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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Experiments on the applicability of MAE techniques for predicting sound diffraction by irregular terrains

The sound field diffracted by a single smooth hill of finite impedance is studied both analytically, within the context of the theory of Matched Asymptotic Expansions (MAE), and experimentally, under laboratory scale modeling conditions. Special attention is given to the sound field on the diffracting surface and throughout the transition region between the illuminated and the shadow zones. The MAE theory yields integral equations that are amenable to numerical computations. Experimental results are obtained with a spark source producing a pulse of 42 microsec duration and about 130 Pa at 1 m. The insertion loss of the hill is inferred from measurements of the acoustic signals at two locations in the field, with subsequent Fourier analysis on an IBM PC/AT. In general, experimental results support the predictions of the MAE theory, and provide a basis for the analysis of more complicated geometries.

Berthelot, Yves H.↗
NASA NTRS1988

I(CO)/N(H2) conversions and molecular gas abundances in spiral and irregular galaxies

Observations of emission in the J = 1-0 rotational transition of interstellar CO are used to obtain column densities and masses of hydrogen. By taking into account the effects of variations in molecular cloud parameters on conversion factors between integrated CO intensity and molecular hydrogen column density, it is shown that conversion factors are very sensitive to the kinetic temperature of the emitting gas. Results indicate that the gas temperatures in systems with high star formation rates can be quite high, and it is suggested that use of a standard conversion factor will lead to systematic overestimation of the amount of molecular gas.

Maloney, Philip↗
NASA NTRS1988

Coherent HF radar backscatter from small-scale irregularities in the dusk sector of the subauroral ionosphere

This paper describes the characteristics of backscatter from decameter-scale ionospheric plasma density irregualarities, observed with an impressive regularity by the Goose Bay (Labrador) high-frequency (HF) radar in the dusk sector of the winter ionosphere, and discusses the relation of the scatter to the midlatitude trough. It is shown that this dusk scatter can be readily distinguished from other types of late afternoon/early evening scatter by the extreme equatorward position of its source region and by the low values of its associated radar Doppler velocities (not above 200 m/s) and spectral widths (not more than 200 m/s). A comparison of the radar observations with nearly simultaneous particle precipitation data obtained with the DMSP F6 satellite demonstrated that the source region of the backscatter lies within the subauroral ionosphere. It is shown that the characteristics of dusk scatter are compatible with the Spiro et al. (1978) model of the electrodynamics of the midlatitude trough.

Ruohoniemi, J. M.↗
NASA NTRS1989

IRAS observations of a large sample of normal irregular galaxies

IRAS 12, 25, 60, and 100 micron data are presented for a sample of Irr galaxies which span a large range in star-formation activity. It is found that the dwarf, giant, and amorphous Irr's generally have similar IR properties. The typical L(IR)/L(H-alpha) ratios of most classes of Irr's, except for many of the luminous blue galaxies, are low compared with those of spiral galaxies and are consistent with the Irr's being relatively transparent systems without large amounts of optically hidden star formation. Compared with spiral galaxies, the Irr's have similar L(IR)/L(B) ratios, warmer S(100)/S(60) ratios, cooler S(25)/S(12) ratios, and lower dust-to-H I gas mass ratios. The temperature, dust-to-H I gas mass ratios, and L(IR)/L(B) ratios do not correlate with the metallicity of the ionized gas of the Irr's. A correlation between the IR fluxes and both the H-alpha and the blue stellar fluxes is found for the Irr's.

Hunter, Deidre A.↗
NASA NTRS1989

Energy dissipation of Alfven wave packets deformed by irregular magnetic fields in solar-coronal arches

The importance of field line geometry for shear Alfven wave dissipation in coronal arches is demonstrated. An eikonal formulation makes it possible to account for the complicated magnetic geometry typical in coronal loops. An interpretation of Alfven wave resonance is given in terms of gradient steepening, and dissipation efficiencies are studied for two configurations: the well-known slab model with a straight magnetic field, and a new model with stochastic field lines. It is shown that a large fraction of the Alfven wave energy flux can be effectively dissipated in the corona.

Similon, Philippe L.↗
NASA NTRS1989

Small scale irregularities in Comet Halley's plasma mantle - An attempt at self-consistent analysis of plasma and magnetic field data

VEGA-1 measurements of the plasma density and magnetic field in the coma of Comet Halley show characteristic signatures over a significant portion of the outbound pass. It is found that the assumption that there is a balance between the thermal and magnetic pressures in these features can be used to obtain estimates of the plasma temperature as a function of distance from the nucleus. These estimates indicate that the ions cool from about 1.5 x 10 to the 6th K at 10 to the 5th km to 2 x 10 to the 5th K at 5 x 10 to the 4th km. The technique used here represents a novel approach whereby temperature measurements can be made in situations where only plasma density and magnetic field data are available.

Vaisberg, O. L.↗
NASA NTRS1989

Reconstructing irregularly sampled images by neural networks

Neural-network-like models of receptor position learning and interpolation function learning are being developed as models of how the human nervous system might handle the problems of keeping track of the receptor positions and interpolating the image between receptors. These models may also be of interest to designers of image processing systems desiring the advantages of a retina-like image sampling array.

