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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Validation of a reduced-order jet model for subsonic and underexpanded hydrogen jets

Much effort has been made to model hydrogen releases from leaks during potential failures of hydrogen storage systems. A reduced-order jet model can be used to quickly characterize these flows, with low computational cost. Notional nozzle models are often used to avoid modeling the complex shock structures produced by the underexpanded jets by determining an “effective” source to produce the observed downstream trends. In our work, the mean hydrogen concentration fields were measured in a series of subsonic and underexpanded jets using a planar laser Rayleigh scattering system. Furthermore, we compared the experimental data to a reduced order jet model for subsonic flows and a notional nozzle model coupled to the jet model for underexpanded jets. The values of some key model parameters were determined by comparisons with the experimental data. Finally, the coupled model was also validated against hydrogen concentrations measurements for 100 and 200 bar hydrogen jets with the predictions agreeing well with data in the literature.

08 HYDROGEN↗
DOE OSTI2014

A magnetohydrodynamic model of the M87 jet. II. Self-consistent quad-shock jet model for optical relativistic motions and particle acceleration

We describe a new paradigm for understanding both relativistic motions and particle acceleration in the M87 jet: a magnetically dominated relativistic flow that naturally produces four relativistic magnetohydrodynamic (MHD) shocks (forward/reverse fast and slow modes). We apply this model to a set of optical super- and subluminal motions discovered by Biretta and coworkers with the Hubble Space Telescope during 1994-1998. The model concept consists of ejection of a single relativistic Poynting jet, which possesses a coherent helical (poloidal + toroidal) magnetic component, at the remarkably flaring point HST-1. We are able to reproduce quantitatively proper motions of components seen in the optical observations of HST-1 with the same model we used previously to describe similar features in radio very long baseline interferometry observations in 2005-2006. This indicates that the quad relativistic MHD shock model can be applied generally to recurring pairs of super/subluminal knots ejected from the upstream edge of the HST-1 complex as observed from radio to optical wavelengths, with forward/reverse fast-mode MHD shocks then responsible for observed moving features. Moreover, we identify such intrinsic properties as the shock compression ratio, degree of magnetization, and magnetic obliquity and show that they are suitable to mediate diffusive shock acceleration of relativistic particles via the first-order Fermi process. We suggest that relativistic MHD shocks in Poynting-flux-dominated helical jets may play a role in explaining observed emission and proper motions in many active galactic nuclei.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Three-dimensional Magnetohydrodynamical Simulations of the Morphology of Head–Tail Radio Galaxies Based on the Magnetic Tower Jet Model

The distinctive morphology of head–tail radio galaxies reveals strong interactions between the radio jets and their intra-cluster environment, the general consensus on the morphology origin of head–tail sources is that radio jets are bent by violent intra-cluster weather. We demonstrate in this paper that such strong interactions provide a great opportunity to study the jet properties and also the dynamics of the intra-cluster medium (ICM). By three-dimensional magnetohydrodynamical simulations, we analyze the detailed bending process of a magnetically dominated jet, based on the magnetic tower jet model. We use stratified atmospheres modulated by wind/shock to mimic the violent intra-cluster weather. Core sloshing is found to be inevitable during the wind-cluster core interaction, which induces significant shear motion and could finally drive ICM turbulence around the jet, making it difficult for the jet to survive. We perform a detailed comparison between the behavior of pure hydrodynamical jets and the magnetic tower jet and find that the jet-lobe morphology could not survive against the violent disruption in all of our pure hydrodynamical jet models. On the other hand, the head–tail morphology is well reproduced by using a magnetic tower jet model bent by wind, in which hydrodynamical instabilities are naturally suppressed and the jet could always keep its integrity under the protection of its internal magnetic fields. Finally, we also check the possibility for jet bending by shock only. We find that shock could not bend the jet significantly, and thus could not be expected to explain the observed long tails in head–tail radio galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

CONFRONTING THE JET MODEL OF Sgr A* WITH THE FARADAY ROTATION MEASURE OBSERVATIONS

Sgr A* is probably the supermassive black hole being investigated most extensively due to its proximity to Earth. Several theoretical models for its steady state emission have been proposed in the past two decades. Both the radiative-inefficient accretion flow and the jet model have been shown to well explain the observed spectral energy distribution. The Faraday rotation measure (RM) has been unambiguously measured at the submillimeter wavelength, but it has only been tested against the accretion flow model. Here we first calculate the RM based on the jet model and find that the predicted value is two orders of magnitude lower than the measured value. We then include an additional contribution from the accretion flow in front of the jet and show that the measured RM may be reconciled with the model under some tight constraints. The main constraint is that the inclination angle should be greater than ∼73°. However, this requirement is not consistent with an existing observational estimate of the inclination angle.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

