SEARCH · Engineering Papers

Results for “regularity”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,009 records · Page 56

All sources NASA NTRS DOE OSTI

NASA NTRS1985

Three-dimensional algorithms for grid restructuring in Free-Lagrangian calculations

Grid restructuring algorithms which lower the price of three-dimensional Free-Lagrange calculations are presented. The algorithms are first given for the case of planar triangulated surfaces embedded in and spanning a three-dimensional region. The tetrahedra generated by this technique form a Delaunay mesh if the interplane spacing is comparable to the resolution within the planes. The algorithm can therefore be used for efficient determinations of Voronoi connections for initial grids. Modifications of the algorithm for the case of closely spaced surfaces are demonstrated in the context of restructuring algorithms which can accommodate colliding surfaces. Then, the restriction to planar surfaces is removed and regular surfaces are examined. The basic algorithm is the same, with an additional operation to project the vertices of one surface onto another. Finally, vertices on the surface are allowed to migrate anywhere in space.

Fritts, M.↗

NASA NTRS1985

Solution adaptivity using a triangular mesh

Solution adaptivity is discussed first from a general perspective and then from the specific viewpoint of triangular meshes. The use of a general connectivity triangular mesh is emphasized. The development of monitor surfaces and their geometric properties is discussed. Mesh point movement is addressed, as is the dynamic restructuring of the connectivity pattern among moving modes. Changes in the number of mesh nodes to obtain suitable refinement for a physical simulation is examined, and the use of locally regular structures to offset the data structure limitation of a general connectivity triangular mesh and thus to obtain an enhanced range of application is considered. To illustrate the basic features of the adaptive triangular mesh strategy, an application to the study of plasma equilibrium is briefly considered.

Eiseman, P. R.↗

NASA NTRS1985

Recent developments in FEM-CFD

The current status of CFD with regard to unstructured grids employing finite element methods and Eulerian frames is reviewed. Algorithms suitable for the computation of large three-dimensional problems involving flow past arbitrary geometries are developed. Adaptive mesh refinement strategy is reviewed, and domain splitting or local time-stepping are briefly addressed. The development of search algorithms of optimal order, variable time-stepping Jacobi smoothers for elliptic problems, and transport concepts for hyperbolics to help achieve good performance for unstructured multigrid processes is discussed. As examples, transient supersonic flow in a channel, regular shock reflection of a wall, viscous flow past a protruberance, potential flow past a cylinder, and Burgers equation are considered.

Loehner, R.↗

NASA NTRS1986

Periodicity of the earth's magnetic reversals

Results are presented from an attempt to perform a relatively comprehensive analysis of the evidence for a periodicity, with harmonics, of the observed regular reversals of the earth's magnetic field. The database considered covers 296 reversals over the past 165 Myr. Histograms with bins 1 Myr apart reveal only 30 Myr reversal patterns. The reversal dates are fitted to a linear periodic function and a spectrum is computed for the residuals at the adopted dates. The possible presence of multiple periodicities is evaluated and over various time intervals. The analysis shows that a recently observed 15 Myr periodicity is probably a harmonic of the 29.5-30.5 Myr period. The calculations do not confirm an inherent magnetic reversal property of the earth. The reversals may arise from tectonic events or from impacts from extraterrestrial objects.

Stothers, R. B.↗

NASA NTRS1985

Progress in knowledge-based flight monitoring

Features and applications of the script-based flight monitor SECURE are described. Implemented on an on-board computer, SECURE treats a flight as a regular sequence of contexts (situations) defined in a knowledge base with a hierarchical structure for successively more finely delineated flight phases, i.e., takeoff, cruise and landing. SECURE provides normalcy references for flight monitoring and allows context identification, which allows the presentation of checklists. An implementation of SECURE, written in MACLISP, on a DC-10 flight simulator is described.

Chen, D. C.↗

NASA NTRS1986

Detection of two satellites in the Cassini division of Saturn's rings

The presence of two about 10-km-radius satellites within a 100-km-wide gap in the Cassini division of Saturn's rings is inferred from the observation of perturbations of nearby ring material. The perturbations are detected as regular fluctuations in the optical depth profile measured during Voyager radio occultation by the rings and are used to estimate the satellite's orbits and masses. The two satellites appear to 'shepherd' a 30-km-wide ringlet within the gap.

Marouf, E. A.↗

NASA NTRS1986

An exact energy corollary for homentropic flow

An exact conservation equation is developed which generalizes the familiar acoustic energy equation. The new relation is valid for arbitrary homentropic disturbances to a homentropic steady flow. It is suggested by a derivation of the acoustic energy equation by means of a regular perturbation expansion of the general energy equation for an ideal compressible fluid. A perturbation energy density and flux are defined and are identified as the exact physical quantities whose leading order representations are the usual acoustic energy density and flux.

