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At least 577 records · Page 32
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NASA NTRS1993

Preliminary design for a reverse Brayton cycle cryogenic cooler

A long life, single stage, reverse Brayton cycle cryogenic cooler is being developed for applications in space. The system is designed to provide 5 W of cooling at a temperature of 65 Kelvin with a total cycle input power of less than 200 watts. Key features of the approach include high speed, miniature turbomachines; an all metal, high performance, compact heat exchanger; and a simple, high frequency, three phase motor drive. In Phase 1, a preliminary design of the system was performed. Analyses and trade studies were used to establish the thermodynamic performance of the system and the performance specifications for individual components. Key mechanical features for components were defined and assembly layouts for the components and the system were prepared. Critical materials and processes were identified. Component and brassboard system level tests were conducted at cryogenic temperatures. The system met the cooling requirement of 5 W at 65 K. The system was also operated over a range of cooling loads from 0.5 W at 37 K to 10 W at 65 K. Input power to the system was higher than target values. The heat exchanger and inverter met or exceeded their respective performance targets. The compresssor/motor assembly was marginally below its performance target. The turboexpander met its aerodynamic efficiency target, but overall performance was below target because of excessive heat leak. The heat leak will be reduced to an acceptable value in the engineering model. The results of Phase 1 indicate that the 200 watt input power requirement can be met with state-of-the-art technology in a system which has very flexible integration requirements and negligible vibration levels.

Swift, Walter L.↗
NASA NTRS1993

On wave-CISK and the evaporation-wind feedback for the Madden-Julian oscillation

The combined effects of Kelvin wave-CISK and the evaporation-wind (E-W) feedback are proposed as a possible mechanism for the Madden-Julian oscillation. A very simple single vertical mode model has been employed to examine the effects of both these processes on moist Kelvin waves. The effects of wave-induced moisture convergence is parameterized by reducing the moist static stability, and CISK occurs when the moist static stability becomes negative. The E-W feedback in the presence of mean easterlies leads to unstable Kelvin modes. The presence of mean westerlies leads to decaying Kelvin modes. When CISK and the E-W feedback work in concert, an unstable Kelvin mode develops that has phase speeds of propagation between 5 m/s and 10 m/s for a large range of parameter values. On the other hand, the E-W feedback mechanism alone, in the case when CISK is not operating, produces the phase speeds of the observed Madden-Julian oscillation for only a very limited range of parameter values.

Kirtman, B.↗
NASA NTRS1993

Multipass diode-pumped solid-state optical amplifier

A new diode-pumped solid-state multipass amplifier produced 38-dB small-signal gain at 1.047 micron in Nd:YLF with 1.6-W pump power and 37 percent extraction efficiency near saturation. The amplifier had a 1:1 confocally reimaging multipass design that generated both high gain and high efficiency. The same amplifier design with 13 W of pump power was tested with Nd:YAG at 1.064 micron, which gave 38-dB small-signal gain and 3.2 W of output power, and with Nd:YVO4, also at 1.064 micron, which gave greater than 50-dB small-signal gain and 4.3 W of output power.

Plaessmann, Henry↗
NASA NTRS1992

A new model for estimating total body water from bioelectrical resistance

Estimation of total body water (T) from bioelectrical resistance (R) is commonly done by stepwise regression models with height squared over R, H(exp 2)/R, age, sex, and weight (W). Polynomials of H(exp 2)/R have not been included in these models. We examined the validity of a model with third order polynomials and W. Methods: T was measured with oxygen-18 labled water in 27 subjects. R at 50 kHz was obtained from electrodes placed on the hand and foot while subjects were in the supine position. A stepwise regression equation was developed with 13 subjects (age 31.5 plus or minus 6.2 years, T 38.2 plus or minus 6.6 L, W 65.2 plus or minus 12.0 kg). Correlations, standard error of estimates and mean differences were computed between T and estimated T's from the new (N) model and other models. Evaluations were completed with the remaining 14 subjects (age 32.4 plus or minus 6.3 years, T 40.3 plus or minus 8 L, W 70.2 plus or minus 12.3 kg) and two of its subgroups (high and low) Results: A regression equation was developed from the model. The only significant mean difference was between T and one of the earlier models. Conclusion: Third order polynomials in regression models may increase the accuracy of estimating total body water. Evaluating the model with a larger population is needed.

