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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Steady flow past sudden expansions at large Reynolds number. I - Boundary layer solutions

A global Newton method is used to obtain accurate finite difference solutions for uniform inflows to several sudden expansion geometries, avoiding the instabilities encountered at large Reynolds numbers. Steady solutions for lambda values greater than those attained previously are obtained only if lambda is below a critical value. The presence of a singularity at this critical value renders invalid the assumptions of the boundary layer model. The results indicate that for uniform inflows and smaller values of the expansion ratio, the eddy length will no longer increase linearly with R when R becomes sufficiently large.

Milos, F. S.↗
NASA NTRS1991

Are Einstein's transition probabilities for spontaneous emission constant in plasmas?

An investigation is conducted with a ruby laser to experimentally confirm the quenching of spontaneous emission coefficients and propose a mechanism for the phenomenon. Results of previous experiments are examined to determine the consistency and validity of interpretations of the spontaneous emissions. For the C IV 3s-3p and 2s-3p transitions, the line-intensity ratios are found to be dependent on the separation of the laser from the target. Density gradients and Stark broadening are proposed to interpret the results in a way that does not invalidate the Einstein A values. The interpretation is extended to C III and N V, both of which demonstrate similar changes in A values in previous experiments. The apparent quenching of Ar II by photon collisions is explained by Rabi oscillations and power broadening in the argon-ion laser cavity. It is concluded that the changes in A values cannot result from dense plasma effects.

Griem, H. R.↗
NASA NTRS1982

System albedo as sensed by satellites - Its definition and variability

System albedo, an important climatological and environmental parameter, is considered. Some of the problems and assumptions involved in evaluating albedo from satellite data are discussed. Clear-sky and cloud albedos over the United Kingdom and parts of northwest Europe are treated. Consideration is given to the spectral, temporal, and spatial variations and the effect of averaging. The implications of these results for those using and archiving albedo values and for future monitoring of system albedo are discussed. Normalization is of especial importance since this correction alters many albedo values. The pronounced difference in spectral albedo of the two visible channels reemphasizes the problem of attempting to calculate integrated albedo values from meteorological satellite data. The assumption of isotropic reflection is seen to be invalid, hindering the computation of accurate albedo values.

Hughes, N. A.↗
NASA NTRS1987

Dynamic performance of squeeze-film bearings

Earlier work has shown that oil-film forces can be modelled by linear coefficients. Identification techniques were used to generate numerical values for these coefficients. This paper has shown the invalidity of applying the perturbation techniques normally used in bearing studies to derive expressions for linearized coefficients to represent a cavitated oil-film. An alternative approach was developed based upon energy techniques to obtain estimates for linearized coefficients. Some current work being undertaken suggests that an alternative analytical approach is possible. These results will be reported in due course.

Burrow, C. R.↗
NASA NTRS1976

Basic corrections to predictions of solar cell performance required by nonlinearities

The superposition principle is used to derive the approximation that the current-voltage characteristic of an illuminated solar cell is the dark current-voltage characteristic shifted by the short-circuit photocurrent. The derivation requires the linearity of the boundary value problems that underlie the electrical characteristics. The shifting approximation is invalid if considerable photocurrent and considerable dark current both occur within the junction space-charge region; it is invalid also if sizable series resistance is present or if high-injection concentrations of holes and electrons exist within the quasi-neutral regions.

Lindholm, F. A.↗
NASA NTRS1973

Ground based planetary research

High spatial resolution spectrophotometric observations made in the wavelength region lambda lambda 0.6 - 2.0 micrometers are used to study the Jovian and Saturnian limb darkening. Limb darkening coefficients (k) of the Minnaert function are derived for the cloud layers of both planets. A value of k = 1.0 is found for Jupiter over the entire disk while values of between 0.75 and 0.90 are found for different latitudes for Saturn. These data are used to derive geometric albedoes (G) for the various belts, zones, spots and regions observed on Jupiter and Saturn. These values of G and k are in turn used to show that an isotropic scattering model is invalid for Jupiter and that at least an asymmetric scattering function, such as the Euler function, is needed to fit the Jovian data. The Jovian scattering function is found to generally vary between 0.960 and 0.994 as a function of wavelength and the feature observed. The Saturn geometric albedoes and values of k indicate that Euler's function fails to adequately model the scattering properties of the Saturnian clouds. As a result it is suggested that simple scattering theory may not apply to the Saturn clouds or that they are better represented by a cumulus cloud model.

