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At least 847 records · Page 47
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NASA NTRS1987

Convenient stability criteria for difference approximations of hyperbolic initial-boundary value problems. II

The results of Goldberg and Tadmor (1985) are extended to achieve improved stability criteria for a large class of approximations to the initial boundary value problem associated with a particular hyperbolic system in a quarter plane. In a stability analysis, it is shown that the entire approximation is stable if and only if the scalar outflow components of its principal part are stable. Thus, the global stability question is reduced to that of a scalar, homogeneous outflow problem. The stability criteria for the reduced problem, which depend both on the basic scheme and the boundary conditions, but very little on the interaction between the two, are stated and used to establish previous examples and new ones, including a host of dissipative and nondissipative examples. There is no difficulty in extending the stability criteria to two-boundary problems and initial-boundary value problems with variable coefficients.

Goldberg, Moshe↗
NASA NTRS1987

Severe winds in the DFW microburst measured from two aircraft

Advanced analytical methods are applied to the digital flight data and Air Traffic Control radar records from two airliners, Delta flight 191 and American flight 539, that penetrated a microburst at the Dallas/Ft. Worth airport on August 2, 1985. The results for Delta 191 (which crashed) show that the aircraft encountered a strong downburst followed by a strong outflow accompanied by large and rapid changes in the vertical wind. The rapid changes in the vertical wind detected near the ground are attributed to vortex-induced turbulence. The next following aircraft, American 539, made a go-around 110 sec after Delta 191 and traversed the microburst at an altitude of about 2500 ft above the ground. The measured winds during this go-around indicate a broad pattern of downflow in the microburst, with regions of upflow at the extreme edges. The combined results indicate a microburst that was increasing in size with vortex-induced turbulence embedded in a strong outflow near the ground. These wind measurements provide a realistic description of the microburst wind environment that can be used in simulator studies and pilot training, and provide a standard of comparison for ongoing experiments and modeling of microbursts involving vortex rings.

Wingrove, R. C.↗
NASA NTRS1986

Convective scale interaction: Arc cloud lines and the development and evolution of deep convection

Information is used from satellite data and research aircraft data to provide new insights concerning the mesoscale development and evolution of deep convection in an atmosphere typified by weak synoptic-scale forcing. The importance of convective scale interaction in the development and evolution of deep convection is examined. This interaction is shown to manifest itself as the merger and intersection of thunderstorm outflow boundaries (arc cloud lines) with other convective lines, areas or boundaries. Using geostationary satellite visible and infrared data convective scale interaction is shown to be responsible for over 85 percent of the intense convection over the southeast U.S. by late afternoon, and a majority of that area's afternoon rainfall. The aircraft observations provided valuable information concerning critically important regions of the arc cloud line: (1) the cool outflow region, (2) the density surge line interface region; and (3) the sub-cloud region above the surge line. The observations when analyzed with rapid scan satellite data, helped in defining the arc cloud line's life cycle as 3 evolving stages.

Purdom, James Francis Whitehurst↗
NASA NTRS1987

Radio and infrared properties of young stars

Observing young stars, or more appropriately, pre-main-sequence (PMS) stars, in the infrared and at radio frequencies has the advantage over optical observation in that the heavy extinction associated with a star forming region is only a minor problem, so that the whole region can be studied thoroughly. Therefore, it means being able to: (1) search for stars and do statistical studies on the rate of star formation; (2) determine their luminosity, hence, to study luminosity functions and initial mass functions down to low masses; and (3) to study their spectra and, thus, to determine the prevailing conditions at and near the surface of a newly born star and its relations with the surrounding environment. The third point is of principal interest. The report limits itself to a consideration of the observations concerning the processes of outflows from, and accretion onto, PMS stars and the theory necessary to interpret them. Section 2 discusses the radiative processes relevant in stellar outflows. The main observational results are presented in Section 3. A discussion of the statistical properties of stellar winds from PMS stars are given in Section 4.

Panagia, Nino↗
NASA NTRS1987

Detection of interstellar PN - The first phosphorus-bearing species observed in molecular clouds

Phosphorus nitride (PN) has been detected in the interstellar medium. The J = 2-1, 3-2, 5-4, and 6-5 rotational lines of this species have been observed toward Orion-KL, and the J = 2-1 transition in Sgr B2 and W51. The PN line profiles in Orion indicate that the molecule's emission arises from the 'plateau' or 'doughnut' region associated with the outflow from IRc2. The species is thus primarily present in hot, dense gas. Column densities derived for PN toward Orion-KL are (3-4) x 10 to the 13th/sq cm, but may be as high as 10 to the 14th/sq cm, if the species is located in a 10-arcsec region. These column densities imply a fractional abundance for PN in the Orion 'plateau' of (1-4) x 10 to the -10th. Such a large abundance for PN is not predicted by quiescent cloud ion-molecule chemistry and suggests that high-temperature processes are responsible for the synthesis of PN in the KL outflow.

