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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 343 records · Page 19
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NASA NTRS1983

Cryogenic wind tunnels for high Reynolds number testing

The present investigation has the objective to provide an overview of the development of cryogenic wind tunnels and their application to high Reynolds number testing. The current need for wind tunnels reflects the fact that many complex three-dimensional flows cannot yet be adequately dealt with analytically. Imperfections of wind tunnels are partly related to the fact that for most wind tunnel tests the Reynolds number is much too low. The best solution to this problem appears to be an operation of the tunnel at cryogenic temperatures (arbitrarily defined as temperatures of 150 K or less. In addition to decreasing the temperature, there are three other approaches for increasing the Reynolds number. These approaches include the use of a heavy gas, an increase in the model size, and an increase in pressure. Attention is given to the variable density tunnel concept, problems with high dynamic pressures, low-speed cryogenic tunnel results, transonic cryogenic tunnel tests, and the injection of liquid nitrogen into the tunnel.

Kilgore, R. A.↗
NASA NTRS2014

The First Stars: A Low-Mass Formation Mode

We perform numerical simulations of the growth of a Population III stellar system under photodissociating feedback. We start from cosmological initial conditions at z = 100, self-consistently following the formation of a minihalo at z = 15 and the subsequent collapse of its central gas to high densities. The simulations resolve scales as small as approx. 1 AU, corresponding to gas densities of 10(exp 16)/cu cm. Using sink particles to represent the growing protostars, we evolve the stellar system for the next 5000 yr. We find that this emerging stellar group accretes at an unusually low rate compared with minihalos which form at earlier times (z = 20-30), or with lower baryonic angular momentum. The stars in this unusual system will likely reach masses ranging from <1Stellar Mass to approx. 5 Stellar Mass by the end of their main-sequence lifetimes, placing them in the mass range for which stars will undergo an asymptotic giant branch (AGB) phase. Based upon the simulation, we predict the rare existence of Population III stars that have survived to the present day and have been enriched by mass overflow from a previous AGB companion.

Low-Mass↗
NASA NTRS1984

Acoustically-induced modulation spectroscopy for ultra-sensitive gas analysis

A new optical technique has been developed for ultra-sensitive attenuation measurements in gaseous media and, in particular, for determination of low levels of smoke emitted from jet engines. It is a variation on direct light transmission where the sample gas density in a cell is modulated acoustically by a speaker. The amplitude variation of the light transmission is proportional to the gas density and is insensitive to window contamination and detector instabilities. Preliminary analysis and experiments indicate that the instrument promises to measure light absorption to less than 1 percent per meter and allow measurment of smoke emissions from 1 to 100 mg/cu m. The technique has been demonstrated through the use of an absorbing gas, viz., 200 ppm of NO2 in N2 which produces 25 percent per meter absorption.

Pitz, R. W.↗
NASA NTRS1994

The use of silica aerogel to collect interplanetary dust in space

We report on impact experiments to evaluate the suitability of silica aerogel as a capture medium for interplanetary dust particles in space. The structure of silica aerogel is such that even micrometer-sized hypervelocity particles may sense it as a target of very low density; consequently shock stresses and temperatures due to impact should be minimized. We performed impact experiments in a light gas gun that employed projectiles manufactured from forsterite, pyrrhotite, and calcite. From these experiments we have developed techniques that will permit the successful extrication of interplanetary dust particles from aerogel. We note a rough correlation between increasing particle penetration 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. We conclude that aerogel track length should not be used even as (crude) velocity indicators. Transmission electron microscope studies show the re covered particulate residues are encased in melted silica aerogel, which possibly protects the residue from some impact damage. Individual minerals within interplanetary dust are expected to exhibit differential survival, and may be vaporized, melted, dehydrated, or structurally reordered. However, some mineral grains should successfully be captured in an unmelted state at encounter velocities less than 8 km/s.

