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NASA NTRS1992

Study Of Dc Modulation Noise In Magnetic Recording Disks

Report discusses dc modulation noise in thin-film magnetic medium on magnetic recording disk. Presents study of statistical and spectral characteristics of noise and describes study of dependence of noise upon applied magnetic field, thickness of magnetic layer, and roughness of surface of layer. DC modulation noise attributed to nucleation of isolated regions of reversal of magnetization in recording medium, and to growth and coalescence of regions with increasing reverse applied magnetic field. Inhomogeneities in magnetic recording media, not roughnesses of surfaces, are dominant sources of noise.

Katti, Romney R.↗

NASA NTRS1991

The evolution of the gravitational radiation from stellar components of galaxies

The evolution is discussed of gravitational waves spectra produced by binary stars, supernova explosions, and coalescences of binary compact stars in outer galaxies. These spectra are integrated over a simple model of the universe to give an estimate of the stochastic gravitational wave background due to astrophysical sources.

Lipunov, V. M.↗

NASA NTRS1992

Aircraft optimization by a system approach: Achievements and trends

Recently emerging methodology for optimal design of aircraft treated as a system of interacting physical phenomena and parts is examined. The methodology is found to coalesce into methods for hierarchic, non-hierarchic, and hybrid systems all dependent on sensitivity analysis. A separate category of methods has also evolved independent of sensitivity analysis, hence suitable for discrete problems. References and numerical applications are cited. Massively parallel computer processing is seen as enabling technology for practical implementation of the methodology.

Sobieszczanski-Sobieski, Jaroslaw↗

NASA NTRS1991

Dynamics of free drops on USML-1

Drop-dynamics experiments are described in terms of scope and methodology that are designed for the USML-1 flight. Included are experiments regarding: (1) the equilibrium shapes of acoustically rotated drops; (2) large-amplitude drop and bubble-shape oscillations; (3) the collision and coalescence of free drops; and (4) the deformation and instability of levitated drops. The space flight is concluded to provide an effective containerless microgravity environment for studying linear and nonlinear dynamics.

Wang, Taylor G.↗

NASA NTRS1991

Cloud condensation nuclei near marine stratus

Extensive airborne measurements of cloud condensation nucleus (CCN) spectra and concentrations of total particles, or condensation nuclei (CN), below, in, and above the stratus cloud decks off the southern California coast point to important aerosol-cloud interactions. Consistently low CCN concentrations below cloud appear to be due to cloud scavenging processes which include Brownian coagulation, nucleation, coalescence, and drizzle. The higher CCN and CN concentrations above cloud are associated with ambient ozone concentrations which suggest a link with continental, probably anthropogenic, sources, even at distances of 500 km from the California coast.

Hudson, James G.↗

NASA NTRS1992

Generic grain size distribution for liquid phase sintering

In order to develop an empirical model to represent a generic grain size distribution for liquid phase sintering, experimental data and theoretical models are analyzed using the plot of standard normal vs normalized size. From the experimental 3D grain size plot, the most representative line is deduced. The line best fits the Lifshitz-Slyozov encounter modified model except for the small size ranges of the lower 15 percent of the distribution. It is shown that grain coalescence might be a cause for this departure from theory.

German, R. M.↗

NASA NTRS1991

Grain growth kinetics in liquid-phase-sintered zinc oxide-barium oxide ceramics

Grain growth of ZnO in the presence of a liquid phase of the ZnO-BaO system has been studied for temperatures from 1300 to 1400 C. The specimens were treated in boiling water and the grains were separated by dissolving the matrix phase in an ultrasonic bath. As a consequence 3D grain size measurements were possible. Microstructural examination shows some grain coalescence with a wide range of neck size ratios and corresponding dihedral angles, however, most grains are isolated. Lognormal grain size distributions show similar shapes, indicating that the growth mechanism is invariant over this time and temperature. All regressions between G exp n and time for n = 2 and 3 proved statistically significant. The rate constants calculated with the growth exponent set to n = 3 are on the same order of magnitude as in metallic systems. The apparent activation energy for growth is estimated between 355 and 458 kJ/mol.

Yang, Sung-Chul↗

NASA NTRS1992

Numerical studies of dilute and dense spray characteristics

Several issues involving the improvement of physical submodels and the computational efficiency in modeling dilute and dense spray combustion are discussed. First, the implementations of a dispersion width approach accounting for turbulent dispersion within each computational parcel is discussed. This is essentially a statistical transport model and the testings of this model confirm the capability of accurately representing dispersion in nearly-homogeneous and inhomogeneous turbulent flows with improved efficiency over the delta function stochastic separated flow model. To account for the dense spray effects, an existing drop collision and coalescence model and a Taylor analogy breakup (TAB) model were employed. These models were incorporated into a state-of-the-art multiphase all-speed transient flow solution procedure. Several examples including nonevaporating, evaporating, and burning dense spray cases were studied. The numerical results show reasonably good comparisons with available experimental data in terms of spray penetration, drop sizes, and overall configuration of a spray flame.