Ahumada, Albert J., Jr.↗
NASA NTRS1990

Observations of CO in the Magellanic irregular galaxy NGC 55

The content of molecular gas in galaxies, mainly H2, is one of the key observations necessary for the understanding of star formation processes and history. As the CO molecule is the most widely distributed molecule after H2 and has easily observable mm lines, it is used as a tracer for the molecular gas. CO was detected towards the direction where the H alpha and 6 cm radio continuum emission is strongest (Hummel et al. 1986). Here, researchers present the Gaussian line parameters in tabular form. The distribution of CO corresponds well with the intense HI cloud near the bar of NGC 55. The extent of the CO cloud is about 975 pc perpendicular to the major axis. As the radio continuum and the H alpha emission also peaks in this region, it is most probably associated with the star forming region in NGC 55. Assuming that the molecular gas is in virial equilibrium, researchers derive a mass of about 8 times 10(exp 7) solar magnitude. The molecular mass found indicates that the conversion factor for the molecular mass in Irr galaxies as inferred from CO line emission is indeed higher by up to a factor of 20 compared to the canonical value for the Galaxy.

Heithausen, Andreas↗
NASA NTRS1991

Similarity scaling applied to sodar observations of the convective boundary layer above an irregular hill

Doppler sodar measurements made during the First ISLSCP Field Experiment of the temperature structure parameter C(T)2 and the standard deviation of vertical velocity fluctuations sigma-w in the convective boundary layer were analyzed. Results indicate that estimates of the surface heat flux H(0) inferred from the profiles of C(T)2 near the surface are smaller than values of H(0) measured by eddy correlation at a tower. The measured values of sigma-w were slightly smaller than estimates based on available parameterizations.

Sorbjan, Zbigniew↗
NASA NTRS1991

Distributed memory compiler methods for irregular problems: Data copy reuse and runtime partitioning

Outlined here are two methods which we believe will play an important role in any distributed memory compiler able to handle sparse and unstructured problems. We describe how to link runtime partitioners to distributed memory compilers. In our scheme, programmers can implicitly specify how data and loop iterations are to be distributed between processors. This insulates users from having to deal explicitly with potentially complex algorithms that carry out work and data partitioning. We also describe a viable mechanism for tracking and reusing copies of off-processor data. In many programs, several loops access the same off-processor memory locations. As long as it can be verified that the values assigned to off-processor memory locations remain unmodified, we show that we can effectively reuse stored off-processor data. We present experimental data from a 3-D unstructured Euler solver run on iPSC/860 to demonstrate the usefulness of our methods.

Das, Raja↗
NASA NTRS1992

Parallelization of irregularly coupled regular meshes

Regular meshes are frequently used for modeling physical phenomena on both serial and parallel computers. One advantage of regular meshes is that efficient discretization schemes can be implemented in a straight forward manner. However, geometrically-complex objects, such as aircraft, cannot be easily described using a single regular mesh. Multiple interacting regular meshes are frequently used to describe complex geometries. Each mesh models a subregion of the physical domain. The meshes, or subdomains, can be processed in parallel, with periodic updates carried out to move information between the coupled meshes. In many cases, there are a relatively small number (one to a few dozen) subdomains, so that each subdomain may also be partitioned among several processors. We outline a composite run-time/compile-time approach for supporting these problems efficiently on distributed-memory machines. These methods are described in the context of a multiblock fluid dynamics problem developed at LaRC.

Chase, Craig↗
NASA NTRS1992

Compiler analysis for irregular problems in FORTRAN D

We developed a dataflow framework which provides a basis for rigorously defining strategies to make use of runtime preprocessing methods for distributed memory multiprocessors. In many programs, several loops access the same off-processor memory locations. Our runtime support gives us a mechanism for tracking and reusing copies of off-processor data. A key aspect of our compiler analysis strategy is to determine when it is safe to reuse copies of off-processor data. Another crucial function of the compiler analysis is to identify situations which allow runtime preprocessing overheads to be amortized. This dataflow analysis will make it possible to effectively use the results of interprocedural analysis in our efforts to reduce interprocessor communication and the need for runtime preprocessing.

Vonhanxleden, Reinhard↗
NASA NTRS1992

Ultraviolet observations of close-binary and pulsating nuclei of planetary nebulae; Winds and shells around low-mass supergiants; The close-binary nucleus of the planetary nebula HFG-1; A search for binary nuclei of planetary nebulae; UV monitoring of irregularly variable planetary nuclei; and The pulsating nucleus of the planetary nebula Lo 4

A brief summary of the research highlights is presented. The topics covered include the following: binary nuclei of planetary nebulae; other variable planetary nuclei; low-mass supergiants; and other IUE-related research.

Bond, Howard E.↗
NASA NTRS1992

IOPS advisor: Research in progress on knowledge-intensive methods for irregular operations airline scheduling

Our research focuses on the problem of recovering from perturbations in large-scale schedules, specifically on the ability of a human-machine partnership to dynamically modify an airline schedule in response to unanticipated disruptions. This task is characterized by massive interdependencies and a large space of possible actions. Our approach is to apply the following: qualitative, knowledge-intensive techniques relying on a memory of stereotypical failures and appropriate recoveries; and quantitative techniques drawn from the Operations Research community's work on scheduling. Our main scientific challenge is to represent schedules, failures, and repairs so as to make both sets of techniques applicable to the same data. This paper outlines ongoing research in which we are cooperating with United Airlines to develop our understanding of the scientific issues underlying the practicalities of dynamic, real-time schedule repair.

Borse, John E.↗
NASA NTRS1993

On solving the irregularly-shaped plate buckling problem

This paper develops methods for finding critical buckling loads of isotropic plates. In particular, this paper is motivated by the lack of tabular data for non-rectangular plates such as general trapezoids. High order displacement functions are used, and boundary conditions are imposed by utilizing the Lagrange multiplier method, which allows for a variety of constraint conditions. The element is derived with the use of minimum potential energy and subparametric mappings. An example is given, and several test cases are presented to show the validity of the approach in the one element case.

Smith, James P.↗