Source record↗
NASA NTRS2014

Comment on 'Shang S. 2012. Calculating actual crop evapotranspiration under soil water stress conditions with appropriate numerical methods and time step. Hydrological Processes 26: 3338-3343. DOI: 10.1002/hyp.8405'

A previous study analyzed errors in the numerical calculation of actual crop evapotranspiration (ET(sub a)) under soil water stress. Assuming no irrigation or precipitation, it constructed equations for ET(sub a) over limited soil-water ranges in a root zone drying out due to evapotranspiration. It then used a single crop-soil composite to provide recommendations about the appropriate usage of numerical methods under different values of the time step and the maximum crop evapotranspiration (ET(sub c)). This comment reformulates those ET(sub a) equations for applicability over the full range of soil water values, revealing a dependence of the relative error in numerical ET(sub a) on the initial soil water that was not seen in the previous study. It is shown that the recommendations based on a single crop-soil composite can be invalid for other crop-soil composites. Finally, a consideration of the numerical error in the time-cumulative value of ET(sub a) is discussed besides the existing consideration of that error over individual time steps as done in the previous study. This cumulative ET(sub a) is more relevant to the final crop yield.

Time step↗
NASA NTRS1973

IR spectrophotometry of Jupiter and Saturn

High spatial resolution spectrophotometric observations (from 0.6 to 2.0 microns) are used to study the Jovian and Saturnian limb darkening at 0 deg phase angle. The limb-darkening coefficients (k) of the Minnaert function are 1.0 over Jupiter's entire disk and between 0.75 and 0.90 for different latitudes on Saturn. These data are used to derive geometric albedoes (G) for the various belts, zones, spots, and regions observed on Jupiter and Saturn. These values of G and k are in turn used to show that an isotropic scattering model is invalid for Jupiter and that at least an asymmetric scattering function, such as the Euler function, is needed to fit the Jovian data. The Jovian scattering function is found to generally vary between 0.960 (1 + 0.85 cos theta) and 0.994 (1 + 0.42 cos theta) as a function of wavelength and the feature observed. The Saturn geometric albedoes and values of k indicate that Euler's function fails to adequately model the scattering properties of Saturnian clouds; they may be better represented by a cumulus cloud model.

Binder, A. B.↗
NASA NTRS2011

Effects of Data Quality on the Characterization of Aerosol Properties from Multiple Sensors

Cross-comparison of aerosol properties between ground-based and spaceborne measurements is an important validation technique that helps to investigate the uncertainties of aerosol products acquired using spaceborne sensors. However, it has been shown that even minor differences in the cross-characterization procedure may significantly impact the results of such validation. Of particular consideration is the quality assurance I quality control (QA/QC) information - an auxiliary data indicating a "confidence" level (e.g., Bad, Fair, Good, Excellent, etc.) conferred by the retrieval algorithms on the produced data. Depending on the treatment of available QA/QC information, a cross-characterization procedure has the potential of filtering out invalid data points, such as uncertain or erroneous retrievals, which tend to reduce the credibility of such comparisons. However, under certain circumstances, even high QA/QC values may not fully guarantee the quality of the data. For example, retrievals in proximity of a cloud might be particularly perplexing for an aerosol retrieval algorithm, resulting in an invalid data that, nonetheless, could be assigned a high QA/QC confidence. In this presentation, we will study the effects of several QA/QC parameters on cross-characterization of aerosol properties between the data acquired by multiple spaceborne sensors. We will utilize the Multi-sensor Aerosol Products Sampling System (MAPSS) that provides a consistent platform for multi-sensor comparison, including collocation with measurements acquired by the ground-based Aerosol Robotic Network (AERONET), The multi-sensor spaceborne data analyzed include those acquired by the Terra-MODIS, Aqua-MODIS, Terra-MISR, Aura-OMI, Parasol-POLDER, and CalipsoCALIOP satellite instruments.

