Chaotic advection and heat transfer enhancement in Stokes flows

International Nuclear Information System (INIS)

Lefevre, A.; Mota, J.P.B.; Rodrigo, A.J.S.; Saatdjian, E.

2003-01-01

The heat transfer rate from a solid boundary to a highly viscous fluid can be enhanced significantly by a phenomenon which is called chaotic advection or Lagrangian turbulence. Although the flow is laminar and dominated by viscous forces, some fluid particle trajectories are chaotic due either to a suitable boundary displacement protocol or to a change in geometry. As in turbulent flow, the heat transfer rate enhancement between the boundary and the fluid is intimately linked to the mixing of fluid in the system. Chaotic advection in real Stokes flows, i.e. flows governed by viscous forces and that can be constructed experimentally, is reviewed in this paper. An emphasis is made on recent new results on 3-D time-periodic open flows which are particularly important in industry

Anchoring Polar Magnetic Field in a Stationary Thick Accretion Disk

Energy Technology Data Exchange (ETDEWEB)

Samadi, Maryam; Abbassi, Shahram, E-mail: samadimojarad@um.ac.ir [Department of Physics, School of Sciences, Ferdowsi University of Mashhad, Mashhad, 91775-1436 (Iran, Islamic Republic of)

2017-08-20

We investigate the properties of a hot accretion flow bathed in a poloidal magnetic field. We consider an axisymmetric viscous-resistive flow in the steady-state configuration. We assume that the dominant mechanism of energy dissipation is due to turbulence viscosity and magnetic diffusivity. A certain fraction of that energy can be advected toward the central compact object. We employ the self-similar method in the radial direction to find a system of ODEs with just one varible, θ in the spherical coordinates. For the existence and maintenance of a purely poloidal magnetic field in a rotating thick disk, we find that the necessary condition is a constant value of angular velocity along a magnetic field line. We obtain an analytical solution for the poloidal magnetic flux. We explore possible changes in the vertical structure of the disk under the influences of symmetric and asymmetric magnetic fields. Our results reveal that a polar magnetic field with even symmetry about the equatorial plane makes the disk vertically thin. Moreover, the accretion rate decreases when we consider a strong magnetic field. Finally, we notice that hot magnetized accretion flows can be fully advected even in a slim shape.

Foundations of Black Hole Accretion Disk Theory

Directory of Open Access Journals (Sweden)

Marek A. Abramowicz

2013-01-01

Full Text Available This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks, Shakura-Sunyaev (thin disks, slim disks, and advection-dominated accretion flows (ADAFs. After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs.

Foundations of Black Hole Accretion Disk Theory.

Science.gov (United States)

Abramowicz, Marek A; Fragile, P Chris

2013-01-01

This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

Correlating non-linear properties with spectral states of RXTE data: possible observational evidences for four different accretion modes around compact objects

Science.gov (United States)

Adegoke, Oluwashina; Dhang, Prasun; Mukhopadhyay, Banibrata; Ramadevi, M. C.; Bhattacharya, Debbijoy

2018-05-01

By analysing the time series of RXTE/PCA data, the non-linear variabilities of compact sources have been repeatedly established. Depending on the variation in temporal classes, compact sources exhibit different non-linear features. Sometimes they show low correlation/fractal dimension, but in other classes or intervals of time they exhibit stochastic nature. This could be because the accretion flow around a compact object is a non-linear general relativistic system involving magnetohydrodynamics. However, the more conventional way of addressing a compact source is the analysis of its spectral state. Therefore, the question arises: What is the connection of non-linearity to the underlying spectral properties of the flow when the non-linear properties are related to the associated transport mechanisms describing the geometry of the flow? This work is aimed at addressing this question. Based on the connection between observed spectral and non-linear (time series) properties of two X-ray binaries: GRS 1915+105 and Sco X-1, we attempt to diagnose the underlying accretion modes of the sources in terms of known accretion classes, namely, Keplerian disc, slim disc, advection dominated accretion flow and general advective accretion flow. We explore the possible transition of the sources from one accretion mode to others with time. We further argue that the accretion rate must play an important role in transition between these modes.

Hydrodynamical wind in magnetized accretion flows with convection

International Nuclear Information System (INIS)

Abbassi, Shahram; Mosallanezhad, Amin

2012-01-01

The existence of outflow and magnetic fields in the inner region of hot accretion flows has been confirmed by observations and numerical magnetohydrodynamic (MHD) simulations. We present self-similar solutions for radiatively inefficient accretion flows (RIAFs) around black holes in the presence of outflow and a global magnetic field. The influence of outflow is taken into account by adopting a radius that depends on mass accretion rate M-dot = M-dot 0 (r/r 0 ) s with s > 0. We also consider convection through a mixing length formula to calculate convection parameter α con . Moreover we consider the additional magnetic field parameters β r,φ,z [ = c 2 r,φ,z /(2c 2 s )], where c 2 r,φ,z are the Alfvén sound speeds in three directions of cylindrical coordinates. Our numerical results show that by increasing all components of the magnetic field, the surface density and rotational velocity increase, but the sound speed and radial infall velocity of the disk decrease. We have also found that the existence of wind will lead to reduction of surface density as well as rotational velocity. Moreover, the radial velocity, sound speed, advection parameter and the vertical thickness of the disk will increase when outflow becomes important in the RIAF. (research papers)

Evolution of accretion disks in tidal disruption events

Energy Technology Data Exchange (ETDEWEB)

Shen, Rong-Feng [Current address: Racah Institute of Physics, Hebrew University of Jerusalem, Israel. (Israel); Matzner, Christopher D., E-mail: rf.shen@mail.huji.ac.il, E-mail: matzner@astro.utoronto.ca [Department of Astronomy and Astrophysics, University of Toronto, M5S 3H4 (Canada)

2014-04-01

During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates that may exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history or, at longer wavelengths, by its own emission. We model the evolution of TDE disk size and accretion rate by accounting for the time-dependent fallback rate, for the influence of wind losses in the early advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure-dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body because of the Lense-Thirring effect, and precession may manifest itself as a quasi-periodic modulation of the light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t = 500 days corresponding to a disk transition from the advective state to the gas-pressure-dominated state.

Inclusion of TCAF model in XSPEC to study accretion flow dynamics around black hole candidates

Science.gov (United States)

Debnath, Dipak; Chakrabarti, Sandip Kumar; Mondal, Santanu

Spectral and Temporal properties of black hole candidates can be well understood with the Chakrabarti-Titarchuk solution of two component advective flow (TCAF). This model requires two accretion rates, namely, the Keplerian disk accretion rate and the sub-Keplerian halo accretion rate, the latter being composed of a low angular momentum flow which may or may not develop a shock. In this solution, the relevant parameter is the relative importance of the halo (which creates the Compton cloud region) rate with respect to the Keplerian disk rate (soft photon source). Though this model has been used earlier to manually fit data of several black hole candidates quite satisfactorily, for the first time we are able to create a user friendly version by implementing additive Table model FITS file into GSFC/NASA's spectral analysis software package XSPEC. This enables any user to extract physical parameters of accretion flows, such as two accretion rates, shock location, shock strength etc. for any black hole candidate. Most importantly, unlike any other theoretical model, we show that TCAF is capable of predicting timing properties from spectral fits, since in TCAF, a shock is responsible for deciding spectral slopes as well as QPO frequencies.

Images and Spectra of Time Dependent Two Component Advective Flow in Presence of Outflows

Science.gov (United States)

Chatterjee, Arka; Chakrabarti, Sandip K.; Ghosh, Himadri; Garain, Sudip K.

2018-05-01

Two Component Advective Flow (TCAF) successfully explains the spectral and temporal properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or CENtrifugal pressure supported Boundary Layer (CENBOL) due to gravitational bending of photons have been studied before. In this paper, we study time dependent images of advective flows around a Schwarzschild black hole which include cooling effects due to Comptonization of soft photons from a Keplerian disks well as the self-consistently produced jets and outflows. We show the overall image of the disk-jet system after convolving with a typical beamwidth. A long exposure image with time dependent system need not show the black hole horizon conspicuously, unless one is looking at a soft state with no jet or the system along the jet axis. Assuming these disk-jet configurations are relevant to radio emitting systems also, our results would be useful to look for event horizons in high accretion rate Supermassive Black Holes in Seyfert galaxies, RL Quasars.

Exploring the accretion model of M87 and 3C 84 with the Faraday rotation measure observations

OpenAIRE

Li, Ya-Ping; Yuan, Feng; Xie, Fu-Guo

2016-01-01

Low-luminosity active galactic nuclei (LLAGNs) are believed to be powered by an accretion-jet model, consisting of an inner advection-dominated accretion flow (ADAF), an outer truncated standard thin disk, and a jet. But model degeneracy still exists in this framework. For example, the X-ray emission can originate from either the ADAF or jet. The aim of the present work is to check these models with the Faraday rotation measure (RM) observations recently detected for two LLAGNs, M87 and 3C 84...

Chaotic advection, diffusion, and reactions in open flows

International Nuclear Information System (INIS)

Tel, Tamas; Karolyi, Gyoergy; Pentek, Aron; Scheuring, Istvan; Toroczkai, Zoltan; Grebogi, Celso; Kadtke, James

2000-01-01

We review and generalize recent results on advection of particles in open time-periodic hydrodynamical flows. First, the problem of passive advection is considered, and its fractal and chaotic nature is pointed out. Next, we study the effect of weak molecular diffusion or randomness of the flow. Finally, we investigate the influence of passive advection on chemical or biological activity superimposed on open flows. The nondiffusive approach is shown to carry some features of a weak diffusion, due to the finiteness of the reaction range or reaction velocity. (c) 2000 American Institute of Physics

Study of magnetized accretion flow with variable Γ equation of state

Science.gov (United States)

Singh, Kuldeep; Chattopadhyay, Indranil

2018-05-01

We present here the solutions of magnetized accretion flow on to a compact object with hard surface such as neutron stars. The magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. We have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. We have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. We found all possible accretion solutions. All accretion solutions terminate with a shock very near to the star surface and the height of this primary shock does not vary much with either the spin period or the Bernoulli parameter of the flow, although the strength of the shock may vary with the period. For moderately rotating central star, there is possible formation of multiple sonic points in the flow and therefore, a second shock far away from the star surface may also form. However, the second shock is much weaker than the primary one near the surface. We found that if rotation period is below a certain value (P*), then multiple critical points or multiple shocks are not possible and P* depends upon the composition of the flow. We also found that cooling effect dominates after the shock and that the cyclotron and the bremsstrahlung cooling processes should be considered to obtain a consistent accretion solution.

Advecting Procedural Textures for 2D Flow Animation

Science.gov (United States)

Kao, David; Pang, Alex; Moran, Pat (Technical Monitor)

2001-01-01

This paper proposes the use of specially generated 3D procedural textures for visualizing steady state 2D flow fields. We use the flow field to advect and animate the texture over time. However, using standard texture advection techniques and arbitrary textures will introduce some undesirable effects such as: (a) expanding texture from a critical source point, (b) streaking pattern from the boundary of the flowfield, (c) crowding of advected textures near an attracting spiral or sink, and (d) absent or lack of textures in some regions of the flow. This paper proposes a number of strategies to solve these problems. We demonstrate how the technique works using both synthetic data and computational fluid dynamics data.

Parametric study of flow patterns behind the standing accretion shock wave for core-collapse supernovae

Energy Technology Data Exchange (ETDEWEB)

Iwakami, Wakana; Nagakura, Hiroki [Yukawa Institute for Theoretical Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan); Yamada, Shoichi, E-mail: wakana@heap.phys.waseda.ac.jp [Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555 (Japan)

2014-05-10

In this study, we conduct three-dimensional hydrodynamic simulations systematically to investigate the flow patterns behind the accretion shock waves that are commonly formed in the post-bounce phase of core-collapse supernovae. Adding small perturbations to spherically symmetric, steady, shocked accretion flows, we compute the subsequent evolutions to find what flow pattern emerges as a consequence of hydrodynamical instabilities such as convection and standing accretion shock instability for different neutrino luminosities and mass accretion rates. Depending on these two controlling parameters, various flow patterns are indeed realized. We classify them into three basic patterns and two intermediate ones; the former includes sloshing motion (SL), spiral motion (SP), and multiple buoyant bubble formation (BB); the latter consists of spiral motion with buoyant-bubble formation (SPB) and spiral motion with pulsationally changing rotational velocities (SPP). Although the post-shock flow is highly chaotic, there is a clear trend in the pattern realization. The sloshing and spiral motions tend to be dominant for high accretion rates and low neutrino luminosities, and multiple buoyant bubbles prevail for low accretion rates and high neutrino luminosities. It is interesting that the dominant pattern is not always identical between the semi-nonlinear and nonlinear phases near the critical luminosity; the intermediate cases are realized in the latter case. Running several simulations with different random perturbations, we confirm that the realization of flow pattern is robust in most cases.

Low-density, radiatively inefficient rotating-accretion flow on to a black hole

Science.gov (United States)

Inayoshi, Kohei; Ostriker, Jeremiah P.; Haiman, Zoltán; Kuiper, Rolf

2018-05-01

We study low-density axisymmetric accretion flows on to black holes (BHs) with two-dimensional hydrodynamical simulations, adopting the α-viscosity prescription. When the gas angular momentum is low enough to form a rotationally supported disc within the Bondi radius (RB), we find a global steady accretion solution. The solution consists of a rotational equilibrium distribution around r ˜ RB, where the density follows ρ ∝ (1 + RB/r)3/2, surrounding a geometrically thick and optically thin accretion disc at the centrifugal radius RC(dominated accretion flows (ρ ∝ r-1/2). In the inner solution, the gas inflow rate decreases towards the centre due to convection (\\dot{M}∝ r), and the net accretion rate (including both inflows and outflows) is strongly suppressed by several orders of magnitude from the Bondi accretion rate \\dot{M}_B. The net accretion rate depends on the viscous strength, following \\dot{M}/\\dot{M}_B∝ (α /0.01)^{0.6}. This solution holds for low accretion rates of \\dot{M}_B/\\dot{M}_Edd≲ 10^{-3} having minimal radiation cooling, where \\dot{M}_Edd is the Eddington accretion rate. In a hot plasma at the bottom (r < 10-3 RB), thermal conduction would dominate the convective energy flux. Since suppression of the accretion by convection ceases, the final BH feeding rate is found to be \\dot{M}/\\dot{M}_B˜ 10^{-3}-10-2. This rate is as low as \\dot{M}/\\dot{M}_Edd˜ 10^{-7}-10-6 inferred for SgrA* and the nuclear BHs in M31 and M87, and can explain their low luminosities, without invoking any feedback mechanism.