Siconolfi, S. F.↗
NASA NTRS1992

The effect of intermittent standing or walking during head down tilt bedrest on peak O2 consumption

The cardiovascular aspect of bedrest deconditioning is manifested by decreases in peak O2 uptake (VO(sub 2 peak)) during minimal exercise. The effect of intermittent standing (+G(z)) or walking (+G(z)W) during 4 days of 7 degree Head Down Tilt bedrest (HDT) on VO(sub 2 peak) was evaluated. Methods: Five protocols were performed by eight male subjects; control (C) consisting of complete bedrest, and 15 minute periods to total 2 or 4 hours daily of standing (+G(z)(exp 2) and +G(z)(exp 4) respectively) or walking at 3.0 MPH (+G(z)W2 and +G(z)W4 respectively). Subjects performed VO(sub 2 peak) tests prior to and on the final day of HDT. VO(sub 2 peak) was determined using open circuit indirect calorimetry during supine leg cycling ergometry. After a 5 minute warmup, three 2 minute incremental loads of 33 W previously determined to elicit VO(sub 2 peak) were given and the subject cycled to volitional fatigue. Results: The C protocol VO(sub 2 peak) decreased by 16 percent (2.71 plus or minus 0.16 to 2.27 plus or minus 0.14 L/min) and 11 percent in +G(z)(exp 4) (2.72 plus or minus 0.15 to 2.43 plus or minus 0.14 L/min). With +G(z)W2 VO(sub 2 peak) decreased by 9 percent (2.71 plus or minus 0.17 to 2.46 plus or minus 0.14 L/min) and with +G(z)W4, VO(sub 2 peak) decreased by 10 percent (2.71 plus or minus 0.14 to 2.43 plus or minus 0.14 L/min). VO(sub 2 peak) in all protocols decreased with HDT (P less than 0.05). The decrease in C VO(sub 2 peak) was significantly greater (P less than 0.05) than the decreases in either +G(z) or +G(z)W protocols. Conclusion: The deconditioning that occurs after only 4 days of HDT was demonstrated by decreases in VO(sub 2 peak). Intermittent +G(z) or +G(z)W attenuated, but did not prevent, the decrease in VO(sub 2 peak) with HDT.

Ertl, A. C.↗
NASA NTRS1995

A method of solving simple harmonic oscillator Schroedinger equation

A usual step in solving totally Schrodinger equation is to try first the case when dimensionless position independent variable w is large. In this case the Harmonic Oscillator equation takes the form (d(exp 2)/dw(exp 2) - w(exp 2))F = 0, and following W.K.B. method, it gives the intermediate corresponding solution F = exp(-w(exp 2)/2), which actually satisfies exactly another equation, (d(exp 2)/dw(exp 2) + 1 - w(exp 2))F = 0. We apply a different method, useful in anharmonic oscillator equations, similar to that of Rampal and Datta, and although it is slightly more complicated however it is also more general and systematic.

Maury, Juan Carlos F.↗
NASA NTRS1994

Galactic cosmic ray modulation and interplanetary medium perturbations due to a long-living active region during October 1989

