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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

3-D GRMHD Simulations of Disk-Jet Coupling and Associated Variabilities and Emission

We have performed the fully three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion disk around a Schwarzschild black hole with a free-falling corona. The initial simulation results show that a bipolar jet is created as shown by previous two-dimensional axisymmetric simulations with mirror symmetry at the equator. The 3-D simulation ran over one hundred light-crossing time units which is considerably longer than the previous simulations. We show that the jet is initially formed as predicted due in part to magnetic pressure from the twisting the initially uniform magnetic field and from gas pressure associated with shock formation in the region around r = 3 r_S. At later times, the accretion disk becomes thick and the jet fades resulting in a wind that is ejected from the surface of the thickened (torus-like) disk. It should be noted that no streaming matter from a donor is included at the outer boundary in the simulation (an isolated black hole not binary black hole). The wind flows outwards with a wider angle than the initial jet. The widening of the jet is consistent with the outward moving torsional Alfven waves (TAWs). This evolution of disk-jet coupling suggests that the jet fades with a thickened accretion disk due to the lack of streaming material from an accompanying star. We will report initial results of emission calculated based on simulations with Kerr metric using a new ray-tracing method developed by S. Feurst and K. Wu. Images include free-free emission, absorption, and electron scattering. Fluorescent iron line emission and its variabilities from black holes will also be calculated based on simulation results.

Nishikawa, K. I.↗
NASA NTRS2005

A General Relativistic Magnetohydrodynamic Simulation of Jet Formation

We have performed a fully three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulation ofjet formation from a thin accretion disk around a Schwarzschild black hole with a free-falling corona. The initial simulation results show that a bipolar jet (velocity approx.0.3c) is created, as shown by previous two-dimensional axi- symmetric simulations with mirror symmetry at the equator. The three-dimensional simulation ran over 100 light crossing time units (T(sub s) = r(sub s)/c, where r(sub s = 2GM/c(sup 2), which is considerably longer than the previous simulations. We show that the jet is initially formed as predicted owing in part to magnetic pressure from the twisting of the initially uniform magnetic field and from gas pressure associated with shock formation in the region around r = 3r(sub s). At later times, the accretion disk becomes thick and the jet fades resulting in a wind that is ejected from the surface ofthe thickened (torus-like) disk. It should be noted that no streaming matter from a donor is included at the outer boundary in the simulation (an isolated black hole not binary black hole). The wind flows outward with a wider angle than the initial jet. The widening of the jet is consistent with the outward-moving torsional Alfven waves. This evolution of disk-jet coupling suggests that the jet fades with a thickened accretion disk because of the iack of streaming materiai from an accompanying star.

Nishikawa, K.-I.↗
NASA NTRS1993

Comparison of hydrodynamic and semi-kinetic treatments for plasma flow along closed field lines

