions (1 GeV/nucleon, 145 keV/microm) at Brookhaven National Laboratory (Upton, NY). Doses of 0.3 and 3 Gy were used for both radiation types. Chromosomes were prematurely condensed by a phosphatase inhibitor (calyculin A) to avoid the population selection bias observed at metaphase as a result of the severe cell cycle delays induced by heavy ions. A total of 1053 karyotypes (G(2) and M phases) were analyzed in irradiated lymphocytes. Results revealed different distribution patterns for chromosomal aberrations after low- and high-LET radiation exposures: Heavy ions induced a much higher fraction of cells with multiple aberrations, while the majority of the aberrant cells induced by low doses of gamma rays contained a single aberration. The high fraction of complex-type exchanges after heavy ions leads to an overestimation of simple-type asymmetrical interchanges (dicentrics) from analysis of Giemsa-stained samples. However, even after a dose of 3 Gy iron ions, about 30% of the cells presented no complex-type exchanges. The involvement of individual chromosomes in exchanges was similar for densely and sparsely ionizing radiation, and no statistically significant evidence of a nonrandom involvement of specific chromosomes was detected.

NASA Center JSC↗
NASA NTRS2000

Metabolic activity of permafrost bacteria below the freezing point

Metabolic activity was measured in the laboratory at temperatures between 5 and -20 degrees C on the basis of incorporation of (14)C-labeled acetate into lipids by samples of a natural population of bacteria from Siberian permafrost (permanently frozen soil). Incorporation followed a sigmoidal pattern similar to growth curves. At all temperatures, the log phase was followed, within 200 to 350 days, by a stationary phase, which was monitored until the 550th day of activity. The minimum doubling times ranged from 1 day (5 degrees C) to 20 days (-10 degrees C) to ca. 160 days (-20 degrees C). The curves reached the stationary phase at different levels, depending on the incubation temperature. We suggest that the stationary phase, which is generally considered to be reached when the availability of nutrients becomes limiting, was brought on under our conditions by the formation of diffusion barriers in the thin layers of unfrozen water known to be present in permafrost soils, the thickness of which depends on temperature.

NASA Center ARC↗
NASA NTRS2009

Large-Aperture Membrane Active Phased-Array Antennas

Large-aperture phased-array microwave antennas supported by membranes are being developed for use in spaceborne interferometric synthetic aperture radar systems. There may also be terrestrial uses for such antennas supported on stationary membranes, large balloons, and blimps. These antennas are expected to have areal mass densities of about 2 kg/sq m, satisfying a need for lightweight alternatives to conventional rigid phased-array antennas, which have typical areal mass densities between 8 and 15 kg/sq m. The differences in areal mass densities translate to substantial differences in total mass in contemplated applications involving aperture areas as large as 400 sq m. A membrane phased-array antenna includes patch antenna elements in a repeating pattern. All previously reported membrane antennas were passive antennas; this is the first active membrane antenna that includes transmitting/receiving (T/R) electronic circuits as integral parts. Other integral parts of the antenna include a network of radio-frequency (RF) feed lines (more specifically, a corporate feed network) and of bias and control lines, all in the form of flexible copper strip conductors on flexible polymeric membranes. Each unit cell of a prototype antenna (see Figure 1) contains a patch antenna element and a compact T/R module that is compatible with flexible membrane circuitry. There are two membrane layers separated by a 12.7-mm air gap. Each membrane layer is made from a commercially available flexible circuit material that, as supplied, comprises a 127-micron-thick polyimide dielectric layer clad on both sides with 17.5-micron-thick copper layers. The copper layers are patterned into RF, bias, and control conductors. The T/R module is located on the back side of the ground plane and is RF-coupled to the patch element via a slot. The T/R module is a hybrid multilayer module assembled and packaged independently and attached to the membrane array. At the time of reporting the information for this article, an 8 16 passive array (not including T/R modules) and a 2 4 active array (including T/R modules) had been demonstrated, and it was planned to fabricate and test larger arrays.

