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At least 235 records · Page 13

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NASA NTRS1988

Scan-independent slot arrays with parasitic wire arrays in stratified medium

The effect of a parasitic wire array on the scan admittance of a slot array has been investigated. Structures considered can consist of an infinite slot array and an arbitrary number of parasitic infinte arrays of piecewise linear wires, all arrays being embedded in a stratified medium. These include, as particular cases, phased arrays of Clavin elements. Expressing the fields from the arrays as plane waves, a procedure similar to the periodic moment method for infinite periodic structures is set up to obtain the the scan admittance of the slot array. Scan admittances are presented for a slot array with monopole arrays in free space, and a slot array with a tilted dipole array in a stratified medium. Blind spots at which the incident energy is mainly reflected rather than transmitted were found. Results obtained indicate the possibility of using parasitic wire arrays for scan compensation of active slot arrays.

Ng, Kwong T.↗

NASA NTRS1988

The small scale structure of the interstellar medium in the Orion association: The flotsam of star formation

The small scale dynamics and structure of the interstellar medium of the Orion OB1b, c association was studied using IUE high dispersion SWP and LWR/LWP spectra. The area surveyed centers on the Belt and encompasses the Orion Cloak. The dynamics and abundances of the ionized and warm neutral gas surrounding the central parts of the association are determined using 35 B stars. Results are compared with the picture of the Orion interstellar medium gained using COPERNICUS, especially for the structure and dynamics of the Orion Cloak.

Sonneborn, G.↗

NASA NTRS1988

Cracking of a layered medium on an elastic foundation under thermal shock

The cladded pressure vessel under thermal shock conditions which is simulated by using two simpler models was studied. The first model (Model 1) assumes that, if the crack size is very small compared to the vessel thickness, the problem can be treated as a semi-infinite elastic medium bonded to a very thin layer of different material. However, if the crack size is of the same order as the vessel thickness, the curvature effects may not be negligible. In this case it is assumed that the relatively thin walled hollow cylinder with cladding can be treated as a composite beam on an elastic foundation (Model 2). In both models, the effect of surface cooling rate is studied by assuming the temperature boundary condition to be a ramp function. The calculated results include the transient temperature, thermal stresses in the uncracked medium and stress intensity factors which are presented as a function of time, and the duration of cooling ramp. The stress intensity factors are also presented as a function of the size and the location of the crack. The problem is solved for two bonded materials of different thermal and mechanical properties. The mathematical formulation results in two singular integral equations which are solved numerically. The results are given for two material pairs, namely an austenitic steel layer welded on a ferritic steel substrate, and a ceramic coating on ferritic steel. In the case of the yielded clad, the stress intensity factors for a crack under the clad are determined by using a plastic strip model and are compared with elastic clad results.

Rizk, Abd El-Fattah A.↗

NASA NTRS1988

The turbulent interstellar medium and pressure-bounded molecular clouds

The existence of turbulence throughout the interstellar medium suggests that an appropriate value for the average pressure may be P/K larger than about 10,000. Negative-index polytropic models of interstellar clouds in equilibrium with an external medium at these pressures are predicted to have sizes, line widths, masses, and size-line width and size-density relations in good agreement with those observed and inferred for dark clouds. Thus these observed features of interstellar clouds do not require that they be completely self-gravitating or 'virialized' in the commonly used sense.

Maloney, Philip↗

NASA NTRS1988

Energy and mass balance in the three-phase interstellar medium

Details of the energy and mass balances are considered in the context of a three-phase interstellar medium. The rates of mass exchange between the different phases are derived based on the pressure variations created by supernova remnant expansions. It is shown that the pressure-confined warm and cold gases have stable temperatures under a variety of interstellar conditions. The three-phase quasi-static configuration is found to be a natural outcome, and both warm and cold phases generally contribute about half of the total mass density to the diffuse interstellar gas. The model is also likely to be self-regulatory in the sense that variations of the input parameters do not strongly alter the general result, which is consistent with most current observations. The consequences of extreme conditions on this model are considered, and the possible implications for interstellar medium in other galaxies are briefly discussed.

Wang, Zhong↗

NASA NTRS1987

The local interstellar medium

The structure of the local interstellar medium is reviewed with emphasis on the new observations of the cold neutral component. The properties of the high latitude molecular clouds are discussed as well as their relationship to the 'infrared cirrus' and atomic gas. The molecular clouds are shown to lie preferentially along the loops and filaments of local H I gas, but a detailed examination shows the CO and H I peaks are offset from one another. An expansion velocity of 7 km/s is derived for one of the shells, suggesting an expansion energy of about 8 x 10 to the 47th ergs, which is somewhat smaller than that expected for a supernova explosion. If the pressure in the clumps of the high latitude clouds is in equilibrium with the surrounding gas, the pressure in the shells is 10,000-100,000/cu cm K, an order of magnitude higher than what is generally argued to exist in the interstellar medium.