Kim, Y. M.↗

NASA NTRS1992

Three-dimensional simulation of a translating strut inlet

A three-dimensional Navier-Stokes code is used to numerically simulate the flow through a translating strut scramjet inlet. The inlet has variable geometry for efficient operation over a wide speed range. Overall flow-field features such as the corner flow, topwall separation, shockwave coalescence, cowl pressure increase, and flow distortion at the throat are investigated. Comparisons are made with experimental results to provide for the assessment of the present analysis. Effects of boundary-layer ingestion on the overall flow features are also investigated.

Singh, D. J.↗

NASA NTRS1992

An experimental study of the fluid mechanics associated with porous walls

The fluid mechanics associated with the blowing phenomenon from porous walls is measured and characterized. The measurements indicate that the flow exiting a porous wall exhibits a lumpy velocity profile caused by the coalescence effects of smaller jets emerging from the surface. The velocity variations are spatially stable and prevail even at low flow rates. The intensity of this pseudoturbulence is found to be directly proportional to the filter rating of the porous wall and to increase linearly with the mean velocity. Beyond a critical mean velocity, the pseudoturbulence intensity shows a leveling trend with increase in the mean velocity. This critical velocity varies inversely as the filter rating and represents the onset of fully developed jetting action in the flow field. Based on the data, a more appropriate length scale for the flow field is proposed and a correlation is developed that can be used to predict the onset of fully developed jets in the flow emerging from a porous wall.

Ramachandran, N.↗

NASA NTRS1992

Polygenetic origin of Hrad Vallis region of Mars

A variety of apparently genetically related landforms in the Hrad Vallis region of Mars is examined. It is found that the outer valley of Hrad and the neighboring valley are composite features formed by karst or thermokarst processes and the growth and coalescence of smaller fault-aligned depressions. The materials in the vicinity may be soluble carbonates or sulfates or clay-rich sediment subject to thixotropic collapse. Water circulating within fault and fissure systems removed materials or altered the stability of materials to allow formation of steep-sided depressions. After the emplacement of surface materials in early Amazonian time, water was probably discharged to the surface. Isolate topographic depressions probably formed synchronously with the early release of water. Open lakes may have existed for a time.

De Hon, Rene A.↗

NASA NTRS1992

Triton - Voyager's finale

The investigation of the Neptunian satellite Triton by the Voyager 2 is described with interpretations of the object's nature and composition. The orbit, seasonal cycle, and southern-hemisphere solstice are described, and the composition of the satellite is discussed. Triton's mass and radius are known, and the objects is made up of about 70 percent rock and organics and 30 percent ice by mass. Triton's interior is warm and geologically active considering its distance from the sun, and large amounts of frozen methane and nitrogen are theorized to contribute to the object's high reflectivity. Also noted in the Voyager color images are creeping ice, cryogenic lava, and dark streaks on the south polar cap from nitrogen gas leaks driven by a type of greenhouse effect. Triton represents a class of satellite that has not been observed previously: a moon-sized body in a retrograde inclined orbit from the class of objects that coalesced to form Neptune.

Brown, R. H.↗

NASA NTRS1992

The evolution of a primordial binary population in a globular cluster

A simple model is presented for the evolution of a primordial binary population in a globular cluster. Monte Carlo simulations are given for an initial population of 50,000 binaries against a fixed background population of 500,000 single stars in a tidally truncated cluster model. Individual histories of all binaries are followed through mass segregation, scattering recoil, escape from the cluster, or coalescence. It is found that most binaries are destroyed by binary-binary interactions, with the rest escaping in the point-mass approximation. In a more realistic model, the majority of the rest merge. At any instant, most of the remaining binaries are drifting in toward the center before their first strong encounter. A typical binary spends most of its active life in or near the cluster core. The few binaries which receive a recoil sufficient to place them in the halo past the half-mass radius remain there long enough to make a significant contribution to the radial binary distribution.

