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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Automatic partitioning of unstructured meshes for the parallel solution of problems in computational mechanics

Most of the recently proposed computational methods for solving partial differential equations on multiprocessor architectures stem from the 'divide and conquer' paradigm and involve some form of domain decomposition. For those methods which also require grids of points or patches of elements, it is often necessary to explicitly partition the underlying mesh, especially when working with local memory parallel processors. In this paper, a family of cost-effective algorithms for the automatic partitioning of arbitrary two- and three-dimensional finite element and finite difference meshes is presented and discussed in view of a domain decomposed solution procedure and parallel processing. The influence of the algorithmic aspects of a solution method (implicit/explicit computations), and the architectural specifics of a multiprocessor (SIMD/MIMD, startup/transmission time), on the design of a mesh partitioning algorithm are discussed. The impact of the partitioning strategy on load balancing, operation count, operator conditioning, rate of convergence and processor mapping is also addressed. Finally, the proposed mesh decomposition algorithms are demonstrated with realistic examples of finite element, finite volume, and finite difference meshes associated with the parallel solution of solid and fluid mechanics problems on the iPSC/2 and iPSC/860 multiprocessors.

Farhat, Charbel↗
NASA NTRS1991

Equilibrium wave spectrum and sea state bias in satellite altimetry

For a well-developed sea at equilibrium with a constant wind, the energy-containing range of the wavenumber spectrum for wind-generated gravity waves is approximated by a generalized power law involving the angular spread function and mu, interpreted as a fractal codimension of a small surface patch. Dependence of mu on the wave age is estimated, and the 'Phillips constant', beta, along with the low-wavenumber boundary, k0, of the inertial subrange are analyzed on the basis of the wave action and energy conservation principles. The resulting expressions are employed to evaluate various non-Gaussian statistics of a weakly nonlinear sea surface, which determine the sea state bias in satellite altimetry. The locally accelerated decay of the spectral density function in a high-wavenumber dissipation subrange is pointed out as an important factor of wave dynamics and the geometrical optics treatment of the sea state bias. The analysis is carried out in the approximation of a unidirectional wave field and confined to the case of a well-developed sea.

Glazman, Roman E.↗
NASA NTRS1993

Input impedance of coaxially fed rectangular microstrip antenna on electrically thick substrate

A full-wave spectral domain analysis has been used to obtain input-impedance results for a probe-fed rectangular-patch antenna, modeling the source as a magnetic-current frill. Multiple modes are used in the probe surface current to account for axial and azimuthal variations. It is established that maximum resistance is dependent on the substrate loss tangent. The axial variation of the probe current must be taken into account for substrate thicknesses greater than about 0.02 wavelengths.

Chen, Wei↗
NASA NTRS1993

Virtual zone Navier-Stokes computations for oscillating control surfaces

A new zoning method called 'virtual zones' has been developed for application to an unsteady finite difference Navier-Stokes code. The virtual zoning method simplifies the zoning and gridding of complex configurations for use with patched multi-zone flow codes. An existing interpolation method has been extensively modified to bring the run time for the interpolation procedure down to the same level as for the flow solver. Unsteady Navier-Stokes computations have been performed for transonic flow over a clipped delta wing with an oscillating control surface. The computed unsteady pressure and response characteristics of the control-surface motion compare well with experimental data.

Klopfer, G. H.↗
NASA NTRS1993

Spatial correlations of interdecadal variation in global surface temperatures

We have analyzed spatial correlation patterns of interdecadal global surface temperature variability from an empirical perspective. Using multitaper coherence estimates from 140-yr records, we find that correlations between hemispheres are significant at about 95 percent confidence for nonrandomness for most of the frequency band in the 0.06-0.24 cyc/yr range. Coherence estimates of pairs of 100-yr grid-point temperature data series near 5-yr period reveal teleconnection patterns consistent with known patterns of ENSO variability. Significant correlated variability is observed near 15 year period, with the dominant teleconnection pattern largely confined to the Northern Hemisphere. Peak-to-peak Delta-T is at about 0.5 deg, with simultaneous warming and cooling of discrete patches on the earth's surface. A global average of this pattern would largely cancel.

Mann, Michael E.↗
NASA NTRS1993

Piezoelectric actuator models for active sound and vibration control of cylinders

Analytical models for piezoelectric actuators, adapted from flat plate concepts, are developed for noise and vibration control applications associated with vibrating circular cylinders. The loadings applied to the cylinder by the piezoelectric actuators for the bending and in-plane force models are approximated by line moment and line force distributions, respectively, acting on the perimeter of the actuator patch area. Coupling between the cylinder and interior acoustic cavity is examined by studying the modal spectra, particularly for the low-order cylinder modes that couple efficiently with the cavity at low frequencies. Within the scope of this study, the in-plane force model produced a more favorable distribution of low-order modes, necessary for efficient interior noise control, than did the bending model.