Blitz, Leo↗

NASA NTRS1989

Some aspects of transient cooling of a radiating rectangular medium

The emission from a gray radiating medium is analyzed for transient cooling in surroundings at a low temperature. The medium is rectangular with no variations in the direction normal to the cross section. The integral equation for the transient temperature distribution is solved numerically using a two-dimensional Gaussian integration subroutine. The emissive ability for a rectangle at uniform temperature is compared with that for transient cooling where the temperature distribution of the region has reached a fully developed shape, as shown by a separation of variables solution. The two solutions provide the upper and lower bounds for the emittance of a rectangle during transient cooling. The emittances for various aspect ratios are presented as a function of the mean length of the rectangle and are compared with results for a plane layer and a cylinder.

Siegel, Robert↗

NASA NTRS1990

Evidence for an inner scale to the density turbulence in the interstellar medium

The form of the density power spectrum is studied. The relationship between the visibility and the properties of density turbulence is discussed. Observational determinations of the power-law index of the interstellar density power spectrum are presented. An inner scale is derived, and the effect of an inner scale in the interstellar plasma turbulence is examined. It is concluded that the host plasma for the density irregularities must be similar to the warm ionized medium in the Mckee-Ostriker model (1979) of the interstellar medium or the low-density envelopes of H II regions.

Spangler, Steven R.↗

NASA NTRS1990

Turbulent mixing layers in the interstellar and intracluster medium

Cold clouds embedded in hot turbulent gas are often observed in the interstellar medium of galaxies and in the intracluster medium. Mixing layers develop around the clouds by the action of the turbulent gas. It is shown that their column density is limited by cooling and their mean temperature by the momentum flux. Provided that mixing is efficient, the mean temperature is the geometric mean of the hot and cold gas temperatures. EUV radiation from mixing layers around filaments in cluster cooling flows photoionizes the colder gas and may produce the commonly observed optical and UV emission lines. Mixing layers around cold galactic clouds can produce absorption by O VI and similar species with a column density consistent with observations.

Begelman, Mitchell C.↗

NASA NTRS1990

Interaction of planetary nebulae with the interstellar medium

The interaction of a moving planetary nebula (PN) with the interstellar medium is considered. The PN shell is compressed first in the direction of the stellar motion. This produces a dipole asymmetry in the surface brightness of the nebula, typically at a nebular density of about 40/cu cm if the nebula is located in the Galactic plane. In the later stages of the interaction, this part of the shell is significantly decelerated with respect to the central star, and the PN becomes strongly asymmetric in shape. This distortion and the subsequent stripping of the nebular gas away from the central star typically occurs at a low nebular density of about 6/cu cm. The morphology of PNs with central stars whose proper motions exceed 0.015 arcsec/yr was examined, and it was found that many of the extended nebulae are interacting with the interstellar medium (ISM). The sample doubles the number of known PNs interacting with the ISM. The morphology of nearby PNs was examined, and a number of strongly asymmetric nebuale were found.

Borkowski, Kazimierz J.↗

NASA NTRS1990

Evolution of a superbubble blastwave in a magnetized medium

Researchers investigate the effects of interstellar magnetic fields on the evolution and structure of interstellar superbubbles, using both analytic and numerical magnetohydrodynamic (MHD) calculations. These cavities of hot gas, surrounded by shells of cold dense material preceded by a shock wave result from the combined action of stellar winds and supernova explosions in OB associations. If the medium in which a superbubble goes off is homogeneous and unmagnetized, the blast wave expands isotropically. As the interstellar gas flows through the shock, it cools significantly and gets strongly compressed such that thermal pressure remains approximately equal to ram pressure. Hence, the swept up material is confined to a very thin shell. However, if the ambient medium is permeated by a uniform magnetic field B sub o approx. 3 mu G (typical value for the interstellar matter (ISM)), the configuration loses its spherical symmetry, and, due to magnetic pressure, the shell of swept up material does not remain thin. Researchers found the following qualitative differences: (1) Except in the immediate vicinity of the magnetic poles, the shell is supported by magnetic pressure. (2) The refraction of field lines at the shock and the thermal pressure gradient along the shell both contribute to accelerating the gas toward the equator. The resulting mass flux considerably decreases the column density at the magnetic poles. (3) Away from the poles, magnetic tension in the shell causes the field lines (particularly the inner boundary) to elongate in the direction of B sub o. In contrast, the shock wave radius increases with increasing theta. (4) The reduced inertia of a parcel in the polar neighborhood makes it easier to decelerate, and accounts for the dimple which appears at the poles in numerical simulations. This dimple also results from the necessity to call on intermediate shocks in order to insure a smooth transition between a purely thermal shock at the poles and a magnetic shock in the rest of the shell. (5) The shock wave propagates faster than in the absence of magnetic field, except near the poles where the reduced mass of the shell allows it to be more efficiently decelerated.

