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NASA NTRS1992

Numerical models for the collapse and fragmentation of centrally condensed molecular cloud cores

The gravitational collapse and fragmentation of centrally condensed molecular cloud cores are investigated using a new hydrodynamical code. The numerical scheme is second-order accurate and uses explicit finite difference methods to advance the fluid variables on a 3D Cartesian grid. Two initial power-law density profiles, rho varies as r exp -1 and rho varies as r exp -2, are considered, as well as two initial density perturbations in the azimuthal coordinate theta, rho-i - rho(1 + a cos 2theta) where a = 0.1 and 0.5. Fragmentation is found to be possible in these centrally condensed cores if the initial conditions also include differential rotation. Models which collapse with initial uniform rotation do not produce fragments. If molecular cloud cores are indeed centrally condensed, as suggested by observations of star-forming regions and by studies of ambipolar diffusion, then differential rotation may be a mechanism for producing binary protostars during gravitational collapse.

Myhill, Elizabeth A.↗

NASA NTRS1992

Excitation efficiency of an optical fiber core source

The exact field solution of a step-index profile fiber is used to determine the excitation efficiency of a distribution of sources in the core of an optical fiber. Previous results of a thin-film cladding source distribution to its core source counterpart are used for comparison. The behavior of power efficiency with the fiber parameters is examined and found to be similar to the behavior exhibited by cladding sources. It is also found that a core-source fiber is two orders of magnitude more efficient than a fiber with a bulk distribution of cladding sources. This result agrees qualitatively with previous ones obtained experimentally.

Egalon, Claudio O.↗

NASA NTRS1992

An improved heat transfer configuration for a solid-core nuclear thermal rocket engine

Interrupted flow, impingement cooling, and axial power distribution are employed to enhance the heat-transfer configuration of a solid-core nuclear thermal rocket engine. Impingement cooling is introduced to increase the local heat-transfer coefficients between the reactor material and the coolants. Increased fuel loading is used at the inlet end of the reactor to enhance heat-transfer capability where the temperature differences are the greatest. A thermal-hydraulics computer program for an unfueled NERVA reactor core is employed to analyze the proposed configuration with attention given to uniform fuel loading, number of channels through the impingement wafers, fuel-element length, mass-flow rate, and wafer gap. The impingement wafer concept (IWC) is shown to have heat-transfer characteristics that are better than those of the NERVA-derived reactor at 2500 K. The IWC concept is argued to be an effective heat-transfer configuration for solid-core nuclear thermal rocket engines.

Clark, John S.↗

NASA NTRS1993

On the nonlinear interfacial instability of rotating core-annular flow

The interfacial stability of rotating core-annular flows is investigated. The linear and nonlinear effects are considered for the case when the annular region is very thin. Both asymptotic and numerical methods are used to solve the flow in the core and film regions which are coupled by a difference in viscosity and density. The long-term behavior of the fluid-fluid interface is determined by deriving its nonlinear evolution in the form of a modified Kuramoto-Sivashinsky equation. We obtain a generalization of this equation to three dimensions. The flows considered are applicable to a wide array of physical problems where liquid films are used to lubricate higher or lower viscosity core fluids, for which a concentric arrangement is desired. Linearized solutions show that the effects of density and viscosity stratification are crucial to the stability of the interface. Rotation generally destabilizes non-axisymmetric disturbances to the interface, whereas the centripetal forces tend to stabilize flows in which the film contains the heavier fluid. Nonlinear affects allow finite amplitude helically travelling waves to exist when the fluids have different viscosities.

Coward, Aidrian V.↗

NASA NTRS1993

Steady induction effects in geomagnetism. Part 1B: Geomagnetic estimation of steady surficial core motions: A non-linear inverse problem

The problem of estimating a steady fluid velocity field near the top of Earth's core which induces the secular variation (SV) indicated by models of the observed geomagnetic field is examined in the source-free mantle/frozen-flux core (SFI/VFFC) approximation. This inverse problem is non-linear because solutions of the forward problem are deterministically chaotic. The SFM/FFC approximation is inexact, and neither the models nor the observations they represent are either complete or perfect. A method is developed for solving the non-linear inverse motional induction problem posed by the hypothesis of (piecewise, statistically) steady core surface flow and the supposition of a complete initial geomagnetic condition. The method features iterative solution of the weighted, linearized least-squares problem and admits optional biases favoring surficially geostrophic flow and/or spatially simple flow. Two types of weights are advanced radial field weights for fitting the evolution of the broad-scale portion of the radial field component near Earth's surface implied by the models, and generalized weights for fitting the evolution of the broad-scale portion of the scalar potential specified by the models.