During October 1989, three very energetic flares were ejected by the same active region at longitudes 9 deg E, 32 deg W, and 57 deg W, respectively. The shape of the galactic cosmic ray variations suggests the presence of large magnetic cloud structures (Nagashima et al., 1990) following the shock-associated perturbations. In spite of long data gaps the interplanetary observations at Interplanetary Monitoring Platform (IMP) 8 (near the Earth) and International Cometary Explorer (ICE)(approximately 1 AU, approximately 65 deg W) confirm this possibility for the event related to the 9 deg E flare; the principal axes analysis shows that the interplanetary magnetic field variations at both spacecraft locations are mainly confined on a meridian plane. This result suggests that the western longitudinal extension of this cloud is indeed very large (greater than or equal to 5 deg). The nonnegligible depression in the cosmic ray intensity observed inside the possible cloud related to the 57 deg W flare indicates that also the eastern extension could be very wide. The analysis of neutron monitor data shows clearly the cosmic ray trapping effect of magnetic clouds; this mechanism seems to be responsible for the enhanced diurnal effect often observed during the recovery phase of Forbush decreases. We give an interpretation for the anisotropic cosmic ray peak occurring in the third event, and, related to that, we suggest that the Forbush decrease modulated region at the Earth's orbit could be somewhat wider than the magnetic cloud, as already anticipated by Nagashima et al. (1990). By this analysis, based mainly on cosmic ray data, we show that it is possible to do reasonable inferences on the large-scale structure of flare-related interplanetary perturbations when interplanetary medium data are not completely present.

Bavassano, B.↗
NASA NTRS1994

Direct measurement of free-energy barrier to nucleation of crystallites in amorphous silicon thin films

A method is introduced to measure the free-energy barrier W(sup *), the activation energy, and activation entropy to nucleation of crystallites in amorphous solids, independent of the energy barrier to growth. The method allows one to determine the temperature dependence of W(sup *), and the effect of the preparation conditions of the initial amorphous phase, the dopants, and the crystallization methds on W(sup *). The method is applied to determine the free-energy barrier to nucleation of crystallites in amorphous silicon (a-Si) thin films. For thermally induced nucleation in a-Si thin films with annealing temperatures in the range of from 824 to 983 K, the free-energy barrier W(sup *) to nucleation of silicon crystals is about 2.0 - 2.1 eV regardless of the preparation conditions of the films. The observation supports the idea that a-Si transforms into an intermediate amorphous state through the structural relaxation prior to the onset of nucleation of crystallites in a-Si. The observation also indicates that the activation entropy may be an insignificant part of the free-energy barrier for the nucleation of crystallites in a-Si. Compared with the free-energy barrier to nucleation of crystallites in undoped a-Si films, a significant reduction is observed in the free-energy barrier to nucleation in Cu-doped a-Si films. For a-Si under irradiation of Xe(2+) at 10(exp 5) eV, the free-energy barrier to ion-induced nucleation of crystallites is shown to be about half of the value associated with thermal-induced nucleation of crystallites in a-Si under the otherwise same conditions, which is much more significant than previously expected. The present method has a general kinetic basis; it thus should be equally applicable to nucleation of crystallites in any amorphous elemental semiconductors and semiconductor alloys, metallic and polymeric glasses, and to nucleation of crystallites in melts and solutions.

Shi, Frank G.↗
NASA NTRS1994

Sea level variability in the eastern tropical Pacific as observed by TOPEX and Tropical Ocean-Global Atmosphere Tropical Atmosphere-Ocean Experiment

Sea surface height measurements from the TOPEX altimeter and dynamic height from Tropical Ocean-Global Atmosphere Tropical Atmosphere-Ocean (TOGA TAO) moorings are used to explore sea level variability in the northeastern tropical Pacific Ocean. Afetr the annual harmonic is removed, there are two distinct bands of variability: one band is centered at 5 deg N to 7 deg N and extends from 165 deg W to 110 deg W, and the other band is centered at 10 deg N to 12 deg N and extends from 120 deg W to the coast of Central America. The correspondence between the two independent observation data sets at 5 deg N is excellent with correlations of about 90%. The variability at 5 deg-7 deg N is identified as instability waves formed just south of the North Equatorial Countercurrent during the months of July and March. Wave amplitudes are largest in the range of longitudes 160 deg-140 deg W, where they can exceed 10 cm. The waves disappear when the equatorial current system weakens, during the months of March and May. The variability at 11 deg N in 1993 has the form of anticyclone eddies. These eddies propagate westward at a speed of about 17 cm/s, consistent with the dispersion characteristics of free Rossby waves. The eddies are shown to have their origin near the coast of central America during northern fall and winter. Their formation seems to result from intense wind bursts across the Gulfs of Tehuantepec and Papagayo which generate strong anticyclonic ocean eddies. The disappearance of the eddies in the summer of 1993 coincidences with the seasonal intensification of equatorial currents. Thus the variability at 11 deg N has very little overlap in time with the variability at 5 deg N.