Coulombic interactions during advection-dominated transport of ions in porous media

DEFF Research Database (Denmark)

Muniruzzaman, Muhammad; Stolze, Lucien; Rolle, Massimo

2017-01-01

bench-scale experiments and numerical simulations. The investigation aims at quantifying the key role of small-scale electrostatic interactions in flow-through systems, especially when advection is the dominant mass-transfer process. Considering dilute solutions of strong electrolytes (e.g., MgCl2......Solute transport of charged species in porous media is significantly affected by the electrochemical migration term resulting from the charge-induced interactions among dissolved ions and with solid surfaces. Therefore, the characterization of such Coulombic interactions and their effect...... on multicomponent ionic transport is of critical importance for assessing the fate of charged solutes in porous media. In this work we present a detailed investigation of the electrochemical effects during conservative multicomponent ionic transport in homogeneous and heterogeneous domains by means of laboratory...

Dynamo dominated accretion and energy flow: The mechanism of active galactic nuclei

Energy Technology Data Exchange (ETDEWEB)

Colgate, S.A.; Li, H.

1998-12-31

An explanation of the magnetic fields of the universe, the central mass concentration of galaxies, the massive black hole of every galaxy, and the AGN phenomena has been an elusive goal. The authors suggest here the outlines of such a theoretical understanding and point out where the physical understanding is missing. They believe there is an imperative to the sequence of mass flow and hence energy flow in the collapse of a galactic mass starting from the first non-linearity appearing in structure formation following decoupling. This first non-linearity of a two to one density fluctuation, the Lyman-{alpha} clouds, ultimately leads to the emission spectra of the phenomenon of AGN, quasars, blazars, etc. The over-arching physical principle is the various mechanisms for the transport of angular momentum. They believe they have now understood the new physics of two of these mechanisms that have previously been illusive and as a consequence they impose strong constraints on the initial conditions of the mechanisms for the subsequent emission of the gravitational binding energy. The new phenomena described are: (1) the Rossby vortex mechanism of the accretion disk {alpha}-viscosity, and (2) the mechanism of the {alpha}-{Omega} dynamo in the accretion disk. The Rossby vortex mechanism leads to a prediction of the black hole mass and rate of energy release and the {alpha}-{Omega} dynamo leads to the generation of the magnetic flux of the galaxy (and the far greater magnetic flux of clusters) and separately explains the primary flux of energy emission as force-free magnetic energy density. This magnetic flux and magnetic energy density separately are the necessary consequence of the saturation of a dynamo created by the accretion disk with a gain greater than unity.

On the importance of aqueous diffusion and electrostatic interactions in advection-dominated transport in saturated porous media

DEFF Research Database (Denmark)

Rolle, Massimo

2015-01-01

to multicomponent ionic dispersion: the dispersive fluxes of the different ions are cross-coupled due to the effects of Coulombic interactions. Such effects are illustrated in flow-through experiments in saturated porous media. Simple strong electrolytes were selected as tracers and their transport was studied...... under different advection-dominated conditions and in homogeneous and heterogeneous porous media. The interpretation of the experimental results requires a multicomponent modeling approach with an accurate description of local hydrodynamic dispersion and explicitly accounting for the cross-coupling...

Journal of Astrophysics and Astronomy | Indian Academy of Sciences

Indian Academy of Sciences (India)

We explore the global structure of the accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. The accretion flow is optically thin and advection dominated. The synchrotron radiation is considered to be the active cooling mechanism in the flow. With this, we obtain ...

Advection and Taylor-Aris dispersion in rivulet flow

Science.gov (United States)

Al Mukahal, F. H. H.; Duffy, B. R.; Wilson, S. K.

2017-11-01

Motivated by the need for a better understanding of the transport of solutes in microfluidic flows with free surfaces, the advection and dispersion of a passive solute in steady unidirectional flow of a thin uniform rivulet on an inclined planar substrate driven by gravity and/or a uniform longitudinal surface shear stress are analysed. Firstly, we describe the short-time advection of both an initially semi-infinite and an initially finite slug of solute of uniform concentration. Secondly, we describe the long-time Taylor-Aris dispersion of an initially finite slug of solute. In particular, we obtain the general expression for the effective diffusivity for Taylor-Aris dispersion in such a rivulet, and discuss in detail its different interpretations in the special case of a rivulet on a vertical substrate.

Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

Science.gov (United States)

Nandi, Anuj; Mandal, S.; Sreehari, H.; Radhika, D.; Das, Santabrata; Chattopadhyay, I.; Iyer, N.; Agrawal, V. K.; Aktar, R.

2018-05-01

We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data (˜166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband (3-150 keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (\\dot{m}d), sub-Keplerian accretion rate (\\dot{m}h), shock location (rs) and black hole mass (M_{bh}) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as L^{obs}_{jet} ˜3-6 ×10^{37} erg s^{-1} during one of the observed radio flares which indicates that jet power corresponds to 8-16% mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate (˜14%) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of 5.2-7.9 M_{⊙} with 90% confidence.

Radiatively-suppressed spherical accretion under relativistic radiative transfer

Science.gov (United States)

Fukue, Jun

2018-03-01

We numerically examine radiatively-suppressed relativistic spherical accretion flows on to a central object with mass M under Newtonian gravity and special relativity. We simultaneously solve both the relativistic radiative transfer equation and the relativistic hydrodynamical equations for spherically symmetric flows under the double iteration process in the case of the intermediate optical depth. We find that the accretion flow is suppressed, compared with the freefall case in the nonrelativistic regime. For example, in the case of accretion on to a luminous core with accretion luminosity L*, the freefall velocity v normalized by the speed of light c under the radiative force in the nonrelativistic regime is β (\\hat{r}) = v/c = -√{(1-Γ _*)/(\\hat{r}+1-Γ _*)}, where Γ* (≡ L*/LE, LE being the Eddington luminosity) is the Eddington parameter and \\hat{r} (= r/rS, rS being the Schwarzschild radius) the normalized radius, whereas the infall speed at the central core is ˜0.7β(1), irrespective of the mass-accretion rate. This is due to the relativistic effect; the comoving flux is enhanced by the advective flux. We briefly examine and discuss an isothermal case, where the emission takes place in the entire space.

Measuring Advection and Diffusion of Colloids in Shear Flow

NARCIS (Netherlands)

Duits, Michael H.G.; Ghosh, Somnath; Mugele, Friedrich Gunther

2015-01-01

An analysis of the dynamics of colloids in shear flow can be challenging because of the superposition of diffusion and advection. We present a method that separates the two motions, starting from the time-dependent particle coordinates. The restriction of the tracking to flow lanes and the

General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes

Energy Technology Data Exchange (ETDEWEB)

McKinney, Jonathan C.; Tchekhovskoy, Alexander; Blandford, Roger D.

2012-04-26

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height H to cylindrical radius R ratio of |H/R| {approx} 0.2-1) accretion flows around BHs with various dimensionless spins (a/M, with BH mass M) and with initially toroidally-dominated ({phi}-directed) and poloidally-dominated (R-z directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough |a/M|, coherent large-scale (i.e. >> H) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. While models with |H/R| {approx} 1 and |a/M| {le} 0.5 do not launch jets due to quenching by mass infall, for sufficiently high |a/M| or low |H/R| the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric 'magnetically choked accretion flow' (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with and 100% efficiency for |a/M| {approx}> 0.9. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk oscillation (JDO) type of quasi-periodic oscillation (QPO) mechanism. The high-frequency QPO has spherical harmonic |m| = 1 mode period of {tau} {approx} 70GM/c{sup 3} for a/M {approx} 0.9 with coherence quality factors Q {approx}> 10. Overall, our models are qualitatively distinct from most prior MHD simulations (typically, |H/R| << 1 and poloidal flux is

Clay with Desiccation Cracks is an Advection Dominated Environment

Science.gov (United States)

Baram, S.; Kurtzman, D.; Sher, Y.; Ronen, Z.; Dahan, O.

2012-04-01

, indicating deep soil evaporation. Daily fluctuation of the air temperature in the desiccation cracks supported thermally induced air convection within the cracks void and could explain the deep soil salinization process. Combination of all the abovementioned observations demonstrated that the formation of desiccation cracks network in dispersive clay sediments generates a bulk advection dominated environment for both air and water flow, and that the reference to clay sediments as "hydrologically safe" should to be reconsidered.

Hydrodynamic Simulations of the Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds

Science.gov (United States)

Ressler, S. M.; Quataert, E.; Stone, J. M.

2018-05-01

We present Athena++ grid-based, hydrodynamic simulations of accretion onto Sagittarius A* via the stellar winds of the ˜30 Wolf-Rayet stars within the central parsec of the galactic center. These simulations span ˜ 4 orders of magnitude in radius, reaching all the way down to 300 gravitational radii of the black hole, ˜32 times further in than in previous work. We reproduce reasonably well the diffuse thermal X-ray emission observed by Chandra in the central parsec. The resulting accretion flow at small radii is a superposition of two components: 1) a moderately unbound, sub-Keplerian, thick, pressure-supported disc that is at most (but not all) times aligned with the clockwise stellar disc, and 2) a bound, low-angular momentum inflow that proceeds primarily along the southern pole of the disc. We interpret this structure as a natural consequence of a few of the innermost stellar winds dominating accretion, which produces a flow with a broad distribution of angular momentum. Including the star S2 in the simulation has a negligible effect on the flow structure. Extrapolating our results from simulations with different inner radii, we find an accretion rate of ˜ a few × 10-8M⊙/yr at the horizon scale, consistent with constraints based on modeling the observed emission of Sgr A*. The flow structure found here can be used as more realistic initial conditions for horizon scale simulations of Sgr A*.

Self-gravity in Magnetized Neutrino-dominated Accretion Disks

Energy Technology Data Exchange (ETDEWEB)

Shahamat, Narjes; Abbassi, Shahram, E-mail: abbassi@um.ac.ir [Department of Physics, School of Science, Ferdowsi University of Mashhad, Mashhad, P.O. Box 91775-1436 (Iran, Islamic Republic of)

2017-08-10

In the present work we study self-gravity effects on the vertical structure of a magnetized neutrino-dominated accretion disk as a central engine for gamma-ray bursts (GRBs). Some of the disk physical timescales that are supposed to play a pivotal role in the late-time evolutions of the disk, such as viscous, cooling, and diffusion timescales, have been studied. We are interested in investigating the possibility of the occurrence of X-ray flares, observed in late-time GRBs’ extended emission through the “magnetic barrier” and “fragmentation” processes in our model. The results lead us to interpret self-gravity as an amplifier for Blandford–Payne luminosity (BP power) and the generated magnetic field, but a suppressor for neutrino luminosity and magnetic barrier processes via highlighting the fragmentation mechanism in the outer disk, especially for the higher mass accretion rates.

Relativistic sonic geometry for isothermal accretion in the Kerr metric

Science.gov (United States)

Arif Shaikh, Md

2018-03-01

We linearly perturb advective isothermal transonic accretion onto rotating astrophysical black holes to study the emergence of the relativistic acoustic spacetime and to investigate how the salient features of this spacetime is influenced by the spin angular momentum of the black hole. We have perturbed three different quantities—the velocity potential, the mass accretion rate and the relativistic Bernoulli’s constant to show that the acoustic metric obtained for these three cases are the same up to a conformal factor. By constructing the required causal structures, it has been demonstrated that the acoustic black holes are formed at the transonic points of the flow and the acoustic white holes are formed at the shock location. The corresponding acoustic surface gravity has been computed in terms of the relevant accretion variables and the background metric elements. We have performed a linear stability analysis of the background stationary flow.

Upper Limit of the Viscosity Parameter in Accretion Flows around a Black Hole with Shock Waves

Science.gov (United States)

Nagarkoti, Shreeram; Chakrabarti, Sandip K.

2016-01-01

Black hole accretion is necessarily transonic; thus, flows must become supersonic and, therefore, sub-Keplerian before they enter into the black hole. The viscous timescale is much longer than the infall timescale close to a black hole. Hence, the angular momentum remains almost constant and the centrifugal force ˜ {l}2/{r}3 becomes increasingly dominant over the gravitational force ˜ 1/{r}2. The slowed down matter piles creating an accretion shock. The flow between shock and inner sonic point is puffed up and behaves like a boundary layer. This so-called Comptonizing cloud/corona produces hard X-rays and jets/outflows and, therefore, is an important component of black hole astrophysics. In this paper, we study steady state viscous, axisymmetric, transonic accretion flows around a Schwarzschild black hole. We adopt a viscosity parameter α and compute the highest possible value of α (namely, {α }{cr}) for each pair of two inner boundary parameters (namely, specific angular momentum carried to horizon, lin and specific energy at inner sonic point, E({x}{in})) which is still capable of producing a standing or oscillating shock. We find that while such possibilities exist for α as high as {α }{cr}=0.3 in very small regions of the flow parameter space, typical {α }{cr} appears to be about ˜0.05-0.1. Coincidentally, this also happens to be the typical viscosity parameter achieved by simulations of magnetorotational instabilities in accretion flows. We therefore believe that all realistic accretion flows are likely to have centrifugal pressure supported shocks unless the viscosity parameter everywhere is higher than {α }{cr}.