Myers, M. K.↗

NASA NTRS1986

Techniques for computing the discrete Fourier transform using the quadratic residue Fermat number systems

The complex integer multiplier and adder over the direct sum of two copies of finite field developed by Cozzens and Finkelstein (1985) is specialized to the direct sum of the rings of integers modulo Fermat numbers. Such multiplication over the rings of integers modulo Fermat numbers can be performed by means of two integer multiplications, whereas the complex integer multiplication requires three integer multiplications. Such multiplications and additions can be used in the implementation of a discrete Fourier transform (DFT) of a sequence of complex numbers. The advantage of the present approach is that the number of multiplications needed to compute a systolic array of the DFT can be reduced substantially. The architectural designs using this approach are regular, simple, expandable and, therefore, naturally suitable for VLSI implementation.

Truong, T. K.↗

NASA NTRS1986

Some background about satellites

Four tables of planetary and satellite data are presented which list satellite discoveries, planetary parameters, satellite orbits, and satellite physical properties respectively. A scheme for classifying the satellites is provided and it is noted that most known moons fall into three general classes: regular satellites, collisional shards, and irregular satellites. Satellite processes are outlined with attention given to origins, dynamical and thermal evolution, surface processes, and composition and cratering. Background material is provided for each family of satellites.

Burns, Joseph A.↗

NASA NTRS1986

SAM II measurements of Antarctic PSC's and aerosols

Measurements by the SAM II satellite instrument show that polar stratospheric clouds (PSC's) are a regular feature of the austral winter season in either nonvolcanically or volcanically disturbed periods. The tops of these clouds are observed above 20 km in early winter and descend in altitude over the course of the season to heights near 15 km in mid September. Typically, PSC's persist in the lowest stratospheric altitudes throughout September. Subsequently, October always represents a relative annual minimum in aerosol extinction above 15 km and in stratospheric column amount. In addition, volcanically produced aerosols in Antarctica peaked in early 1983 and, if linearly related to ozone losses, are probably not a contributing factor to the continued loss of total ozone in the Antarctic spring in 1984 and 1985.

Mccormick, M. P.↗

NASA NTRS1986

Sleep, sleepiness, and circadian rhythmicity in aircrews operating on transatlantic routes

A two-phase study was performed on B-747 crew members operating on regular passenger flights with 9-h time difference. In phase I, sleep-log surveys were obtained. The results for the layover period indicate congruent sleep patterns with shifts in sleep onset less than 9 h; sleep duration was prolonged. Phase II consisted of polygraphic sleep recordings and multiple sleep latency tests (MSLTs) applied to four cockpit crews in a baseline period, during the layover, and after return to home base. During the layover, mean bed times were shifted by about 4.5 h, and sleep was disturbed by early and prolonged awakenings which led to a reduction of sleep efficiency. The ECG and rectal temperature recordings gave evidence for a desynchronization of the circadian system and an internal dissociation of different body functions.

Wegmann, Hans M.↗

NASA NTRS1986

Time-dependent numerical modeling of dust halo formation at comets

The evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic-hydrodynamic method. In the inner coma the time-dependent continuity, momentum, and energy equations of the dusty gas flow were solved simultaneously using 12 dust sizes. Beyond 300 km a three-dimensional kinetic model was used to calculate the trajectory of each individual dust grain. It was found that following the onset of the comet outburst a gas-dust blast wave propagates outward in the inner coma. About 15 minutes after the increased gas and dust production was initiated at the nucleus, a new equilibrium was reached in the inner coma. The most important feature of this new steady state was the significant increase of the dust terminal velocities. These higher terminal velocity values resulted in larger apex distances for dust particles emitted during the outburst. The dust particles spend a relatively long time near their apex points; therefore, the outburst generates long-lasting distinct dust envelopes in front of the regular dust coma.

Gombosi, T. I.↗

NASA NTRS1986

Narrowband signal detection in the SETI field test

Various methods for detecting narrow-band signals are evaluated. The characteristics of synchronized and unsynchronized pulses are examined. Synchronous, square law, regular pulse, and the general form detections are discussed. The CW, single pulse, synchronous, and four pulse detections are analyzed in terms of false alarm rate and threshold relative to average noise power. Techniques for saving memory and retaining sensitivity are described. Consideration is given to nondrifting CW detection, asynchronous pulse detection, interpolative and extrapolative pulse detectors, and finite and infinite pulses.