Hut, Piet↗

NASA NTRS1992

The kinematics of intermediate-redshift gaseous galaxy halos

High-resolution spectroscopy of the absorption lines associated with four intermediate-redshift galaxies are presented. These observations are combined with existing spectroscopy of the redshifted 21-cm absorption lines toward 0235 + 164 and 1229 - 021. Evidence is presented to the effect that the lines of sight through the absorbers toward 0235 + 164 and 1229 - 021 penetrate material for which the velocity fields are dominated by systematic rather than by random motions, and the line profiles are found to be consistent with what is expected for rotating ensembles of clouds. It is suggested that for these two absorbers the absorption arises in individual clouds within single galaxy halos rather than, for example, in collections of coalescing subgalactic fragments, for which the expected velocity fields are more turbulent, or as a result of galaxy-galaxy clustering. It is concluded that lines of sight that impact absorbing regions at small impact parameters intercept many clouds, sample large velocity intervals, and are likely to show redshifted 21-cm absorption, whereas lines of sight that impact absorbing regions at large impact parameters intercept few clouds, sample small velocity intervals, and are unlikely to show redshifted 21-cm absorption.

Lanzetta, Kenneth M.↗

NASA NTRS1992

High-entropy fireballs and jets in gamma-ray burst sources

Two mechanisms whereby compact coalescing binaries can produce relatively 'clean' fireballs via neutrino-antineutrino annihilation are proposed. Preejected mass due to tidal heating will collimate the fireball into jets. The resulting anisotropic gamma-ray emission can be efficient and intense enough to provide an acceptable model for gamma-ray bursts, if these originate at cosmological distances.

Meszaros, P.↗

NASA NTRS1992

Studies on fuel spray characteristics in high-pressure environment

The present study deals with several issues involving the improvement of physical submodels and the computational efficiency in modeling dense fuel sprays. To improve the computational efficiency, a parcel PDF approach is implemented which can account for turbulent dispersion within each computational parcel. The advantage of a parcel PDF tracking method is to reduce the number of computational parcels representing the spray dynamics as well as to obtain grid-independent solutions for two-phase flows. To account for the dense spray effects, an existing drop collision and coalescence model, two breakup models, and a Reitz's wave instability model were used. These models were incorporated into a state-of-the-art multiphase all-speed transient flow solution procedure. Comparative performance for two breakup models as well as the turbulence modulation effects are also studied. Validation cases include the nonevaporating and evaporating solid-cone dense sprays. The predictions show a reasonably good agreement with available experimental results in terms of spray penetration, drop sizes, gas and drop mean velocities, and gas and drop rms velocities. The numerical results indicate that the present parcel PDF model has the capability of accurately representing drop dispersion in dense sprays with manageable number of computational parcels.

Shang, H. M.↗

NASA NTRS1992

Analytic and numerical models of wave-CISK with conditional heating

Wave-CISK with conditional heating is investigated in the equatorial zonal-height plane by analytic and numerical techniques. For two- and three-level models, previous results are extended to give additional evidence that the most unstable mode has a single wet region of infinitesimal width. A three-level model has qualitatively similar behavior as the two-level model except that propagating solutions are possible due to coalescence of internal vertical modes. Phase speeds with conditional heating are found to be slightly greater than those for unconditional heating. The structure has one circulation cell in the vertical and is asymmetric in longitude with stronger motion on the leading edge. Growth rate is inversely proportional to the width of the single wet region. That width can be limited by second-order diffusion. A general integral relationship between growth rate, viscosity, phase speed, and heating is derived. The main conclusion is that the linear wave-CISK catastrophe is modified by conditional heating but not eliminated. The preferred mode of instability has one wet region, but it occurs on the smallest possible scale. It is likely that numerical models that use conditional heating are sensitive to resolution, especially for the commonly used spectral truncations, unless there is sufficiently strong damping at the smallest scales.

Crum, Francis X.↗

NASA NTRS1992

Al Umchaimin depression, Western Iraq: An impact structure?

Al Umchaimin, in Arabic 'hiding place' or 'place of ambush', is located at latitude 32 degrees 35.5 N and longitude 39 degrees 25 E. The nearly circular depression averages 2.75 km in diameter and is 33-42 m deep. It is floored with fine-grained, clay-rich deposits, estimated to be 36 m thick, the surface of which shows well-developed desiccation fissures or mudcracks when dry. Because of its nearly circular planimetric shape and its apparent isolation from other surface and subsurface features, it has been considered by some to be a possible meteorite impact structure and by others to be a surface collapse feature that originated following removal of magma from the subsurface as the magma extruded elsewhere. Al Umchaimin was listed in the U.S. Geological Survey tabulation of 110 structures worldwide for which a meteorite impact origin had been suggested. It was placed in Category 6 Structures for which more data are required for classification. It is concluded that, on the basis of the studies that have been made of Al Umchaimin and on the basis of the brief site visit made, Al Umchaimin probably is not an impact structure but most likely resulted from the enlargement and coalescence of sink holes and eventual collapse of the roof material into the resulting cavity.

Underwood, James R., Jr.↗