Status of the accretion flow solution in the Golden Jubilee year of the discovery of extra-solar X-ray sources

Science.gov (United States)

Chakrabarti, S. K.

Fifty years have just passed since the first discovery of the extra-solar X-ray sources by Giacconi and his team which we know today to be some stellar mass black holes. By 1973, not only a catalog of these enigmatic objects were made, and their spectra were obtained. Today, forty years have passed since the revolutionary idea of the thin, axisymmetric, Keplerian, disk model by Shakura and Sunyaev was published. Yet, the complete predictability of their radiative properties remains as illusive as ever. The only available and self-consistent solution to date is the generalized viscous transonic flow solutions where both heating and cooling effects are included. I demonstrate that the latest `Avatar' of the accretion/outflow picture, the Generalized Two Component Advective Flow (GTCAF), is capable of explaining almost all the black hole observational results, when the results of the time dependent simulation of viscous and radiative processes are also taken into consideration. I also discuss the problems with predictability and argue that understanding companion's behaviour in terms of its habit of mass loss, ellipticity of its orbit, magnetic properties, etc. is extremely important for the prediction of emission properties of the accretion flow.

Properties of two-temperature dissipative accretion flow around black holes

Science.gov (United States)

Dihingia, Indu K.; Das, Santabrata; Mandal, Samir

2018-04-01

We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields that leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain the two-temperature global accretion solutions in terms of dissipation parameters, namely, viscosity (α) and accretion rate ({\\dot{m}}), and find for the first time in the literature that such solutions may contain standing shock waves. Solutions of this kind are multitransonic in nature, as they simultaneously pass through both inner critical point (xin) and outer critical point (xout) before crossing the black hole horizon. We calculate the properties of shock-induced global accretion solutions in terms of the flow parameters. We further show that two-temperature shocked accretion flow is not a discrete solution, instead such solution exists for wide range of flow parameters. We identify the effective domain of the parameter space for standing shock and observe that parameter space shrinks as the dissipation is increased. Since the post-shock region is hotter due to the effect of shock compression, it naturally emits hard X-rays, and therefore, the two-temperature shocked accretion solution has the potential to explain the spectral properties of the black hole sources.

THERMAL AND DYNAMICAL PROPERTIES OF GAS ACCRETING ONTO A SUPERMASSIVE BLACK HOLE IN AN ACTIVE GALACTIC NUCLEUS

International Nuclear Information System (INIS)

Mościbrodzka, M.; Proga, D.

2013-01-01

We study stability of gas accretion in active galactic nuclei (AGNs). Our grid-based simulations cover a radial range from 0.1 to 200 pc, which may enable linking the galactic/cosmological simulations with small-scale black hole (BH) accretion models within a few hundreds of Schwarzschild radii. Here, as in previous studies by our group, we include gas radiative cooling as well as heating by a sub-Eddington X-ray source near the central supermassive BH of 10 8 M ☉ . Our theoretical estimates and simulations show that for the X-ray luminosity, L X ∼ 0.008 L Edd , the gas is thermally and convectively unstable within the computational domain. In the simulations, we observe that very tiny fluctuations in an initially smooth, spherically symmetric, accretion flow, grow first linearly and then nonlinearly. Consequently, an initially one-phase flow relatively quickly transitions into a two-phase/cold-hot accretion flow. For L X = 0.015 L Edd or higher, the cold clouds continue to accrete but in some regions of the hot phase, the gas starts to move outward. For L X Edd , the cold phase contribution to the total mass accretion rate only moderately dominates over the hot phase contribution. This result might have some consequences for cosmological simulations of the so-called AGN feedback problem. Our simulations confirm the previous results of Barai et al. who used smoothed particle hydrodynamic (SPH) simulations to tackle the same problem. Here, however, because we use a grid-based code to solve equations in one dimension and two dimensions, we are able to follow the gas dynamics at much higher spacial resolution and for longer time compared with the three-dimensional SPH simulations. One of the new features revealed by our simulations is that the cold condensations in the accretion flow initially form long filaments, but at the later times, those filaments may break into smaller clouds advected outward within the hot outflow. Therefore, these simulations may serve as

Characteristics of transonic spherical symmetric accretion flow in Schwarzschild-de Sitter and Schwarzschild anti-de Sitter backgrounds, in pseudo-general relativistic paradigm

Science.gov (United States)

Ghosh, Shubhrangshu; Banik, Prabir

2015-07-01

dominant (CD) galaxies directly from ambient intercluster medium (ICM). However, for high mass accretion rate, the influence of cosmological constant on Bondi accretion dynamics, generically, diminishes. As active galactic nuclei (AGN)/ICM feedback can be advertently linked to Bondi type spherical accretion, any proper modeling of AGN feedback or megaparsecs-scale jet dynamics or accretion flow from ICM onto the central regions of host galaxies should take into account the relevant information of repulsive Λ, especially in context to supergiant elliptical galaxies or CD galaxies present in rich galaxy clusters. This could also explore the feasibility to limit the value of Λ, from the kinematics in local galactic-scales.

Spectral and evolutionary analysis of advection-diffusion equations and the shear flow paradigm

International Nuclear Information System (INIS)

Thyagaraja, A.; Loureiro, N.; Knight, P.J.

2002-01-01

Advection-diffusion equations occur in a wide variety of fields in many contexts of active and passive transport in fluids and plasmas. The effects of sheared advective flows in the presence of irreversible processes such as diffusion and viscosity are of considerable current interest in tokamak and astrophysical contexts, where they are thought to play a key role in both transport and the dynamical structures characteristic of electromagnetic plasma turbulence. In this paper we investigate the spectral and evolutionary properties of relatively simple, linear, advection-diffusion equations. We apply analytical approaches based on standard Green's function methods to obtain insight into the nature of the spectra when the advective and diffusive effects occur separately and in combination. In particular, the physically interesting limit of small (but finite) diffusion is studied in detail. The analytical work is extended and supplemented by numerical techniques involving a direct solution of the eigenvalue problem as well as evolutionary studies of the initial value problem using a parallel code, CADENCE. The three approaches are complementary and entirely consistent with each other when appropriate comparison is made. They reveal different aspects of the properties of the advection-diffusion equation, such as the ability of sheared flows to generate a direct cascade to high wave numbers transverse to the advection and the consequent enhancement of even small amounts of diffusivity. The invariance properties of the spectra in the low diffusivity limit and the ability of highly sheared, jet-like flows to 'confine' transport to low shear regions are demonstrated. The implications of these properties in a wider context are discussed and set in perspective. (author)

Global hydromagnetic simulations of a planet embedded in a dead zone: Gap opening, gas accretion, and formation of a protoplanetary jet

Energy Technology Data Exchange (ETDEWEB)

Gressel, O. [NORDITA, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm (Sweden); Nelson, R. P. [Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS (United Kingdom); Turner, N. J. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Ziegler, U., E-mail: oliver.gressel@nordita.org, E-mail: r.p.nelson@qmul.ac.uk, E-mail: neal.j.turner@jpl.nasa.gov, E-mail: uziegler@aip.de [Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482, Potsdam (Germany)

2013-12-10

We present global hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations with mesh refinement of accreting planets embedded in protoplanetary disks (PPDs). The magnetized disk includes Ohmic resistivity that depends on the overlying mass column, leading to turbulent surface layers and a dead zone near the midplane. The main results are: (1) the accretion flow in the Hill sphere is intrinsically three-dimensional for HD and MHD models. Net inflow toward the planet is dominated by high-latitude flows. A circumplanetary disk (CPD) forms. Its midplane flows outward in a pattern whose details differ between models. (2) The opening of a gap magnetically couples and ignites the dead zone near the planet, leading to stochastic accretion, a quasi-turbulent flow in the Hill sphere, and a CPD whose structure displays high levels of variability. (3) Advection of magnetized gas onto the rotating CPD generates helical fields that launch magnetocentrifugally driven outflows. During one specific epoch, a highly collimated, one-sided jet is observed. (4) The CPD's surface density is ∼30 g cm{sup −2}, small enough for significant ionization and turbulence to develop. (5) The accretion rate onto the planet in the MHD simulation reaches a steady value 8 × 10{sup –3} M {sub ⊕} yr{sup –1} and is similar in the viscous HD runs. Our results suggest that gas accretion onto a forming giant planet within a magnetized PPD with a dead zone allows rapid growth from Saturnian to Jovian masses. As well as being relevant for giant planet formation, these results have important implications for the formation of regular satellites around gas giant planets.

THE EFFECTS OF ACCRETION FLOW DYNAMICS ON THE BLACK HOLE SHADOW OF SAGITTARIUS A*

Energy Technology Data Exchange (ETDEWEB)

Pu, Hung-Yi; Asada, Keiichi [Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Taipei 10617, Taiwan (China); Akiyama, Kazunori [Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886 (United States)

2016-11-01

A radiatively inefficient accretion flow (RIAF), which is commonly characterized by its sub-Keplerian nature, is a favored accretion model for the supermassive black hole at the Galactic center, Sagittarius A*. To investigate the observable features of an RIAF, we compare the modeled shadow images, visibilities, and spectra of three flow models with dynamics characterized by (i) a Keplerian shell that is rigidly rotating outside the innermost stable circular orbit (ISCO) and infalling with a constant angular momentum inside ISCO, (ii) a sub-Keplerian motion, and (iii) a free-falling motion with zero angular momentum at infinity. At near-millimeter wavelengths, the emission is dominated by the flow within several Schwarzschild radii. The energy shift due to these flow dynamics becomes important and distinguishable, suggesting that the flow dynamics are an important model parameter for interpreting the millimeter/sub-millimeter very long baseline interferometric observations with the forthcoming, fully assembled Event Horizon Telescope (EHT). As an example, we demonstrate that structural variations of Sagittarius A* on event horizon-scales detected in previous EHT observations can be explained by the non-stationary dynamics of an RIAF.

Emergent scar lines in chaotic advection of passive directors

Science.gov (United States)

Hejazi, Bardia; Mehlig, Bernhard; Voth, Greg A.

2017-12-01

We examine the spatial field of orientations of slender fibers that are advected by a two-dimensional fluid flow. The orientation field of these passive directors are important in a wide range of industrial and geophysical flows. We introduce emergent scar lines as the dominant coherent structures in the orientation field of passive directors in chaotic flows. Previous work has identified the existence of scar lines where the orientation rotates by π over short distances, but the lines that were identified disappeared as time progressed. As a result, earlier work focused on topological singularities in the orientation field, which we find to play a negligible role at long times. We use the standard map as a simple time-periodic two-dimensional flow that produces Lagrangian chaos. This class of flows produces persistent patterns in passive scalar advection and we find that a different kind of persistent pattern develops in the passive director orientation field. We identify the mechanism by which emergent scar lines grow to dominate these patterns at long times in complex flows. Emergent scar lines form where the recent stretching of the fluid element is perpendicular to earlier stretching. Thus these scar lines can be labeled by their age, defined as the time since their stretching reached a maximum.

Temporal evolution of photon energy emitted from two-component advective flows: origin of time lag

Science.gov (United States)

Chatterjee, Arka; Chakrabarti, Sandip K.; Ghosh, Himadri

2017-12-01

X-ray time lag of black hole candidates contains important information regarding the emission geometry. Recently, study of time lags from observational data revealed very intriguing properties. To investigate the real cause of this lag behavior with energy and spectral states, we study photon paths inside a two-component advective flow (TCAF) which appears to be a satisfactory model to explain the spectral and timing properties. We employ the Monte Carlo simulation technique to carry out the Comptonization process. We use a relativistic thick disk in Schwarzschild geometry as the CENtrifugal pressure supported BOundary Layer (CENBOL) which is the Compton cloud. In TCAF, this is the post-shock region of the advective component. Keplerian disk on the equatorial plane which is truncated at the inner edge i.e. at the outer boundary of the CENBOL, acts as the soft photon source. Ray-tracing code is employed to track the photons to a distantly located observer. We compute the cumulative time taken by a photon during Comptonization, reflection and following the curved geometry on the way to the observer. Time lags between various hard and soft bands have been calculated. We study the variation of time lags with accretion rates, CENBOL size and inclination angle. Time lags for different energy channels are plotted for different inclination angles. The general trend of variation of time lag with QPO frequency and energy as observed in satellite data is reproduced.

FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

International Nuclear Information System (INIS)

Kirk, Helen; Myers, Philip C.; Bourke, Tyler L.; Gutermuth, Robert A.; Wilson, Grant W.; Hedden, Abigail

2013-01-01

One puzzle in understanding how stars form in clusters is the source of mass—is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of ∼30 M ☉ Myr –1 (inferred from the N 2 H + velocity gradient along the filament), and radially contracting onto the filament at ∼130 M ☉ Myr –1 (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

Chaotic advection and heat transfer in two similar 2-D periodic flows and in their corresponding 3-D periodic flows

Science.gov (United States)

Vinsard, G.; Dufour, S.; Saatdjian, E.; Mota, J. P. B.

2016-03-01

Chaotic advection can effectively enhance the heat transfer rate between a boundary and fluids with high Prandtl number. These fluids are usually highly viscous and thus turbulent agitation is not a viable solution since the energy required to mix the fluid would be prohibitive. Here, we analyze previously obtained results on chaotic advection and heat transfer in two similar 2-D periodic flows and on their corresponding 3-D periodic flows when an axial velocity component is superposed. The two flows studied are the flow between eccentric rotating cylinders and the flow between confocal ellipses. For both of these flows the analysis is simplified because the Stokes equations can be solved analytically to obtain a closed form solution. For both 2-D periodic flows, we show that chaotic heat transfer is enhanced by the displacement of the saddle point location during one period. Furthermore, the enhancement by chaotic advection in the elliptical geometry is approximately double that obtained in the cylindrical geometry because there are two saddle points instead of one. We also explain why, for high eccentricity ratios, there is no heat transfer enhancement in the cylindrical geometry. When an axial velocity component is added to both of these flows so that they become 3-D, previous work has shown that there is an optimum modulation frequency for which chaotic advection and heat transfer enhancement is a maximum. Here we show that the optimum modulation frequency can be derived from results without an axial flow. We also explain by physical arguments other previously unanswered questions in the published data.

