Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Multiphase”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Superbubbles in the multiphase ISM and the loading of galactic winds

We use numerical simulations to analyze the evolution and properties of superbubbles (SBs), driven by multiple supernovae (SNe), that propagate into the two-phase (warm/cold), cloudy interstellar medium (ISM). We consider a range of mean background densities n{sub avg}=0.1--10 cm{sup −3} and intervals between SNe Δt{sub SN}=0.01--1 Myr, and follow each SB until the radius reaches ∼(1--2)H, where H is the characteristic ISM disk thickness. Except for embedded dense clouds, each SB is hot until a time t{sub sf,m} when the shocked warm gas at the outer front cools and forms an overdense shell. Subsequently, diffuse gas in the SB interior remains at T{sub h}∼10{sup 6}--10{sup 7} K, with an expansion velocity v{sub h}∼10{sup 2}--10{sup 3} km s{sup −1} (both highest for low Δt{sub SN}). At late times, the warm shell gas velocities are several tens to ∼100 km s{sup −1}. While shell velocities are too low to escape from a massive galaxy, they are high enough to remove substantial mass from dwarfs. Dense clouds are also accelerated, reaching a few to tens of km s{sup −1}. We measure the mass in hot gas per SN, M-hat {sub h}, and the total radial momentum of the bubble per SN, p-hat {sub b}. After t{sub sf,m}, M-hat {sub h}∼10--100 M{sub ⊙} (highest for low n{sub avg}), while p-hat {sub b}∼0.7--3×10{sup 5} M{sub ⊙} km s{sup −1} (highest for high Δt{sub SN}). If galactic winds in massive galaxies are loaded by the hot gas in SBs, we conclude that the mass-loss rates would generally be lower than star formation rates. Only if the SN cadence is much higher than usual in galactic disks, as may occur for nuclear starbursts, can SBs breakout while hot and expel up to 10 times the mass locked up in stars. The momentum injection values, p-hat {sub b}, are consistent with requirements to control star formation rates within galaxies at observed levels.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Pairing of Accreting Massive Black Holes in Multiphase Circumnuclear Disks: the Interplay Between Radiative Cooling, Star Formation, and Feedback Processes

We study the orbital decay of a pair of massive black holes (BHs) with masses 5×10{sup 5} and 10{sup 7} M{sub ⊙}, using hydrodynamical simulations of circumnuclear disks (CNDs) with the alternating presence of sub-grid physics, such as radiative cooling, star formation, supernova feedback, BH accretion, and BH feedback. In the absence of such processes, the orbit of the secondary BH decays over timescales of ∼10 Myr to the center of the CND, where the primary BH resides. When strong dissipation operates in CNDs, fragmentation into massive objects the size of giant molecular clouds with densities in the range 10{sup 4}–10{sup 7} amu cm{sup −3} occurs, causing stochastic torques and hits that can eject the secondary BH from the midplane. Outside the plane, the low-density medium provides only weak drag, and the BH return is governed by inefficient dynamical friction. In rare cases, clump–BH interactions can lead to a faster decay. Feedback processes lead to outflows, but do not significantly change the overall density of the CND midplane. However, with a spherically distributed BH feedback, a hot bubble is generated behind the secondary, which almost shuts off dynamical friction. We dub this phenomenon “wake evacuation.” It leads to delays in the decay, possibly of ∼0.3 Gyr. We discuss the non-trivial implications on the discovery space of the eLISA telescope. Our results suggest that the largest uncertainty in predicting BH merger rates lies in the potentially wide variety of galaxy host systems, with different degrees of gas dissipation and heating, yielding decay timescales from ∼10 to ∼300 Myr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Quantifying Supernovae-driven Multiphase Galactic Outflows

Galactic outflows are observed everywhere in star-forming disk galaxies and are critical for galaxy formation. Supernovae (SNe) play the key role in driving the outflows, but there is no consensus as to how much energy, mass, and metal they can launch out of the disk. We perform 3D, high-resolution hydrodynamic simulations to study SNe-driven outflows from stratified media. Assuming the SN rate scales with gas surface density Σ{sub gas} as in the Kennicutt–Schmidt relation, we find that the mass loading factor, η {sub m}, defined as the mass outflow flux divided by the star formation surface density, decreases with increasing Σ{sub gas} as η{sub m}∝Σ{sub gas}{sup −0.61}. Approximately Σ{sub gas} ≲ 50 M {sub ⊙} pc{sup −2} marks when η {sub m} ≳ 1. About 10%–50% of the energy and 40%–80% of the metals produced by SNe end up in the outflows. The tenuous hot phase (T > 3 × 10{sup 5} K), which fills 60%–80% of the volume at the midplane, carries the majority of the energy and metals in the outflows. We discuss how various physical processes, including the vertical distribution of SNe, photoelectric heating, external gravitational field, and SN rate, affect the loading efficiencies. The relative scale height of gas and SNe is a very important factor in determining the loading efficiencies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Probing the Outflowing Multiphase Gas ∼1 kpc below the Galactic Center

