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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Ultrasonic transducer with a two-dimensional Gaussian field profile

A transducer is described which generates a two-dimensional Gaussian field by controlling both the position of multiple circular electrodes and the voltage applied to each electrode. The transducer is constructed by depositing concentric rings electrodes onto one flat surface of a circular piezoelectric crystal disk and attaching the rings to an impedance matching network which acts as a voltage divider. Geometrical inter-ring separations and electrical inter-ring impedances are designed to minimize the error between the generated acoustic field, modeled as a piecewise linear function, and the desired Gaussian distribution. Total mean squared error between the averaged far-field data and a Gaussian shape is less than two percent.

Claus, R. O.↗
NASA NTRS1984

APART/PADE analytical evaluation of the diffuse infrared background experiment for NASA's cosmic background explorer

The stray light performance of NASA's Diffuse Infrared Background Experiment (DIRBE) has been calculated using the APART/PADE code. That code has been upgraded to handle off-axis optical systems such as DIRBE. Under observing conditions, sunlight is attenuated by 21 to 29 orders-of-magnitude; uniform diffuse illumination, by 6 to 9 orders of magnitude. The sunlight is attenuated by multiple diffraction at an external sun shield and by a forebaffle at the entrance to the DIRBE. Stray light is nearly a linear function of the Bidirectional Reflectance Distribution Function (BRDF) of the primary mirror at all wavelengths - or it can be made to be so by reducing the size of the field defining stop from 0.866 to 0.7 degrees square. For uniform diffuse illumination, half of the stray radiation comes from within 5 degrees of the center of the field-of-view. Particulate contamination of the primary mirror is expected to be a problem and special polishing and cleaning procedures are recommended - before mirror overcoating and again before flight.

Evans, D. C.↗
NASA NTRS1986

A perturbation approach to cosmic ray transients in interplanetary space

A perturbation approach is used to model the linear response of the cosmic ray distribution as a function of perturbations in the solar wind transport parameters. The analytical technique permits examination of the effects of the different solar wind parameters, i.e., velocity, drift velocity and the diffusion tensor, which are additive. Cosmic ray changes in the solar wind are neglected. Variations in the diffusion coefficient and the convection-diffusion equation are applied to describing Forbush decreases and the 11-yr solar cycle variations. The treatment is limited to energies above 100 MeV, yet is considered valid enough to have identified a hysteresis effect in the 11-yr variation, wherein high-energy particles lead low-energy particles to a magnitude that increases with increasing heliocentric distance within the inner heliosphere. The calculations also indicate that the Forbush decreases are a cumulative effect of precipitous precursors accompanied by slow recoveries from travelling solar wind perturbations.

Chih, P. P.↗
NASA NTRS1986

The generation of low-frequency electrostatic waves in association with auroral kilometric radiation

Observations of the electron distribution in the source region of auroral kilometric radiation (AKR) indicate that it can have positive gradients with respect to both v-perpendicular and v-parallel. It is shown that this type of distribution is unstable to both the electron cyclotron maser instability, which is responsible for AKR, and the bump-in-tail instability which generates electron acoustic waves in the hiss band. Simulations indicate that the two instabilities do not develop independently but compete for the available free energy due to the reduction in the positive gradients of the distribution caused by the quasi-linear diffusion associated with each instability. The dominant instability depends on the ratio of the density of the energetic electrons n(E) to that of the background electrons n(b). For n(E)/n(b) greater than about 1, the maser instability dominates, and the bump-in-tail instability is suppressed, whereas the reverse is true if n(E)/n(b) is smaller than about 1. Comparison with observations indicates that probably n(E)/n(b) is greater than about 1 in the source region of AKR.

Winglee, R. M.↗
NASA NTRS1990

Simulation studies of plasma waves in the electron foreshock - The transition from reactive to kinetic instability

Particle simulation experiments were used to analyze the electron beam-plasma instability. It is shown that there is a transition from the reactive state of the electron beam-plasma instability to the kinetic instability of Langmuir waves. Quantitative tests, which include an evaluation of the dispersion relation for the evolving non-Maxwellian beam distribution, show that a quasi-linear theory describes the onset of this transition and applies again fully to the kinetic stage. This stage is practically identical to the late stage seen in simulations of plasma waves in the electron foreshock described by Dum (1990).