VISCOUS BOUNDARY LAYERS OF RADIATION-DOMINATED, RELATIVISTIC JETS. II. THE FREE-STREAMING JET MODEL

We analyze the interaction of a radiation-dominated jet and its surroundings using the equations of radiation hydrodynamics in the viscous limit. In a previous paper we considered the two-stream scenario, which treats the jet and its surroundings as distinct media interacting through radiation viscous forces. Here we present an alternative boundary layer model, known as the free-streaming jet model—where a narrow stream of fluid is injected into a static medium—and present solutions where the flow is ultrarelativistic and the boundary layer is dominated by radiation. It is shown that these jets entrain material from their surroundings and that their cores have a lower density of scatterers and a harder spectrum of photons, leading to observational consequences for lines of sight that look “down the barrel of the jet.” These jetted outflow models may be applicable to the jets produced during long gamma-ray bursts and super-Eddington phases of tidal disruption events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A TWO-COMPONENT JET MODEL FOR THE TIDAL DISRUPTION EVENT SWIFT J164449.3+573451

We analyze both the early- and late-time radio and X-ray data of the tidal disruption event (TDE) Swift J1644+57. The data at early times (≲ 5 days) necessitate separation of the radio and X-ray emission regions, either spatially or in velocity space. This leads us to suggest a two-component jet model, in which the inner jet is initially relativistic with Lorentz factor Γ ≈ 15, while the outer jet is trans-relativistic, with Γ ≲ 1.2. This model enables a self-consistent interpretation of the late-time radio data, both in terms of peak frequency and flux. We solve the dynamics, radiative cooling, and expected radiation from both jet components. We show that while during the first month synchrotron emission from the outer jet dominates the radio emission, at later times, radiation from ambient gas collected by the inner jet dominates. This provides a natural explanation to the observed re-brightening, without the need for late-time inner engine activity. After 100 days, the radio emission peak is in the optically thick regime, leading to a decay of both the flux and peak frequency at later times. Our model's predictions for the evolution of radio emission in jetted TDEs can be tested by future observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CONSTRAINING THE BULK LORENTZ FACTOR OF GAMMA-RAY BURST OUTFLOW IN THE MAGNETIC-DOMINATED JET MODEL

Recent observations by the Fermi-LAT showed that there are delayed arrivals of GeV photons relative to the onset of MeV photons in some gamma-ray bursts (GRBs). In order to avoid a large optical depth, the minimal value of the Lorentz factor has been estimated to be higher than 1000 in some of the brightest bursts. In this paper, we present a detailed calculation of the time delay between the MeV and GeV photons in the framework of the magnetic-dominated jet model. We find that the time delay strongly depends on the saturated bulk Lorentz factor of the jet. Inspired by this fact, we use this model to calculate the Lorentz factors of the four brightest Fermi bursts. The results indicate that the Lorentz factors are much smaller than those obtained from the 'single-zone' scenario. The short burst GRB 090510 has a minimal Lorentz factor of 385, while the three long bursts, GRB 080916c, GRB 090902b, and GRB 090926, have almost the same Lorentz factors with an average value near 260. Another interesting result is that, for long bursts, GeV photons are emitted after the bulk Lorentz factor saturates. For the short GRB, however, MeV and GeV photons are emitted at the same phase, i.e., either in the expansion phase or in the coasting phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

An accretion-jet model for M87: interpreting the spectral energy distribution and faraday rotation measure

M87 is arguably the best supermassive black hole (BH) to explore jet and/or accretion physics, due to its proximity and fruitful high-resolution multi-waveband observations. We model the multi-wavelength spectral energy distribution (SED) of the M87 core that observed at a scale of 0.4 arcsec (∼10{sup 5} R {sub g}, R {sub g} is gravitational radius), as recently presented by Prieto et al. Similar to Sgr A*, we find that the millimeter bump as observed by the Atacama Large Millimeter/submillimeter Array can be modeled by the synchrotron emission of the thermal electrons in an advection-dominated accretion flow (ADAF), while the low-frequency radio emission and X-ray emission may predominantly come from the jet. The millimeter radiation from ADAF predominantly comes from the region within 10R {sub g}, which is roughly consistent with the recent very long baseline interferometry observations at 230 GHz. We further calculate the Faraday rotation measure (RM) from both ADAF and jet models, and find that the RM predicted from the ADAF is roughly consistent with the measured value, while the RM predicted from the jet is much higher if jet velocity close to the BH is low or moderate (e.g., v {sub jet} ≲ 0.6 c). With the constraints from the SED modeling and RM, we find that the accretion rate close to the BH horizon is ∼(0.2--1) ×10{sup −3} M{sub ⊙} yr{sup −1}≪ M-dot {sub B}∼0.2 M{sub ⊙} yr{sup −1} ( M-dot {sub B} is Bondi accretion rate), where the electron density profile, n {sub e} ∝ r {sup ∼−1}, in the accretion flow, is consistent with that determined from X-ray observation inside the Bondi radius and recent numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Benchmark studies of thermal jet mixing in SFRs using a two-jet model