Lester, Harold C.↗
NASA NTRS1993

Emission-line objects near R Coronae Australis

New CCD images show additional emission-line objects close to R Coronae Australis in the Corona Australis cloud. Two faint knots, strong in forbidden S II, are seen against the receding CO flow associated with R CrA. They are closely aligned with HH 104 A,B and R CrA and have thus been labeled HH 104 C,D. The position angle corresponds to that of the transient spike described by Hartigan and Graham (1987). HH 104 C,D lie on the edge of a diffuse patch of nebulosity, strongest in H-alpha. It is suggested that this is illuminated by light scattered from R CrA in the direction of the receding CO flow. The emission-line star 1-100 observed by Hartigan and Graham has a variable spectrum. There is a forbidden S II knot 17 arc sec W, 9 arcsec S of this star. A faint 19th magnitude emission-line star is identified. A change in the morphology of the HH 100 reflection nebula between 1973 and 1983 is noted.

Graham, J. A.↗
NASA NTRS1993

A numerical study of the effect of geometry variation, turbulence models, and dissipation on the flow past control surfaces

The steady 3D viscous flow past the ONERA M6 wing and a slender delta wing-body with trailing edge control surfaces has been computed. A cell-centered finite-volume Navier-Stokes patched zonal method has been used for the numerical simulation. Both diagonalized and LUSGS schemes have been implemented. Besides the standard nonplanar zonal interfacing techniques, a new virtual zone capability has been employed. For code validation, the transonic flow past the ONERA M5 wing is calculated for angles-of-attack of 3.06 deg and 5.06 deg and compared with the available experiments. The wing-body computational results are compared with experimental data for both trailing-edge flaps deflected. The experimental flow conditions are M subinfinity = 0.4, a turbulent Reynolds number of 5.41 million based on a mean aerodynamic chord of 25.959 inches, adiabatic wall, and angles-of-attack varying from 0 deg to 23.85 deg. The computational results are presented for the 23.85 deg angle-of-attack case. The effects of the base flow due to a model sting, the varying second and fourth order numerical dissipation, and the turbulence model are all considered.

Chaussee, Denny S.↗
NASA NTRS1993

Unsteady, three-dimensional, Navier-Stokes simulations of multistage turbomachinery flows

Hub corner stall regions, endwall boundary layers, tip leakage flows and airfoil wakes combine to form the 3D flowfields found in axial turbomachines. Turbomachinery flow fields are also inherently unsteady because of the relative motion between rotor and stator airfoils. This relative motion causes complex time-varying aerodynamic interactions to occur between the different aerodynamic structures and the rotor and stator airfoils. It is necessary to understand the 3D unsteady aerodynamics associated with these interactions in order to design turbomachines that are both lightweight and compact as well as reliable and efficient. The current study uses a time-accurate 3D thin-layer Navier-Stokes zonal approach to investigate the unsteady aerodynamics of multistage turbines and compressors. Relative motion between rotor and stator airfoils is accounted for by the use of systems of patched and overlaid grids. Time-averaged surface pressures, surface flow visualizations, and time-averaged flow field contours have been computed for a 1 1/2-stage turbine and are in good agreement with experimental data. This favorable comparison represents an initial validation of the current method for unsteady computations of multistage turbomachinery flows.

Gundy-Burlet, Karen L.↗
NASA NTRS1993

Application of GPS attitude determination to gravity gradient stabilized spacecraft

Recent advances in the Global Positioning System (GPS) technology have initiated a new era in aerospace navigation and control. GPS receivers have become increasingly compact and affordable, and new developments have made attitude determination using subcentimeter positioning among two or more antennas feasible for real-time applications. GPS-based attitude control systems will become highly portable packages which provide time, navigation, and attitude information of sufficient accuracy for many aerospace needs. A typical spacecraft application of GPS attitude determination is a gravity gradient stabilized satellite in low Earth orbit that employs a GPS receiver and four body mounted patch antennas. The coupled, linearized equations of motion enable complete position and attitude information to be extracted from only two antennas. A discussion of the various error sources for spaceborne GPS attitude measurement systems is included. Attitude determination of better than 0.3 degrees is possible for 1 meter antenna separation. Suggestions are provided to improve the accuracy of the attitude solution.