Karasik, Boris↗
NASA NTRS2007

Noise-Canceling Helmet Audio System

A prototype helmet audio system has been developed to improve voice communication for the wearer in a noisy environment. The system was originally intended to be used in a space suit, wherein noise generated by airflow of the spacesuit life-support system can make it difficult for remote listeners to understand the astronaut s speech and can interfere with the astronaut s attempt to issue vocal commands to a voice-controlled robot. The system could be adapted to terrestrial use in helmets of protective suits that are typically worn in noisy settings: examples include biohazard, fire, rescue, and diving suits. The system (see figure) includes an array of microphones and small loudspeakers mounted at fixed positions in a helmet, amplifiers and signal-routing circuitry, and a commercial digital signal processor (DSP). Notwithstanding the fixed positions of the microphones and loudspeakers, the system can accommodate itself to any normal motion of the wearer s head within the helmet. The system operates in conjunction with a radio transceiver. An audio signal arriving via the transceiver intended to be heard by the wearer is adjusted in volume and otherwise conditioned and sent to the loudspeakers. The wearer s speech is collected by the microphones, the outputs of which are logically combined (phased) so as to form a microphone- array directional sensitivity pattern that discriminates in favor of sounds coming from vicinity of the wearer s mouth and against sounds coming from elsewhere. In the DSP, digitized samples of the microphone outputs are processed to filter out airflow noise and to eliminate feedback from the loudspeakers to the microphones. The resulting conditioned version of the wearer s speech signal is sent to the transceiver.

Seibert, Marc A.↗
NASA NTRS2012

Gamma-Ray Light Curves from Pulsar Magnetospheres with Finite Conductivity

The Fermi Large Area Telescope has provided an unprecedented database for pulsar emission studies that includes gamma-ray light curves for over 100 pulsars. Modeling these light curves can reveal and constrain the geometry of the particle accelerator, as well as the pulsar magnetic field structure. We have constructed 3D magnetosphere models with finite conductivity, that bridge the extreme vacuum and force-free solutions used in previous light curves modeling. We are investigating the shapes of pulsar gamma-ray light curves using these dissipative solutions with two different approaches: (l) assuming geometric emission patterns of the slot gap and outer gap, and (2) using the parallel electric field provided by the resistive models to compute the trajectories and . emission of the radiating particles. The light curves using geometric emission patterns show a systematic increase in gamma-ray peak phase with increasing conductivity, introducing a new diagnostic of these solutions. The light curves using the model electric fields are very sensitive to the conductivity but do not resemble the observed Fermi light curves, suggesting that some screening of the parallel electric field, by pair cascades not included in the models, is necessary

Harding, A. K.↗
NASA NTRS2011

A Hybrid Lyot Coronagraph for the Direct Imaging and Spectroscopy of Exoplanet Systems: Recent Results and Prospects

We report our best laboratory contrast demonstrations achieved to date. We review the design, fabrication, performance, and future prospects of a hybrid focal plane occulter for exoplanet coronagraphy. Composed of thickness-profiled metallic and dielectric thin films vacuum deposited on a fused silica substrate, the hybrid occulter uses two superimposed thin films for control over both the real and imaginary parts of the complex attenuation pattern. Together with a deformable mirror for adjustment of wavefront phase, the hybrid Lyot coronagraph potentially exceeds billion-to one contrast over dark fields extending to within angular separations of 3 lambda/D (3 x the cosmological constant / diameter of the telescope) from the central star, over spectral bandwidths of 20 percent or more, and with throughput efficiencies up to 60 percent. We report laboratory contrasts of 3 x 10 (sup -10) degrees over 2 percent bandwidths, 6 x 10 (sup -10) degrees over 10 percent bandwidths, and 2 x 10 (sup -9) over 20 percent bandwidths, achieved across high contrast fields extending from an inner working angle of 3 lambda/D to a radius of 15 lambda/D. Occulter performance is analyzed in light of recent experiments and optical models, and prospects for further improvements are summarized. The science capabilities of the hybrid Lyot coronagraph are compared with requirements of the ACCESS mission, a representative exoplanet space telescope concept study for the direct imaging and spectroscopy of exoplanet systems. This work has been supported by NASA's Technology Demonstration for Exoplanet Missions (TDEM) program.

coronagraph↗
NASA NTRS2007

The ENSO Effect on the Temporal and Spatial Distribution of Global Lightning Activity