Truncation of the Inner Accretion Disk Around a Black Hole at Low Luminosity

Science.gov (United States)

Tomsick, John A.; Yamoka, Kazutaka; Corbel, Stephane; Kaaret, Philip; Kalemci, Emrah; Migliari, Simone

2011-01-01

Most black hole binaries show large changes in X-ray luminosity caused primarily by variations in mass accretion rate. An important question for understanding black hole accretion and jet production is whether the inner edge of the accretion disk recedes at low accretion rate. Measurements of the location of the inner edge (R(sub in)) can be made using iron emission lines that arise due to fluorescence of iron in the disk, and these indicate that R(sub in) is very close to the black hole at high and moderate luminosities (greater than or equal to 1% of the Eddington luminosity, L(sub Edd). Here, we report on X-ray observations of the black hole GX 339-4 in the hard state by Suzaku and the Rossi X-ray Timing Explorer that extend iron line studies to 0.14% L(sub Edd) and show that R(sub in) increases by a factor of greater than 27 over the value found when GX 339-4 was bright. The exact value of R(sub in) depends on the inclination of the inner disk (i), and we derive 90% confidence limits of R(sub in) greater than 35 R(sub g) at i = 0 degrees and R(sub in) greater than 175 R(sub g) at i = 30 degrees. This provides direct evidence that the inner portion of the disk is not present at low luminosity, allowing for the possibility that the inner disk is replaced by advection- or magnetically dominated accretion flows.

Assessing lateral flows and solute transport during floods in a conduit-flow-dominated karst system using the inverse problem for the advection-diffusion equation

Science.gov (United States)

Cholet, Cybèle; Charlier, Jean-Baptiste; Moussa, Roger; Steinmann, Marc; Denimal, Sophie

2017-07-01

The aim of this study is to present a framework that provides new ways to characterize the spatio-temporal variability of lateral exchanges for water flow and solute transport in a karst conduit network during flood events, treating both the diffusive wave equation and the advection-diffusion equation with the same mathematical approach, assuming uniform lateral flow and solute transport. A solution to the inverse problem for the advection-diffusion equations is then applied to data from two successive gauging stations to simulate flows and solute exchange dynamics after recharge. The study site is the karst conduit network of the Fourbanne aquifer in the French Jura Mountains, which includes two reaches characterizing the network from sinkhole to cave stream to the spring. The model is applied, after separation of the base from the flood components, on discharge and total dissolved solids (TDSs) in order to assess lateral flows and solute concentrations and compare them to help identify water origin. The results showed various lateral contributions in space - between the two reaches located in the unsaturated zone (R1), and in the zone that is both unsaturated and saturated (R2) - as well as in time, according to hydrological conditions. Globally, the two reaches show a distinct response to flood routing, with important lateral inflows on R1 and large outflows on R2. By combining these results with solute exchanges and the analysis of flood routing parameters distribution, we showed that lateral inflows on R1 are the addition of diffuse infiltration (observed whatever the hydrological conditions) and localized infiltration in the secondary conduit network (tributaries) in the unsaturated zone, except in extreme dry periods. On R2, despite inflows on the base component, lateral outflows are observed during floods. This pattern was attributed to the concept of reversal flows of conduit-matrix exchanges, inducing a complex water mixing effect in the saturated zone

FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

Energy Technology Data Exchange (ETDEWEB)

Kirk, Helen; Myers, Philip C.; Bourke, Tyler L. [Radio and Geoastronomy Division, Harvard Smithsonian Center for Astrophysics, MS-42, Cambridge, MA, 02138 (United States); Gutermuth, Robert A.; Wilson, Grant W. [Department of Astronomy, University of Massachusetts Amherst, Amherst, MA 01003 (United States); Hedden, Abigail, E-mail: kirkh@mcmaster.ca [Army Research Labs, Adelphi, MD 20783 (United States)

2013-04-01

One puzzle in understanding how stars form in clusters is the source of mass-is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of {approx}30 M{sub Sun} Myr{sup -1} (inferred from the N{sub 2}H{sup +} velocity gradient along the filament), and radially contracting onto the filament at {approx}130 M{sub Sun} Myr{sup -1} (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

A SIMPLE TOY MODEL OF THE ADVECTIVE-ACOUSTIC INSTABILITY. I. PERTURBATIVE APPROACH

International Nuclear Information System (INIS)

Foglizzo, T.

2009-01-01

Some general properties of the advective-acoustic instability are described and understood using a toy model, which is simple enough to allow for analytical estimates of the eigenfrequencies. The essential ingredients of this model, in the unperturbed regime, are a stationary shock and a subsonic region of deceleration. For the sake of analytical simplicity, the two-dimensional unperturbed flow is parallel and the deceleration is produced adiabatically by an external potential. The instability mechanism is determined unambiguously as the consequence of a cycle between advected and acoustic perturbations. The purely acoustic cycle, considered alone, is proven to be stable in this flow. Its contribution to the instability can be either constructive or destructive. A frequency cutoff is associated with the advection time through the region of deceleration. This cutoff frequency explains why the instability favors eigenmodes with a low frequency and a large horizontal wavelength. The relation between the instability occurring in this highly simplified toy model and the properties of standing accretion shock instability observed in the numerical simulations of stellar core collapse is discussed. This simple setup is proposed as a benchmark test to evaluate the accuracy, in the linear regime, of numerical simulations involving this instability. We illustrate such benchmark simulations in a companion paper.

Accretion dynamics and polarized x-ray emission of magnetized neutron stars

International Nuclear Information System (INIS)

Arons, J.

1991-01-01

The basic ideas of accretion onto magnetized neutron stars are outlined. These are applied to a simple model of the structure of the plasma mound sitting at the magnetic poles of such as star, in which upward diffusion of photons is balanced by their downward advection. This steady flow model of the plasma's dynamical state is used to compute the emission of polarized X-rays from the optically thick, birefringent medium. The linear polarization of the continuum radiation emerging from the quasi-static mound is found to be as much as 40% at some rotation phases, but is insensitive to the geometry of the accretion flow. The role of the accretion shock, whose detailed polarimetric and spectral characteristics have yet to be calculated, is emphasized as the final determinant of the properties of the emerging X-rays. Some results describing the fully time dependent dynamics of the flow are also presented. In particular, steady flow onto a neutron star is shown to exhibit formation of ''photon bubbles,'' regions of greatly reduced plasma density filled with radiation which form and rise on millisecond time scales. The possible role of these complex structures in the flow for the formation of the emergent spectrum is briefly outlined

Accretion dynamics and polarized X-ray emission of magnetized neutron stars

Science.gov (United States)

Arons, Jonathan

1991-01-01

The basic ideas of accretion onto magnetized neutron stars are outlined. These are applied to a simple model of the structure of the plasma mound sitting at the magnetic poles of such a star, in which upward diffusion of photons is balanced by their downward advection. This steady flow model of the plasma's dynamical state is used to compute the emission of polarized X-raysfrom the optically thick, birefringent medium. The linear polarization of the continuum radiation emerging from the quasi-static mound is found to be as much as 40 percent at some rotation phases, but is insensitive to the geometry of the accretion flow. The role of the accretion shock, whose detailed polarimetric and spectral characteristics have yet to be calculated, is emphasized as the final determinant of the properties of the emerging X-rays. Some results describing the fully time dependent dynamics of the flow are also presented. In particular, steady flow onto a neutron star is shown to exhibit formation of 'photon bubbles', regions of greatly reduced plasma density filled with radiation which form and rise on millisecond time scale. The possible role of these complex structures in the flow for the formation of the emergent spectrum is briefly outlined.

CSI 2264: characterizing accretion-burst dominated light curves for young stars in NGC 2264

International Nuclear Information System (INIS)

Stauffer, John; Cody, Ann Marie; Rebull, Luisa; Carey, Sean; Baglin, Annie; Alencar, Silvia; Hillenbrand, Lynne A.; Carpenter, John; Findeisen, Krzysztof; Venuti, Laura; Bouvier, Jerome; Turner, Neal J.; Plavchan, Peter; Terebey, Susan; Morales-Calderón, María; Micela, Giusi; Flaccomio, Ettore; Song, Inseok; Gutermuth, Rob; Hartmann, Lee

2014-01-01

Based on more than four weeks of continuous high-cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high-quality, multi-wavelength light curves for young stellar objects whose optical variability is dominated by short-duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief—several hours to one day—brightenings at optical and near-infrared wavelengths with amplitudes generally in the range of 5%-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a 30 day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u – g versus g – r color-color diagram with the largest UV excesses. These stars also have large Hα equivalent widths, and either centrally peaked, lumpy Hα emission profiles or profiles with blueshifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Among the stars with the largest UV excesses or largest Hα equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.

Study of magnetized accretion flow with cooling processes

Indian Academy of Sciences (India)

Kuldeep Singh

2018-02-09

Feb 9, 2018 ... 2University of Delhi, South Campus, Delhi 110 021, India. ∗ ... Abstract. We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with .... where Ap is the area of cross-section of the flux tube.

Bondi or not Bondi: the impact of resolution on accretion and drag force modelling for Supermassive Black Holes

Science.gov (United States)

Beckmann, R. S.; Slyz, A.; Devriendt, J.

2018-04-01

Whilst in galaxy-size simulations, supermassive black holes (SMBH) are entirely handled by sub-grid algorithms, computational power now allows the accretion radius of such objects to be resolved in smaller scale simulations. In this paper, we investigate the impact of resolution on two commonly used SMBH sub-grid algorithms; the Bondi-Hoyle-Lyttleton (BHL) formula for accretion onto a point mass, and the related estimate of the drag force exerted onto a point mass by a gaseous medium. We find that when the accretion region around the black hole scales with resolution, and the BHL formula is evaluated using local mass-averaged quantities, the accretion algorithm smoothly transitions from the analytic BHL formula (at low resolution) to a supply limited accretion (SLA) scheme (at high resolution). However, when a similar procedure is employed to estimate the drag force it can lead to significant errors in its magnitude, and/or apply this force in the wrong direction in highly resolved simulations. At high Mach numbers and for small accretors, we also find evidence of the advective-acoustic instability operating in the adiabatic case, and of an instability developing around the wake's stagnation point in the quasi-isothermal case. Moreover, at very high resolution, and Mach numbers above M_∞ ≥ 3, the flow behind the accretion bow shock becomes entirely dominated by these instabilities. As a result, accretion rates onto the black hole drop by about an order of magnitude in the adiabatic case, compared to the analytic BHL formula.

EXPLORING THE ACCRETION MODEL OF M87 AND 3C 84 WITH THE FARADAY ROTATION MEASURE OBSERVATIONS

Energy Technology Data Exchange (ETDEWEB)

Li, Ya-Ping; Yuan, Feng; Xie, Fu-Guo, E-mail: fyuan@shao.ac.cn [Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030 (China)

2016-10-20

Low-luminosity active galactic nuclei (LLAGNs) are believed to be powered by an accretion-jet model, consisting of an inner advection-dominated accretion flow (ADAF), an outer truncated standard thin disk, and a jet; however, model degeneracy still exists in this framework. For example, the X-ray emission can originate from either the ADAF or the jet. The aim of the present work is to check these models with the Faraday rotation measure (RM) observations recently detected for two LLAGNs, M87 and 3C 84, in the sub-mm band. For M87, we find that the RM predicted by the model in which the X-ray emission originates from the ADAF is larger than the observed upper limit of RM by over two orders of magnitude, while the model in which the X-ray emission originates from the jet predicts a RM lower than the observed upper limit. For 3C 84, the sub-mm emission is found to be dominated by the jet component, while the Faraday screen is attributed to the ADAFs. This scenario can naturally explain the observed external origin of the RM and why the RM is found to be stable during a two-year interval although the sub-mm emission increases at the same period.

EXPLORING THE ACCRETION MODEL OF M87 AND 3C 84 WITH THE FARADAY ROTATION MEASURE OBSERVATIONS

International Nuclear Information System (INIS)

Li, Ya-Ping; Yuan, Feng; Xie, Fu-Guo

2016-01-01

Low-luminosity active galactic nuclei (LLAGNs) are believed to be powered by an accretion-jet model, consisting of an inner advection-dominated accretion flow (ADAF), an outer truncated standard thin disk, and a jet; however, model degeneracy still exists in this framework. For example, the X-ray emission can originate from either the ADAF or the jet. The aim of the present work is to check these models with the Faraday rotation measure (RM) observations recently detected for two LLAGNs, M87 and 3C 84, in the sub-mm band. For M87, we find that the RM predicted by the model in which the X-ray emission originates from the ADAF is larger than the observed upper limit of RM by over two orders of magnitude, while the model in which the X-ray emission originates from the jet predicts a RM lower than the observed upper limit. For 3C 84, the sub-mm emission is found to be dominated by the jet component, while the Faraday screen is attributed to the ADAFs. This scenario can naturally explain the observed external origin of the RM and why the RM is found to be stable during a two-year interval although the sub-mm emission increases at the same period.