To guide the modeling, simulations and design of Sodium Fast Reactors (SFRs), we explore and compare the predictive capabilities of two numerical solvers COMSOL and OpenFOAM in the thermal jet mixing of two buoyant jets typical of the outlet flow from a SFR tube bundle. This process will help optimize on-going experimental efforts at obtaining high resolution data for V and V of CFD codes as anticipated in next generation nuclear systems. Using the k-{epsilon} turbulence models of both codes as reference, their ability to simulate the turbulence behavior in similar environments was first validated for single jet experimental data reported in literature. This study investigates the thermal mixing of two parallel jets having a temperature difference (hot-to-cold) {Delta}T{sub hc}= 5 deg. C, 10 deg. C and velocity ratios U{sub c}/U{sub h} = 0.5, 1. Results of the computed turbulent quantities due to convective mixing and the variations in flow field along the axial position are presented. In addition, this study also evaluates the effect of spacing ratio between jets in predicting the flow field and jet behavior in near and far fields. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2014

Modeling jet and outflow feedback during star cluster formation

Powerful jets and outflows are launched from the protostellar disks around newborn stars. These outflows carry enough mass and momentum to transform the structure of their parent molecular cloud and to potentially control star formation itself. Despite their importance, we have not been able to fully quantify the impact of jets and outflows during the formation of a star cluster. The main problem lies in limited computing power. We would have to resolve the magnetic jet-launching mechanism close to the protostar and at the same time follow the evolution of a parsec-size cloud for a million years. Current computer power and codes fall orders of magnitude short of achieving this. In order to overcome this problem, we implement a subgrid-scale (SGS) model for launching jets and outflows, which demonstrably converges and reproduces the mass, linear and angular momentum transfer, and the speed of real jets, with ∼1000 times lower resolution than would be required without the SGS model. We apply the new SGS model to turbulent, magnetized star cluster formation and show that jets and outflows (1) eject about one-fourth of their parent molecular clump in high-speed jets, quickly reaching distances of more than a parsec, (2) reduce the star formation rate by about a factor of two, and (3) lead to the formation of ∼1.5 times as many stars compared to the no-outflow case. Most importantly, we find that jets and outflows reduce the average star mass by a factor of ∼ three and may thus be essential for understanding the characteristic mass of the stellar initial mass function.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

X-RAY AND GAMMA-RAY POLARIZATION IN LEPTONIC AND HADRONIC JET MODELS OF BLAZARS

We present a theoretical analysis of the expected X-ray and {gamma}-ray polarization signatures resulting from synchrotron self-Compton emission in leptonic models compared to the polarization signatures from proton synchrotron and cascade synchrotron emission in hadronic models for blazars. Source parameters resulting from detailed spectral-energy-distribution modeling are used to calculate photon-energy-dependent upper limits on the degree of polarization, assuming a perfectly organized mono-directional magnetic field. In low-synchrotron-peaked blazars, hadronic models exhibit substantially higher maximum degrees of X-ray and gamma-ray polarization than leptonic models, which may be within reach of existing X-ray and {gamma}-ray polarimeters. In high-synchrotron-peaked blazars (with electron-synchrotron-dominated X-ray emission), leptonic and hadronic models predict the same degree of X-ray polarization but substantially higher maximum {gamma}-ray polarization in hadronic models than leptonic ones. These predictions are particularly relevant in view of the new generation of balloon-borne X-ray polarimeters (and possibly GEMS, if revived), and the ability of Fermi-LAT to measure {gamma}-ray polarization at <200 MeV. We suggest observational strategies combining optical, X-ray, and {gamma}-ray polarimetry to determine the degree of ordering of the magnetic field and to distinguish between leptonic and hadronic high-energy emissions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Modelling jets, tori and flares in pulsar wind nebulae

