Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “outflows”

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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Magnetically Driven Neutron-rich Ejecta Unleashed: Global 3D Neutrino–General Relativistic Magnetohydrodynamic Simulations of Collapsars Probe the Conditions for r -process Nucleosynthesis

Abstract Collapsars—rapidly rotating stellar cores that form black holes—can power gamma-ray bursts and are proposed to be key contributors to the production of heavy elements in the Universe via the rapid neutron capture process ( r -process). Previous neutrino-transport collapsar simulations have been unable to unbind neutron-rich material from the disk. However, these simulations have not included sufficiently strong magnetic fields and the black hole (BH), both of which are essential for launching mass outflows. We present ν h-amr , a novel neutrino-transport general relativistic magnetohydrodynamic ( ν GRMHD) code, which we use to perform the first 3D global ν GRMHD collapsar simulations. We find a self-consistent formation of a weakly magnetized dense accretion disk, which has sufficient time to neutronize. Eventually, substantial magnetic flux accumulates near the BH, becomes dynamically important, leads to a magnetically arrested disk (MAD), and unbinds some of the neutron-rich material. However, the strong flux also hinders accretion, lowers density, and increases neutrino-cooling timescale, which prevents further disk neutronization. Typical collapsar progenitors with mass accretion rates, M ̇ ∼ 0.1 − 1 M ⊙ s - 1 , do not produce significant neutron-rich ( Y e < 0.25) ejecta. However, we find that MADs at higher mass accretion rates, M ̇ ≳ few M ⊙ s - 1 (e.g., for more centrally concentrated progenitors), can unbind M ej ≲ M ⊙ of neutron-rich ejecta. The outflows inflate a shocked cocoon that mixes with the infalling neutron-poor stellar gas and raises the final outflow Y e ; however, the final r -process yield may be determined earlier at the point of neutron capture freeze-out. Future work will explore under what conditions more typical collapsar engines become r -process factories.

Issa, Danat (ORCID:0009000524787631)↗
NASA NTRS1975

A comparison of the temperature and density structure in high and low speed thermal proton flows

Steady-state altitude profiles of H(+) density, drift velocity, and temperature and O(+) density and temperature were deduced for a wide range of H(+) outflow velocities from subsonic to supersonic flow for plasma densities typical of both undisturbed and trough regions of the ionsophere. Allowance was made for the effects of inertia, parallel stress, and the velocity dependence of the H(+) collision frequencies. It was found that at supersonic outflow velocities there is a decrease in H(+) temperature with increasing outflow velocity. The H(+) temperatures are substantially increased above the O(+) temperatures when H(+) is flowing, with T(H+)/T(O+) reaching a maximum ratio of about 3:1.

Raitt, W. J.↗
NASA NTRS1978

One-dimensional outgassing problem

A predominantly numerical study, using the direct-simulation Monte Carlo method, of the flow from outgassing surfaces with plane, cylindrical, or spherical geometry is presented. The numerical results for the plane flow case are compared with existing theoretical and experimental results and also with a new formulation of the jump conditions across the Knudsen layer between the surface and the continuum outflow. This outflow is sonic for the free expansion case but subsonic in the presence of back-pressure effects. The self-scattered return flux is 19% of the outgassed flux in the sonic outflow case and increases at lower Mach numbers. Thermal accomodation and reflection of this return flux produces an outgassing cooling effect. The cylindrical and spherical geometries introduce an additional Knudsen number defined by the ratio of the mean free path in the effusing gas to the radius. The plane flow results apply when this Knudsen number is less than 0.001 and, when it is of order unity or above, the return flux is inversely proportional to it.

Tran Cong, T.↗
NASA NTRS1978

Metastable states and mass loss in Be stars

Three strong asymmetric absorption features with short-wavelength wings extending 350 km/s from their respective line centers at 2061.6, 2068.2, and 2079.0 A appear in high-resolution UV spectral data (0.1 A) for the Be star Phi Per (B2 Ve4) obtained with the Balloon-borne Ultraviolet Stellar Spectrometer (BUSS) on the evening of September 16, 1976. These transitions are not resonance lines and have been identified as transitions of Fe III (UV 48) originating from a metastable state with an excitation of 5.01 eV. The relative populations of the metastable and ground states are independent of the dilution factor, allowing calculation of the mass-outflow rate for a generalized model. Thus, a mass-outflow rate of 5 hundred-billionths of a solar mass per year is obtained. These transitions are observable with IUE and represent an important tool for studying mass outflow in Be and shell stars.