Exploring the Accretion Model of M87 and 3C 84 with the Faraday Rotation Measure Observations

Science.gov (United States)

Li, Ya-Ping; Yuan, Feng; Xie, Fu-Guo

2016-10-01

Low-luminosity active galactic nuclei (LLAGNs) are believed to be powered by an accretion-jet model, consisting of an inner advection-dominated accretion flow (ADAF), an outer truncated standard thin disk, and a jet; however, model degeneracy still exists in this framework. For example, the X-ray emission can originate from either the ADAF or the jet. The aim of the present work is to check these models with the Faraday rotation measure (RM) observations recently detected for two LLAGNs, M87 and 3C 84, in the sub-mm band. For M87, we find that the RM predicted by the model in which the X-ray emission originates from the ADAF is larger than the observed upper limit of RM by over two orders of magnitude, while the model in which the X-ray emission originates from the jet predicts a RM lower than the observed upper limit. For 3C 84, the sub-mm emission is found to be dominated by the jet component, while the Faraday screen is attributed to the ADAFs. This scenario can naturally explain the observed external origin of the RM and why the RM is found to be stable during a two-year interval although the sub-mm emission increases at the same period.

X-ray pulsars: accretion flow deceleration

International Nuclear Information System (INIS)

Miller, G.S.

1987-01-01

X-ray pulsars are thought to be neutron stars that derive the energy for their x-ray emission by accreting material onto their magnetic polar caps. The accreting material and the x-ray pulsar atmospheres were idealized as fully ionized plasmas consisting only of electrons and protons. A high magnetic field (∼ 5 x 10 12 Gauss) permeates the atmospheric plasma, and causes the motion of atmospheric electrons perpendicular to the field to be quantized into discrete Landau levels. All atmospheric electrons initially lie in the Landau ground state, but in the author's calculations of Coulomb collisions between atmospheric electrons and accreting protons, he allows for processes that leave the electrons in the first excited Landau level. He also considers interactions between accreting protons and the collective modes of the atmospheric plasma. Division of the electromagnetic interaction of a fast proton with a magnetized plasma into single particle and collective effects is described in detail in Chapter 2. Deceleration of the accretion flow due to Coulomb collisions with atmospheric electrons and collective plasma effects was studied in a number of computer simulations. These simulations, along with a discussion of the physical state of the atmospheric plasma and its interactions with a past proton, are presented in Chapter 3. Details of the atmospheric model and a description of the results of the simulations are given in Chapter 4. Chapter 5 contains some brief concluding remarks, and some thoughts on future research

Accretion in Radiative Equipartition (AiRE) Disks

Energy Technology Data Exchange (ETDEWEB)

Yazdi, Yasaman K.; Afshordi, Niayesh, E-mail: yyazdi@pitp.ca, E-mail: nafshordi@pitp.ca [Perimeter Institute for Theoretical Physics, 31 Caroline Street N, Waterloo, ON N2L 2Y5 (Canada)

2017-07-01

Standard accretion disk theory predicts that the total pressure in disks at typical (sub-)Eddington accretion rates becomes radiation pressure dominated. However, radiation pressure dominated disks are thermally unstable. Since these disks are observed in approximate steady state over the instability timescale, our accretion models in the radiation-pressure-dominated regime (i.e., inner disk) need to be modified. Here, we present a modification to the Shakura and Sunyaev model, where the radiation pressure is in equipartition with the gas pressure in the inner region. We call these flows accretion in radiative equipartition (AiRE) disks. We introduce the basic features of AiRE disks and show how they modify disk properties such as the Toomre parameter and the central temperature. We then show that the accretion rate of AiRE disks is limited from above and below, by Toomre and nodal sonic point instabilities, respectively. The former leads to a strict upper limit on the mass of supermassive black holes as a function of cosmic time (and spin), while the latter could explain the transition between hard and soft states of X-ray binaries.

Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence

Science.gov (United States)

Klotz, L.; Lemoult, G.; Frontczak, I.; Tuckerman, L. S.; Wesfreid, J. E.

2017-04-01

We present an experimental setup that creates a shear flow with zero mean advection velocity achieved by counterbalancing the nonzero streamwise pressure gradient by moving boundaries, which generates plane Couette-Poiseuille flow. We obtain experimental results in the transitional regime for this flow. Using flow visualization, we characterize the subcritical transition to turbulence in Couette-Poiseuille flow and show the existence of turbulent spots generated by a permanent perturbation. Due to the zero mean advection velocity of the base profile, these turbulent structures are nearly stationary. We distinguish two regions of the turbulent spot: the active turbulent core, which is characterized by waviness of the streaks similar to traveling waves, and the surrounding region, which includes in addition the weak undisturbed streaks and oblique waves at the laminar-turbulent interface. We also study the dependence of the size of these two regions on Reynolds number. Finally, we show that the traveling waves move in the downstream (Poiseuille) direction.

On the absence of winds in ADAFs

OpenAIRE

Abramowicz, Marek A.; Lasota, Jean-Pierre; Igumenshchev, Igor V.

2000-01-01

We show that recently published assertions that advection dominated accretion flows (ADAFs) require the presence of strong winds are unfounded because they assume that low radiative efficiency in flows accreting at low rates onto black holes implies vanishing radial energy and angular momentum fluxes through the flow (which is also formulated in terms of the `Bernoulli function' being positive). This, however, is a property only of self-similar solutions which are an inadequate representation...

Advection diffusion model for particles deposition in Rayleigh-Benard turbulent flows

International Nuclear Information System (INIS)

Oresta, P.; Lippolis, A.; Verzicco, R.; Soldati, A.

2005-01-01

In this paper, Direct Numerical Simulation (DNS) and Lagrangian Particle Tracking are used to precisely investigate the turbulent thermally driven flow and particles dispersion in a closed, slender cylindrical domain. The numerical simulations are carried out for Rayleigh (Ra) and Prandtl numbers (Pr) equal to Ra = 2X10 8 and Pr = 0.7, considering three sets of particles with Stokes numbers, based on Kolmogorov scale, equal to St k 1.3, St k 0.65 and St k = 0.13. This data are used to calculate a priori the drift velocity and the turbulent diffusion coefficient for the Advection Diffusion model. These quantities are function of the Stokes, Froude, Rayleigh and Prandtl numbers only. One dimensional, time dependent, Advection- Diffusion Equation (ADE) is presented to predict particles deposition in Rayleigh-Benard flow in the cylindrical domain. This archetype configuration models flow and aerosol dynamics, produced in case of accident in the passive containment cooling system (PCCS) of a nuclear reactor. ADE results show a good agreement with DNS data for all the sets of particles investigated. (author)

Floc size distributions of suspended kaolinite in an advection transport dominated tank: measurements and modeling

Science.gov (United States)

Shen, Xiaoteng; Maa, Jerome P.-Y.

2017-11-01

In estuaries and coastal waters, floc size and its statistical distributions of cohesive sediments are of primary importance, due to their effects on the settling velocity and thus deposition rates of cohesive aggregates. The development of a robust flocculation model that includes the predictions of floc size distributions (FSDs), however, is still in a research stage. In this study, a one-dimensional longitudinal (1-DL) flocculation model along a streamtube is developed. This model is based on solving the population balance equation to find the FSDs by using the quadrature method of moments. To validate this model, a laboratory experiment is carried out to produce an advection transport-dominant environment in a cylindrical tank. The flow field is generated by a marine pump mounted at the bottom center, with its outlet facing upward. This setup generates an axially symmetric flow which is measured by an acoustic Doppler velocimeter (ADV). The measurement results provide the hydrodynamic input data required for this 1-DL model. The other measurement results, the FSDs, are acquired by using an automatic underwater camera system and the resulting images are analyzed to validate the predicted FSDs. This study shows that the FSDs as well as their representative sizes can be efficiently and reasonably simulated by this 1-DL model.

Fast multigrid solution of the advection problem with closed characteristics

Energy Technology Data Exchange (ETDEWEB)

Yavneh, I. [Israel Inst. of Technology, Haifa (Israel); Venner, C.H. [Univ. of Twente, Enschede (Netherlands); Brandt, A. [Weizmann Inst. of Science, Rehovot (Israel)

1996-12-31

The numerical solution of the advection-diffusion problem in the inviscid limit with closed characteristics is studied as a prelude to an efficient high Reynolds-number flow solver. It is demonstrated by a heuristic analysis and numerical calculations that using upstream discretization with downstream relaxation-ordering and appropriate residual weighting in a simple multigrid V cycle produces an efficient solution process. We also derive upstream finite-difference approximations to the advection operator, whose truncation terms approximate {open_quotes}physical{close_quotes} (Laplacian) viscosity, thus avoiding spurious solutions to the homogeneous problem when the artificial diffusivity dominates the physical viscosity.

A filtering technique for solving the advection equation in two-phase flow problems

International Nuclear Information System (INIS)

Devals, C.; Heniche, M.; Bertrand, F.; Tanguy, P.A.; Hayes, R.E.

2004-01-01

The aim of this work is to develop a numerical strategy for the simulation of two-phase flow in the context of chemical engineering applications. The finite element method has been chosen because of its flexibility to deal with complex geometries. One of the key points of two-phase flow simulation is to determine precisely the position of the interface between the two phases, which is an unknown of the problem. In this case, the interface can be tracked by the advection of the so-called color function. It is well known that the solution of the advection equation by most numerical schemes, including the Streamline Upwind Petrov-Galerkin (SUPG) method, may exhibit spurious oscillations. This work proposes an approach to filter out these oscillations by means of a change of variable that is efficient for both steady state and transient cases. First, the filtering technique will be presented in detail. Then, it will be applied to two-dimensional benchmark problems, namely, the advection skew to the mesh and the Zalesak's problems. (author)

Multidimensional flux-limited advection schemes

International Nuclear Information System (INIS)

Thuburn, J.

1996-01-01

A general method for building multidimensional shape preserving advection schemes using flux limiters is presented. The method works for advected passive scalars in either compressible or incompressible flow and on arbitrary grids. With a minor modification it can be applied to the equation for fluid density. Schemes using the simplest form of the flux limiter can cause distortion of the advected profile, particularly sideways spreading, depending on the orientation of the flow relative to the grid. This is partly because the simple limiter is too restrictive. However, some straightforward refinements lead to a shape-preserving scheme that gives satisfactory results, with negligible grid-flow angle-dependent distortion

THE ROLE OF THE ACCRETION DISK, DUST, AND JETS IN THE IR EMISSION OF LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI

Energy Technology Data Exchange (ETDEWEB)

Mason, R. E. [Gemini Observatory, Northern Operations Center, 670 N. A' ohoku Place, Hilo, HI 96720 (United States); Ramos Almeida, C. [Instituto de Astrofísica de Canarias, C/Vía Láctea, s/n, E-38205 La Laguna, Tenerife (Spain); Levenson, N. A. [Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena (Chile); Nemmen, R. [NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States); Alonso-Herrero, A., E-mail: rmason@gemini.edu [Instituto de Física de Cantabria, CSIC-UC, Avenida de los Castros s/n, E-39005 Santander (Spain)

2013-11-10

We use recent high-resolution infrared (IR; 1-20 μm) photometry to examine the origin of the IR emission in low-luminosity active galactic nuclei (LLAGN). The data are compared with published model fits that describe the spectral energy distribution (SED) of LLAGN in terms of an advection-dominated accretion flow, truncated thin accretion disk, and jet. The truncated disk in these models is usually not luminous enough to explain the observed IR emission, and in all cases its spectral shape is much narrower than the broad IR peaks in the data. Synchrotron radiation from the jet appears to be important in very radio-loud nuclei, but the detection of strong silicate emission features in many objects indicates that dust must also contribute. We investigate this point by fitting the IR SED of NGC 3998 using dusty torus and optically thin (τ{sub mid-IR} ∼ 1) dust shell models. While more detailed modeling is necessary, these initial results suggest that dust may account for the nuclear mid-IR emission of many LLAGN.

PBH mass growth through radial accretion during the radiation dominated era

Energy Technology Data Exchange (ETDEWEB)

Lora-Clavijo, F.D. [Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 70-264, Distrito Federal 04510 (Mexico); Guzmán, F.S.; Cruz-Osorio, A., E-mail: fdlora@astro.unam.mx, E-mail: guzman@ifm.umich.mx, E-mail: alejandro@ifm.umich.mx [Instituto de Física y Matemáticas, Universidad Michoacana de San Nicolás de Hidalgo, Edificio C-3, Cd. Universitaria, 58040 Morelia, Michoacán (Mexico)

2013-12-01

We model the radial accretion of radiation on Primordial Black Holes (PBH) by numerically solving Einstein's equations coupled to an ultrarelativistic ideal gas with equation of state p = ρ/3. We calculate the final mass of a black hole by the integration of the accreted radiation energy density during the leptonic era between t ∼ 10{sup −4}s to t ∼ 10{sup 2}s after the Big Bang. Our results indicate that small PBHs with initial masses between 10{sup −4} to 1M{sub ⊙} may grow up to hundreds of solar masses, and thus can be SMBH seeds. On the other hand, PBHs formed at t ∼ 1s with initial mass between 900 and ∼ 980M{sub ⊙}, by the time t ∼ 100s show masses of 10{sup 4} to 10{sup 6}M{sub ⊙} which are masses of seeds or already formed SMBHs. The fact that we consider only radial flow implies that our results work well as limiting cases, and it is expected that under more general scenarios the accretion rates may change significantly. Nevertheless we show that it is possible that SMBHs can be PBHs that grew due to the accretion of radiation.

MHD Flows in Compact Astrophysical Objects Accretion, Winds and Jets

CERN Document Server

Beskin, Vasily S

2010-01-01

Accretion flows, winds and jets of compact astrophysical objects and stars are generally described within the framework of hydrodynamical and magnetohydrodynamical (MHD) flows. Analytical analysis of the problem provides profound physical insights, which are essential for interpreting and understanding the results of numerical simulations. Providing such a physical understanding of MHD Flows in Compact Astrophysical Objects is the main goal of this book, which is an updated translation of a successful Russian graduate textbook. The book provides the first detailed introduction into the method of the Grad-Shafranov equation, describing analytically the very broad class of hydrodynamical and MHD flows. It starts with the classical examples of hydrodynamical accretion onto relativistic and nonrelativistic objects. The force-free limit of the Grad-Shafranov equation allows us to analyze in detail the physics of the magnetospheres of radio pulsars and black holes, including the Blandford-Znajek process of energy e...