Ziurys, L. M.↗
NASA NTRS1988

Luminosity excesses in low-mass young stellar objects - A statistical study

This paper presents a statistical study in which the observed total luminosity is compared quantitatively with an estimate of the stellar luminosity for a sample of 59 low-mass young stellar objects (YSOs) in the Taurus-Auriga complex. In 13 of the analyzed YSOs, luminosity excesses greater than 0.20 are observed together with greater than 0.6 IR excesses, which typically contribute the bulk of the observed excess luminosity and are characterized by spectral energy distributions which are flat or rise toward long wavelengths. The analysis suggests that YSOs showing the largest luminosity excesses typically power optical jets and/or molecular outflows or have strong winds, as evidenced by the presence of O I emission, indicating a possible correlation between accretion and mass-outflow properties.

Strom, Karen M.↗
NASA NTRS1988

On wind-type flows in astrophysical jets. III - Temporal evolution of perturbations and the formation of shocks

The temporal evolution of disturbances in a spherically symmetric polytropic wind from a central object is studied. Such disturbances may be due to localized momentum addition/subtraction, as, for example, by MHD waves, heating/cooling mechanisms in the outflow, or localized deviations from spherical symmetric expansion. The evolution of an initial perturbed state to a continuous or discontinuous final equilibrium state, as predicted by previous analytic calculations for stationary flows, is followed. It is shown that some of the predicted discontinuous equilibrium states are not physically accessible, while the attainment of the remaining equilibrium states depends on both the temporal and the spatial parameters characterizing the perturbation. The results are derived for solar conditions, but in fact can be applied to outflows in other astrophysical systems. In particular, applications to the solar wind and flows in astrophysical jets are discussed.

Trussoni, E.↗
NASA NTRS1988

Mass loss from rapidly rotating magnetic protostars

It is proposed that bipolar outflows from young stellar objects originate from a protostar rotating at breakup at its equator because it is being spun up by an adjoining accretion disk. Mass outflow at an appreciable fraction of the infall rate from a surrounding molecular cloud core onto the star and disk can be driven centrifugally if the protostar has a sufficiently strong magnetic field. The expansion of the flow toward the rotational poles may provide a collimation mechanism for focusing an ordinary stellar wind into optical jets.

Shu, Frank H.↗
NASA NTRS1988

Comet P/Encke's nongravitational force

A nearly direct measurement of the nongravitational force on a cometary nucleus is presented. IUE spectra of comet P/Encke show significant Greenstein effects even when centered on the nucleus. These imply net heliocentric components of outflow of 1.6 and 0.3 km/s for the OH radicals before and after perihelion near r = 0.8 AU. This implies a large asymmetry in the radial nongravitational acceleration (A1) about perihelion, which must be taken into account in order to derive the mass of the nucleus. The asymmetry is consistent with Sekanina's (1988) latest model for the Encke nucleus, but the measurements suggest an outflow velocity near 2 km/s, significantly greater than generally accepted values. A tentative value for the mass of the Encke nucleus is proposed, although improved modeling of the nongravitational acceleration is required to obtain a definitive value.

A'Hearn, Michael F.↗
NASA NTRS1988

Chaos in a spatially-developing plane mixing layer

A spatially-developing plane mixing layer was analyzed for chaotic behavior. A direct numerical simulation of the Navier-Stokes equations in a 2-D domain infinite in y and having inflow-outflow boundary conditions in x was used for data. Spectra, correlation dimension and the largest Lyapunov exponent were computed as functions of downstream distance x. When forced at a single (fundamental) frequency with maximum amplitude, the flow is periodic at the inflow but becomes aperiodic with increasing x. The aperiodic behavior is caused by the presence of a noisy subharmonic caused by the feedback between the necessarily nonphysical inflow and outflow boundary conditions. In order to overshadow this noise the flow was also studied with the same fundamental forcing and added random forcing of amplitude upsilon prime sub R/delta U = 0.01 at the inlet. Results were qualitatively the same in both cases: for small x, spectral peaks were sharp and dimension was nearly 1, but as x increased a narrowband spectral peak grew, spectra decayed exponentially at high frequencies and dimension increased to greater than 3. Based on these results, the flow appears to exhibit deterministic chaos. However, at no location was the largest Lyapunov exponent found to be significantly greater than zero.

Broze, J. G.↗
NASA NTRS1988

Inflow/outflow boundary conditions and global dynamics of spatial mixing layers

The numerical simulation of incompressible spatially-developing shear flows poses a special challenge to computational fluid dynamicists. The Navier-Stokes equations are elliptic and boundary equations need to be specified at the inflow and outflow boundaries in order to compute the fluid properties within the region of interest. It is, however, difficult to choose inflow and outflow conditions corresponding to a given experimental situation. Furthermore the effects that changes in the boundary conditions or in the size of the computational domain may induce on the global dynamics of the flow are presently unknown. These issues are examined in light of recent developments in hydrodynamic stability theory. The particular flow considered is the spatial mixing layer but it was expected that similar phenomena were bound to occur in other cases such as channel flow, the boundary layer, etc. A short summary of local/global and absolute/convective instability concepts is given. The results of numerical simulations are presented which strongly suggest that global resonances may be triggered in domains of finite streamwise extent although the evolution of the perturbation vorticity field is everywhere locally convective. A relationship between finite domains and pressure sources which might help in devising a scheme to eliminate these difficulties is discussed.