Bruhweiler, F. C.↗
NASA NTRS1981

Electron temperature anisotropy in the polar wind

The steady state flow of a fully ionized H(+)-O(+)-electron plasma along geomagnetic field lines in the high-latitude topside ionosphere is investigated theoretically, with emphasis on the electron temperature anisotropy and heat flow in the polar wind. The 13-moment system of transport equations developed by Schunk (1975, 1977), which contains a continuity, momentum, internal energy, stress tensor and heat flow equation for each species, is employed to represent the electrons, with a simplified set of transport equations used for the ions, and the transport equations are solved at altitudes from 1500 to 12,000 km for both subsonic and supersonic H(+) outflows. For subsonic H(+) outflows, the electron gas is found to remain collision dominated to high altitudes. For supersonic H(+) outflows, the electron gas is also found to be collision dominated at altitudes below 2500 km, with an anisotropy in electron temperature distribution developing such that the temperature of the perpendicular electrons is greater than that of the parallel electrons at higher altitudes. In addition, the magnitude of downward electron heat flux at 1500 km is shown to have a dramatic effect on the individual parallel and perpendicular electron temperatures.

Schunk, R. W.↗
NASA NTRS1981

Boundary conditions for subsonic compressible Navier-Stokes calculations

A systematic study of inflow and outflow boundary conditions for the numerical solution of the compressible Navier-Stokes equations is presented. Combinations of several representative inflow and outflow boundary conditions are applied in the solution of subsonic flow over a flat plate in a finite computational domain. These boundary conditions are evaluated in terms of their effect on the accuracy of the solution and the rate of convergence to a steady state. It is shown that errors in the data specified at the inflow boundary can produce significant errors in the computed flow field. It is also shown that a non-reflecting outflow boundary condition can significantly reduce the total computational time required.

Rudy, D. H.↗
NASA NTRS1982

Solutions to the equations for corotating magnetospheric convection

The equations of centrifugally driven convection appropriate to a treatment of the transport of Io plasma in Jupiter's magnetosphere are solved for two special cases of the source distribution. The results show that, in the corotating frame of Jupiter, the mass flows outward from the source, forming spirallike streamlines. It is found that the pitch of a spiral depends on the amount of resident mass and the ionospheric conductivity and that the outflow speed depends on the size of the source region, the resident mass, and the source strength of new ions. An example of a wide source and slow outflow is presented. It is noted that, in the limit of a narrow source, the outflow is rapid and the streamlines are nearly radial.

Summers, D.↗
NASA NTRS1984

Numerical Investigation of unsteady inlet flow fields

The flow field within an unsteady, two-dimensional inlet is studied numerically, using a two dimensional Navier Stokes and a one-dimensional inviscid model. Unsteadiness is introduced by varying the outflow pressure boundary condition. The cases considered include outflow pressure variations which were a single pressure pulse, a rapid increase and a sine function. The amplitude of the imposed exit plane pressure disturbance varied between 1 percent and 20 percent of the mean exit pressure. At the higher levels of pressure fluctuation, the viscous flow field results bore little resemblance to the inviscid ones. The viscous solution included such phenomena as shock trains and bifurcating separation pockets. The induced velocity at the outflow plane predicted by the viscous model differs significantly from accoustical theory or small perturbation results.

Hsieh, T.↗
NASA NTRS1984

Numerical simulation of flow through inlets/diffusers with terminal shocks

Two-dimensional Euler and Navier-Stokes solutions of the flow through three inlet/diffuser configurations with terminal shock systems are reported. Calculations without bleed indicate that the terminal shock location is very sensitive to the outflow back pressure. For cases where there are no available experimental results, it becomes difficult to estimate the back pressure that will result in a terminal shock. Estimates based on quasi-one-dimensional analysis are not found adequate for complex two-dimensional flows. It is found that since the flow downstream of the terminal shock is subsonic, and what happens at the outflow boundary affects the flow inside the inlet, enough of the subsonic diffuser must be modeled to accurately predict the terminal shock region. The diffuser portion should be fairly long with the outflow boundary occurring in a region of more or less uniform flow to be able to prescribe a uniform back pressure. The third configuration studied was investigated with and without incorporating bleed in the code. It is found that the use of bleed stabilizes the shock location and allows solutions which without bleed result in unstarting of the inlet. Comparisons are made with available experimental data.

Talcott, N. A., Jr.↗
NASA NTRS1985

Magnetic Effects in the Heating and Modification of Flows in the Outer Stellar Atmospheres with Application to Early Type Star Case

Possible magnetic effects in the heating and modification of flows in early type star atmospheres are discussed by referring to the physically related phenomena dealt with for late type stars, young stars, and close binary systems. It is pointed out as the result of that the magnetic field may play important roles also in early type star atmospheres in converting the energy of the radiatively driven outflow into heat, or in modifying the outflow by nozzling or by initial modification of temperature and/or momentum by which the effect of the radiation pressure may be made the most of in accelerating the outflow.