Hydrodynamic and semi-kinetic treatments of plasma flow along closed geomagnetic field lines are compared. The hydrodynamic treatment is based on a simplified 16-moment set of transport equations as the equations for the heat flows are not solved; the heat flows are treated heuristically. The semi-kinetic treatment is based on a particle code. The comparison deals with the distributions of the plasma density, flow velocity, and parallel and perpendicular temperatures as obtained from the two treatments during the various stages of the flow. In the kinetic treatment, the appropriate boundary condition is the prescription of the velocity distribution functions for the particles entering the flux tubes at the ionospheric boundaries; those particles leaving the system are determined by the processes occurring in the flux tube. The prescribed distributions are half-Maxwellian with temperature T(sub 0) and density n(sub 0). In the hydrodynamic model, the prescribed boundary conditions are on density (n(sub 0)), flow velocity (V(sub 0)) and temperature (T(sub 0). It was found that results from the hydrodynamic treatment critically depend on V(sub 0); for early stages of the flow this treatment yields results in good agreement with those from the kinetic treatment, when V(sub 0) = square root of (kT(sub 0)/2 (pi)m), which is the average velocity of particles moving in a given direction for a Maxwellian distribution. During this early stage, the flows developing form the conjugate ionospheres show some distinct transitions. For the first hour or so, the flows are highly supersonic and penetrate deep into the opposite hemispheres, and both hydrodynamics and kinetic treatments yield almost similar features. It is found that during this period heatflow effects are negligibly small. When a flow penetrates deep into the opposite hemisphere, the kinetic treatment predicts reflection and setting up of counterstreaming. In contrast, the hydrodynamic treatment yields a shock in the flow. The reasons for this difference in the two treatments is discussed, showing that in view of the relatively warm ions, the coupling of ion beams and the consequent shock formation in the offequatorial region are not likely due to the enhancements in the beam temperatures. The counterstreaming in the kinetic treatment and the shock in the hydrodynamic treatment first advance upward to the equator and then downward to the ionospheric boundary from where the flow originated. The transit time for this advancement is found to be about 1 hour for the respective models. After 2 hours or so, both models predict that the flows from the ionospheric boundaries are generally subsonic with respect to the local ion-sound speed. At late stages of the flow, when a substantial fraction of ions entering the flux tube begin to return back in the kinetic treatment, the hydrodynamic treatment with the boundary condition V(sub 0) = square root of (kT(sub 0)/2(pi)m) yields an over-refilling, and the choice of V(sub 0) becomes uncertain.

Singh, Nagendra↗
DOE OSTI2026

Violent mergers can explain the inflated state of some of the fastest stars in the Galaxy

A significant number of hypervelocity stars with velocities between 1500 − 2500 km s −1 have recently been observed. The only plausible explanation so far is that they were produced through thermonuclear supernovae in white dwarf binaries. Since these stars are thought to be surviving donors of Type Ia supernovae, a surprising finding was that these stars are inflated, with radii an order of magnitude higher than expected for Roche-lobe-filling donors. Recent attempts at explaining them have combined 3D hydrodynamical supernova explosion simulations with 1D stellar modelling to explain the impact of supernova shocks on runaway white dwarfs. However, only the hottest and most compact of those runaway stars can so far marginally be reproduced by detailed models of runaways from supernova explosions. In this and a companion paper, we introduce a new AREPO simulation of two massive CO white dwarfs that explode via a violent merger. During the merger, the primary white dwarf ignites when the secondary is on its last orbit and plunging towards the primary. In the corresponding aftermath, the core of the secondary white dwarf of 0.16 M ⊙ remains bound, moving at a velocity of ∼2800 km s −1 . We mapped this object into MESA and show that this runaway star can explain the observations of two hypervelocity stars that were dubbed D6-1 and D6-3 based on their original discovery motivated by the D6 scenario, though the violent merger scenario presented here is somewhat distinct from the D6 scenario.

Astronomy and AstroPhysics↗
DOE OSTI2021

Comments on the SURFplus reactive burn model

The SURFplus reactive burn model is intended for heterogeneous solid HE that produce an excess amount of carbon. It utilizes a fast hotspot reaction (SURF model) plus a second slow reaction for the energy release from carbon clustering. The fast reaction dominates shock initiation, though the fast rate parameters are slightly affected by the late energy release from the slow reaction. The slow reaction has a significant affect on propagating detonation waves. Propagating detonation waves are characterized by the curvature effect; i.e., detonation speed $D_n$ as function of the local front curvature κ. The $D_n(κ)$ curves have a qualitative different shape. This results from the sensitivity of the slope of the $D_n(κ)$ curve to the reaction-zone width up to the sonic point, and the sonic point moving as κ increases from the end of the slow reaction to the neighborhood of the end of the fast reaction. For PBX 9502, the steep slope for small curvature is determined by the slow reaction, while the lower slope for larger curvature is determined by the fast reaction rate in the high detonation pressure regime. The remainder of this paper is organized as follows. In section 2 we review the assumptions of the Shaw and Johnson carbon cluster model used by the SURFplus model for the slow reaction. Section 3 describes in detail the mixture model for a partly burned HE. The form of the slow rate used by the SURFplus model is described in section 4. We conclude with a brief discussion of applying a burn model with 2 rates to either a metalized or a composite PBX.