Zolensky, Michael E.↗
NASA NTRS2013

Numerical Study of Transmission Loss Through a Slow Gas Layer Adjacent to a Plate

This paper describes a systematic numerical investigation of the sound transmission loss through a multilayer system consisting of a bagged gas and lightweight panel. The goal of the study is to better understand the effect of the gas on transmission loss and determine whether a gas with a slow speed of sound is beneficial for noise control applications. As part of the study, the density and speed of sound of the gas are varied independently to assess the impact of each on transmission loss. Results show that near grazing incidence the plane wave transmission loss through the multilayer system is more sensitive to the speed of sound than the density of the gas. In addition, it was found that a slow wave speed in the bagged gas provides more low-frequency transmission loss benefit than a fast wave speed. At low angles of incidence, close to the plate normal, the benefit is due to the reduction of the characteristic impedance of the gas. At high angles of incidence, the benefit is attributed to the fact that the incident waves at the air/gas interface are bent towards the surface normal. Since transmission loss is angle dependent, refraction in the slow gas layer results in a significant improvement in the transmission loss at high angles of incidence.

Schiller, Noah H.↗
NASA NTRS1984

Low-Density High-Strength Foamed Materials

Molten bubbles of metal or plastic coalesce into strong, lightweight materials that look like solidified foam. Bubbles formed in compartment that receives molten material and compressed gas that fills bubbles. Compartment has matrix of nozzles. Leaving nozzles, bubbles fall into acoustic chamber and coalesce; then drop through funnel and are cast into desired shape by extrusion or molding. Materials used for construction, extruded into molds, sawed, nailed, and generally handled as wood.

Wang, T.↗
NASA NTRS1994

Lick optical spectra of quasar HS 1946+7658 at 10 kilometers per second resolution Lyman-alpha forest and metal absorption systems

We present optical spectra of the most luminous known quasi stellar object (QSO) HS 1946+7658 (z(sub em) = 3.051). Our spectra have both full wavelength coverage, 3240-10570 A, and in selected regions, either high signal-to-noise ratio, SNR approximately equals 40-100, or unusually high approximately 10 km/sec resolution, and in parts of the Ly alpha forest and to the red of Ly alpha emission they are among the best published. We find 113 Ly alpha systems and six metal-line systems, three of which are new. The metal systems at z(sub abs) = 2.844 and 3.050 have complex velocity structure with four and three prominent components, respectively. We find that the system at z(sub abs) = 2.844 is a damped Ly alpha absorption (DLA) system, with a neutral hydrogen column density of log N(H I) = 20.2 +/- 0.4, and it is the cause of the Lyman limit break at lambda approximately equals 3520 A. We believe that most of the H I column density in this system is in z(sub abs) = 2.8443 component which shows the strongest low-ionization absorption lines. The metal abundance in the gas phase of the system is (M/H) approximately equals -2.6 +/- 0.3, with a best estimate of (M/H) = -2.8, with ionizaion parameter log gamma = -2.75, from a photoionization model. The ratios of the logarithmic abundances of C, O, Al, and Si are all within a factor of 2 of solar, which is important for two reasons. First, we believe that the gas abundances which we measure are close to the total abundances, because the ratio of aluminum to other elements is near cosmic, and Al is a refractory element which depletes very readily like chromium, in the interstellar medium. Second, we do not see the enhancement of O with respect to C of (O/C) approximately equals 0.5-0.9 reported in three partial Lyman limit systems by Reimers et al. (1992) and Vogel & Reimers (1993); we measure (O/C) = -0.06 for observed ions and (O/C) approximately equals 0.2 after ionization corrections, which is consistent with solar abundances. We see C II*(lambda 1335) offset by 15 km/sec with respect to C II(lambda 1334), presumably because the gas density varies from 2 to 8 cm(exp -3) with changing velocity in the DLA system. These densities imply that the damped component is 6-25 pc thick, which is reasonable for a single cloud in a cold spiral disk. They also imply that the cloud is relatively highly ionized with more C III than C II, more O III than O I, and log N(H I) = 20.72, which is 3 times the H I column. The system at z(abs) = 1.7382 is also believed to be damped with N(H I) approximately equals 10(exp 21) cm(exp -2), because we see Cr II, but its Ly alpha line will never be seen because it is below the Lyman limit of the other DLA system. We see a 2.6 sigma lack of Ly alpha forest lines well away from the QSO redshift, which may be a chance fluctuation. We also see a correlation between column density N(H I) and Doppler parameter b for 96 unsaturated Ly Alpha forest absorption lines, and although this correlation persists in the 36 Ly alpha lines which lie in regions where the SNR approximately equals 8-16, we agree with Rauch et al. (1993) that it is probably a bogus effect of low supernova remnant (SNR). The same applies to lines with very low b values: in regions where SNR less than or equal to 8 we see many Ly alpha lines which appear to have 10 less than or equal to b less than or equal to 20, but when 8 less than or equal to SNR less than or equal to 16 we see only one line with b less than or equal to 15 km/sec, and two others which we believe have b less than or equal to 20, with values of 20 and 16 km/sec. Traditional Ly alpha line samples which include all lines which have W/sigma(W) greater than or equal to 4 are not adequate to explore the distribution of the properties of individual clouds, because we need much higher (W/sigma(W)) and SNR to avoid the strong biases.

