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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Soil temperature extrema recovery rates after precipitation cooling

From a one dimensional view of temperature alone variations at the Earth's surface manifest themselves in two cyclic patterns of diurnal and annual periods, due principally to the effects of diurnal and seasonal changes in solar heating as well as gains and losses of available moisture. Beside these two well known cyclic patterns, a third cycle has been identified which occurs in values of diurnal maxima and minima soil temperature extrema at 10 cm depth usually over a mesoscale period of roughly 3 to 14 days. This mesoscale period cycle starts with precipitation cooling of soil and is followed by a power curve temperature recovery. The temperature recovery clearly depends on solar heating of the soil with an increased soil moisture content from precipitation combined with evaporation cooling at soil temperatures lowered by precipitation cooling, but is quite regular and universal for vastly different geographical locations, and soil types and structures. The regularity of the power curve recovery allows a predictive model approach over the recovery period. Multivariable linear regression models alloy predictions of both the power of the temperature recovery curve as well as the total temperature recovery amplitude of the mesoscale temperature recovery, from data available one day after the temperature recovery begins.

Welker, J. E.↗
NASA NTRS1986

Modified Denavit-Hartenberg parameters for better location of joint axis systems in robot arms

The Denavit-Hartenberg parameters define the relative location of successive joint axis systems in a robot arm. A recent justifiable criticism is that one of these parameters becomes extremely large when two successive joints have near-parallel rotational axes. Geometrically, this parameter then locates a joint axis system at an excessive distance from the robot arm and, computationally, leads to an ill-conditioned transformation matrix. In this paper, a simple modification (which results from constraining a transverse vector between successive joint rotational axes to be normal to one of the rotational axes, instead of both) overcomes this criticism and favorably locates the joint axis system. An example is given for near-parallel rotational axes of the elbow and shoulder joints in a robot arm. The regular and modified parameters are extracted by an algebraic method with simulated measurement data. Unlike the modified parameters, extracted values of the regular parameters are very sensitive to measurement accuracy.

Barker, L. K.↗
NASA NTRS1986

Cosmic ray sources, acceleration and propagation

A review is given of selected papers on the theory of cosmic ray (CR) propagation and acceleration. The high isotropy and a comparatively large age of galactic CR are explained by the effective interaction of relativistic particles with random and regular electromagnetic fields in interstellar medium. The kinetic theory of CR propagation in the Galaxy is formulated similarly to the elaborate theory of CR propagation in heliosphere. The substantial difference between these theories is explained by the necessity to take into account in some cases the collective effects due to a rather high density of relativisitc particles. In particular, the kinetic CR stream instability and the hydrodynamic Parker instability is studied. The interaction of relativistic particles with an ensemble of given weak random magnetic fields is calculated by perturbation theory. The theory of CR transfer is considered to be basically completed for this case. The main problem consists in poor information about the structure of the regular and the random galactic magnetic fields. An account is given of CR transfer in a turbulent medium.

Ptuskin, V. S.↗
NASA NTRS1985

Development and testing of a simple assimilation technique to derive average wind fields from simulated scatterometer data

A simple algorithm is developed and tested to derive a regularly spaced wind field in a limited area from simulated multiorbit scatterometer data. The data are generated by sampling a time-varying known wind field, the 1000-mb FGGE data, from a simulated scatterometer. A simple assimilation technique is used to derive a regularly spaced (100-km grid) wind field representation of two-day averages from the simulated data. Several test cases are considered, and it is noted that the assimilation technique might be applicable to large-scale ocean or atmospheric models.

Legler, D. M.↗
NASA NTRS1985

Markov-Tree model of intrinsic transport in Hamiltonian systems

A particle in a chaotic region of phase space can spend a long time near the boundary of a regular region since transport there is slow. This 'stickiness' of regular regions is thought to be responsible for previous observations in numerical experiments of a long-time algebraic decay of the particle survival probability, i.e., survival probability approximately t to the (-z) power for large t. This paper presents a global model for transport in such systems and demonstrates the essential role of the infinite hierarchy of small islands interspersed in the chaotic region. Results for z are discussed.

Meiss, J. D.↗
NASA NTRS1985

Mesh-connected processor arrays for the transitive closure problem

The main purpose in this paper is to lay a theoretical foundation for the design of mesh-connected processor arrays for the transitive closure problem. Using a simple path-algebraic formulation of the problem and observing its similarity to certain well-known smoothing problems that occur in digital signal processing, it is shown how to draw upon existing techniques from the signal processing literature to derive regular iterative algorithms for determining the transitive closure of the graph. The regular iterative algorithms that are derived using these considerations, are then analyzed and synthesized on mesh-connected processor arrays. Among the vast number of mesh-connected processor arrays that can be designed using this unified approach, the systolic arrays reported in the literature for this problem are shown to be special cases.