Buell, J. C.↗
NASA NTRS1988

The polar wind

The classical polar wind is considered to be an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes. The paper reviews the characteristic features of this flow, with emphasis placed on recent theoretical predictions. Consideration is given to H(+) and He(+) outflow characteristics both with and without convection electric field effects, the collisionless polar wind characteristics (including the formation of temperature anisotropies), and the time-dependent polar wind behavior in response to localized density perturbations and localized heat sources.

Schunk, R. W.↗
NASA NTRS1989

The constancy of the ratio of the molecular hydrogen lines at 3.8 microns in Orion

The 1-0 O(7) and 0-0 S(13) lines of H2, at 3.807 and 3.846 microns, have been mapped over the region of the Orion molecular outflow. The intensity ratio of these lines is found to be independent of position in the outflow. From this it is inferred that the structure of the shocks and their cooling flows in Orion may be more akin to hydrodynamic shocks than the low-temperature C-shocks that are currently favored.

Brand, P. W. J. L.↗
NASA NTRS1989

High-excitation lines of molecular hydrogen: A discriminant between shock models

The results of column densities of molecular hydrogen, calculated from nineteen infrared line intensities, are discussed. They were measured at peak 1 of the outflow of the Orion molecular cloud OMC-1. The 1-0 O(7) and 0-0 S(13) lines of H2, at 3.8 microns, are mapped over the source. Their intensity ratio is found to be independent of position in the outflow. These observations are well fitted by a simple cooling-flow model of the line emitting region, but seem to be at variance with predictions of C-shocks current in the literature.

Burton, M.↗
NASA NTRS1989

Fluvial valleys and Martian palaeoclimates

Theoretical models of early Martian atmospheric evolution describe the maintenance of a dense CO2 atmosphere and a warm, wet climate until the end of the heavy-bombardment phase of impacting. However, the presence of very young, earthlike fluvial valleys on the northern flank of Alba Patera conflicts with this scenario. Whereas the widespread ancient Martian valleys generally have morphologies indicative of sapping erosion by the slow outflow of subsurface water, the local Alba valleys were probably formed by surface-runoff processes. Because subsurface water flow might be maintained by hydrothermal energy inputs and because surface-runoff valleys developed late in Martian history, it is not necessary to invoke drastically different planet-wide climatic conditions to explain valley development on Mars. The Alba fluvial valleys can be explained by hydrothermal activity or outflow-channel discharges that locally modified the atmosphere, including precipitation and local overland flow on low-permeability volcanic ash.

Gulick, Virginia C.↗
NASA NTRS1989

Modeling mass loss from B(e) stars

It was suggested by Zickgraf et al. (1986) that the outer atmospheres of some B(e) stars have a two-component structure: a fast, radiation-driven wind from the pole, and a dense, slow outflow from the equator. Poe et al. (1989) developed this theory to explain the momentum problem associated with WR stars. This paper uses the multiforce wind theory of Poe et al. to model the B(e) outflow phenomenon. Two general questions are investigated: (1) whether B(e) stars can be rotating near critical speed, and if so, (2) what constraints can be placed on the parameters that determine the two-component flow structure.

Cassinelli, J. P.↗
NASA NTRS1990

Relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1, FGGE

FGGE Level III-b analyses, produced by the Goddard Laboratory for Atmospheres, NASA, are used to investigate the relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1 (January 5 - March 4, 1979). The mean state of two 15-day periods is examined, as well as day-to-day variations for the entire 59-day period. In Period 1 (January 6-20), the central South Pacific was extremely active convectively, while in Period 2 (February 3-17), convective activity over the western Indian Ocean was enhanced. Episodes of strong outflow in the tropics, as measured by the upper tropospheric velocity potential, were found to be well correlated with the strengthening and propagation of westerly wind maxima in the subtropics. The average location of the westerly maximum over the South Pacific and Indian oceans was about 16 deg latitudes south, and slightly east, of its corresponding heat source. For a cyclone case study which is presented, however, this distance was considerably less. The response time between the upper level tropical outflow and subtropical westerly enhancement appears to be less than 12 hours; however, an exact temporal scale was difficult to identify.

Hurrel, James W.↗
NASA NTRS1989

PAH formation in carbon-rich circumstellar envelopes

While there is growing observational evidence that some fraction of interstellar carbon is in polycyclic aromatic hydrocarbons (PAH's), the mechanisms by which these molecules might be formed have not been extensively studied. A detailed investigation of PAH production in the outflowing molecular envelopes of carbon-rich red giant star is presented. The gasphase kinetics of a chemical reaction mechanism developed to study soot production in hydrocarbon flames is modified to apply in circumstellar environments. It was found that astrophysically significant quantities of PAH's can be formed in carbon star envelopes provided the gas is sufficiently dense and resides for a long time in the temperature range of 900 to 1100 k. The precise yield of PAH's is very sensitive to astronomical parameters of the envelope (e.g., mass loss rate, outflow velocity, and acetylene abundance) and certain poorly determined chemical reaction rates.

Feigelson, Eric D.↗