Fan, Xiao-Ming↗
NASA NTRS1994

Explosions of infalling comets in Jupiter's atmosphere

In view of the expected collision of comet Shoemaker-Levy 9 (1993e) with Jupiter in 1994 July, we calculate basic properties of the initial interaction for a simplified Jovian atmosphere. The comet is expected to impact Jupiter at 60 km/sec and at an angle of 45 deg to the zenith. The shock wave generated by the bolide should be optically thick once it has penetrated to an atmospheric density approximately 10(exp -6) gr/cm(exp 3), and we calculate the post-shock conditions assuming local thermodynamic equilibrium (LTE) for shock velocities v(sub sh) in the range 10 to 60 km/sec and preshock densities rho(sub a) = 10(exp -6) to 10(exp -2) gr/cm(exp 3). Our shock calculations include molecular hydrogen, atomic hydrogen, ionized hydrogen, neutral helium, and singly ionized helium. Even at the highest shock velocity, the gas is only partially ionized and the postshock temperature rises with preshock density in order to maintain the ionization. The value of the effective shock adiabatic index gamma(sub sh) varies from 1.17 (at low v(sub sh) and rho(sub a) to 1.40 (at high v(sub sh) and rho(sub a). The ablation rate is limited by the radiative flux that reaches the bolide surface. We argue that the ablated gas does not efficiently transfer its kinetic energy to the atmosphere, and it ultimately slows in a similar fashion to the comet material. As the bolide initially falls through the atmosphere, the character of the shock emission changes. At rho(sub a) approximately 10(exp -8) gr/cm (exp 3), the gas is optically thin and we expect line emission; in the optical spectrum, Balmer emission is expected from the shocked atmosphere and low-ionization metal lines from ablated cometary material. At rho(sub a) approximately 10(exp -6) gr/m(exp 3), the shocked gas is optically thick and the shock front near the bolide produces a blackbody spectrum. The temperature is favorable for ultraviolet (1000 to 3000 A) emission and the luminosity may be approximately 5 x 10(exp 23) ergs/sec for approximately 0.6 sec for a bolide 1 km in radius. At rho(sub a) approximately 10(exp -4) gr/cm(exp 3), the bolide has passed below the ultraviolet photosphere. The shock front emits considerable ionizing radiation, but it is absorbed in a narrow preshock region. The bolometric correction for the optical luminosity is large and we expect a 3000 to 8000 A luminosity of approximately 3 x 10(exp 23) ergs/sec for approximately 1 sec. The optical emission is strongly peaked in the vicinity of the bolide. The bolide does have a somewhat less luminous, optically thick trail extending greater than or equal to 10 km, but the radiation is characterized by a temperature of 4000 to 5000 K. From the fragmentation model of Chyba, Thomas, & Zahnle (1993), the bolide deposits most of its kinetic energy at rho(sub a) approximately 10(exp -3) gr/cm(exp 3) and this is the effective explosion site. The shock wave from such an explosion can move up about one density scale height. We examine the breakout of the shock front from the Jovian atmosphere and find that the shock acceleration in the decreasing density region is slow, so that the energy flux in the shock front is small. Higher velocities might be generated by shock acceleration along the channel left by the bolide if the shock motion can occur before the channel closes off as a result of radiative cooling. Hot gas created by the explosion ultimately rises due to buoyancy on a timescale of a minute. The luminosity is highest when the bubble first rises into the optically thin part of the atmosphere and may be approximately 1 x 10(exp 25) ergs/sec in the near-infrared. Roughly 1% of the initial bolide energy may be radiated in this way; the rest of the energy is lost to sound waves from the initial explosion and to work done by the bubble on the surrounding atmosphere.