Rao, S. K.↗
NASA NTRS1986

Complex and singular solutions of KdV and MKdV equations

The Korteweg-de Vries (KdV) and the modified Korteweg-de Vries (MKdV) equations are shown to have, besides the regular real solutions, exact regular complex as well as singular solutions. The singular solution for the KdV is real but for the MKdV it is pure imaginary. Implications of the complex solutions are discussed.

Buti, B.↗
NASA NTRS1987

Pumping the stellar hydroxyl maser

IRAS far-IR flux data for 163 OH maser stars were analyzed to quantify the contributions 35 and 53 microns inversions make to pumping of the hydroxyl maser. The 35 microns transition is from the 3,3 ground state to the 1,5 rotationally excited level and subsequent decay; the 53 microns transition is a change from the ground state to the 1,3 excited level and relaxation. The stars examined included Mira, short period semi-regular and long-period semi-regular variables. Both transition lines had rough parity in contributing to the approximately 8 percent pumping efficiency at 1612 MHz. However, the individual contributions of the lines could not be determined for the stellar population studied.

Dickinson, Dale F.↗
NASA NTRS1987

Narrow-angle tail radio sources and the distribution of galaxy orbits in Abell clusters

The present data on the orientations of the tails with respect to the cluster centers of a sample of 70 narrow-angle-tail (NAT) radio sources in Abell clusters show the distribution of tail angles to be inconsistent with purely radial or circular orbits in all the samples, while being consistent with isotropic orbits in (1) the whole sample, (2) the sample of NATs far from the cluster center, and (3) the samples of morphologically regular Abell clusters. Evidence for very radial orbits is found, however, in the sample of NATs near the cluster center. If these results can be generalized to all cluster galaxies, then the presence of radial orbits near the center of Abell clusters suggests that violent relaxation may not have been fully effective even within the cores of the regular clusters.

O'Dea, Christopher P.↗
NASA NTRS1988

Unsteady three-dimensional flow separation

A concise mathematical framework is constructed to study the topology of steady 3-D separated flows of an incompressible, or a compressible viscous fluid. Flow separation is defined by the existence of a stream surface which intersects with the body surface. The line of separation is itself a skin-friction line. Flow separation is classified as being either regular or singular, depending respectively on whether the line of separation contains only a finite number of singular points or is a singular line of the skin-friction field. The special cases of 2-D and axisymmetric flow separation are shown to be of singular type. In regular separation it is shown that a line of separation originates from a saddle point of separation of the skin-friction field and ends at nodal points of separation. Unsteady flow separation is defined relative to a coordinate system fixed to the body surface. It is shown that separation of an unsteady 3-D incompressible viscous flow at time t, when viewed from such a frame of reference, is topologically the same as that of the fictitious steady flow obtained by freezing the unsteady flow at the instant t. Examples are given showing effects of various forms of flow unsteadiness on flow separation.

Hui, W. H.↗
NASA NTRS1987

Array architectures for iterative algorithms

Regular mesh-connected arrays are shown to be isomorphic to a class of so-called regular iterative algorithms. For a wide variety of problems it is shown how to obtain appropriate iterative algorithms and then how to translate these algorithms into arrays in a systematic fashion. Several 'systolic' arrays presented in the literature are shown to be specific cases of the variety of architectures that can be derived by the techniques presented here. These include arrays for Fourier Transform, Matrix Multiplication, and Sorting.

Jagadish, Hosagrahar V.↗
NASA NTRS1989

Mandrels For Microtextured Small-Vessel Implants

Research shows artificial blood-vessel and heart-valve implants made more compatible with their biological environments by use of regularly microtextured surfaces. In new manufacturing process, ion beam etches patterned array of small pillars on mandrel used to mold tubular plastic implant. Pillars create tiny regularly spaced holes in inner surface of tube. Holes expected to provide sites for attachment of healthy lining. Polytetrafluoroethylene (PTFE) used as mandrel material because it can be etched by ion beam.

Deininger, William D.↗
NASA NTRS1988

Airborne measurements of tropospheric ozone destruction and particulate bromide formation in the Arctic