42 ENGINEERING↗
NASA NTRS2000

Evaluation of Thermo-Mechanical Stability of COTS Dual-Axis MEMS Accelerometers for Space Applications

Microelectromechanical systems in MEMS is one of the fastest growing technologies in microelectronics, and is of great interest for military and aerospace applications. Accelerometers are the earliest and most developed representatives of MEMS. First demonstrated in 1979, micromachined accelerometers were used in automobile industry for air bag crash- sensing applications since 1990. In 1999, N4EMS accelerometers were used in NASA-JPL Mars Microprobe. The most developed accelerometers for airbag crash- sensing are rated for a full range of +/- 50 G. The range of sensitivity for accelerometers required for military or aerospace applications is much larger, varying from 20,000 G (to measure acceleration during gun and ballistic munition launches), and to 10(exp -6) G, when used as guidance sensors (to measure attitude and position of a spacecraft). The presence of moving parts on the surface of chip is specific to MEMS, and particularly, to accelerometers. This characteristic brings new reliability issues to micromachined accelerometers, including cyclic fatigue cracking of polysilicon cantilevers and springs, mechanical stresses that are caused by packaging and contamination in the internal cavity of the package. Studies of fatigue cracks initiation and growth in polysilicon showed that the fatigue damage may influence MEMS device performance, and the presence of water vapor significantly enhances crack initiation and growth. Environmentally induced failures, particularly, failures due to thermal cycling and mechanical shock are considered as one of major reliability concerns in MEMS. These environmental conditions are also critical for space applications of the parts. For example, the Mars pathfinder mission had experienced 80 mechanical shock events during the pyrotechnic separation processes.

Sharma, Ashok K.↗
DOE OSTI2021

Shear Strength and Release of Large Cryogenic Pellets from the Barrel of a Shattered Pellet Injector for Disruption Mitigation

Shattered pellet injection (SPI) has been chosen as the baseline disruption mitigation system on ITER due to its ability to rapidly inject material deep into the plasma to greatly increase the plasma density and radiate the thermal energy. SPI utilizes a mechanical punch or high-pressure gas to release and accelerate a pellet that has been cryogenically desublimated in the barrel of a pipe gun. Various material injection combinations could possibly be implemented during different phases of a disruption event to radiate plasma energy, reduce electromagnetic loads on machine components, avoid the formation of runaway electrons, or to dissipate runaway electrons that form. Each injection phase could possibly utilize combinations of deuterium, neon, or argon. In this paper we outline experimental measurements of pellet material shear strength at SPI operating temperatures to understand the force needed to release SPI pellets. Deuterium, neon, argon, and deuterium-neon mixture pellets with diameters of 8.5, 12.5, and 15.7 mm are formed at a range of relevant gas pressures and temperatures and dislodged from the cold zone with a slow-moving piston driven by a motor. The slow-moving piston is kept above the triple point temperature of the material while the pellet is forming, then cooled to below the triple point temperature before contacting the pellet to minimize any thermal conduction to the pellet. The piston incorporates a load cell to measure the force applied when the pellet breaks away from the cold zone in the barrel.The ability of the gas and punch methods to exceed the shear strength of the studied pellet materials for release has been analyzed. High pressure gas delivered by fast opening valves produce pressure shock to the pellet due to supersonic expansion of the propellant gas. Pressure (and therefore, force) oscillations are present due to transverse density propagation throughout the breech volume. Mechanical punches deliver an impact force through a high-kinetic energy impact. The effect of the mechanical shock on the pellet has been explored and will be presented in this paper. Scaling to larger ITER size SPI pellets will be described.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1991

Sources of mineral fragments in impact melts 15445 and 15455 - Toward the origin of low-K Fra Mauro basalt

The compositions of major and minor elements in mineral clasts in low-K Fra Mauro (LKFM) are studied in order to gain a deeper understanding of the genesis of LKFM and its primogenitors. The fragments are picked up as the impact melt moves outward from the center of the target. It is suggested that most of the mineral debris found in the melts is derived from troctolites of the Mg-suite. The chemical end member that dominates the LKFM melt matrics is not present as either mineral or lithic clastic debris in these melt rocks. It is proposed that the missing chemical component of LKFM could have been a magma, existing within the crust 3.9 Ga ago, and was either ejected intact to form LKFM melt rocks or was mixed with other shock-melted crusted and mantle rocks to produce the LKFM compositions. Assimilation of lithic and mineral clasts into the melt after breccian assembly is not a significant process and cannot be adduced to explain the LKFM melt/clast dichotomy.