Chevalier, Roger A.↗
NASA NTRS1984

Advanced onboard storage concepts for natural gas-fueled automotive vehicles

The evaluation of several advanced concepts for storing natural gas at reduced pressure is presented. The advanced concepts include adsorption on high surface area carbon, adsorption in high porosity zeolite, storage in clathration compounds, and storage by dissolution in liquid solvents. High surface area carbons with high packing density are the best low pressure storage mediums. A simple mathematical model is used to compare adsorption storage on a state of the art carbon with compression storage. The model indicates that a vehicle using adsorption storage of natural gas at 3.6 MPa will have 36 percent of the range, on the EPA city cycle, of a vehicle operating on a compression storage system having the same physical size and a peak storage pressure of 21 MPa. Preliminary experiments and current literature suggest that the storage capacity of state of the art carbons could be improved by as much as 50 percent, and that adsorption systems having a capacity equal to compression storage at 14 MPa are possible without exceeding a maximum pressure of 3.6 MPa.

Remick, R. J.↗
NASA NTRS1984

Free stream turbulence and density ratio effects on the interaction region of a jet in a cross flow

Jets of low temperature air are introduced into the aft sections of gas turbine combustors for the purpose of cooling the high temperature gases and quenching the combustion reactions. Research studies, motivated by this complex flow field, have been executed by introducing a heated jet into the cross stream of a wind tunnel. The investigation by Kamotani and Greber stands as a prime example of such investigations and it serves as the principal reference for the present study. The low disturbance level of the cross stream, in their study and in similar research investigations, is compatible with an interest in identifying the basic features of this flow field. The influence of the prototypes' strongly disturbed cross flow is not, however, made apparent in these prior investigations.

Wark, C. E.↗
NASA NTRS1989

Superbubble blowout dynamics

Multiple supernovae and stellar winds from OB associations carve large holes filled with hot gas in the galactic disk. These superbubbles sweep up H I into cold, thin, dense shells and eventually grow large enough to blow completely out of the galactic H I disk. When superbubbles blow out of the disk, they vent hot gas and supernova energy into the galactic corona. In this paper ZEUS, a two-dimensional hydrodynamics code, is used to model the blowout of a superbubble from exponential and Gaussian models for the vertical density stratification. The results are compared to those from the Kompaneets (thin-shell) approximation. It is found that this approximation works very well, and that most of the mass of the shell remains in the plane, with 5 percent of it accelerating upward. The venting of the hot gas and the stability of the shell depends strongly on the model of the density distribution. It is suggested that the low galactic halo actually consists of a froth of merged superbubbles.

Mac Low, Mordecai-Mark↗
NASA NTRS1993

A ROSAT observation of NGC 5548

We present a detailed spectral and temporal analysis of a long ROSAT PSPC pointing centered on the bright Seyfert 1 galaxy NGC 5548. The spectrum is particularly complex, with a two-component model favored, confirming the existence of the soft excess in this source. In addition, an absorption feature arising from highly ionized oxygen in the line of sight has been detected, consistent with an origin in the 'warm absorber' which has been found to be a common feature in Seyfert galaxies observed by Ginga. The energy and depth of the feature should allow us to constrain the density and geometry of the absorbing gas, which may lie close to the central continuum source. NGC 5548 was in a low X-ray state during the observation, brightening by a factor of about 2 on a time-scale of about 2 d. Time-resolved spectral analysis shows a change in the ratio of the hard and soft fluxes, consistent with reprocessing models.