Dum, C. T.↗
NASA NTRS1991

Polarized radio emission from the edge-on spiral galaxies NGC 891 and NGC 4565

Results are presented, at a resolution of 20 arcsec, of observations of the distribution of radio continuum intensity and linear polarization with the VLA in two nearby edge-on spiral galaxies, NGC 891 and NGC 4565, at 6 and 20 cm, respectively. A unified model is presented to account for the main features of the radio polarization in these two galaxies. The model geometry is determined from recent observations of face-on galaxies where the polarized emission is found to be strongest in the dark inter-arm and outer parts of the disks. A substantial Z-thickness is ascribed to this polarized emission. It is shown that the exceptionally strong wavelength dependence of this type of Faraday depolarization can result in edge-on galaxies becoming rapidly 'Faraday thick' at decimeter wavelengths, thereby obliterating the polarization from regions on the dark side of the disk. The degree of polarization observed in both galaxies increases strongly with increasing Z-distance from the plane.

Sukumar, S.↗
NASA NTRS1996

A Computational and Experimental Study of Nonlinear Aspects of Induced Drag

Despite the 80-year history of classical wing theory, considerable research has recently been directed toward planform and wake effects on induced drag. Nonlinear interactions between the trailing wake and the wing offer the possibility of reducing drag. The nonlinear effect of compressibility on induced drag characteristics may also influence wing design. This thesis deals with the prediction of these nonlinear aspects of induced drag and ways to exploit them. A potential benefit of only a few percent of the drag represents a large fuel savings for the world's commercial transport fleet. Computational methods must be applied carefully to obtain accurate induced drag predictions. Trefftz-plane drag integration is far more reliable than surface pressure integration, but is very sensitive to the accuracy of the force-free wake model. The practical use of Trefftz plane drag integration was extended to transonic flow with the Tranair full-potential code. The induced drag characteristics of a typical transport wing were studied with Tranair, a full-potential method, and A502, a high-order linear panel method to investigate changes in lift distribution and span efficiency due to compressibility. Modeling the force-free wake is a nonlinear problem, even when the flow governing equation is linear. A novel method was developed for computing the force-free wake shape. This hybrid wake-relaxation scheme couples the well-behaved nature of the discrete vortex wake with viscous-core modeling and the high-accuracy velocity prediction of the high-order panel method. The hybrid scheme produced converged wake shapes that allowed accurate Trefftz-plane integration. An unusual split-tip wing concept was studied for exploiting nonlinear wake interaction to reduced induced drag. This design exhibits significant nonlinear interactions between the wing and wake that produced a 12% reduction in induced drag compared to an equivalent elliptical wing at a lift coefficient of 0.7. The performance of the split-tip wing was also investigated by wing tunnel experiments. Induced drag was determined from force measurements by subtracting the estimated viscous drag, and from an analytical drag-decomposition method using a wake survey. The experimental results confirm the computational prediction.

Smith, Stephen C.↗
NASA NTRS1959

Analysis and Evaluation of Supersonic Underwing Heat Addition

The linearized theory for heat addition under a wing has been developed to optimize wing geometry, heat addition, and angle of attack. The optimum wing has all of the thickness on the underside of the airfoil, with maximum-thickness point well downstream, has a moderate thickness ratio, and operates at an optimum angle of attack. The heat addition is confined between the fore Mach waves from under the trailing surface of the wing. By linearized theory, a wing at optimum angle of attack may have a range efficiency about twice that of a wing at zero angle of attack. More rigorous calculations using the method of characteristics for particular flow models were made for heating under a flat-plate wing and for several wings with thickness, both with heat additions concentrated near the wing. The more rigorous calculations yield in practical cases efficiencies about half those estimated by linear theory. An analysis indicates that distributing the heat addition between the fore waves from the undertrailing portion of the wing is a way of improving the performance, and further calculations appear desirable. A comparison of the conventional ramjet-plus wing with underwing heat addition when the heat addition is concentrated near the wing shows the ramjet to be superior on a range basis up to Mach number of about B. The heat distribution under the wing and the assumed ramjet and airframe performance may have a marked effect on this conclusion. Underwing heat addition can be useful in providing high-altitude maneuver capability at high flight Mach numbers for an airplane powered by conventional ramjets during cruise.

wings↗
NASA NTRS2001

Disturbance Dynamics in Transitional and Turbulent Flows

The dynamics of an ensemble of linear disturbances with a known probability distribution associated with the initial mode amplitudes are studied in boundary-layer flows through an analysis of the transport equations for the mean disturbance kinetic energy and disturbance energy dissipation rate. Effects of adverse and favorable pressure-gradients on the disturbance dynamics are also included in the analysis. Unlike the fully turbulent regime where nonlinear phase scrambling of the fluctuations affects the flow field in proximity to the wall, the laminar regime fluctuations studied here are influenced across the boundary layer by the solid boundary. In addition to the low Reynolds number, early stage transition regime, the dynamics of these disturbance fields can be related in some respects to the near-wall dynamics of the fully turbulent regime.