Spudis, P. D.↗
NASA NTRS1953

Rocket-Model Investigation of the Longitudinal Stability, Drag, and Duct Performance Characteristics of the North American MX-770 (X-10) Missile at Mach Numbers from 0.80 to 1.70

A free-flight 0.12-scale rocket-boosted model of the North American MX-770 (X-10) missile has been tested in flight by the Pilotless Aircraft Research Division of the Langley Aeronautical Laboratory. Drag, longitudinal stability, and duct performance data were obtained at Mach numbers from 0.8 to 1.7 covering a Reynolds number range of about 9 x 10(exp 6) to 24 x 10(exp 6) based on wing mean aerodynamic chord. The lift-curve slope, static stability, and damping-in-pitch derivatives showed similar variations with Mach number, the parameters increasing from subsonic values in the transonic region and decreasing in the supersonic region. The variations were for the most part fairly smooth. The aerodynamic center of the configuration shifted rearward in the transonic region and moved forward gradually in the supersonic region. The pitching effectiveness of the canard control surfaces was maintained throughout the flight speed range, the supersonic values being somewhat greater than the subsonic. Trim values of angle of attack and lift coefficient changed abruptly in the transonic region, the change being associated with variations in the out-of-trim pitching moment, control effectiveness, and aerodynamic-center travel in this speed range. Duct total-pressure recovery decreased with increase in free-stream Mach number and the values were somewhat less than normal-shock recovery. Minimum drag data indicated a supersonic drag coefficient about twice the subsonic drag coefficient and a drag-rise Mach number of approximately 0.90. Base drag was small subsonically but was about 25 percent of the minimum drag of the configuration supersonically.

Bond, Aleck C.↗
NASA NTRS1988

The observed characteristics of flare energy release. II - High-speed soft X-ray fronts

Flare-associated large-scale brightenings of magnetic loop structures have recently been shown to be related to the propagation of soft X-ray fronts, moving at speeds of the order of 1000 km/s. These are also linked with the brightening of remote H-alpha patches and, in many cases, with type II or U radio emission. A detailed study of the best example found in the Solar Maximum Mission's Hard X-ray Imaging Spectrometer data was performed and with the help of numerical simulations and additional information provided by H-alpha records, it is shown that all together the three energy transport processes proposed by previous authors, namely high-energy particles, conduction fronts, and shocks, play significant roles in the redistribution of flare energy within the loops. The observable evidence of thermal flux limitation and the implication of these and previous results on the efficiency ratio between thermal and nonthermal processes in flares are discussed. Finally, these results are placed under the perspective of the interacting loop model of flares discussed in previous papers, to show that only about 10 percent of the total energy conversion occurs at the interface between loops. The bulk of the flare energy seems to be released internally within one of the bipolar loop structures.

Machado, Marcos E.↗
NASA NTRS2018

Series Elastic Tether Management for Rappelling Rovers

The Axel rappelling rover was designed to enable access to intriguing and important science sites that lie in difficult terrains that are inaccessible to conventional rovers. Extended autonomous rappelling calls for careful control of tethertension,precisemanagementoftetherspooling,andsome measure of shock tolerance. This paper covers the design and testing of a first-generation tether management system (TMS) for Axel. The system uses a double bull-wheel capstan driven by a low-stiffness series elastic actuator (SEA) to provide tension control and decouple internal spooling tension from external tether tension. The benefit of the series elastic actuator is two-fold. First, it permits closed-loop tether tension control with lower mass and power consumption than a comparable torque motor. Second, it improves the shock/drop tolerance of the rappelling system both while moving and when the system is inactive with the motors locked. Experiments on the new TMS show that this design performs well in keeping nearly constant spooling tension while rejecting large dynamic disturbances at the output. While the SEA is very effective at maintaining a given tension contribution, the additional effects of friction and the unique mechanical properties of the tether result in substantial errors in the measured output tension. Upcoming field trials will be used to evaluate the effectiveness and sufficiency of this system when integrated in Axel.