Petrenko, Maksym↗
NASA NTRS1982

Wave-driven winds from cool stars. II - Models for T Tauri stars

The Alfven wave-driven wind theory of Hartmann and MacGregor is applied to T Tauri variables, including modifications which permit the calculation of wind temperatures. It is shown that large wave fluxes generate low-temperature winds, which can radiate strongly in Balmer and other optical emission lines. If wave fluxes are restricted to be less than the stellar luminosity, mass loss rates are restricted to values less than or equal to 10 to the -8th solar mass per year. Although these mass loss rates are low in comparison to many previous estimates, it is shown that the wind models produce optical and ultraviolet emission roughly consistent with observations. The theory predicts large wave amplitudes, so that 'turbulent' velocities are generally comparable to, or larger than, local expansion velocities in the optical line-emitting regions, thus making the Sobolev approximation invalid. It is suggested that the discrepancy between the mass loss rates predicted here and previously estimated values is due to the inapplicability of the Sobolev approximation in T Tauri winds.

Hartmann, L.↗
NASA NTRS1993

Deriving the intermediate spectra and photocycle kinetics from time-resolved difference spectra of bacteriorhodopsin. The simpler case of the recombinant D96N protein

The bacteriorhodopsin photocycle contains more than five spectrally distinct intermediates, and the complexity of their interconversions has precluded a rigorous solution of the kinetics. A representation of the photocycle of mutated D96N bacteriorhodopsin near neutral pH was given earlier (Varo, G., and J. K. Lanyi. 1991. Biochemistry. 30:5008-5015) as BRhv-->K<==>L<==>M1-->M2--> BR. Here we have reduced a set of time-resolved difference spectra for this simpler system to three base spectra, each assumed to consist of an unknown mixture of the pure K, L, and M difference spectra represented by a 3 x 3 matrix of concentration values between 0 and 1. After generating all allowed sets of spectra for K, L, and M (i.e., M1 + M2) at a 1:50 resolution of the matrix elements, invalid solutions were eliminated progressively in a search based on what is expected, empirically and from the theory of polyene excited states, for rhodopsin spectra. Significantly, the average matrix values changed little after the first and simplest of the search criteria that disallowed negative absorptions and more than one maximum for the M intermediate. We conclude from the statistics that during the search the solutions strongly converged into a narrow region of the multidimensional space of the concentration matrix. The data at three temperatures between 5 and 25 degrees C yielded a single set of spectra for K, L, and M; their fits are consistent with the earlier derived photocycle model for the D96N protein.

NASA Program Exobiology↗
NASA NTRS1979

Examination of Lambertian and Non-lambertian Models for Simulating the Topographic Effect on Remotely Sensed Data

As a preliminary step to developing a technique to eliminate the topographic effect from remotely sensed data, two radiance simulation models were examined and compared. A Lambertian and a non-Lambertian model were tested using hand-held radiometer measurements from a uniform surface at different slope angle aspect orientations. Linear correlation coefficients for the non-Lambertian model and the field spectra were calculated to be greater than 0.92 for all cases; whereas correlation coefficients for the Lambertian model ranged from 0.06 to 0.98. An assumption regarding an empirical constant within the non-Lambertian equation was found to be invalid and the model was improved by using subsets of the data to derive the empirical value.

Justice, C.↗
NASA NTRS1988

Current sheet crossings in the distant magnetotail

A computer search of crossings of the field reversal region in the middle of the distant magnetotail has been done with ISEE-3 data. It is found that the instantaneous direction of the magnetic field at the crossings Bz is northward on average at a distance of 220 earth radii, with moderate values of the plasma flow. This result suggests that the magnetic field lines are closed; since the flow is tailward, the Dungey (1961) model with a steady state reconnection line across the magnetotail earthward of the spacecraft is invalidated. The present result gives strong support to a model in which tailward boundary layer flow provides the viscous interaction proposed by Axford and Hines (1961). At higher values of the plasma velocity, negative values of Bz are found.