Ferriere, Katia M.↗

NASA NTRS1989

Stochastic histories of dust grains in the interstellar medium

The purpose is to study an evolving system of refractory dust grains within the Interstellar Medium (ISM). This is done via a combination of Monte Carlo processes and a system of partial differential equations, where refractory dust grains formed within supernova remnants and ejecta from high mass loss stars are subjected to the processes of sputtering and collisional fragmentation in the diffuse media and accretion within the cold molecular clouds. In order to record chemical detail, the authors take each new particle to consist of a superrefractory core plus a more massive refractory mantle. The particles are allowed to transfer to and fro between the different phases of the interstellar medium (ISM) - on a time scale of 10(exp 8) years - until either the particles are destroyed or the program finishes at a Galaxy time of 6x10(exp 9) years. The resulting chemical and size spectrum(s) are then applied to various astrophysical problems with the following results. For an ISM which has no collisional fragmentation of the dust grains, roughly 10 percent by mass of the most refractory material survives the rigors of the ISM intact, which leaves open the possibility that fossilized isotopically anomalous material may have been present within the primordial solar nebula. Stuctured or layered refractory dust grains within the model cannot explain the observed interstellar depletions of refractory material. Fragmentation due to grain-grain collisions in the diffuse phase plus the accretion of material in the molecular cloud phase can under certain circumstances cause a bimodal distribution in grain size.

Liffman, Kurt↗

NASA NTRS1991

Supernova remnant evolution in an interstellar medium with evaporating clouds

A new similarity solution that describes the evolution of an SNR expanding into a cloudy interstellar medium is expounded. The solution incorporates a reasonable model of the conductive evaporation of cold clouds embedded in the hot gas behind the shock. The model has two new parameters in addition to those describing the usual Sedov solution for a uniform interstellar medium. The X-ray, infrared, and optical luminosities of remnants with evaporating clouds are calculated, and some of the expected spectral characteristics in the various wavebands are discussed. The effects described may explain the class of remnants observed to have centrally peaked X-ray emission and shell-like radio emission. The total H-alpha luminosity from evaporating clouds predicted by this model is less than that observed for SNRs with centrally peaked thermal X-ray emission. A detailed calculation of the X-ray and optical line emission from evaporating clouds would be very useful in testing this model against the observations.

White, Richard L.↗

NASA NTRS1991

Flux-limited diffusion in a scattering medium

A diffusion equation (FDT) is presented with a coefficient that reduces to the appropriate limiting form in the streaming and near thermodynamic limits for a moving fluid in which the dominant source of opacity is Thomson scattering. The present results are compared to those obtained with the corresponding equations for an absorptive medium. It is found that FDT for a scattering medium is accurate to better than less than about 17 percent over the range of optical depths of tau in the range of about 0 to 3.

Melia, Fulvio↗

NASA NTRS1992

Radiative transfer solutions for cylindrical coordinates with emitting, absorbing, and anisotropic scattering medium

The radiative transfer equation is solved by the S-N discrete ordinates method in one-dimensional radial and two-dimensional r-z coordinates systems. The walls of the enclosures are diffuse, and the participating medium absorbs, emits and anisotropically scatters the radiative energy. Diffuse wall incidence, isothermal medium emission and collimated incidence problems are considered. Effects of the scattering phase functions on average incident radiation and net radiative heat fluxes are studied. In addition, the effects of scattering albedo, optical thickness and the wall emissivity are briefly discussed.

Jendoubi, Slah↗

NASA NTRS1992

Suitability of silica aerogel as a capture medium for interplanetary dust

Impact experiments using silica aerogel as a deceleration and capture medium for interplanetary dust are reported. A rough correlation is noted between increasing particle track lengths and decreasing aerogel density, and there is a poor correlation of track lengths with impact velocity at laboratory attainable velocities of 5-7 km/s. It is concluded that aerogel track lengths should not be used as velocity indicators. Chemical analyses are also reported of aerogel samples used in this study in order to assess the risks concerning contamination of interplanetary dust particles by the silica aerogel capture medium. It is demonstrated that this material is impressively clean.

Barrett, R. A.↗

NASA NTRS1992

Optimal Trajectories For Orbital Transfers Using Low And Medium Thrust Propulsion Systems

For many problems it is reasonable to expect that the minimum time solution is also the minimum fuel solution. However, if one allows the propulsion system to be turned off and back on, it is clear that these two solutions may differ. In general, high thrust transfers resemble the well-known impulsive transfers where the burn arcs are of very short duration. The low and medium thrust transfers differ in that their thrust acceleration levels yield longer burn arcs which will require more revolutions, thus making the low thrust transfer computational intensive. Here, we consider optimal low and medium thrust orbital transfers.

Cobb, Shannon S.↗

NASA NTRS1993

An MHD study of the interaction between the solar wind and the interstellar medium

The overall objective of this research program is to obtain a better understanding of the interaction between the solar wind and the interstellar medium through the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations. The simulated results will be compared with observations where possible and with the results from previous analytic and numerical studies. The primary progress during the first two years has been to develop codes for 2-D models in both spherical and cylindrical coordinates and to apply them to the solar wind-interstellar medium interaction. Computations have been carried out for both a relatively simple gas-dynamic interaction and a flow-aligned interstellar magnetic field. The results have been shown to compare favorably with models that use more approximations and to modify and extend the previous results as would be expected. Work has also been initiated on the development of a 3-D MHD code in spherical coordinates.

Steinolfson, R. S.↗