The recently reprocessed (1997-2006) OTD/LIS database is used to investigate the global lightning climatology in response to the ENSO cycle. A linear correlation map between lightning anomalies and ENSO (NINO3.4) identifies areas that generally follow patterns similar to precipitation anomalies. We also observed areas where significant lightning/ENSO correlations are found and are not accompanied of significant precipitation/ENSO correlations. An extreme case of the strong decoupling between lightning and precipitation is observed over the Indonesian peninsula (Sumatra) where positive lightning/NINO3.4 correlations are collocated with negative precipitation/NINO3.4 correlations. Evidence of linear relationships between the spatial extent of thunderstorm distribution and the respective NINO3.4 magnitude are presented for different regions on the Earth. Strong coupling is found over areas remote to the main ENSO axis of influence and both during warm and cold ENSO phases. Most of the resulted relationships agree with the tendencies of precipitation related to ENSO empirical maps or documented teleconnection patterns. Over the Australian continent, opposite behavior in terms of thunderstorm activity is noted for warm ENSO phases with NINO3.4 magnitudes with NINO3.4>+l.08 and 0<NqNO3.4<I.08. Finally, we investigate the spatial distribution of areas that consistently portrayed enhanced lightning activity during the main warm/cold (El Nino/La Nina) ENSO episodes of the past decade. The observed patterns show no spatial overlapping and identify areas that in their majority are in agreement with empirical precipitation/ENSO maps. The areas that appear during the warm ENSO phase are found over regions that have been identified as anomalous Hadley circulation ENSO-related patterns. The areas that appear during the cold ENSO phase are found predominantly around the west hemisphere equatorial belt and are in their majority identified by anomalous Walker circulation.

Chronis, Themis G.↗
NASA NTRS1988

Phase-Locked Semiconductor Lasers With Separate Contacts

Individual current feeds enable better uniformity and flexible control. Separate contacts for lasers in array enable control of output radiation pattern and compensation of manufacturing nonuniformities among lasers. Concept of separate current control described for two-laser array in "Semiconductor Laser Phased Array" (NPO-15963).

Katz, Joseph↗
NASA NTRS1993

Multiple flow regimes in the Northern Hemisphere winter. I - Methodology and hemispheric regimes. II - Sectorial regimes and preferred transitions

Multivariate pdfs in the phase space of large-scale atmospheric motions are examined in order to identify recurrent and persistent flow patterns. The extent to which intraseasonal variablity can be described and understood in terms of multiple flow regimes is clarified. It is shown that the synoptically intriguing and statistically significant inhomogeneities exhibited by the 2D phase plane on the periphery of the distribution are due to the existence of persistent and recurrent anomaly patterns. It is argued that these inhomgeneities are obscured when PDFs are examined in a smaller-dimensional subspace than dynamically desired. A synoptic characterization of onsets and breaks for the flow regimes is given by compositing. In situ evolutions of anomaly patterns, slow westward shifts of high-latitude anomaly centers, and successive downstream increase of anomaly magnitudes are the typical signatures of such events.

Kimoto, Masahide↗
NASA NTRS1993

Topographic mapping using a monopulse SAR system

Terrain height variations in mountainous areas cause two problems in the radiometric correction of SAR images: the first being that the wrong elevation angle may be used in correcting for the radiometric variation of the antenna pattern; the second that the local incidence angle used in correcting the projection of the pixel area from slant range to ground range coordinates may vary from that given by the flat earth assumption. We propose a novel design of a SAR system which exploits the monopulse principle to determine the elevation angle and thus the height at the different parts of the image. The key element of such a phase monopulse system is an antenna, which can be divided into a lower and upper half in elevation using a monopulse comparator. In addition to the usual sum pattern, the elevation difference pattern can be generated by a -pi phase shift on one half of the antenna. From the ratios of images radiometrically modulated by the difference and sum antenna pattern in cross-track direction, we can derive the appropriate elevation angle at any point in the image. Together with the slant range we can calculate the height of the platform above this point using information on the antenna pointing and the platform attitude. This operation, repeated at many locations throughout the image, allows us to build up a topographic map of the height of the aircraft above each location. Inversion of this map, using the precisely determined aircraft altitude and the accurate flight path, leads to the actual topography of the imaged surface. The precise elevation of one point in the image could also be used to convert the height map to a topographic map. In this paper, we present design considerations for a corresponding airborne SAR system in X-Band and give estimates of the error due to system noise and azimuth ambiguities as well as the expected performance and precision in topographic mapping.