Cullers, D. Kent↗

NASA NTRS1986

Hopf bifurcation with dihedral group symmetry - Coupled nonlinear oscillators

The theory of Hopf bifurcation with symmetry developed by Golubitsky and Stewart (1985) is applied to systems of ODEs having the symmetries of a regular polygon, that is, whose symmetry group is dihedral. The existence and stability of symmetry-breaking branches of periodic solutions are considered. In particular, these results are applied to a general system of n nonlinear oscillators coupled symmetrically in a ring, and the generic oscillation patterns are described. It is found that the symmetry can force some oscillators to have twice the frequency of others. The case of four oscillators has exceptional features.

Golubitsky, Martin↗

NASA NTRS1987

Gravitational lenses and dark matter - Observations

Following a few general comments on gravitational lenses from an observer's perspective, the currently available observations of the six known gravitational lenses are summarized. Attention is called to some regularities and peculiarities of the properties of the known lenses and to how they might be interpreted. The most important conclusions, relevant to the dark matter problem, which can be obtained from current observations are that the distributions of mass and light appear to be quite different in at least some of the lensing objects and that objects with projected mass/brightness values about 10 times larger than those ordinarily associated with galaxies exist and are not too rare.

Turner, Edwin L.↗

NASA NTRS1987

Equatorial long-wave characteristics determined from satellite sea surface temperature and in situ data

SST maps and imagery derived from the NOAA 6 satellite AVHRR for June and July 1981 in the eastern tropical Pacific portray the wavelike structure of the cool water along the equator from 93 deg W to 125 deg W. Cusped waves of approximately 1000-km zonal wavelength and 25-day period propagated westward with a phase speed of 40 km/day. The observed meridional extent between the crest and trough of the waves is about 300 km. Details in the imagery show cooler water at the cusps advected north and then east with the north equatorial countercurrent (NECC), consistent with the suggestion of a series of anticyclonic eddies occupying the shear zone between the NECC and the westward flowing south equatorial current. Absolute SST estimates from the AVHRR data agree to within 0.6 C with shipboard data taken along 110 deg W between 5 deg N and 5 deg S. The wavelike structures in the SST maps are also in agreement at the surface with a vertical expendable bathythermograph temperature section made along the equator between 93 deg W and 125 deg W, which shows the phase of the waves tilting westward with increasing depth over the upper 75 m. Such a phase shift, if it extended 100-200 km meridionally in either direction from the equator, would be associated with an equatorward flux of heat. Similar phase shifts appear in temperature time series at depths of 20 and 50 m, from a mooring at 0-deg 33-min N, 110-deg 30-min W. Near-surface currents measured at this and a second mooring on the equator at 109-deg 40-min W indicate a regular pattern of northward advection when wave cusps pass them, followed by southwest flow during the passage of wave troughs, again consistent with an equatorward flux of heat, as well as with earlier theoretical and drift buoy findings.

Pullen, Patricia E.↗

NASA NTRS1987

The IHW island network

Early astronomical photography of comets at perihelion encouraged the establishment of an International Halley Watch (IHW) Team for regularly photographing the Comet. The February 1986 period was particularly troublesome due to the limitations of cometary visibility in the Southern Hemisphere. Schmidt cameras were placed on Tahiti, Easter Island, Faraday Station on the Antarctic Peninsula, Reunion Island and in South Africa. Blue- and red-filter B/W images were obtained every night and color prints were occasionally shot. Each night's images were examined before the next night's photography. Several interesting anecdotes are recounted from shipping, manning and operation of the telescopes.

Niedner, Malcolm B., Jr.↗

NASA NTRS1987

Comment on the paper 'On the influx of small comets into the earth's upper atmosphere'

The possibility that comets containing up to 100 tons of ice encounter the earth's atmosphere at a rate of one every 20 min is discussed. Cometary 'hail storms' were proposed to explain observed regular transient decreases in the atmospheric UV dayglow intensity. The decreases take the form of dark 'holes' up to 50 km across. The probability that clouds of objects assumed to be as dark as the nucleus of Comet Halley between the earth and moon would be detectable by ground-based electrooptical deep space telescopes is considered. Conflicting projections of the number of objects which would be detected per hour are examined. High correlations are noted between cometary passages (Comets Encke, Tuttle, Tempel) and intervals of meteor showers (Taurids, Leonids, Geminis, etc.). The holes, however, are not correlated or coincident with the showers. It is suggested that dedicated searches for the unclassified dark objects be carried out in November, when cometary fluxes are high.

Soter, Steven↗