Giese, Benjamin S.↗
NASA NTRS1995

Comparison of tropical pacific temperature and current simulations with two vertical mixing schemes embedded in an ocean general circulation model and reference to observations

The Pacanowski-Philander (PP) and Mellor-Yamada (MY) parameterization models of vertical mixing by turbulent processes were embedded in the Geophysical Fluid Dynamics Laboratory high-resolution ocean general circulation model of the tropical Pacific Ocean. All other facets of the numerical simulations were the same. Simulations were made for the 1987-1988 period. At the equator the MY simulation produced near-surface temperatures more uniform with depth, a deeper thermocline, a deeper core speed of the Equatorial Undercurrent, and a South Equatorial Current with greater vertical thickness compared with that computed with the PP method. Along 140 deg W, between 5 deg N and 10 deg N, both simulations were the same. Moored buoy current and temperature observations had been recorded by the Pacific Marine Environmental Laboratory at three sites (165 deg E, 140 deg W, 110 deg W) along the equator and at three sites (5 deg N, 7 deg N, 9 deg N) along 140 deg W. Simulated temperatures were lower than those observed in the near-surface layer and higher than those observed in the thermocline. Temperature simulations were in better agreement with observations compared to current simulations. At the equator, PP current and temperature simulations were more representative of the observations than MY simulations.

Halpern, David↗
NASA NTRS1994

Observations of the Earth's Radiation Budget in relation to atmospheric hydrology. 4: Atmospheric column radiative cooling over the world's oceans

This paper introduces a simple method for deriving climatological values of the longwave flux emitted from the clear sky atmosphere to the ice-free ocean surface. It is shown using both theory and data from simulations how the ratio of the surface to top-of-atmosphere (TOA) flux is a simple function of water vapor (W) and a validation of the simple relationship is presented based on a limited set of surface flux measurements. The rms difference between the retrieved surface fluxes and the simulated surface fluxes is approximately 6 W/sq m. The clear sky column cooling rate of the atmosphere is derived from the Earth Radiation Budget Experiment (ERBE) values of the clear sky TOA flux and the surface flux retrieved using Special Scanning Microwave Imager (SSM/I) measurements of w together with ERBE clear sky fluxes. The relationship between this column cooling rate, w, and the sea surface temperature (SST) is explored and it is shown how the cooling rate systematically increases as both w and SST increase. The uncertainty implied in these estmates of cooling are approximately +/- 0.2 K/d. The effects of clouds on this longwave cooling are also explored by placing bounds on the possible impact of clouds on the column cooling rate based on certain assumptions about the effect of clouds on the longwave flux to the surface. It is shown how the longwave effects of clouds in a moist atmosphere where the column water vapor exceeds approximately 30 kg/sq m may be estimated from presently available satellite data with an uncertainty estimated to be approximately 0.2 K/d. Based on an approach described in this paper, we show how clouds in these relatively moist regions decrease the column cooling by almost 50% of the clear sky values and the existence of significant longitudinal gradients in column radiative heating across the equatorial and subtropical Pacific Ocean.

Stephens, Graeme L.↗
NASA NTRS1996

Effective Numerical Methods for Solving Elliptical Problems in Strengthened Sobolev Spaces

Fourth-order elliptic boundary value problems in the plane can be reduced to operator equations in Hilbert spaces G that are certain subspaces of the Sobolev space W(sub 2)(exp 2)(Omega) is identical with G(sup (2)). Appearance of asymptotically optimal algorithms for Stokes type problems made it natural to focus on an approach that considers rot w is identical with (D(sub 2)w - D(sub 1)w) is identical with vector of u as a new unknown vector-function, which automatically satisfies the condition div vector of u = 0. In this work, we show that this approach can also be developed for an important class of problems from the theory of plates and shells with stiffeners. The main mathematical problem was to show that the well-known inf-sup condition (normal solvability of the divergence operator) holds for special Hilbert spaces. This result is also essential for certain hydrodynamics problems.