Zink, M.↗
NASA NTRS1994

Multiscale low-frequency circulation modes in the global atmosphere

In this paper, fundamental multiscale circulation modes in the global atmosphere are identified with the objective of providing better understanding of atmospheric low-frequency variabilities over a wide range of spatial and temporal scales. With the use of a combination of rotated principal component technique, singular spectrum analysis, and phase space portraits, three categories of basic multiscale modes in the atmosphere are found. The first is the interannual-mode (IAM), which is dominated by time scales longer than a year and can be attributed to heating and circulation anomalies associated with the coupled tropical ocean-atmosphere, in particular the El Nino-Southern Oscillation. The second is a set of tropical intraseasonal modes consisting of three separate multiscale patterns (ISO-1, -2, -3) related to tropical heating that can be identified with the different phases of the Madden-Julian Oscillation (MJO), including its teleconnection to the extratropics. The ISO spatial and temporal patterns suggest that the extratropical wave train in the North Pacific and North America is related to heating over the Maritime Continent and that the evolution of the MJO around the equator may require forcing from the extratropics spawning convection over the Indian Ocean. The third category represents extratropical intraseasonal oscillations arising from internal dynamics of the basic-state circulation. In the Northern Hemisphere, there are two distinct circulation modes with multiple frequencies in this category: the Pacific/North America (PNA) and the North Atlantic/Eurasia (NAE). In the Southern Hemisphere, two phase-locked modes (PSA-1 and PSA-2) are found depicting an eastward propagating wave train from eastern Australia, via the Pacific South America to the South Atlantic. The extratropical modes exhibit temporal characteristics such as phase locking and harmonic oscillations possibly associated with quadratically nonlinear dynamical systems. Additionally, the observed monthly and seasonal anomalies arise from a complex interplay of the various multiscale low-frequency modes. The relative dominance of the different modes varies widely from month to month and from year to year. On the monthly time scale, while one or two mechanisms may dominate in one year, no single mechanism seems to dominate for all years. There are indications that when the IAM, that is, ENSO heating patterns are strong, the extratropical modes may be suppressed and vice versa. For the seasonal mean, the interannual mode tends to dominate and the contribution from the PNA remains quite significant.

Lau, K.-M.↗
NASA NTRS2012

Theory of a Traveling Wave Feed for a Planar Slot Array Antenna

Planar arrays of waveguide-fed slots have been employed in many radar and remote sensing applications. Such arrays are designed in the standing wave configuration because of high efficiency. Traveling wave arrays can produce greater bandwidth at the expense of efficiency due to power loss in the load or loads. Traveling wave planar slot arrays may be designed with a long feed waveguide consisting of centered-inclined coupling slots. The feed waveguide is terminated in a matched load, and the element spacing in the feed waveguide is chosen to produce a beam squinted from the broadside. The traveling wave planar slot array consists of a long feed waveguide containing resonant-centered inclined coupling slots in the broad wall, coupling power into an array of stacked radiating waveguides orthogonal to it. The radiating waveguides consist of longitudinal offset radiating slots in a standing wave configuration. For the traveling wave feed of a planar slot array, one has to design the tilt angle and length of each coupling slot such that the amplitude and phase of excitation of each radiating waveguide are close to the desired values. The coupling slot spacing is chosen for an appropriate beam squint. Scattering matrix parameters of resonant coupling slots are used in the design process to produce appropriate excitations of radiating waveguides with constraints placed only on amplitudes. Since the radiating slots in each radiating waveguide are designed to produce a certain total admittance, the scattering (S) matrix of each coupling slot is reduced to a 2x2 matrix. Elements of each 2x2 S-matrix and the amount of coupling into the corresponding radiating waveguide are expressed in terms of the element S11. S matrices are converted into transmission (T) matrices, and the T matrices are multiplied to cascade the coupling slots and waveguide sections, starting from the load end and proceeding towards the source. While the use of non-resonant coupling slots may provide an additional degree of freedom in the design, resonant coupling slots simplify the design process. The amplitude of the wave going to the load is set at unity. The S11 parameter, r of the coupling slot closest to the load, is assigned an arbitrary value. A larger value of r will reduce the power dissipated in the load while increasing the reflection coefficient at the input port. It is now possible to obtain the excitation of the radiating waveguide closest to the load and the coefficients of the wave incident and reflected at the input port of this coupling slot. The next coupling slot parameter, r , is chosen to realize the excitation of that radiating waveguide. One continues this process moving towards the source, until all the coupling slot parameters r and hence the S11 parameter of the 4-port coupler, r, are known for each coupling slot. The goal is to produce the desired array aperture distribution in the feed direction. From an interpolation of the computed moment method data for the slot parameters, all the coupling slot tilt angles and lengths are obtained. From the excitations of the radiating waveguides computed from the coupling values, radiating slot parameters may be obtained so as to attain the desired total normalized slot admittances. This process yields the radiating slot parameters, offsets, and lengths. The design is repeated by choosing different values of r for the last coupling slot until the percentage of power dissipated in the load and the input reflection coefficient values are satisfactory. Numerical results computed for the radiation pattern, the tilt angles and lengths of coupling slots, and excitation phases of the radiating waveguides, are presented for an array with uniform amplitude excitation. The design process has been validated using computer simulations. This design procedure is valid for non-uniform amplitude excitations as well.

Rengarajan, Sembiam↗