NUMERICAL AND ANALYTICAL SOLUTIONS OF NEUTRINO-DOMINATED ACCRETION FLOWS WITH A NON-ZERO TORQUE BOUNDARY CONDITION AND ITS APPLICATIONS IN GAMMA-RAY BURSTS

Energy Technology Data Exchange (ETDEWEB)

Xie, Wei; Lei, Wei-Hua; Wang, Ding-Xiong, E-mail: leiwh@hust.edu.cn [School of Physics, Huazhong University of Science and Technology, Wuhan 430074 (China)

2016-12-20

A stellar-mass black hole (BH) surrounded by a neutrino-dominated accretion flow (NDAF) has been discussed in a number of works as the central engine of gamma-ray bursts (GRBs). It is widely believed that NDAF cannot liberate enough energy for bright GRBs. However, these works have been based on the assumption of a “no torque” boundary condition, which is invalid when the disk is magnetized. In this paper, we present both numerical and analytical solutions for NDAFs with non-zero boundary stresses and reexamine their properties. We find that an NDAF with such a boundary torque can be powerful enough to account for those bright short GRBs, energetic long GRBs, and ultra-long GRBs. The disk becomes viscously unstable, which makes it possible to interpret the variability of GRB prompt emission and the steep decay phase in the early X-ray afterglow. Finally, we study the gravitational waves radiated from a processing BH-NDAF. We find that the effects of the boundary torque on the strength of the gravitational waves can be ignored.

A BABCOCK–LEIGHTON SOLAR DYNAMO MODEL WITH MULTI-CELLULAR MERIDIONAL CIRCULATION IN ADVECTION- AND DIFFUSION-DOMINATED REGIMES

Energy Technology Data Exchange (ETDEWEB)

Belucz, Bernadett; Forgács-Dajka, Emese [Eötvös University, Department of Astronomy, 1518 Budapest, Pf. 32 (Hungary); Dikpati, Mausumi, E-mail: bbelucz@astro.elte.hu, E-mail: dikpati@ucar.edu [High Altitude Observatory, National Center for Atmospheric Research, 3080 Center Green, Boulder, CO 80307-3000 (United States)

2015-06-20

Babcock–Leighton type-solar dynamo models with single-celled meridional circulation are successful in reproducing many solar cycle features. Recent observations and theoretical models of meridional circulation do not indicate a single-celled flow pattern. We examine the role of complex multi-cellular circulation patterns in a Babcock–Leighton solar dynamo in advection- and diffusion-dominated regimes. We show from simulations that the presence of a weak, second, high-latitude reverse cell speeds up the cycle and slightly enhances the poleward branch in the butterfly diagram, whereas the presence of a second cell in depth reverses the tilt of the butterfly wing to an antisolar type. A butterfly diagram constructed from the middle of convection zone yields a solar-like pattern, but this may be difficult to realize in the Sun because of magnetic buoyancy effects. Each of the above cases behaves similarly in higher and lower magnetic diffusivity regimes. However, our dynamo with a meridional circulation containing four cells in latitude behaves distinctly differently in the two regimes, producing solar-like butterfly diagrams with fast cycles in the higher diffusivity regime, and complex branches in butterfly diagrams in the lower diffusivity regime. We also find that dynamo solutions for a four-celled pattern, two in radius and two in latitude, prefer to quickly relax to quadrupolar parity if the bottom flow speed is strong enough, of similar order of magnitude as the surface flow speed.

A BABCOCK–LEIGHTON SOLAR DYNAMO MODEL WITH MULTI-CELLULAR MERIDIONAL CIRCULATION IN ADVECTION- AND DIFFUSION-DOMINATED REGIMES

International Nuclear Information System (INIS)

Belucz, Bernadett; Forgács-Dajka, Emese; Dikpati, Mausumi

2015-01-01

Babcock–Leighton type-solar dynamo models with single-celled meridional circulation are successful in reproducing many solar cycle features. Recent observations and theoretical models of meridional circulation do not indicate a single-celled flow pattern. We examine the role of complex multi-cellular circulation patterns in a Babcock–Leighton solar dynamo in advection- and diffusion-dominated regimes. We show from simulations that the presence of a weak, second, high-latitude reverse cell speeds up the cycle and slightly enhances the poleward branch in the butterfly diagram, whereas the presence of a second cell in depth reverses the tilt of the butterfly wing to an antisolar type. A butterfly diagram constructed from the middle of convection zone yields a solar-like pattern, but this may be difficult to realize in the Sun because of magnetic buoyancy effects. Each of the above cases behaves similarly in higher and lower magnetic diffusivity regimes. However, our dynamo with a meridional circulation containing four cells in latitude behaves distinctly differently in the two regimes, producing solar-like butterfly diagrams with fast cycles in the higher diffusivity regime, and complex branches in butterfly diagrams in the lower diffusivity regime. We also find that dynamo solutions for a four-celled pattern, two in radius and two in latitude, prefer to quickly relax to quadrupolar parity if the bottom flow speed is strong enough, of similar order of magnitude as the surface flow speed

Axisymmetric accretion flows very near black holes and Rosen-collapsed objects

International Nuclear Information System (INIS)

Stoeger, W.R.

1979-01-01

Motivated by the need for stronger observational leverage on the black hole hypothesis and for a more detailed characterization of axisymmetric accretion flows across the marginally stable circular orbit rsub(ms), a general approach for describing the non-Keplerian accretion in the region rsub(H) 0 , where rsub(H) = radius of the event horizon and r 0 > = rsub(ms) is developed. The procedure possesses many advantages, including easily imposed consistency with the Keplerian for r > rsub(o), the avoidance of ad hoc boundary conditions at rsub(ms) and/or at rsub(H) and its application also to accretion in Rosen's bimetric theory, whose spherically symmetric solution has the same qualitative orbital topography as that of general relativity. It becomes apparent, furthermore, that the particular viscosity law chosen in this procedure will have a crucial bearing on the flow in the region rsub(ms) 0 . (author)

Bondi flow from a slowly rotating hot atmosphere

Science.gov (United States)

Narayan, Ramesh; Fabian, Andrew C.

2011-08-01

A supermassive black hole in the nucleus of an elliptical galaxy at the centre of a cool-core group or cluster of galaxies is immersed in hot gas. Bondi accretion should occur at a rate determined by the properties of the gas at the Bondi radius and the mass of the black hole. X-ray observations of massive nearby elliptical galaxies, including M87 in the Virgo cluster, indicate a Bondi accretion rate ? which roughly matches the total kinetic power of the jets, suggesting that there is a tight coupling between the jet power and the mass accretion rate. While the Bondi model considers non-rotating gas, it is likely that the external gas has some angular momentum, which previous studies have shown could decrease the accretion rate drastically. We investigate here the possibility that viscosity acts at all radii to transport angular momentum outwards so that the accretion inflow proceeds rapidly and steadily. The situation corresponds to a giant advection-dominated accretion flow (ADAF) which extends from beyond the Bondi radius down to the black hole. We find solutions of the ADAF equations in which the gas accretes at just a factor of a few less than ?. These solutions assume that the atmosphere beyond the Bondi radius rotates with a sub-Keplerian velocity and that the viscosity parameter is large, α≥ 0.1, both of which are reasonable for the problem at hand. The infall time of the ADAF solutions is no more than a few times the free-fall time. Thus, the accretion rate at the black hole is closely coupled to the surrounding gas, enabling tight feedback to occur. We show that jet powers of a few per cent of ? are expected if either a fraction of the accretion power is channelled into the jet or the black hole spin energy is tapped by a strong magnetic field pressed against the black hole by the pressure of the accretion flow. We discuss the Bernoulli parameter of the flow, the role of convection and the possibility that these as well as magnetohydrodynamic effects

The profiles of Fe K α line from the inhomogeneous accretion flow

Science.gov (United States)

Yu, Xiao-Di; Ma, Ren-Yi; Li, Ya-Ping; Zhang, Hui; Fang, Tao-Tao

2018-05-01

The clumpy disc, or inhomogeneous accretion flow, has been proposed to explain the properties of accreting black hole systems. However, the observational evidence remains to be explored. In this work, we calculate the profiles of Fe K α lines emitted from the inhomogeneous accretion flow through the ray-tracing technique, in order to find possible observable signals of the clumps. Compared with the skewed double-peaked profile of the continuous standard accretion disc, the lines show a multipeak structure when the emissivity index is not very steep. The peaks and wings are affected by the position and size of the cold clumps. When the clump is small and is located in the innermost region, due to the significant gravitational redshift, the blue wing can overlap with the red wing of the outer cold disc/clump, forming a fake peak or greatly enhancing the red peak. Given high enough resolution, it is easier to constrain the clumps around the supermassive black holes than the clumps in stellar mass black holes due to the thermal Doppler effect.

IRON OPACITY BUMP CHANGES THE STABILITY AND STRUCTURE OF ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI

Energy Technology Data Exchange (ETDEWEB)

Jiang, Yan-Fei [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Davis, Shane W. [Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325 (United States); Stone, James M. [Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)

2016-08-10

Accretion disks around supermassive black holes have regions where the Rosseland mean opacity can be larger than the electron scattering opacity due to the large number of bound–bound transitions in iron. We study the effects of this iron opacity “bump” on the thermal stability and vertical structure of radiation-pressure-dominated accretion disks, utilizing three-dimensional radiation magnetohydrodynamic (MHD) simulations in the local shearing box approximation. The simulations self-consistently calculate the heating due to MHD turbulence caused by magneto-rotational instability and radiative cooling by using the radiative transfer module based on a variable Eddington tensor in Athena. For a 5 × 10{sup 8} solar mass black hole with ∼3% of the Eddington luminosity, a model including the iron opacity bump maintains its structure for more than 10 thermal times without showing significant signs of thermal runaway. In contrast, if only electron scattering and free–free opacity are included as in the standard thin disk model, the disk collapses on the thermal timescale. The difference is caused by a combination of (1) an anti-correlation between the total optical depth and the midplane pressure, and (2) enhanced vertical advective energy transport. These results suggest that the iron opacity bump may have a strong impact on the stability and structure of active galactic nucleus (AGN) accretion disks, and may contribute to a dependence of AGN properties on metallicity. Since this opacity is relevant primarily in UV emitting regions of the flow, it may help to explain discrepancies between observation and theory that are unique to AGNs.

Chaotic cold accretion on to black holes in rotating atmospheres

Science.gov (United States)

Gaspari, M.; Brighenti, F.; Temi, P.

2015-07-01

The fueling of black holes is one key problem in the evolution of baryons in the universe. Chaotic cold accretion (CCA) profoundly differs from classic accretion models, as Bondi and thin disc theories. Using 3D high-resolution hydrodynamic simulations, we now probe the impact of rotation on the hot and cold accretion flow in a typical massive galaxy. In the hot mode, with or without turbulence, the pressure-dominated flow forms a geometrically thick rotational barrier, suppressing the black hole accretion rate to ~1/3 of the spherical case value. When radiative cooling is dominant, the gas loses pressure support and quickly circularizes in a cold thin disk; the accretion rate is decoupled from the cooling rate, although it is higher than that of the hot mode. In the more common state of a turbulent and heated atmosphere, CCA drives the dynamics if the gas velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor number Tat 1), the broadening of the distribution and the efficiency of collisions diminish, damping the accretion rate ∝ Tat-1, until the cold disk drives the dynamics. This is exacerbated by the increased difficulty to grow TI in a rotating halo. The simulated sub-Eddington accretion rates cover the range inferred from AGN cavity observations. CCA predicts inner flat X-ray temperature and r-1 density profiles, as recently discovered in M 87 and NGC 3115. The synthetic Hα images reproduce the main features of cold gas observations in massive ellipticals, as the line fluxes and the filaments versus disk morphology. Such dichotomy is key for the long-term AGN feedback cycle. As gas cools, filamentary CCA develops and boosts AGN heating; the cold mode is thus reduced and the rotating disk remains the sole cold structure. Its consumption leaves the atmosphere in hot mode with suppressed accretion and feedback, reloading the cycle.

The multiwavelength spectrum of NGC 3115: hot accretion flow properties

Science.gov (United States)

Almeida, Ivan; Nemmen, Rodrigo; Wong, Ka-Wah; Wu, Qingwen; Irwin, Jimmy A.

2018-04-01

NGC 3115 is the nearest galaxy hosting a billion solar mass black hole and is also a low-luminosity active galactic nucleus (LLAGN). X-ray observations of this LLAGN are able to spatially resolve the hot gas within the sphere of gravitational influence of the supermassive black hole. These observations make NGC 3115 an important test bed for black hole accretion theory in galactic nuclei since they constrain the outer boundary conditions of the hot accretion flow. We present a compilation of the multiwavelength spectral energy distribution (SED) of the nucleus of NGC 3115 from radio to X-rays. We report the results from modelling the observed SED with radiatively inefficient accretion flow (RIAF) models. The radio emission can be well-explained by synchrotron emission from the RIAF without the need for contribution from a relativistic jet. We obtain a tight constraint on the RIAF density profile, ρ (r) ∝ r^{-0.73 _{-0.02} ^{+0.01}}, implying that mass-loss through subrelativistic outflows from the RIAF is significant. The lower frequency radio observation requires the synchrotron emission from a non-thermal electron population in the RIAF, similarly to Sgr A*.