Lightsey, E. G.↗
NASA NTRS1989

Advanced solar-propelled cargo spacecraft for Mars missions

At the University of Washington, three concepts for an unmanned, solar powered, cargo spacecraft for Mars-support missions have been investigated. These spacecraft are designed to carry a 50,000 kg payload from a low Earth orbit to a low Mars orbit. Each design uses a distinctly different propulsion system: a solar radiation absorption (SRA) system, a solar-pumped laser (SPL) system, and a solar powered mangetoplasmadynamic (MPD) arc system. The SRA directly converts solar energy to thermal energy in the propellant through a novel process developed at the University of Washington. A solar concentrator focuses sunlight into an absorption chamber. A mixture of hydrogen and potassium vapor absorbs the incident radiation and is heated to approximately 3700 K. The hot propellant gas exhausts through a nozzle to produce thrust. The SRA has an I(sub sp) of approximately 1000 sec and produces a thrust of 2940 N using two thrust chambers. In the SPL system, a pair of solar-pumped, multi-megawatt, CO2 lasers in sun-synchronous Earth orbit converts solar energy to laser energy. The laser beams are transmitted to the spacecraft via laser relay satellites. The laser energy heats the hydrogen propellant through a plasma breakdown process in the center of an absorption chamber. Propellant flowing through the chamber, heated by the plasma core, expands through a nozzle to produce thrust. The SPL has an I(sub sp) of 1285 sec and produces a thrust of 1200 N using two thrust chambers. The MPD system uses indium phosphide solar cells to convert sunlight to electricity, which powers the propulsion system. In this system, the argon propellant is ionized and electromagnetically accelerated by a magnetoplasmadynamic arc to produce thrust. The MPD spacecraft has an I(sub sp) of 2490 sec and produces a thrust of 100 N. Various orbital transfer options are examined for these concepts. In the SRA system, the mother ship transfers the payload into a very high Earth orbit and a small auxiliary propulsion system boosts the payload into a Hohmann transfer to Mars. The SPL spacecraft releases the payload as the spacecraft passes by Mars. Both the SRA-powered spacecraft and the SPL-powered spacecraft return to Earth for subsequent missions. The MPD-propelled spacecraft, however, remains at Mars as an orbiting space station. A patched conic approximation was used to determine a heliocentric interplanetary transfer orbit for the MPD propelled spacecraft. All three solar-powered spacecraft use an aerobrake procedure to place the payload into a low Mars parking orbit. The payload delivery times range from 160 days to 873 days (2.39 years).

Auziasdeturenne, J.↗
NASA NTRS1993

Galileo imaging results from the second Earth-Moon flyby: Lunar Maria and related units

The second flyby of the Earth-Moon System by Galileo occurred on December 7, 1992, on its trajectory toward Jupiter. The flyby took the spacecraft over the lunar north polar region from the dark farside and continued across the illuminated nearside. This provided the first opportunity to observe northern and northeastern limb regions with a modern, multispectral imaging system with high spatial resolution (up to 1.1 km/pixel). Scientific objectives included compositional assessment of previously uncharacterized mare regions, study of various light plains materials, and assessment of dark mantle deposits (DMD) and dark halo craters (DHC). Color composite images were prepared from ratios of Galileo SSI filter data (0.76/0.41 yields red; 0.76/0.99 yields green; 0.41/0.76 yields blue) and used for preliminary comparison of units. The 0.41/0.76 ratio has been empirically correlated to Ti content of mare soils (blue is relatively high, red is relatively low). The relative strengths of the ferrous one micron absorption in mafic minerals can be compared using the 0.76/0.99 ratio. In addition, relative ages of units analyzed spectrally were determined from crater statistics using Lunar Orbiter images following the techniques of Neukum et al. Mare deposits analyzed include Mare Humboldtianum, central and eastern Mare Frigoris, Mare Crisium and other deposits in the Crisium Basin, and isolated mare patches on the northeastern lunar limb. Preliminary results show a diversity of 0.41/0.76 micron signatures, implying a wide range of titanium contents. Some light plains units are similar to units found at the Apollo 16 site; others may be ancient mare materials. Dark mantle deposits (DMD) analyzed also are available.