Uchida, Y.↗
NASA NTRS1984

Star Formation in Globules

The low mass star formation process was studied by application of submillimeter and supplementing far-infrared continuum observations by molecular line observations to the discovery and analysis of compact cores in molecular clouds with low luminosity. These dense regions are likely to be the sites where a cloud has recently collapsed or is still collapsing to form a star. Observations show that nonisotropic effects must play an important role in star formation. Four low luminosity sources embedded in the clouds L1551, B335, L1455 and L723 are presented. All these sources exhibit collimated bipolar molecular outflows of varying strengths. Data on the outflows are combined with determinations of mass, luminosity and temperature of the dust in the dense cores surrounding these embedded energy sources, to infer the evolutionary stage of the object. The possible role of the cores in channeling or generating the outflow is examined.

Davidson, J. A.↗
NASA NTRS1986

Radiative transfer in circumstellar 1612 MHz OH masers

A detailed investigation of radiative transfer in the 1612 MHz OH maser in the expanding envelopes around evolved stars is made in the context of the standard model in which the mass outflow is windlike. The most sensitive diagnostic of the models is the maser emission at the velocity of the star for which VLA and MERLIN data exist. Fully saturated models are analytically eliminated. Detailed numerical modeling also eliminates partially saturated models. The need to examine alternative models for the mass outflow is suggested by this conclusion. A model for the mass outflow from these stars is advocated in which all mass is lost by the ejection of blobs of material. These blobs, acted on by radiation pressure on grains, evolve into platelike structures in the OH maser region. It is shown that the peculiar difficulties of the standard model for the OH masers do not occur in this picture.

Alcock, C.↗
NASA NTRS1986

MM and subMM molecular line observations of the southwest lobe of L1551: Evidence of a shell structure

Observations have been made of the southwest outflow lobe of L1551 in several millimeter and submillimeter molecular lines. Maps have been made in the J=3-2 and J=2-1 transitions of CO over areas of 7.5 by 2.5 arc minutes and 5 by 5 arc minutes respectively at UKIRT. More detailed maps have also been made in the J=2-1 CO transition over an area of about 6 by 3.5 arc minutes at the NRAO 12m telescope. Additional observations of the J=4-3 transitions of HCN, HCO(+) abd H(13)CO(+) were made at selected positions. The HC(+) J=4-3 transition was detected at several positions along the outflow axis and at the position of IRS 5. Similarly the HCN J=4-3 transition was detected at the position of IRS 5 and also at a position close to HH29. However, the J=4-3 transition of H(13)CO(+) was bit detected at the position of IRS 5 even through it was observed at the position close to HH29 with a peak corrected antenna temperature of 0.23K at a V(LSR) of 1 km s(-1). The detection of the J=4-3 transitions of both HCO(+) and H(13)CO(+) close to the position of HH29 suggest the presence of very dense gas in this region. LVG analysis of the various molecular lines observed give a kinetic temperature between 10 and 15K and a density from 10(5) to 10(6) cm(-3) at the position of IRS 5 at the ambient cloud velocity. At the position close to HH29 LVG analysis of the CO observations gives a density between 10(3) and 10(4) cm(-3) at a kinetic temperature of 25k for a V(LSR) of 0 km s(-1). To the southwest of HH29 there is a large decrease in both the linewidth and intensity of CO emission. This may be due to the interaction between the outflow and a dense clump of gas which gives rise to HH29. The maps of the CO J=3-2 and CO J=2-1 emission integrated in 3.25 km s intervals show the shell structure postulated by Snell and Schloerb (1985).

Rainey, R.↗
NASA NTRS1986

Velocity-resolved observations of water in Comet Halley

High resolution (lambda/delta lambda approx. = 3 x 10 to the 5th power) near-infrared observations of H2O emission from Comet Halley were acquired at the time of maximum post-perihelion geocentric Doppler shift. The observed widths and absolute positions of the H2O line profiles reveal characteristics of the molecular velocity field in the coma. These results support H2O outflow from a Sun-lit hemisphere or the entire nucleus, but not from a single, narrow jet emanating from the nucleus. The measured pre- and post-perihelion outflow velocities were 0.9 + or - 0.2 and 1.4 + or - 0.2 km/s, respectively. Temporal variations in the kinematic properties of the outflow were inferred from changes in the spectral line shapes. These results are consistent with the release of H2O into the coma from multiple jets.