Influence of fast advective flows on pattern formation of Dictyostelium discoideum

Science.gov (United States)

Bae, Albert; Zykov, Vladimir; Bodenschatz, Eberhard

2018-01-01

We report experimental and numerical results on pattern formation of self-organizing Dictyostelium discoideum cells in a microfluidic setup under a constant buffer flow. The external flow advects the signaling molecule cyclic adenosine monophosphate (cAMP) downstream, while the chemotactic cells attached to the solid substrate are not transported with the flow. At high flow velocities, elongated cAMP waves are formed that cover the whole length of the channel and propagate both parallel and perpendicular to the flow direction. While the wave period and transverse propagation velocity are constant, parallel wave velocity and the wave width increase linearly with the imposed flow. We also observe that the acquired wave shape is highly dependent on the wave generation site and the strength of the imposed flow. We compared the wave shape and velocity with numerical simulations performed using a reaction-diffusion model and found excellent agreement. These results are expected to play an important role in understanding the process of pattern formation and aggregation of D. discoideum that may experience fluid flows in its natural habitat. PMID:29590179

Research on dynamic characteristics of new chaotic-advection fins

International Nuclear Information System (INIS)

Kong Songtao; Dong Qiwu; Liu Minshan; Zhu Qing

2007-01-01

Analysis and the numerical simulation has confirmed that the flow is of the chaotic advection in the flow channel of the new fin. The chaotic advection results in stronger mixing under low Re, and thus enhances the heat transfer and anti-scaling ability. The new fin provides the beneficial exploration to the concept of chaotic advection which applies to the plate-fin heat exchanger. (authors)

Black holes in radiation-dominated gas: an analogue of the Bondi accretion problem

International Nuclear Information System (INIS)

Begelman, M.C.

1978-01-01

Black holes, unlike other compact objects, are able to accrete matter more rapidly than their Eddington rate (dM/dt)sub(E) = Lsub(E)/c 2 . Nevertheless, at such a high dM/dt, radiation will probably be emitted by the in-falling gas in copious enough quantities to have a profound influence on the flow. To aid in understanding the nature of this influence a study is made of the steady flow, on to a stationary Schwarzchild black hole, of a uniform, non-relativistic gas in which radiation pressure swamps thermal pressure at infinity, and in which Thomson scattering provides the only radiation-gas couple. Asymptotic radiation pressure p (infinity) and matter density rho(infinity) determine an asymptotic sound speed c(infinity), from which one can derive an accretion rate (dM/dt)sub(B) corresponding to the adiabatic flow of a γ = 4/3 gas. The actual accretion rate depends on the optical depth tausub(B) of a column of unperturbed gas spanning the Bondi radius, rsub(B) = GM/c 2 (infinity). If tau > ((square root of 2)/3) (c/c(infinity)), then the flow is adiabatic, and dM/dt (dM/dt)sub(B). For a somewhat smaller tausub(B), diffusion is efficient enough for the radiation to leak out of the gas as it moves towards the trans-sonic point. As a result, the sound speed decreases inwards in the subsonic region, while the density must increase steeply to maintain pressure balance. dM/dt may then exceed (dM/dt)sub(B) by a factor of up to ((square root of 2)/3) (c/tausub(B)c (infinity)), although this effect can be limited by thermal pressure. Finally, for small enough tausub(B) the diffusion approximation breaks down, and radiation drag limits an otherwise thermally-determined dM/dt. Our boundary conditions occur within supermassive (M/M (solar mass) approximately >10 2 ) stars, and in the pre- and post-recombination universe. (author)

Fulltext PDF

Indian Academy of Sciences (India)

user1

Self-Similar Solutions for Viscous and Resistive Advection Dominated Accretion. Flows (Kazem Faghei), 9. Photoionization Cross-Section of Chlorine-like Iron ... Photoelectric Radial Velocities, Paper XX: 45 Years' Monitoring of the Radial. Velocities of the Redman K Stars (R. F. Griffin & A. Stroe), 245. On the Maximum ...

Chaotic advection in the ocean

Energy Technology Data Exchange (ETDEWEB)

Koshel' , Konstantin V; Prants, Sergei V [V.I. Il' ichev Pacific Oceanological Institute, Far-Eastern Division of the Russian Academy of Sciences, Vladivostok (Russian Federation)

2006-11-30

The problem of chaotic advection of passive scalars in the ocean and its topological, dynamical, and fractal properties are considered from the standpoint of the theory of dynamical systems. Analytic and numerical results on Lagrangian transport and mixing in kinematic and dynamic chaotic advection models are described for meandering jet currents, topographical eddies in a barotropic ocean, and a two-layer baroclinic ocean. Laboratory experiments on hydrodynamic flows in rotating tanks as an imitation of geophysical chaotic advection are described. Perspectives of a dynamical system approach in physical oceanography are discussed. (reviews of topical problems)

Accretion torques due to three-dimensional channelled flows in magnetic cataclysmic variables

International Nuclear Information System (INIS)

Campbell, C.G.

1986-01-01

Angular momentum transfer due to three-dimensional magnetically channelled accretion flows in cataclysmic binaries is considered. The white dwarf experiences a torque due to the twist in that part of its magnetic field which interacts with the accretion stream. The channelling process can also enhance angular momentum exchange between the stream and the orbit by increasing the gravitational torques. The components of the accretion torque are calculated for an arbitrary static magnetic orientation of the white dwarf, and their variation with orientation is presented. For high inclinations of the accreting pole to the orbital plane the component of the accretion torque parallel to this plane can be comparable to its perpendicular component. It is shown that the parallel component of the torque is still significant relative to the perpendicular component if material links to the white dwarf's magnetic field well away from the L 1 region. (author)

Relativistic low angular momentum accretion: long time evolution of hydrodynamical inviscid flows

Science.gov (United States)

Mach, Patryk; Piróg, Michał; Font, José A.

2018-05-01

We investigate relativistic low angular momentum accretion of inviscid perfect fluid onto a Schwarzschild black hole. The simulations are performed with a general-relativistic, high-resolution (second-order), shock-capturing, hydrodynamical numerical code. We use horizon-penetrating Eddington–Finkelstein coordinates to remove inaccuracies in regions of strong gravity near the black hole horizon and show the expected convergence of the code with the Michel solution and stationary Fishbone–Moncrief toroids. We recover, in the framework of relativistic hydrodynamics, the qualitative behavior known from previous Newtonian studies that used a Bondi background flow in a pseudo-relativistic gravitational potential with a latitude-dependent angular momentum at the outer boundary. Our models exhibit characteristic ‘turbulent’ behavior and the attained accretion rates are lower than those of the Bondi–Michel radial flow. For sufficiently low values of the asymptotic sound speed, geometrically thick tori form in the equatorial plane surrounding the black hole horizon while accretion takes place mainly through the poles.

Possible Outflow Formation in the Central Engine of GRBs Tong Liu ...

Indian Academy of Sciences (India)

be geometrically thick when advection becomes dominant. In this paper, we dis- cuss the enhanced neutrino annihilation luminosity and the possible formation of the accretion flow. 2. Results. Figure 1(a) shows that there exists a narrow empty funnel along the rotation axis. Thus the volume above the disk shrinks and the ...

Radiative, two-temperature simulations of low-luminosity black hole accretion flows in general relativity

Science.gov (United States)

Sądowski, Aleksander; Wielgus, Maciek; Narayan, Ramesh; Abarca, David; McKinney, Jonathan C.; Chael, Andrew

2017-04-01

We present a numerical method that evolves a two-temperature, magnetized, radiative, accretion flow around a black hole, within the framework of general relativistic radiation magnetohydrodynamics. As implemented in the code KORAL, the gas consists of two sub-components - ions and electrons - which share the same dynamics but experience independent, relativistically consistent, thermodynamical evolution. The electrons and ions are heated independently according to a prescription from the literature for magnetohydrodynamical turbulent dissipation. Energy exchange between the particle species via Coulomb collisions is included. In addition, electrons gain and lose energy and momentum by absorbing and emitting synchrotron and bremsstrahlung radiation and through Compton scattering. All evolution equations are handled within a fully covariant framework in the relativistic fixed-metric space-time of the black hole. Numerical results are presented for five models of low-luminosity black hole accretion. In the case of a model with a mass accretion rate dot{M}˜ 4× 10^{-8} dot{M}_Edd, we find that radiation has a negligible effect on either the dynamics or the thermodynamics of the accreting gas. In contrast, a model with a larger dot{M}˜ 4× 10^{-4} dot{M}_Edd behaves very differently. The accreting gas is much cooler and the flow is geometrically less thick, though it is not quite a thin accretion disc.

New advection schemes for free surface flows

International Nuclear Information System (INIS)

Pavan, Sara

2016-01-01

The purpose of this thesis is to build higher order and less diffusive schemes for pollutant transport in shallow water flows or 3D free surface flows. We want robust schemes which respect the main mathematical properties of the advection equation with relatively low numerical diffusion and apply them to environmental industrial applications. Two techniques are tested in this work: a classical finite volume method and a residual distribution technique combined with a finite element method. For both methods we propose a decoupled approach since it is the most advantageous in terms of accuracy and CPU time. Concerning the first technique, a vertex-centred finite volume method is used to solve the augmented shallow water system where the numerical flux is computed through an Harten-Lax-Van Leer-Contact Riemann solver. Starting from this solution, a decoupled approach is formulated and is preferred since it allows to compute with a larger time step the advection of a tracer. This idea was inspired by Audusse, E. and Bristeau, M.O. [13]. The Monotonic Upwind Scheme for Conservation Law, combined with the decoupled approach, is then used for the second order extension in space. The wetting and drying problem is also analysed and a possible solution is presented. In the second case, the shallow water system is entirely solved using the finite element technique and the residual distribution method is applied to the solution of the tracer equation, focusing on the case of time-dependent problems. However, for consistency reasons the resolution of the continuity equation must be considered in the numerical discretization of the tracer. In order to get second order schemes for unsteady cases a predictor-corrector scheme is used in this work. A first order but less diffusive version of the predictor-corrector scheme is also introduced. Moreover, we also present a new locally semi-implicit version of the residual distribution method which, in addition to good properties in

Instability, Turbulence, and Enhanced Transport in Collisionless Black-Hole Accretion Flows

Science.gov (United States)

Kunz, Matthew

Many astrophysical plasmas are so hot and diffuse that the collisional mean free path is larger than the system size. Perhaps the best examples of such systems are lowluminosity accretion flows onto black holes such as Sgr A* at the center of our own Galaxy, or M87 in the Virgo cluster. To date, theoretical models of these accretion flows are based on magnetohydrodynamics (MHD), a collisional fluid theory, sometimes (but rarely) extended with non-MHD features such as anisotropic (i.e. magnetic-field-aligned) viscosity and thermal conduction. While these extensions have been recognized as crucial, they require ad hoc assumptions about the role of microscopic kinetic instabilities (namely, firehose and mirror) in regulating the transport properties. These assumptions strongly affect the outcome of the calculations, and yet they have never been tested using more fundamental (i.e. kinetic) models. This proposal outlines a comprehensive first-principles study of the plasma physics of collisionless accretion flows using both analytic and state-of-the-art numerical models. The latter will utilize a new hybrid-kinetic particle-in-cell code, Pegasus, developed by the PI and Co-I specifically to study this problem. A comprehensive kinetic study of the 3D saturation of the magnetorotational instability in a collisionless plasma will be performed, in order to understand the interplay between turbulence, transport, and Larmor-scale kinetic instabilities such as firehose and mirror. Whether such instabilities alter the macroscopic saturated state, for example by limiting the transport of angular momentum by anisotropic pressure, will be addressed. Using these results, an appropriate "fluid" closure will be developed that can capture the multi-scale effects of plasma kinetics on magnetorotational turbulence, for use by the astrophysics community in building evolutionary models of accretion disks. The PI has already successfully performed the first three-dimensional kinetic

Estimation of bipolar jets from accretion discs around Kerr black holes

Science.gov (United States)

Kumar, Rajiv; Chattopadhyay, Indranil

2017-08-01

We analyse flows around a rotating black hole and obtain self-consistent accretion-ejection solutions in full general relativistic prescription. Entire energy-angular momentum parameter space is investigated in the advective regime to obtain shocked and shock-free accretion solutions. Jet equations of motion are solved along the von Zeipel surfaces computed from the post-shock disc, simultaneously with the equations of accretion disc along the equatorial plane. For a given spin parameter, the mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. Interestingly, we obtain all types of possible jet solutions, for example, steady shock solution with multiple critical points, bound solution with two critical points and smooth solution with single critical point. Multiple critical points may exist in jet solution for spin parameter as ≥ 0.5. The jet terminal speed generally increases if the accretion shock forms closer to the horizon and is higher for corotating black hole than the counter-rotating and the non-rotating one. Quantitatively speaking, shocks in jet may form for spin parameter as > 0.6 and jet shocks range between 6rg and 130rg above the equatorial plane, while the jet terminal speed vj∞ > 0.35 c if Bernoulli parameter E≥1.01 for as > 0.99.

Stunted accretion growth of black holes by combined effect of the flow angular momentum and radiation feedback

Science.gov (United States)

Sugimura, Kazuyuki; Hosokawa, Takashi; Yajima, Hidenobu; Inayoshi, Kohei; Omukai, Kazuyuki

2018-05-01

Accretion on to seed black holes (BHs) is believed to play a crucial role in formation of supermassive BHs observed at high-redshift (z > 6). Here, we investigate the combined effect of gas angular momentum and radiation feedback on the accretion flow, by performing 2D axially symmetric radiation hydrodynamics simulations that solve the flow structure across the Bondi radius and the outer part of the accretion disc simultaneously. The accreting gas with finite angular momentum forms a rotationally-supported disc inside the Bondi radius, where the accretion proceeds by the angular momentum transport due to assumed α-type viscosity. We find that the interplay of radiation and angular momentum significantly suppresses accretion even if the radiative feedback is weakened in an equatorial shadowing region. The accretion rate is O(α) ˜ O(0.01 - 0.1) times the Bondi value, where α is the viscosity parameter. By developing an analytical model, we show that such a great reduction of the accretion rate persists unless the angular momentum is so small that the corresponding centrifugal radius is ≲ 0.04 times the Bondi radius. We argue that BHs are hard to grow quickly via rapid mass accretion considering the angular momentum barrier presented in this paper.

Parsec-Scale Accretion and Winds Irradiated by a Quasar

Science.gov (United States)

Dorodnitsyn, A.; Kallman, T.; Proga, D.