Grosch, C. E.↗
NASA NTRS2006

M(sub W) = 7.2-7.4 Estimated for A.D. 900 Seattle Fault Earthquake by Modeling the Uplift of a Lidar-Mapped Marine Terrace

Inverse modeling of slip on the Seattle fault system, constrained by elevations of uplifted marine terraces, provides a well-constrained estimate of the magnitude of the largest known upper-crust earthquake in the Puget Sound region within the past 2500 years. The terrace elevations that constrain the slip inversion are extracted from elevation and slope images generated from LIDAR surveys of the Puget Sound collected in 1996-2002. The images reveal a single uplifted terrace, dated to 1000 cal yr B.P. near Restoration Point, which is morphologically continuous along the southern shoreline of Bainbridge Island and is visible at comparable elevations within a 25 km by 12 km region encompassing coastlines of West Seattle, Bremerton, East Bremerton, Port Orchard, and Waterman Point. Considering sea level changes since A.D. 900, the maximum uplift magnitudes of shoreline inner edges approach 9 m and are located at the southernmost coastline of Bainbridge Island and the northern tip of Waterman Point, while tilt magnitudes are modest - approaching 0.1 degrees. For each of several different Seattle fault geometry interpretations, we use a linear inversion code to solve for distributed slip on the fault surfaces. Moment magnitudes of 7.2 to 7.4 are calculated directly from the different slip solutions. In general, the greatest slip of the A.D. 900 event was confined to the frontal thrust of the Seattle fault system and was centered beneath Puget Sound between Restoration Point and Alki Point.

Jordan R. Muller↗
NASA NTRS2006

Ensemble Weight Enumerators for Protograph LDPC Codes

Recently LDPC codes with projected graph, or protograph structures have been proposed. In this paper, finite length ensemble weight enumerators for LDPC codes with protograph structures are obtained. Asymptotic results are derived as the block size goes to infinity. In particular we are interested in obtaining ensemble average weight enumerators for protograph LDPC codes which have minimum distance that grows linearly with block size. As with irregular ensembles, linear minimum distance property is sensitive to the proportion of degree-2 variable nodes. In this paper the derived results on ensemble weight enumerators show that linear minimum distance condition on degree distribution of unstructured irregular LDPC codes is a sufficient but not a necessary condition for protograph LDPC codes.

enumerators↗
NASA NTRSDate not supplied

Quantifying radiation quality for space relevant radiation types: Fitting excess risk models to outbred mice data

Accurately quantifying the differences in radiation quality between space and terrestrial environments is important for predicting health risks for astronauts.Recently, Edmundson et al. 2020[1] provided valuable new results from out-bred mice linking tumor induction and genetic background aer exposure to low and high-LET radiation. The goal of the current study is to more rigorously estimate a relative biological effectiveness (RBE) factor by leveraging the solid tumor data from Edmundson et al. 2020. Excess relative risk (ERR) models and excess absolute risk (EAR) models were fit using Poisson regression similar to the models that the Radiation Effects Research Foundation uses to fit atomic bomb survivor data. Linear ERR and EAR slopes were simulated usingBayesian analyses, and RBE values were calculated from the ratio of the heavy ion linear slope to the gamma linear slope using the full posterior distribution.

Lori J. Chappell↗
NASA NTRS2002

Multidimensional indexing structure for use with linear optimization queries

Linear optimization queries, which usually arise in various decision support and resource planning applications, are queries that retrieve top N data records (where N is an integer greater than zero) which satisfy a specific optimization criterion. The optimization criterion is to either maximize or minimize a linear equation. The coefficients of the linear equation are given at query time. Methods and apparatus are disclosed for constructing, maintaining and utilizing a multidimensional indexing structure of database records to improve the execution speed of linear optimization queries. Database records with numerical attributes are organized into a number of layers and each layer represents a geometric structure called convex hull. Such linear optimization queries are processed by searching from the outer-most layer of this multi-layer indexing structure inwards. At least one record per layer will satisfy the query criterion and the number of layers needed to be searched depends on the spatial distribution of records, the query-issued linear coefficients, and N, the number of records to be returned. When N is small compared to the total size of the database, answering the query typically requires searching only a small fraction of all relevant records, resulting in a tremendous speedup as compared to linearly scanning the entire dataset.

Bergman, Lawrence David↗
NASA NTRS1973

Synchrotron sources. I - Extension of theory for small pitch angles.