Nandra, K.↗
NASA NTRS2021

Photoionization Models for High Density Gas

Relativistically broadened and redshifted 6.4 – 6.9 keV iron K lines are observed from many accretion powered objects, including X-ray binaries and active galactic nuclei(AGN). Existence of gas close to the central engine implies large radiation intensities and correspondingly large gas densities if the gas is to remain partially ionized. Simple estimates indicate that high gas densities are needed to allow survival of iron against ionization. These are high enough that rates for many atomic processes are affected by mechanisms related to interactions with nearby ions and electrons. Radiation intensities are high enough that stimulated processes can be important. Most models currently in use for interpreting relativistic lines use atomic rate coefficients designed for use at low densities and neglect stimulated processes. In our work so far we have presented atomic structure calculations with the goal of providing physically appropriate models at densities consistent with line-emitting gas near compact objects. In this paper we apply these rates to photoionization calculations, and produce ionization balance curves and X-ray emissivities and opacities which are appropriate for high densities and high radiation intensities. The final step in our program will be presented in a subsequent paper: Model atmosphere calculations which incorporate these rates into synthetic spectra.

T Kallman↗
NASA NTRS1989

Kinetic theory model for the flow of a simple gas from a two-dimensional nozzle

A system of nonlinear integral equations equivalent to the Krook kinetic equation for the steady state is the mathematical basis used to develop a computer code to model the flowfields for low-thrust two-dimensional nozzles. The method of characteristics was used to solve numerically by an iteration process the approximated Boltzmann equation for the number density, temperature, and velocity profiles of a simple gas as it exhausts into a vacuum. Results predict backscatter and show the effect of the inside wall boundary layer on the flowfields external to the nozzle.

Riley, B. R.↗
NASA NTRS1991

Kinetic theory model for the flow of a simple gas from a three-dimensional axisymmetric nozzle

A system of nonlinear integral equations equivalent to the Krook kinetic equations for the steady state is the mathematical basis used to develop a computer code to model the flowfields for low-thrust three-dimensional axisymmetric nozzles. The method of characteristics is used to solve numerically by an iteration process the approximated Boltzmann equation for the number density, temperature, and velocity profiles of a simple gas as it expands into a vacuum. Results predict backscatter and show the effect of the nozzle wall boundary layer on the external flowfields.

Riley, B. R.↗
NASA NTRS1983

Self-regulated star formation in the galaxy

Assuming that star formation regions are supported against gravity by winds from low mass young objects, the stellar birthrate obtained for winds interacting in the momentum conservation stage is correlated with the molecular gas density of the parent fragment as n to the 13/8 power or n to the 5/8 power, respectively, for rates/unit volume or rates/unit mass. Birthrates derived from protostellar rotationally driven winds are in good agreement with the observed star production in the cloud B18. With the aid of observed Taurus-Auriga complex properties, the present model is extrapolated to the Galaxy as a whole, yielding a Milky Way predicted average rate that is in good agreement with standard estimates based on observations of the solar neighborhood.

Franco, J.↗
NASA NTRS1988

Low-mass spiral galaxies with little molecular gas and prodigious star formation

A comparison of CO and H I properties is used here to demonstrate that many CO-poor low-mass Virgo spiral galaxies are rich in atomic gas, which implies that the lack of CO emission from them is due, at least partly, to a lack of molecular gas. Despite the paucity of molecular gas, these H I-rich, CO-poor, low-mass spiral galaxies are undergoing extensive massive star formation. A column density of 10 to the 21st nuclei/sq cm is a necessary but insufficient condition for the creation of an H2-dominated interstellar medium.

Kenney, Jeffrey D.↗
NASA NTRS1993

The effect of massive stars on the ionized medium of extragalactic HII regions

Giant extragalactic H2 regions (GEHR) are centers of active star formation, ionized by populous clusters of OB stars. These objects are characterized by their low electron density, linear dimensions of order of 10(exp 2) - 10(exp 3) pc, varied morphology, and inhomogeneous distribution of the gas. They are ideal laboratories to study the processes of interaction between the gas and the stars, that could then be extended to the study of H2 and starburst galaxies. An observational program on a selected group of giant H2 regions with the use of one- and two-dimensional spectroscopy is being conducted. Our aim is two-fold: to understand the internal structure of the regions and to study the kinematics and dynamics of the ionized gas. A short report of our ongoing research is presented.

Castaneda, Hector O.↗