D'yakonov, Eugene G.↗
NASA NTRS1994

Satellite Communications Applications Research

This report describes work performed by SDL in the development of monolithically-integrated master oscillator power amplifiers under the NASA-SCAR research contract. The goals of the program were to demonstrate high power diffraction-limited operation from a semiconductor laser with the following characteristics: (1) Output power of 0.5 to 1.0 W continuous-wave (cw); (2) Single spectral mode with greater than 20 dB of side mode suppression; (3) Single spatial mode operation; (4) Modulation capability of 0.5 GHz; and (5) Operating wavelength between 830-860 nm. Cw diffraction-limited output powers as high as 2.2 W were achieved while maintaining a single longitudinal mode. Side mode suppression ratios greater than 20 dB are typically obtained. The single spatial mode output has a Strehl ratio estimated at 0.85, and a high-speed modulation capability was demonstrated with a small-signal 3-dB roll-off at 1.0 GHz. Finally, all of this performance was obtained in the 860 nm wavelength band, which corresponds to the peak sensitivity of Si detectors. During the first option period, three deliverables were fabricated and delivered that included 1 distributed Bragg reflector (DBR) laser operating at 970 nm, 1 DBR laser operating at 860 nm, and one monolithic flared amplifier master oscillator power amplifier (MFA-MOPA) providing 0.5 W cw diffraction-limited output. During the second option period, a total of 5 MFA-MOPA devices were delivered, each operating to better than 1 W cw diffraction-limited at 860 nm.

Lang, Robert J.↗
NASA NTRS1995

Interpretation of Extinction in Gaussian-Beam Scattering

The extinction efficiency for the interaction of a plane wave with a large nonabsorbing spherical particle is approximately 2.0. When a Gaussian beam of half-width w(sub 0) is incident upon a spherical particle of radius a with w(sub 0)/a less than 1, the extinction efficiency attains unexpectedly high or low values, contrary to intuitive expectations. The reason for this is associated with the so-called compensating term in the scattered field, which cancels the field of the Gaussian beam behind the particle, thereby producing the particle's shadow. I introduce a decomposition of the total exterior field into incoming and outgoing portions that are free of compensating terms. It is then shown that a suitably defined interaction efficiency has the intuitively expected asymptotic values of 2.0 for w(sub 0)/a much greater than 1 and 1.0 for w(sub 0)/a much less than 1.

Lock, James A.↗
NASA NTRS1997

A Well-Calibrated Ocean Algorithm for Special Sensor Microwave/Imager

I describe an algorithm for retrieving geophysical parameters over the ocean from special sensor microwave/imager (SSM/I) observations. This algorithm is based on a model for the brightness temperature T(sub B) of the ocean and intervening atmosphere. The retrieved parameters are the near-surface wind speed W, the columnar water vapor V, the columnar cloud liquid water L, and the line-of-sight wind W(sub LS). I restrict my analysis to ocean scenes free of rain, and when the algorithm detects rain, the retrievals are discarded. The model and algorithm are precisely calibrated using a very large in situ database containing 37,650 SSM/I overpasses of buoys and 35,108 overpasses of radiosonde sites. A detailed error analysis indicates that the T(sub B) model rms accuracy is between 0.5 and 1 K and that the rms retrieval accuracies for wind, vapor, and cloud are 0.9 m/s, 1.2 mm, and 0.025 mm, respectively. The error in specifying the cloud temperature will introduce an additional 10% error in the cloud water retrieval. The spatial resolution for these accuracies is 50 km. The systematic errors in the retrievals are smaller than the rms errors, being about 0.3 m/s, 0.6 mm, and 0.005 mm for W, V, and L, respectively. The one exception is the systematic error in wind speed of -1.0 m/s that occurs for observations within +/-20 deg of upwind. The inclusion of the line-of-sight wind W(sub LS) in the retrieval significantly reduces the error in wind speed due to wind direction variations. The wind error for upwind observations is reduced from -3.0 to -1.0 m/s. Finally, I find a small signal in the 19-GHz, horizontal polarization (h(sub pol) T(sub B) residual DeltaT(sub BH) that is related to the effective air pressure of the water vapor profile. This information may be of some use in specifying the vertical distribution of water vapor.