Some of the features of synchrotron radiation are examined which occur when relativistic particles move at small pitch angles in a magnetic field. The emission from individual particles is computed. The general properties of the emission from distributions of particles with small pitch angles is discussed, and the emissivities and degrees of polarization for several specific source models are presented. For sources which mainly contain particles with small pitch angles, the degree of circular polarization will normally be high, whereas, if the magnetic field is somewhat irregular, the degree of linear polarization will be small. If the particle distribution also has a low-energy cutoff, the emissivity will vary linearly with frequency at low frequencies.

Epstein, R. I.↗
NASA NTRS1992

Large shield volcanos on Venus: The effect of neutral buoyancy zone development on evolution and altitude distribution

The Magellan mission to Venus has emphasized the importance of volcanism in shaping the surface of the planet. Volcanic plains make up 80 percent of the terrain and hundreds of regions of localized eruptions have been identified. Large volcanos, defined as edifices with diameters greater than 100 km, are the sites of some of the most voluminous eruptions. Head et al. have identified 158 of these structures. Their spatial distribution is neither random nor arranged in linear chains as on the Earth; large volcanos on Venus are concentrated in two large, near-equatorial clusters that are also the site of many other forms of volcanic activity. The set of conditions that must be met on Venus that controls the change from widespread, distributed volcanism to focused, shield-building volcanism is not well understood. Future studies of transitional features will help to address this problem. It is likely, however, that the formation and evolution of a neutral buoyancy zone (NBZ) plays an important role in both determining the style of the volcanism and the development of the volcanic feature once it has begun to erupt. Head and Wilson have suggested that the high surface pressure on Venus may inhibit volatile exsolution, which may influence the density distribution of the upper crust and hence control the nature and location of a NBZ. The extreme variations in pressure with elevation may result in significantly different characteristics of such a NBZ at different locations on the planet. In order to test these ideas regarding the importance of NBZ development in the evolution of a large shield and to determine the style of volcanism, three large volcanos that occur at different basal elevations were examined and the distribution of large volcanos as a function of altitude was determined.

Keddie, S.↗
NASA NTRS1984

Nonlinear evolution of the resistive tearing mode

The nonlinear behavior of the tearing instability is investigated with the aid of numerical solutions of the resistive incompressible magnetohydrodynamic equations. This study is directed toward solar applications, such as flares. Simulations were carried out for values of the Lundquist number S from 100 to 1,000,000 and wavelength parameter alpha from 0.042 to 0.5. The influence of linear conditions, such as the constancy of psi, on nonlinear growth characteristics is investigated. For high S and low alpha, secondary-flow vortices, opposite in direction to the linear vortices, were found to generate a new magnetic island centered at the initial X point. The nonlinear spatial distributions of the physical variables differ greatly from the linear behavior, and growth slows considerably once nonlinear effects become important.

Steinolfson, R. S.↗
NASA NTRS2001

Orbits of Saturn's Inner Moons and Other Observations Connected within the 1995-1996 Saturnian Ring Plane Crossing

We reduced and analyzed radio images of Saturn taken in 1990-1995 at 0.35, 2.0, 3.6 and 6.1 cm wavelength. The latitudinal brightness distribution varies substantially over time. Detailed modeling of these data shows that NH4SH condenses at the thermochemical equilibrium temperature of 235.5 K only at Northern mid-latitudes. Over most of the planet, condensation does not occur until the gas cools to 190 +/- 5 K. Supersaturation may also cause the dark equatorial region seen in 1995 at 6.1 cm. Observations of the rings show that the West (dusk) ansa is brighter than the East (dawn) ansa by factors up to 2. The magnitude of the asymmetry increases with increasing wavelength and with decreasing distance to the planet. This East-West asymmetry may be due to multiple scattering in gravitational wakes. We improved the Monte Carlo scattering code used to model the microwave emission from Saturn's rings at multiple wavelengths. The basic code outlined in Dunn, Molnar, and Fix describes the rings as a uniform, infinite layer of ice spheres with a finite vertical extent. The particle sizes follow a power law distribution and scatter light according to a linear combination of Mie and isotropic phase functions. These parameters may be adjusted to reflect the physical properties in the scattering properties of the ring particles. This is used to construct a model which can be directly compared to Very Large Array data. The code has been extended by including anisotropic particle distributions, primarily in the form of the wake structures that are likely to exist in the enhancements. The vertical extent of the wakes equals that of the ring layer. Between the wakes are regions of diminished density so that the optical depth integrated over many wakes matches that of the Voyager occultation experiments. This allows us to model the brightness temperature as a function of azimuth and look for asymmetric scattering behavior in the rings. By changing the width and relative density of the wakes, we can alter the nature of the gross scattering properties of the rings to match what is seen in our data.

Lissauer, Jack J.↗