Voorhies, Coerte V.↗

NASA NTRS1992

Globular cluster photometry with the Hubble Space Telescope. I - Description of the method and analysis of the core of 47 Tuc

Accurate photometry for individual post-main-sequence stars in the core of the Galactic globular cluster 47 Tuc is presented and analyzed using an empirical point spread function model and Monte Carlo simulations. A V vs. V-I color-magnitude diagrams is constructed which shows several distinct stellar types, including RGB and HB stars. Twenty-four blue straggler stars are detected in 47 Tuc, more concentrated toward the center of the cluster than the giants. This supports the hypothesis is that the stragglers are either coalesced stars or members of binary systems that are more massive than single stars. The radial profile of the projected stellar density is flat in the central region of 47 Tuc with a core radius of 23 +/- 2 arcsec. No signature of a collapsed core is evident. The observed radial cumulative distribution of stars rules out the presence of a massive compact object in the center.

Guhathakurta, Puragra↗

NASA NTRS1990

Elliptical-core two-mode optical-fiber sensor implementation methods

We present experimental methods for the practical implementation of few-mode elliptical-core sensors. Techniques for desensitizing the lead-in and lead-out fibers are discussed and results of a vibration sensor embedded in a graphite-epoxy composite are presented. A scheme using a single-mode elliptical-core fiber as the lead-in fiber and an offset circular-core single-mode fiber as the lead-out fiber is successfully implemented. Detection techniques for few-mode fiber sensors are reviewed and a novel fringe-counting method to unambiguously detect the amplitude and direction of dynamic strain is reported for the first time.

Murphy, Kent A.↗

NASA NTRS1993

On partial mixing zones in horizontal-branch stellar cores

The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution considers a criterion for the existence of a partial mixing zone in a general stellar model. This criterion implies that there must be a zone with a composition gradient. Attention is given to the consequences of the assumption for the behavior of the core close to the central He exhaustion; with the degree of mixing implied by the constraint on the temperature gradient, core opacity tends to a limit late in evolution, and then declines. The central opacity and flux decreases give the convective region a strong tendency to shrink.

Dorman, Ben↗

NASA NTRS1993

Ultraviolet properties of individual hot stars in globular cluster cores. I - NGC 1904 (M79)

As part of an observing program using the IUE satellite to investigate the properties of stars within the cores of Galactic globular clusters, we have obtained three spectra of the cluster NGC 1904 (M79). All three were long-integration-time, short-wavelength (SWP) spectra obtained at the so-called 'center-of-light', and all three showed evidence of multiple sources within the IUE large aperture. We describe the analysis of these spectra and present evidence that the UV sources represent individual hot stars in the post-horizontal-branch stage of evolution. We see more UV-bright objects in the core of this cluster than expected from surveys of similar objects discovered in the outer regions of other globulars, leading us to conclude that dynamical effects in the core may significantly alter the path of evolution off the horizontal branch. The spectra also appear to be fitted more closely by models using Population I metal abundances than by Population II abundance models.

Altner, B.↗

NASA NTRS1958

Local Instability of the Elements of a Truss-core Sandwich Plate

The charts presented give the compressive buckling coefficients for a single-truss-core and a double-truss-core sandwich plate. These charts cover a wide range of sandwich proportions and may be used for sandwiches with unequal faces. They apply to inplane compressive loads acting parallel or perpendicular to the core direction or for various combinations of these loads. (author)

Anderson, Melvin S↗

NASA NTRS1993

Simultaneous solution for core magnetic field and fluid flow beneath an electrically conducting mantle