Nesnas, Issa↗
NASA NTRS1990

Dynamical experiments on models of colliding disk galaxies

Collisions between galaxies can induce large morphological changes in the participants and, in the case of colliding disk galaxies, bridges and tails are often formed. Observations of such systems indicate a wide variation in color (see Larson and Tinsley, 1978) and that some of the particpants are experiencing enhanced rates of star formation, especially in their central regions (Bushouse 1986, 1987; Kennicutt et al., 1987, Bushouse, Lamb, and Werner, 1988). Here the authors describe progress made in understanding some of the dynamics of interacting galaxies using N-body stellar dynamical computer experiments, with the goal of extending these models to include a hydrodynamical treatment of the gas so that a better understanding of globally enhanced star formation will eventually be forthcoming. It was concluded that close interactions between galaxies can produce large perturbations in both density and velocity fields. The authors measured, via computational experiments that represent a galaxy's stars, average radial velocity flows as large as 100 km/sec and 400 percent density increases. These can occur in rings that move outwards through the disk of a galaxy, in roughly homologous inflows toward the nucleus, and in off center, non-axisymmetric regions. Here the authors illustrate where the gas is likely to flow during the early stages of interaction and in future work they plan to investigate the fate of the gas more realistically by using an N-body/Smoothed Particle Hydrodynamics code to model both the stellar and gaseous components of a disk galaxy during a collision. Specifically, they will determine the locations of enhanced gas density and the strength and location of shock fronts that form during the interaction.

Gerber, Richard A.↗
NASA NTRS1991

Solar wind dynamic pressure variations and possible ground signatures of flux transfer events

The effects of a series of solar wind dynamic pressure pulses are followed through the solar wind, across the bow shock and magnetosheath, into the magnetosphere, and down to the dayside ionosphere. Each pressure pulse had an intrinsic solar wind origin, i.e., was not generated by processes occurring at the earth's bow shock. Both upstream and downstream of the earth's bow shock, the pulses were identified by depressed magnetic field strengths and enhanced plasma densities. The pulses compressed the magnetosheath and magnetosphere, causing satellites in the magnetosphere to briefly observe enhanced magnetic field strengths, and/or enter the low-latitude boundary layer or magnetosheath. Satellites already in the magnetosheath observed a brief burst of enhanced magnetosheath flow caused by the relative inward motion of magnetosheath flow patterns. Alfven and fast mode compressional waves propagated rapidly down to the polar ionosphere, where they produced transient ionospheric flows at latitudes equatorward of the convection reversal boundary. The flow patterns moved westward around the auroral oval.

Sibeck, D. G.↗
NASA NTRS1996

Plasma spray nozzle with low overspray and collimated flow

An improved nozzle for reducing overspray in high temperature supersonic plasma spray devices comprises a body defining an internal passageway having an upstream end and a downstream end through which a selected plasma gas is directed. The nozzle passageway has a generally converging/diverging Laval shape with its upstream end converging to a throat section and its downstream end diverging from the throat section. The upstream end of the passageway is configured to accommodate a high current cathode for producing an electrical arc in the passageway to heat and ionize the gas flow to plasma form as it moves along the passageway. The downstream end of the nozzle is uniquely configured through the methodology of this invention to have a contoured bell-shape that diverges from the throat to the exit of the nozzle. Coating material in powder form is injected into the plasma flow in the region of the bell-shaped downstream end of the nozzle and the powder particles become entrained in the flow. The unique bell shape of the nozzle downstream end produces a plasma spray that is ideally expanded at the nozzle exit and thus virtually free of shock phenomena, and that is highly collimated so as to exhibit significantly reduced fanning and diffusion between the nozzle and the target. The overall result is a significant reduction in the amount of material escaping from the plasma stream in the form of overspray and a corresponding improvement in the cost of the coating operation and in the quality and integrity of the coating itself.