In this contribution we review the recent progress in the modelling of Pulsar Wind Nebulae (PWN). We start with a brief overview of the relevant physical processes in the magnetosphere, the wind-zone and the inflated nebula bubble. Radiative signatures and particle transport processes obtained from 3D simulations of PWN are discussed in the context of optical and X-ray observations. We then proceed to consider particle acceleration in PWN and elaborate on what can be learned about the particle acceleration from the dynamical structures called GwispsG observed in the Crab nebula. We also discuss recent observational and theoretical results of gamma-ray flares and the inner knot of the Crab nebula, which had been proposed as the emission site of the flares. Here, we extend the discussion to GeV flares from binary systems in which the pulsar wind interacts with the stellar wind from a companion star. The chapter concludes with a discussion of solved and unsolved problems posed by PWN.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

X-RAY EMISSION FROM TRANSIENT JET MODEL IN BLACK HOLE BINARIES

While the non-thermal radio through at least near-infrared emission in the hard state in X-ray binaries (XRBs) is known to originate in jets, the source of the non-thermal X-ray component is still uncertain. We introduce a new model for this emission, which takes into account the transient nature of outflows, and show that it can explain the observed properties of the X-ray spectrum. Rapid radiative cooling of the electrons naturally accounts for the break often seen below around 10 keV, and for the canonical spectral slope F{sub {nu}}{proportional_to}{nu}{sup -1/2} observed below the break. We derive the constraints set by the data for both synchrotron- and Compton-dominated models. We show that for the synchrotron-dominated case, the jet should be launched at radii comparable to the inner radius of the disk ({approx}few 100 r{sub s} for the 2000 outburst of XTE J1118+480), with typical magnetic field B {approx}> 10{sup 6} G. We discuss the consequences of our results for the possible connection between the inflow and outflow in the hard state of XRBs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE FERMI BUBBLES. II. THE POTENTIAL ROLES OF VISCOSITY AND COSMIC-RAY DIFFUSION IN JET MODELS

The origin of the Fermi bubbles recently detected by the Fermi Gamma-ray Space Telescope in the inner Galaxy is mysterious. In the companion paper Guo and Mathews (Paper I), we use hydrodynamic simulations to show that they could be produced by a recent powerful active galactic nucleus (AGN) jet event. Here, we further explore this scenario to study the potential roles of shear viscosity and cosmic-ray (CR) diffusion on the morphology and CR distribution of the bubbles. We show that even a relatively low level of viscosity ({mu}{sub visc} {approx}> 3 g cm{sup -1} s{sup -1}, or {approx}0.1%-1% of Braginskii viscosity in this context) could effectively suppress the development of Kelvin-Helmholtz instabilities at the bubble surface, resulting in smooth bubble edges as observed. Furthermore, viscosity reduces circulating motions within the bubbles, which would otherwise mix the CR-carrying jet backflow near bubble edges with the bubble interior. Thus viscosity naturally produces an edge-favored CR distribution, an important ingredient to produce the observed flat gamma-ray surface brightness distribution. Generically, such a CR distribution often produces a limb-brightened gamma-ray intensity distribution. However, we show that by incorporating CR diffusion that is strongly suppressed across the bubble surface (as inferred from sharp bubble edges) but is close to canonical values in the bubble interior, we obtain a reasonably flat gamma-ray intensity profile. The similarity of the resulting CR bubble with the observed Fermi bubbles strengthens our previous result in Paper I that the Fermi bubbles were produced by a recent AGN jet event. Studies of the nearby Fermi bubbles may provide a unique opportunity to study the potential roles of plasma viscosity and CR diffusion on the evolution of AGN jets and bubbles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

First principles transport coefficients and reaction rates of Ar{sub 2}{sup +} ions in argon for cold plasma jet modeling

Momentum-transfer collision cross-sections and integral collision cross-sections for the collision-induced dissociation are calculated for collisions of ionized argon dimers with argon atoms using a nonadiabatic semiclassical method with the electronic Hamiltonian calculated on the fly via a diatomics-in-molecules semiempirical model as well as inverse-method modeling based on simple isotropic rigid-core potential. The collision cross-sections are then used in an optimized Monte Carlo code for evaluations of the Ar{sub 2}{sup +} mobility in argon gas, longitudinal diffusion coefficient, and collision-induced dissociation rates. A thorough comparison of various theoretical calculations as well as with available experimental data on the Ar{sub 2}{sup +} mobility and collision cross-sections is performed. Good agreement is found between both theoretical approaches and the experiment. Analysis of the role of inelastic processes in Ar{sub 2}{sup +}/Ar collisions is also provided.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