The effects of laterally homogeneous mantle electrical conductivity were included in steady, frozen-flux core surface flow estimation along with refinements in method and weighting. The refined method allows simultaneous solution for both the initial radial geomagnetic field component at the core-mantle boundary (CMB) and the sub-adjacent fluid motion; it also features Gauss' method for solving the non-linear inverse problem associated with steady motional induction. The tradeoff between spatial complexity of the derived flows and misfit to the weighted Definitive Geomagnetic Reference Field models (DGRF's) is studied for various mantle conductivity profiles. For simple flow and a fixed initial geomagnetic condition, a fairly high deep-mantle conductivity performs better than either insulating or weakly conducting profiles; however, a thin, very high conductivity layer at the base of the mantle performs almost as well. Simultaneous solution for both initial geomagnetic field and flow reduces the misfit per degree of freedom even more than does changing the mantle conductivity profile. Moreover, when both core field and flow are estimated, the performance of the solutions and the derived flows become insensitive to the conductivity profile.

Voorhies, Coerte V.↗

NASA NTRS1993

Overturn of magma ocean ilmenite cumulate layer: Implications for lunar magmatic evolution and formation of a lunar core

We explore a model for the chemical evolution of the lunar interior that explains the origin and evolution of lunar magmatism and possibly the existence of a lunar core. A magma ocean formed during accretion differentiates into the anorthositic crust and chemically stratified cumulate mantle. The cumulative mantle is gravitationally unstable with dense ilmenite cumulate layers overlying olivine-orthopyroxene cumulates with Fe/Mg that decreases with depth. The dense ilmenite layer sinks to the center of the moon forming the core. The remainder of the gravitationally unstable cumulate pile also overturns. Any remaining primitive lunar mantle rises to its level of neutral buoyancy in the cumulate pile. Perhaps melting of primitive lunar mantle due to this decompression results in early lunar Mg-rich magmatism. Because of its high concentration of incompatible heat producing elements, the ilmenite core heats the overlying orthopyroxene-bearing cumulates. As a conductively thickening thermal boundary layer becomes unstable, the resulting mantle plumes rise, decompress, and partially melt to generate the mare basalts. This model explains both the timing and chemical characteristics of lunar magmatism.

Hess, P. C.↗

NASA NTRS1993

Manson impact structure, Iowa: First geochemical results for drill core M-1

The Manson Impact Structure is a large complex impact crater centered ca. S km north of the town of Manson, Iowa. It is the largest intact impact structure recognized in the United States (35 km in diameter). Its Ar-40/Ar-39 age is indistinguishable from that of the Cretaceous-Tertiary (K-T) boundary. The Manson structure may be one element of the events at the K-T boundary. The crater is completely covered by Quaternary glacial sedimentary deposits that are normally underlain by Cretaceous clastic sediments and flat-lying carbonate sediments of Phanerozoic age, as well as Proterozoic red clastic, metamorphic, volcanic, and plutonic rock sequences. The study of a reflection seismic profile, provided by Amoco, was critical in interpreting the structure. In the 35 km diameter zone that marks the extension of the crater the normal rock sequence is disturbed due to the impact, and at the center of the structure granitic basement rocks are present that have been uplifted from about 4 km depth. Our studies consist of detailed petrological and geochemical characterization of all cores, with emphasis on a detailed description of all rock types found in the core samples and their relationship to target rocks. Geochemical data on samples from the Manson M-1 core are presented.

Koeberl, Christian↗

NASA NTRS1991

Contributions from geomagnetic inverse theory to the study of hydromagnetic conditions near the core-mantle boundary

The Final Report on contributions from geomagnetic inverse theory to the study of hydromagnetic conditions near the core-mantle boundary (CMB) is presented. The original proposal was to study five questions concerning what the surface and satellite magnetic data imply about hydromagnetic and electromagnetic conditions near the CMB. The five questions are: (1) what do the surface and satellite data imply about the geomagnetic field B near the surface of the earth; (2) how does one extrapolate B down through the conducting mantle to the CMB; (3) if B on the CMB is visible, how accurately does it satisfy the frozen-flux approximation; (4) if frozen flux is a good approximation on the CMB, what can be inferred about the fluid velocity v in the upper core; and (5) if v at the CMB is visible, does it suggest any dynamical properties of the core, such as vertical advection, Alfven-inertial waves, link instabilities, or mantle effects. A summary of the research is provided.