Wentz, Frank J.↗
NASA NTRS1997

Work Functions for Models of Scandate Surfaces

The electronic structure, surface dipole properties, and work functions of scandate surfaces have been investigated using the fully relativistic scattered-wave cluster approach. Three different types of model surfaces are considered: (1) a monolayer of Ba-Sc-O on W(100), (2) Ba or BaO adsorbed on Sc2O3 + W, and (3) BaO on SC2O3 + WO3. Changes in the work function due to Ba or BaO adsorption on the different surfaces are calculated by employing the depolarization model of interacting surface dipoles. The largest work function change and the lowest work function of 1.54 eV are obtained for Ba adsorbed on the Sc-O monolayer on W(100). The adsorption of Ba on Sc2O3 + W does not lead to a low work function, but the adsorption of BaO results in a work function of about 1.6-1.9 eV. BaO adsorbed on Sc2O3 + WO3, or scandium tungstates, may also lead to low work functions.

Mueller, Wolfgang↗
NASA NTRS1998

Chemistry and dynamics of the lower troposphere over North America and the North Atlantic Ocean in fall 1997 observed using an airborne UV DIAL system

The NASA Langley Research Center's airborne UV Differential Absorption Lidar (DIAL) system participated in the Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) mission from October 13 to November 12, 1997. The purpose of the mission was to study the upper troposphere/lower stratosphere in and near the North Atlantic flight corridor to better understand this region of the atmosphere and how civilian air travel in the corridor might be affecting the atmospheric chemistry. Bases of operations included NASA Ames, California (37.4 deg N, 122.1 deg W); Bangor, Maine (44.8 deg N, 68.8 deg W); Shannon, Ireland (52.7 deg N, 8.9 deg W); and Lajes, Terceira Island, Azores (38.8 deg N, 27.1 deg W). Since the UV DIAL system observes in the nadir as well as the zenith, aerosol and ozone data were obtained from near the Earth's surface to the lower stratosphere. A number of interesting features were noted relating to both chemistry and dynamics of the troposphere, which are reported here.

Grant, William B.↗
NASA NTRS1999

SeaWiFS Postlaunch Technical Report Series: The SeaBOARR-98 Field Campaign - Volume 3

This report documents the scientific activities during the first Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Bio-Optical Algorithm Round-Robin (SeaBOARR-98) experiment, which took place from 5-17 July 1998, at the Acqua Alta Oceanographic Tower (AAOT) in the northern Adriatic Sea off the coast of Italy. The ultimate objective of the SeaBOARR activity is to evaluate the effect of different measurement protocols on bio-optical algorithms using data from a variety of field campaigns. The SeaBOARR-98 field campaign was concerned with collecting a high quality data set of simultaneous in-water and above-water radiometric measurements. The deployment goals documented in this report were to: a) use four different surface glint correction methods to compute water-leaving radiances, L W (lambda), from above-water data; b) use two different in-water profiling systems and three different methods to compute L W (lambda) from in-water data (one making measurements at a fixed distance from the tower, 7.5 m, and the other at variable distances up to 29 m away); c) use instruments with a common calibration history to minimize intercalibration uncertainties; d) monitor the calibration drift of the instruments in the field with a second generation SeaWiFS Quality Monitor (SQM-II), to separate differences in methods from changes in instrument performance; and e) compare the L W (lambda) values estimated from the above-water and in-water measurements. In addition to describing the instruments deployed and the data collected, a preliminary analysis of the data is presented, and the kind of follow-on work that is needed to completely assess the estimation of L W (lambda) from above-water and in-water measurements is discussed.

Zibordi, Giuseppe↗