Beason, Jr., George P.↗
NASA NTRS1976

Computer simulation of earthquakes

Two computer simulation models of earthquakes were studied for the dependence of the pattern of events on the model assumptions and input parameters. Both models represent the seismically active region by mechanical blocks which are connected to one another and to a driving plate. The blocks slide on a friction surface. In the first model elastic forces were employed and time independent friction to simulate main shock events. The size, length, and time and place of event occurrence were influenced strongly by the magnitude and degree of homogeniety in the elastic and friction parameters of the fault region. Periodically reoccurring similar events were frequently observed in simulations with near homogeneous parameters along the fault, whereas, seismic gaps were a common feature of simulations employing large variations in the fault parameters. The second model incorporated viscoelastic forces and time-dependent friction to account for aftershock sequences. The periods between aftershock events increased with time and the aftershock region was confined to that which moved in the main event.

Cohen, S. C.↗
NASA NTRS1990

Direct simulation Monte Carlo with ionization and radiation

Improvements in the modeling of radiation in low density shock waves with Direct Simulation Monte Carlo (DSMC) are the subject of this study. The physical processes which determine the amount of radiation in a shock wave were investigated and the way in which they were modeled with DSMC was evaluated. Three physical processes were identified for which an improvement in the modeling technique could result in improved radiation predictions. New physical modeling schemes are introduced in this report for the three processes. First a method for determining the electric field and its effect on the flow is introduced. Second, a two step reaction process for electron impact ionization reactions is evaluated. Finally, a new scheme to determine the relaxation collision numbers for excitation of electronic states is proposed. Each new scheme attempts to move the DSMC method toward more accurate physics or more reliance on experimental data. The new schemes are all compared to the current modeling techniques and the differences in the results are evaluated. In all cases the results agree with the available data as well as, or better than the results from earlier schemes.

Carlson, Ann B.↗
DOE OSTI2020

Validation of the Mach Stem Triple Point

Among the more complex characteristics of an explosion’s shock front is the creation of the Mach stem and its triple point. The scientific footage of the US Atmospheric Nuclear Weapons Testing Program has a film that may show the locus of points formed by the triple point, allowing us to visually investigate the trajectory of these shockwaves. During a nuclear blast, once the X-ray diffusion process ends, an incident shockwave forms in all directions. When the incident shockwave contacts the ground, a reflected shockwave is created. The overall magnitude and impact of these two shockwaves are proportional to the increasing total pressure and temperature, allowing the disturbances to move faster than the local speed of sound.

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗
NASA NTRS1976

Accretion onto magnetized neutron stars - Structure and interchange instability of a model magnetosphere

A self-consistent model is analyzed for the spherical infall of weakly magnetized plasma into the magnetosphere of a slowly rotating, strongly magnetized neutron star. It is shown that spherical infall is probably a good approximation for X-ray sources which accrete from a stellar wind. The location of the standoff shock which halts the hypersonic infall is estimated along with the emission from the shocked layer. The location of the equilibrium magnetopause and the structure of the magnetic field within it are calculated; it is found that the magnetic poles are true cusps and that the entry of gas due to equilibrium flow across a cusp is almost certainly dominated by the interchange instability near the magnetic equator. The energy principle is applied to derive necessary conditions for the occurrence of this instability. The results indicate that the strong magnetic-pressure gradient stabilizes the gas unless moderately strong radiative cooling takes place and that the cooled plasma enters the magnetosphere as long filaments capable of moving between field lines. The rate at which the equilibrium magnetopause can 'absorb' mass and momentum is derived, the validity of the approximations employed is discussed, and the likely evolution of the sinking filaments is outlined to show that the spatial distribution of the plasma is determined mainly by the dynamics and thermodynamics of the filaments rather than the magnetic-field structure.

Arons, J.↗