Greeley, R.↗
NASA NTRS1992

Measuring dry plant residues in grasslands: A case study using AVIRIS

Grasslands, savannah, and hardwood rangelands are critical ecosystems and sensitive to disturbance. Approximately 20 percent of the Earth's surface are grasslands and represent 3 million ha. in California alone. Developing a methodology for estimating disturbance and the effects of cumulative impacts on grasslands and rangelands is needed to effectively monitor these ecosystems. Estimating the dry biomass residue remaining on rangelands at the end of the growing season provides a basis for evaluating the effectiveness of land management practices. The residual biomass is indicative of the grazing pressure and provides a measure of the system capacity for nutrient cycling since it represents the maximum organic matter available for decomposition, and finally, provides a measure of the erosion potential for the ecosystem. Remote sensing presents a possible method for measuring dry residue. However, current satellites have had limited application due to the coarse spatial scales (relative to the patch dynamics) and insensitivity of the spectral coverage to resolve dry plant material. Several hypotheses for measuring the biochemical constituents of dry plant material, particularly cellulose and lignin, using high spectral resolution sensors were proposed. The use of Airborne Visible/Infrared Imaging Spectrometers (AVIRIS) to measure dry plant residues over an oak savannah on the eastern slopes of the Coast Range in central California was investigated and it was asked what spatial and spectral resolutions are needed to quantitatively measure dry plant biomass in this ecosystem.

Fitzgerald, Michael↗
NASA NTRS1993

Phase holograms in PMMA with proximity effect correction

Complex computer generated phase holograms (CGPH's) have been fabricated in PMMA by partial e-beam exposure and subsequent partial development. The CGPH was encoded as a sequence of phase delay pixels and written by the JEOL JBX-5D2 E-beam lithography system, a different dose being assigned to each value of phase delay. Following carefully controlled partial development, the pattern appeared rendered in relief in the PMMA, which then acts as the phase-delay medium. The exposure dose was in the range 20-200 micro-C/sq cm, and very aggressive development in pure acetone led to low contrast. This enabled etch depth control to better than plus or minus lambda(sub vis)/60. That result was obtained by exposing isolated 50 micron square patches and measuring resist removal over the central area where the proximity effect dose was uniform and related only to the local exposure. For complex CGPH's with pixel size of the order of the e-beam proximity effect radius, the patterns must be corrected for the extra exposure caused by electrons scattered back up out of the substrate. This has been accomplished by deconvolving the two-dimensional dose deposition function with the desired dose pattern. The deposition function, which plays much the same role as an instrument response function, was carefully measured under the exact conditions used to expose the samples. The devices fabricated were designed with 16 equal phase steps per retardation cycle, were up to 1 cm square, and consisted of up to 100 million 0.3-2.0 micron square pixels. Data files were up to 500 MB long and exposure times ranged to tens of hours. A Fresnel phase lens was fabricated that had diffraction limited optical performance with better than 85 percent efficiency.

Maker, Paul D.↗
NASA NTRS1993

Efficient simulation of incompressible viscous flow over multi-element airfoils

The incompressible, viscous, turbulent flow over single and multi-element airfoils is numerically simulated in an efficient manner by solving the incompressible Navier-Stokes equations. The solution algorithm employs the method of pseudo compressibility and utilizes an upwind differencing scheme for the convective fluxes, and an implicit line-relaxation scheme. The motivation for this work includes interest in studying high-lift take-off and landing configurations of various aircraft. In particular, accurate computation of lift and drag at various angles of attack up to stall is desired. Two different turbulence models are tested in computing the flow over an NACA 4412 airfoil; an accurate prediction of stall is obtained. The approach used for multi-element airfoils involves the use of multiple zones of structured grids fitted to each element. Two different approaches are compared; a patched system of grids, and an overlaid Chimera system of grids. Computational results are presented for two-element, three-element, and four-element airfoil configurations. Excellent agreement with experimental surface pressure coefficients is seen. The code converges in less than 200 iterations, requiring on the order of one minute of CPU time on a CRAY YMP per element in the airfoil configuration.