Backus, George E.↗

NASA NTRS1993

Core formation in the Moon: The mystery of the excess depletion of Mo, W and P

We have evaluated siderophile element depletion models for the Moon in light of our improved statistical treatment of siderophile element abundance data and new information on the physics of core formation. If core formation occurred in the Moon at the large degrees of partial melting necessary for metal segregation, according to recent estimates, then a significant inconsistency (not seen in the eucrite parent body) exists in the depletion of the incompatible siderophile elements Mo, W, and P, compared to other siderophile elements in the Moon. The siderophile data, with the exception of Mo, are most consistent with terrestrial initial siderophile abundances and segregation of a very small core in the Moon. Our improved abundance estimates and possible explanations for these discrepancies are discussed.

Newsom, H. E.↗

NASA NTRS1993

Core formation by giant impacts: Conditions for intact melt region formation

Among the many effects of high-speed, giant impacts is widescale melting that can potentially trigger catastrophic core formation. If the projectile is sufficiently large, the melt pools to form an intact melt region. The dense phase then segregates from the melt, forming a density anomoly at the melt region's base. If the anomoly produces a differential stress larger than a certain minimum, it overcomes the mantle's long-term elastic strength and rapidly forms a core. It was previously shown that giant impacts effectively trigger core formation in silicate bodies by the time they grow to the mass of Mercury and in icy bodies by the time they grow larger than Triton. In order for this process to be viable, an intact melt region must be formed. Conditions under which this occurs is examined in more detail than previously published.

Tonks, W. B.↗

NASA NTRS1993

Measurement of intact-core length of atomizing liquid jets by image deconvolution

The investigation of liquid jet breakup and spray development is critical to the understanding of combustion phenomena in liquid propellant rocket engines. Much work has been done to characterize low-speed liquid jet breakup and dilute sprays, but atomizing jets and dense sprays have yielded few quantitative measurements due to their high liquid load fractions and hence their optical opacity. Focus was on a characteristic of the primary breakup process of round liquid jets, namely the length of the intact-liquid core. The specific application considered is that of shear-coaxial-type rocket engine injectors in which liquid oxygen is injected through the center post while high velocity gaseous hydrogen is injected through a concentric annulus, providing a shear force to the liquid jet surface. Real-time x ray radiography, capable of imaging through the dense two-phase region surrounding the liquid core, is used to make the measurements. The intact-liquid-core length data were obtained and interpreted using two conceptually different methods to illustrate the effects of chamber pressure, gas-to-liquid momentum ratio, and cavitation.

Woodward, Roger↗

NASA NTRS1993

Steady induction effects in geomagnetism. Part 1C: Geomagnetic estimation of steady surficial core motions: Application to the definitive geomagnetic reference field models

In the source-free mantle/frozen-flux core magnetic earth model, the non-linear inverse steady motional induction problem was solved using the method presented in Part 1B. How that method was applied to estimate steady, broad-scale fluid velocity fields near the top of Earth's core that induce the secular change indicated by the Definitive Geomagnetic Reference Field (DGRF) models from 1945 to 1980 are described. Special attention is given to the derivation of weight matrices for the DGRF models because the weights determine the apparent significance of the residual secular change. The derived weight matrices also enable estimation of the secular change signal-to-noise ratio characterizing the DGRF models. Two types of weights were derived in 1987-88: radial field weights for fitting the evolution of the broad-scale portion of the radial geomagnetic field component at Earth's surface implied by the DGRF's, and general weights for fitting the evolution of the broad-scale portion of the scalar potential specified by these models. The difference is non-trivial because not all the geomagnetic data represented by the DGRF's constrain the radial field component. For radial field weights (or general weights), a quantitatively acceptable explication of broad-scale secular change relative to the 1980 Magsat epoch must account for 99.94271 percent (or 99.98784 percent) of the total weighted variance accumulated therein. Tolerable normalized root-mean-square weighted residuals of 2.394 percent (or 1.103 percent) are less than the 7 percent errors expected in the source-free mantle/frozen-flux core approximation.

Voorhies, Coerte V.↗