Larson, Harold P.↗
NASA NTRS1988

A simple, analytic 3-dimensional downburst model based on boundary layer stagnation flow

A simple downburst model is developed for use in batch and real-time piloted simulation studies of guidance strategies for terminal area transport aircraft operations in wind shear conditions. The model represents an axisymmetric stagnation point flow, based on velocity profiles from the Terminal Area Simulation System (TASS) model developed by Proctor and satisfies the mass continuity equation in cylindrical coordinates. Altitude dependence, including boundary layer effects near the ground, closely matches real-world measurements, as do the increase, peak, and decay of outflow and downflow with increasing distance from the downburst center. Equations for horizontal and vertical winds were derived, and found to be infinitely differentiable, with no singular points existent in the flow field. In addition, a simple relationship exists among the ratio of maximum horizontal to vertical velocities, the downdraft radius, depth of outflow, and altitude of maximum outflow. In use, a microburst can be modeled by specifying four characteristic parameters, velocity components in the x, y and z directions, and the corresponding nine partial derivatives are obtained easily from the velocity equations.

Oseguera, Rosa M.↗
NASA NTRS1988

Fluvial processes on Mars: Erosion and sedimentation

One of the most important discoveries of the Mariner 9 and Viking missions to Mars was evidence of change of the Martian surface by the action of liquid water. From the standpoint of a Mars Rover/Sample Return Mission, fluvial activity on Mars is important in two ways: (1) channel formation has deeply eroded the Martian crust, providing access to relatively undisturbed subsurface units; and (2) much of the material eroded from channels may have been deposited in standing bodies of liquid water. The most striking fluvial erosion features on Mars are the outflow channels. A second type of channel apparently caused by flow of liquid water is the valley systems. These are similar to terrestial drainage systems. The sedimentary deposits of outflow channels are often difficult to identfy. No obvious deposits such as deltaic accumulations are visible in Viking images. Another set of deposits that may be water lain and that date approx. from the epoch of outflow channels are the layered deposits in the Valles Marineris. From the standpoint of a Mars Rover/Sample Return mission, the problem with all of these water-lain sediments is their age, or rather the lack of it.

Squyres, Steven W.↗
NASA NTRS1989

Newborn stars and stellar winds in Barnard 228

Imaging and spectroscopic observations of premain-sequence stars in the B228 molecular cloud have identified three sites of extended, shock-excited nebulosity. A highly collimated, blueshifted jet is associated with the T Tauri star Sz68 (CoD -33:10685 deg). A candidate Herbig-Haro object is found near Sz68 and the T Tauri star Sz69. A third region of shock-excited nebulosity is identified near the embedded newborn star IRAS 15398-3359. IR excesses observed toward these stellar sources may imply a relationship between outflow activity and the presence of a circumstellar disk. Only one of the two outflow sources with well-defined orientations is aligned with the large-scale, magnetic-field geometry of the cloud in contrast to the global alignment of outflows in the Taurus cloud complex. Data for three previously unidentified emission-line stars are presented. A faint 18th magnitude dMe star located 48 arcsecs west of Sz68 is identified.

Heyer, Mark H.↗
NASA NTRS1990

Properties of large scale plasma flow during the early stage of the plasmaspheric refilling

The objective is to better characterize the macroscopic properties of the interhemisphere plasma flow by solving a more complete set of hydrodynamic equations than that solved previously. Specifically, the ion continuity, momentum and energy equations were solved for the plasma flow along the closed magnetic field lines. During the initial stage of the supersonic outflow in the equatorial region, the ions cool substantially. Using the hydrodynamic model for the large-scale plasma flow, the dynamics of shocks was examined which form in the geomagnetic flux tubes during the early stages of refilling. These shocks are more like those forming in neutral gases than the electrostatic shocks driven by microinstabilities involving ion-ion interaction. Therefore, the shocks seen in the hydrodynamic model are termed as hydrodynamic shocks. Such shocks are generally unsteady and therefore the usual shock jump conditions given by Rankine-Hugoniot relations are not strictly applicable to them. The density, flow velocity and temperature structures associated with the shocks are examined for both asymmetrical and symmetrical flows. In the asymmetrical flow the outflow from one of two conjugate ionospheres is dominant. On the other hand, in the symmetrical case outflows from the two ionospheric sources are identical. Both cases are treated by a two-stream model. In the late type of flow, the early-time refilling shows a relaxation type of oscillation, which is driven by the large-scale interactions between the two identical streams. After this early stage, the resulting temperature structure shows some interesting features. In the equatorial region the streams are isothermal, but in the off-equatorial regions the streams have quite different temperatures, and also densities and flow velocities. The dense and slow stream is found to be warmer than the low-density fast stream. In the late stage of refilling, the temperature is found to steadily increase from the conjugate ionospheres towards the equator; the equatorial temperature is found to be as high as about 8000 K compared to the ionospheric temperature of 3600 K.

Singh, Nagendra↗