Rogers, Stuart E.↗
NASA NTRS1993

Near grazing scattering from non-Gaussian ocean surfaces

We investigate the behavior of the scattered electromagnetic waves from non-Gaussian ocean surfaces at near grazing incidence. Even though the scattering mechanisms at moderate incidence angles are relatively well understood, the same is not true for near grazing rough surface scattering. However, from the experimental ocean scattering data, it has been observed that the backscattering cross section of a horizontally polarized wave can be as large as the vertical counterpart at near grazing incidence. In addition, these returns are highly intermittent in time. There have been some suggestions that these unexpected effects may come from shadowing or feature scattering. Using numerical scattering simulations, it can be shown that the horizontal backscattering cannot be larger than the vertical one for the Gaussian surfaces. Our main objective of this study is to gain a clear understanding of scattering mechanisms underlying the near grazing ocean scattering. In order to evaluate the backscattering cross section from ocean surfaces at near grazing incidence, both the hydrodynamic modeling of ocean surfaces and an accurate near grazing scattering theory are required. For the surface modeling, we generate Gaussian surfaces from the ocean surface power spectrum which is derived using several experimental data. Then, weakly nonlinear large scale ocean surfaces are generated following Longuet-Higgins. In addition, the modulation of small waves by large waves is included using the conservation of wave action. For surface scattering, we use MOM (Method of Moments) to calculate the backscattering from scattering patches with the two scale shadowing approximation. The differences between Gaussian and non-Gaussian surface scattering at near grazing incidence are presented.

Kim, Yunjin↗
NASA NTRS1993

Multi-band reflector antenna with double-ring element frequency selective subreflector

Frequency selective subreflectors (FSS) are often employed in the reflector antenna system of a communication satellite or a deep space exploration vehicle for multi-frequency operations. In the past, FSS's have been designed for diplexing two frequency bands. For example, the Voyager FSS was designed to diplex S and X bands and the TDRSS FSS was designed to diplex S and Ku bands. Recently, NASA's CASSINI project requires an FSS to multiplex four frequency (S/X/Ku/Ka) bands. Theoretical analysis and experimental verifications are presented for a multi-band flat pannel FSS with double-ring elements. Both the exact formulation and the thin-ring approximation are described for analyzing and designing this multi-ring patch element FSS. It is found that the thin-ring approximation fails to predict the electrically wide ring element FSS's performance. A single screen double-ring element FSS is demonstrated for the tri-band system that reflects the X-band signal while transmitting through the S- and Ku-band signals. In addition, a double screen FSS with non-similar double-ring elements is presented for the Cassini's four-band system which reflects the X- and Ka-band signals while passing the S- and Ku-band signals. To accurately predict the FSS effects on a dual reflector antenna's radiation pattern, the FSS subreflector's transmitted/reflected field variation as functions of the polarization and incident angles with respect to the local coordinates was taken into account. An FSS transmission/reflection coefficient table is computed for TE and TM polarizations at various incident angles based on the planar FSS model. Next, the hybrid Geometric Optics (GO) and Physical Optics (PO) technique is implemented with linearly interpolating the FSS table to efficiently determine the FSS effects in a dual reflector antenna.

Wu, Te-Kao↗
NASA NTRS1993

Mobile antenna development at JPL

The Jet Propulsion Laboratory (JPL), under the sponsorship of NASA, has pioneered the development of land vehicle antennas for commercial mobile satellite communications. Several novel antennas have been developed at L-band frequencies for the Mobile Satellite (MSAT) program initiated about a decade ago. Currently, two types of antennas are being developed at K- and Ka-band frequencies for the ACTS (Advanced Communications Technology Satellite) Mobile Terminal (AMT) project. For the future, several hand-held antenna concepts are proposed for the small terminals of the Ka-band Personal Access Satellite System (PASS). For the L-band MSAT program, a number of omni-directional low-gain antennas, such as the crossed drooping-dipoles, the higher-order-mode circular microstrip patch, the quadrifilar helix, and the wrapped-around microstrip 'mast' array, have been developed for lower data rate communications. Several medium-gain satellite tracking antennas, such as the electronically scanned low-profile phased array, the mechanically steered tilted microstrip array, the mechanically steered low-profile microstrip Yagi array, and the hybrid electronically/mechanically steered low-profile array, have been developed for the MSAT's higher data rate and voice communications. To date, for the L-band vehicle application, JPL has developed the world's lowest-profile phased array (1.8 cm height), as well as the lowest-profile mechanically steered antenna (3.7 cm height). For the 20/30 GHz AMT project, a small mechanically steered elliptical reflector antenna with a gain of 23 dBi has recently been developed to transmit horizontal polarization at 30 GHz and receive vertical polarization at 20 GHz. Its hemispherical radome has a height of 10 cm and a base diameter of 23 cm. In addition to the reflector, a mechanically steered printed MMIC active array is currently being developed to achieve the same electrical requirements with a low profile capability. These AMT antenna developments, along with other Ka-band technologies, will lead to the development of several compact hand-held terminals for the PASS program. A few antenna concepts, such as the lap-top or desk-top terminal's printed array, the hand-held phased array, and the head-mounted low-profile array, have been proposed to achieve a future vision for the personal access communications system.

Huang, J.↗