Heikkila, W. J.↗
NASA NTRS1971

Two examples in the time optimal control theory of distributed parameter systems.

The behavior of hyperbolic and parabolic partial differential equations is contrasted by studying the point-to-point time-optimal control problem for the equation of heat conduction and the equation of motion of a vibrating string. A maximal principle is obtained for the time-optimal control of the one-dimensional heat equation, and it is proven that time optimal controls are weakly bang-bang. The bang-bang principle is proven to be invalid for hyperbolic equations because of the finite speed of wave propagation. In the case of boundary value control of the vibrating spring, the latter is demonstrated by deriving an explicit formula for the time optimal control.

Quinn, J. P.↗
NASA NTRS1972

Electrical conductivity and velocity of highly ionized plasma flows - Theory and experiment.

Use of an immersible, three-coil, magnetic-induction probe, previously tested in a low-density supersonic argon jet, to measure electrical conductivity and velocity profiles of a highly ionized high-density nitrogen jet in the continuum flow regime where effects due to probe bow shocks and boundary layers might not be negligible. Measured centerline values of electrical conductivity and velocity were compared with predictions based on a theoretical analysis previously developed to study the gas as it expanded adiabatically and inviscidly from an equilibrium sonic state to the nozzle exit. The resulting numerical exit plane values for electron density and electron temperature were then substituted into the Spitzer-Haerm conductivity formula to compute a theoretical conductivity which agreed within 40% of the measured conductivity, while the calculated and experimental velocity values differed by as much as 50%. The lack of agreement was attributed to the possible use of invalid assumptions and boundary conditions in the computer analysis or to the unknown effects of shocks on the probe data.

Vendell, E. W.↗
NASA NTRS2012

Implementing the Mars Science Laboratory Terminal Descent Sensor Field Test Campaign

Overall, the MSL TDS Field Test campaign was very successful. TDS was shown to perform extremely well over the required operational envelope. Early BB TDS field tests uncovered a number of issues, but none that invalidated the TDS design or implementation. EM TDS tests uncovered minor things of interest, but nothing of concern. Value of testing hardware in the field was demonstrated and significantly contributed to the overall TDS V&V effort. Over the 5-plus year field test campaign, numerous lessons were learned that will inform future field test efforts.

Terminal Descent Sensor↗
NASA NTRS1974

The non-existence of the Oort cometary shell

The procedures adopted as theory for a shell of comets are shown to be invalid. Any plot of numbers of long-period comets against 1/a will automatically exhibit a peak at small values of this parameter, and cannot be inverted to demonstrate a high volume-density of aphelia in space. The positions of actual aphelion-points show no sign of any concentration at any range. Further, the aphelion-distance undergoes large almost random changes owing to planetary perturbations at each return, and present values can yield no indication of original positions. A recent attempt to save the shell-theory by making use solely of comets with large perihelion-distance is shown to rest on exactly the same errors as have done all the earlier presentations. The plain conclusions emerge that the shell-theory is devoid of any support by facts, and that the alleged shell of comets is non-existent.

Lyttleton, R. A.↗
NASA NTRS2006

Radiometric Calibration Assessment of Commercial High Spatial Resolution Multispectral Image Products

Radiometric calibration of commercial imaging satellite products is required to ensure that science and application communities can better understand their properties. Inaccurate radiometric calibrations can lead to erroneous decisions and invalid conclusions and can limit intercomparisons with other systems. To address this calibration need, satellite at-sensor radiance values were compared to those estimated by each independent team member to determine the sensor's radiometric accuracy. The combined results of this evaluation provide the user community with an independent assessment of these commercially available high spatial resolution sensors' absolute calibration values.

Holekamp, Kara↗