Comparison of interstellar medium (ISM) absorption in the UV spectrum of LS 4825, a B1 Ib−II star d = 21 ± 5 kpc from the Sun toward l = 1.°67 and b = −6.°63, with ISM absorption toward an aligned foreground star at d < 7.0 ± 1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph show low-ionization absorption out to d < 7 kpc (e.g., O i, C ii, Mg ii, Si ii, Fe ii, S ii) only between 0 and 40 km s{sup −1}, while absorption at d > 7 kpc, ∼1 kpc below the Galactic plane, is complex and spans −290 to +94 km s{sup −1}. The intermediate and high ions Si iii, C iv, Si iv, and N v show extremely strong absorption with multiple components from −283 to 107 km s{sup −1}, implying that the ISM ∼1 kpc below the Galactic center has a substantial reservoir of plasma and more gas containing C iv and N v than in the Carina OB1 association at z = 0 kpc. Abundances and physical conditions are presented for many absorption components. The high ion absorption traces cooling transition temperature plasma probably driven by the outflowing hot gas, while the extraordinarily large thermal pressure, p / k ∼ 10{sup 5} cm{sup −3} K{sup −1}, in an absorption component at −114 km s{sup −1} probably arises from the ram pressure of the outflowing hot gas. The observations are consistent with a flow whose ionization structure in the high ions can be understood through a combination of nonequilibrium radiative cooling and turbulent mixing.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Multiphase eruptions associated with the craters Tycho and Aristarchus.

In 1967 part of Tycho and all of Aristarchus were photographed from Orbiter 5 with a ground resolution of about 5 m. Geologic units of Tycho are discussed together with crater counts in and near to Tycho, counts of small craters near to Surveyor 7, geologic units of Aristarchus, and crater counts in and near to Aristarchus. It is concluded that both Tycho and Aristarchus probably originated in the same manner, although there were differences in their later development. Extensive lava flows issued from the inner and outer rims of both craters. Flows from small craters in the vicinity of the Tycho and Aristarchus structures show that these small craters are volcanic. This establishes that one class of lunar volcano, at least, resembles an impact crater.

Strom, R. G.↗
NASA NTRS1974

A method for determining optimum phasing of a multiphase propulsion system for a single-stage vehicle with linearized inert weight

A general analytical treatment is presented of a single-stage vehicle with multiple propulsion phases. A closed-form solution for the cost and for the performance and a derivation of the optimal phasing of the propulsion are included. Linearized variations in the inert weight elements are included, and the function to be minimized can be selected. The derivation of optimal phasing results in a set of nonlinear algebraic equations for optimal fuel volumes, for which a solution method is outlined. Three specific example cases are analyzed: minimum gross lift-off weight, minimum inert weight, and a minimized general function for a two-phase vehicle. The results for the two-phase vehicle are applied to the dual-fuel rocket. Comparisons with single-fuel vehicles indicate that dual-fuel vehicles can have lower inert weight either by development of a dual-fuel engine or by parallel burning of separate engines from lift-off.

Martin, J. A.↗
NASA NTRS1978

Multiphase dispersions by crystallization processing

The interaction between second-phase particles and crystal-liquid interfaces in the solidification process has been studied in ground-based experiments and experiments under microgravity conditions. An analysis of these data with a view toward the solidification processing of composite materials reveals a pronounced difference between high entropy of fusion and low entropy of fusion matrix materials, as well as a pronounced effect of differences in the thermal conductivity of particles and liquid on the behavior of materials. Effects of liquid viscosity surface topography of the particles, particle chemistry, and overall particle shape examined.

Uhlmann, D. R.↗
NASA NTRS1978

Effect of concentration dependence of the diffusion coefficient on homogenization kinetics in multiphase binary alloy systems

Diffusion calculations were performed to establish the conditions under which concentration dependence of the diffusion coefficient was important in single, two, and three phase binary alloy systems. Finite-difference solutions were obtained for each type of system using diffusion coefficient variations typical of those observed in real alloy systems. Solutions were also obtained using average diffusion coefficients determined by taking a logarithmic average of each diffusion coefficient variation considered. The constant diffusion coefficient solutions were used as reference in assessing diffusion coefficient variation effects. Calculations were performed for planar, cylindrical, and spherical geometries in order to compare the effect of diffusion coefficient variations with the effect of interface geometries. In most of the cases considered, the diffusion coefficient of the major-alloy phase was the key parameter that controlled the kinetics of interdiffusion.

Tenney, D. R.↗
NASA NTRS1980

Fast ion transport in silver halide solid solutions and multiphase systems

The incorporation of homovalent ions, e.g., I(-) in AgBr, leads to a substantial increase in ionic conductivity sigma. The charge compensation concept does not explain the enhancement. AgBr + 30 mol. % AgI exhibits sigma approximately 7/omega/cm at 380 C, which is approximately 170% larger than that of alpha-AgI, the best known superionic conductor, at its melting point. The purely elastic displacement caused by the foreign ion is suggested to be the origin of such a unique behavior. Furthermore, AgI-AgBr two-phase systems display sigma approximately 10 to the 3rd times higher than predicted by the classical theories.

Shahi, K.↗
NASA NTRS1983

Multiphase combustion experimentation in microgravity

This paper examines the need for and implementation of microgravity combustion studies of two phase media. Experimental and analytical aspects of several heterogeneous kinetic systems are discussed. These include: flame propagation and extinction for quiescent clouds of uniformly premixed fuel particulates in an oxidizing atmosphere; autoignition of clouds of uniformly premixed fuel particulates in a quiescent oxidizing atmosphere; and the roles of catalytically significant surfaces in gaseous autoignition processes.

Berlad, A. L.↗
NASA NTRS1988

Multiphase flow in porous media

A development history and current status evaluation are presented for the theory of permeability and percolation. The microscale phenomena treated in this field have proven difficult to analyze due both to their tortuous geometry and the influence of capilarity. Capilary effects may be not only important but predominant, and are differentiated into those at the fluid-fluid interface, and those involving the existence of a contact line between the solid substrate and this interface. Percolation theory has been borrowed from physics and adapted to the two-phase engineering context.

Adler, Pierre M.↗