Aircraft profiles of O3 concentrations over the Arctic ice pack in spring exhibit a depletion of O3 beneath the surface temperature inversion. One such profile from the NOAA WP-3D Arctic Gas and Aerosol Sampling Program (AGASP) flights in April, 1986 north of Alert, NWT (YLT, 82.5 N) is shown. The gradient of O3 across the temperature inversion, which is essentially a step function from tropospheric values (35 to 40 ppbv) to 0, is somewhat masked by a 1-min running mean applied to the data. Evidence is presented that O3 destruction beneath the Arctic temperature inversion is the result of a photochemical reaction between gaseous Br compounds and O3 to produce particulate Br aerosol. It is noted that in springtime, O3 at the Alert Baseline Station regularly decreases from 30 to 40 ppbv to near 0 over the period of a few hours to a day. At the same time, there is a production of particulate Br with a near 1.0 anti-correlation to O3 concentration. Surface concentrations of bromoform in the Arctic exhibit a rapid decrease following polar sunrise. AGASP aircraft measurements of filterable bromine particulates in the Arctic (March-April, 1983 and 1986) are shown. The greatest concentrations of Br aerosol (shown as enrichment factors relative to to Na in seawater, EFBR (Na)) were observed in samples collected beneath the surface temperature inversion over ice. Samples collected at the same altitude over open ocean (off Spitzbergen) labeled Marine did not exhibit similar Br enrichments. A second region of particulate Br enrichment was observed in the lower stratosphere, which regularly descends to below 500 mb (5.5 km) in the high Arctic. The NOAA WP-3D flew in the stratosphere on all AGASP flights and occasionally measured O3 concentrations in excess of 300 ppbv.

Schnell, Russell C.↗
NASA NTRS1989

Global behavior of the height/seasonal structure of tides between 40 deg and 60 deg latitude

The radars utilized are meteor (2), medium frequency (2) and the new low frequency (1) systems: analysis techniques were exhaustively studied internally and comparatively and are not thought to affect the results. Emphasis is placed upon the new height-time contours of 24-, 12-h tidal amplitudes and phases, which best display height and seasonal structures; where possible high resolution (10 d) is used (Saskatoon), but all stations provide monthly mean resolution. At these latitudes the diurnal tide is generally smaller than the semidiurnal, and displays more variability. However, there is a tendency for vertical wavelengths and amplitudes to be larger during summer months. On occasions in winter and fall, wavelengths may be less than 50 km. The dominant semidiurnal tide shows significant regular season structure; wavelengths are generally small (about 50 km) in winter, large in summer (equal to or greater than 100 km), and these states are separated by rapid equinoctial transitions. There is some evidence for less regularity toward 40 deg. Coupling with mean winds is apparent. Data from earlier ATMAP campaigns are mentioned, and reasons for their inadequacies presented.

Manson, A. H.↗
NASA NTRS1989

A family of new efficient arrays for matrix multiplication

The authors present a regular iterative algorithm for matrix multiplication and show that several well-known matrix multiplication arrays are directly obtained from it, differing only in the choice of iteration vector. They then present a regular iterative algorithm for matrix multiplication using the method of Winograd (1968) and show in detail how to derive one array from this algorithmic description. Other arrays in the same family can similarly be obtained for different choices of the iteration space. The new arrays compute the product of two matrices faster than available conventional arrays and use a smaller number of processor cells.

Jagadish, H. V.↗
NASA NTRS1988

Terminal attractors for addressable memory in neural networks

A new type of attractors - terminal attractors - for an addressable memory in neural networks operating in continuous time is introduced. These attractors represent singular solutions of the dynamical system. They intersect (or envelope) the families of regular solutions while each regular solution approaches the terminal attractor in a finite time period. It is shown that terminal attractors can be incorporated into neural networks such that any desired set of these attractors with prescribed basins is provided by an appropriate selection of the weight matrix.

Zak, Michail↗
NASA NTRS1989

On an efficient and accurate method to integrate restricted three-body orbits

This work is a quantitative analysis of the advantages of the Bulirsch-Stoer (1966) method, demonstrating that this method is certainly worth considering when working with small N dynamical systems. The results, qualitatively suspected by many users, are quantitatively confirmed as follows: (1) the Bulirsch-Stoer extrapolation method is very fast and moderately accurate; (2) regularization of the equations of motion stabilizes the error behavior of the method and is, of course, essential during close approaches; and (3) when applicable, a manifold-correction algorithm reduces numerical errors to the limits of machine accuracy. In addition, for the specific case of the restricted three-body problem, even a small eccentricity for the orbit of the primaries drastically affects the accuracy of integrations, whether regularized or not; the circular restricted problem integrates much more accurately.

Murison, Marc A.↗
NASA NTRS1989

Theories of the origin and evolution of the giant planets

Following the accretion of solids and gases in the solar nebula, the giant planets contracted to their present sizes over the age of the solar system. It is presently hypothesized that this contraction was rapid, but not hydrodynamic; at a later stage, a nebular disk out of which the regular satellites formed may have been spun out of the outer envelope of the contracting giant planets due to a combination of total angular momentum conservation and the outward transfer of specific angular momentum in the envelope. If these hypotheses are true, the composition of the irregular satellites directly reflects the composition of planetesimals from which the giant planets formed, while the composition of the regular satellites is indicative of the composition of the less volatile components of the outer envelopes of the giant planets.

Pollack, J. B.↗