2016-01-01

We present numerical simulations of properties of a parsec-scale torus exposed to illumination by the central black hole in an active galactic nucleus (AGN). Our physical model allows to investigate the balance between the formation of winds and accretion simultaneously. Radiation-driven winds are allowed by taking into account radiation pressure due to UV and IR radiation along with X-ray heating and dust sublimation. Accretion is allowed through angular momentum transport and the solution of the equations of radiative, viscous radiation hydrodynamics. Our methods adopt flux-limited diffusion radiation hydrodynamics for the dusty, infrared pressure driven part of the flow, along with X-ray heating and cooling. Angular momentum transport in the accreting part of the flow is modeled using effective viscosity. Our results demonstrate that radiation pressure on dust can play an important role in shaping AGN obscuration. For example, when the luminosity illuminating the torus exceeds L greater than 0.01 L(sub Edd), where L(sub Edd) is the Eddington luminosity, we find no episodes of sustained disk accretion because radiation pressure does not allow a disk to form. Despite the absence of the disk accretion, the flow of gas to smaller radii still proceeds at a rate 10(exp -4)-10(exp -1)M dot yr(exp -1) through the capturing of the gas from the hot evaporative flow, thus providing a mechanism to deliver gas from a radiation-pressure dominated torus to the inner accretion disk. As L L(sub edd) increases, larger radiation input leads to larger torus aspect ratios and increased obscuration of the central black hole. We also find the important role of the X-ray heated gas in shaping the obscuring torus.

Linking Chaotic Advection with Subsurface Biogeochemical Processes

Science.gov (United States)

Mays, D. C.; Freedman, V. L.; White, S. K.; Fang, Y.; Neupauer, R.

2017-12-01

This work investigates the extent to which groundwater flow kinematics drive subsurface biogeochemical processes. In terms of groundwater flow kinematics, we consider chaotic advection, whose essential ingredient is stretching and folding of plumes. Chaotic advection is appealing within the context of groundwater remediation because it has been shown to optimize plume spreading in the laminar flows characteristic of aquifers. In terms of subsurface biogeochemical processes, we consider an existing model for microbially-mediated reduction of relatively mobile uranium(VI) to relatively immobile uranium(IV) following injection of acetate into a floodplain aquifer beneath a former uranium mill in Rifle, Colorado. This model has been implemented in the reactive transport code eSTOMP, the massively parallel version of STOMP (Subsurface Transport Over Multiple Phases). This presentation will report preliminary numerical simulations in which the hydraulic boundary conditions in the eSTOMP model are manipulated to simulate chaotic advection resulting from engineered injection and extraction of water through a manifold of wells surrounding the plume of injected acetate. This approach provides an avenue to simulate the impact of chaotic advection within the existing framework of the eSTOMP code.

GLOBAL STRUCTURE OF THREE DISTINCT ACCRETION FLOWS AND OUTFLOWS AROUND BLACK HOLES FROM TWO-DIMENSIONAL RADIATION-MAGNETOHYDRODYNAMIC SIMULATIONS

International Nuclear Information System (INIS)

Ohsuga, Ken; Mineshige, Shin

2011-01-01

We present the detailed global structure of black hole accretion flows and outflows through newly performed two-dimensional radiation-magnetohydrodynamic simulations. By starting from a torus threaded with weak toroidal magnetic fields and by controlling the central density of the initial torus, ρ 0 , we can reproduce three distinct modes of accretion flow. In model A, which has the highest central density, an optically and geometrically thick supercritical accretion disk is created. The radiation force greatly exceeds the gravity above the disk surface, thereby driving a strong outflow (or jet). Because of mild beaming, the apparent (isotropic) photon luminosity is ∼22L E (where L E is the Eddington luminosity) in the face-on view. Even higher apparent luminosity is feasible if we increase the flow density. In model B, which has moderate density, radiative cooling of the accretion flow is so efficient that a standard-type, cold, and geometrically thin disk is formed at radii greater than ∼7 R S (where R S is the Schwarzschild radius), while the flow is radiatively inefficient otherwise. The magnetic-pressure-driven disk wind appears in this model. In model C, the density is too low for the flow to be radiatively efficient. The flow thus becomes radiatively inefficient accretion flow, which is geometrically thick and optically thin. The magnetic-pressure force, together with the gas-pressure force, drives outflows from the disk surface, and the flow releases its energy via jets rather than via radiation. Observational implications are briefly discussed.

Analysis of passive scalar advection in parallel shear flows: Sorting of modes at intermediate time scales

Science.gov (United States)

Camassa, Roberto; McLaughlin, Richard M.; Viotti, Claudio

2010-11-01

The time evolution of a passive scalar advected by parallel shear flows is studied for a class of rapidly varying initial data. Such situations are of practical importance in a wide range of applications from microfluidics to geophysics. In these contexts, it is well-known that the long-time evolution of the tracer concentration is governed by Taylor's asymptotic theory of dispersion. In contrast, we focus here on the evolution of the tracer at intermediate time scales. We show how intermediate regimes can be identified before Taylor's, and in particular, how the Taylor regime can be delayed indefinitely by properly manufactured initial data. A complete characterization of the sorting of these time scales and their associated spatial structures is presented. These analytical predictions are compared with highly resolved numerical simulations. Specifically, this comparison is carried out for the case of periodic variations in the streamwise direction on the short scale with envelope modulations on the long scales, and show how this structure can lead to "anomalously" diffusive transients in the evolution of the scalar onto the ultimate regime governed by Taylor dispersion. Mathematically, the occurrence of these transients can be viewed as a competition in the asymptotic dominance between large Péclet (Pe) numbers and the long/short scale aspect ratios (LVel/LTracer≡k), two independent nondimensional parameters of the problem. We provide analytical predictions of the associated time scales by a modal analysis of the eigenvalue problem arising in the separation of variables of the governing advection-diffusion equation. The anomalous time scale in the asymptotic limit of large k Pe is derived for the short scale periodic structure of the scalar's initial data, for both exactly solvable cases and in general with WKBJ analysis. In particular, the exactly solvable sawtooth flow is especially important in that it provides a short cut to the exact solution to the

Differentiating transpiration from evaporation in seasonal agricultural wetlands and the link to advective fluxes in the root zone

International Nuclear Information System (INIS)

Bachand, P.A.M.; Bachand, S.; Fleck, J.; Anderson, F.; Windham-Myers, L.

2014-01-01

The current state of science and engineering related to analyzing wetlands overlooks the importance of transpiration and risks data misinterpretation. In response, we developed hydrologic and mass budgets for agricultural wetlands using electrical conductivity (EC) as a natural conservative tracer. We developed simple differential equations that quantify evaporation and transpiration rates using flow rates and tracer concentrations at wetland inflows and outflows. We used two ideal reactor model solutions, a continuous flow stirred tank reactor (CFSTR) and a plug flow reactor (PFR), to bracket real non-ideal systems. From those models, estimated transpiration ranged from 55% (CFSTR) to 74% (PFR) of total evapotranspiration (ET) rates, consistent with published values using standard methods and direct measurements. The PFR model more appropriately represents these non-ideal agricultural wetlands in which check ponds are in series. Using a flux model, we also developed an equation delineating the root zone depth at which diffusive dominated fluxes transition to advective dominated fluxes. This relationship is similar to the Peclet number that identifies the dominance of advective or diffusive fluxes in surface and groundwater transport. Using diffusion coefficients for inorganic mercury (Hg) and methylmercury (MeHg) we calculated that during high ET periods typical of summer, advective fluxes dominate root zone transport except in the top millimeters below the sediment–water interface. The transition depth has diel and seasonal trends, tracking those of ET. Neglecting this pathway has profound implications: misallocating loads along different hydrologic pathways; misinterpreting seasonal and diel water quality trends; confounding Fick's First Law calculations when determining diffusion fluxes using pore water concentration data; and misinterpreting biogeochemical mechanisms affecting dissolved constituent cycling in the root zone. In addition, our understanding of

Effect of advection on variations in zooplankton at a single location near Cabo Nazca, Peru

Energy Technology Data Exchange (ETDEWEB)

Smith, S L; Brink, K H; Santander, H; Cowles, T J; Huyer, A

1980-04-01

Temporal variations in the biomass and species composition of zooplankton at a single midshelf station in an upwelling area off Peru can be explained to a large extent by onshore-offshore advection in the upper 20 m of the water column. During periods of strong or sustained near-surface onshore flow, peaks in biomass of zooplankton were observed at midshelf and typically oceanic species of copepod were collected. In periods of offshore flow at the surface, a copepod capable of migrating into oxygen-depleted layers deeper than 30 m was collected. A simple translocation model of advection applied to the cross-shelf distribution of Paracalanus parvus suggests that the fluctuations in P. pavus observed in the midshelf time-series were closely related to onshore-offshore flow in the upper 20 m. Fluctuations in abundance of the numerically dominant copepod, Acartia tonsa, were apparently affected by near surface flow also. The population age-structure suggests that A. tonsa was growing at maximal rates, due in part to its positive feeding response to the dinoflagellate/diatom assemblage of phytoplankton.

The kinetically dominated quasar 3C 418

Science.gov (United States)

Punsly, Brian; Kharb, Preeti

2017-06-01

The existence of quasars that are kinetically dominated, where the jet kinetic luminosity, Q, is larger than the total (infrared to X-ray) thermal luminosity of the accretion flow, Lbol, provides a strong constraint on the fundamental physics of relativistic jet formation. Since quasars have high values of Lbol by definition, only ˜10 kinetically dominated quasars (with \\overline{Q}/L_{bol}>1) have been found, where \\overline{Q} is the long-term time-averaged jet power. We use low-frequency (151 MHz-1.66 GHz) observations of the quasar 3C 418 to determine \\overline{Q}≈ 5.5 ± 1.3 × 10^{46} {erg s^{-1}}. Analysis of the rest-frame ultraviolet spectrum indicates that this equates to 0.57 ± 0.28 times the Eddington luminosity of the central supermassive black hole and \\overline{Q}/L_{bol} ≈ 4.8 ± 3.1, making 3C 418 one of the most kinetically dominated quasars found to date. It is shown that this maximal \\overline{Q}/L_{bol} is consistent with models of magnetically arrested accretion of jet production in which the jet production reproduces the observed trend of a decrement in the extreme ultraviolet continuum as the jet power increases. This maximal condition corresponds to an almost complete saturation of the inner accretion flow with vertical large-scale magnetic flux (maximum saturation).

THE NuSTAR X-RAY SPECTRUM OF HERCULES X-1: A RADIATION-DOMINATED RADIATIVE SHOCK

Energy Technology Data Exchange (ETDEWEB)

Wolff, Michael T.; Wood, Kent S. [Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352 (United States); Becker, Peter A. [Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030-4444 (United States); Gottlieb, Amy M.; Marcu-Cheatham, Diana M.; Pottschmidt, Katja [Department of Physics and Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, MD 21250 (United States); Fürst, Felix [Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States); Hemphill, Paul B. [Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0424 (United States); Schwarm, Fritz-Walter; Wilms, Jörn [Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr, 7, D-96049 Bamberg (Germany)

2016-11-10

We report on new spectral modeling of the accreting X-ray pulsar Hercules X-1. Our radiation-dominated radiative shock model is an implementation of the analytic work of Becker and Wolff on Comptonized accretion flows onto magnetic neutron stars. We obtain a good fit to the spin-phase-averaged 4–78 keV X-ray spectrum observed by the Nuclear Spectroscopic Telescope Array during a main-on phase of the Her X-1 35 day accretion disk precession period. This model allows us to estimate the accretion rate, the Comptonizing temperature of the radiating plasma, the radius of the magnetic polar cap, and the average scattering opacity parameters in the accretion column. This is in contrast to previous phenomenological models that characterized the shape of the X-ray spectrum, but could not determine the physical parameters of the accretion flow. We describe the spectral fitting details and discuss the interpretation of the accretion flow physical parameters.

Non-linear thermal engineering, chaotic advection and mixing; Thermique non-lineaire, melange et advection chaotique

Energy Technology Data Exchange (ETDEWEB)

NONE

1999-12-31

This conference day was jointly organized by the `university group of thermal engineering (GUT)` and the French association of thermal engineers. This book of proceedings contains 7 papers entitled: `energy spectra of a passive scalar undergoing advection by a chaotic flow`; `analysis of chaotic behaviours: from topological characterization to modeling`; `temperature homogeneity by Lagrangian chaos in a direct current flow heat exchanger: numerical approach`; ` thermal instabilities in a mixed convection phenomenon: nonlinear dynamics`; `experimental characterization study of the 3-D Lagrangian chaos by thermal analogy`; `influence of coherent structures on the mixing of a passive scalar`; `evaluation of the performance index of a chaotic advection effect heat exchanger for a wide range of Reynolds numbers`. (J.S.)

Non-linear thermal engineering, chaotic advection and mixing; Thermique non-lineaire, melange et advection chaotique

Energy Technology Data Exchange (ETDEWEB)

NONE

1998-12-31

This conference day was jointly organized by the `university group of thermal engineering (GUT)` and the French association of thermal engineers. This book of proceedings contains 7 papers entitled: `energy spectra of a passive scalar undergoing advection by a chaotic flow`; `analysis of chaotic behaviours: from topological characterization to modeling`; `temperature homogeneity by Lagrangian chaos in a direct current flow heat exchanger: numerical approach`; ` thermal instabilities in a mixed convection phenomenon: nonlinear dynamics`; `experimental characterization study of the 3-D Lagrangian chaos by thermal analogy`; `influence of coherent structures on the mixing of a passive scalar`; `evaluation of the performance index of a chaotic advection effect heat exchanger for a wide range of Reynolds numbers`. (J.S.)

ACCRETION DISKS WITH A LARGE SCALE MAGNETIC FIELD AROUND BLACK HOLES

Directory of Open Access Journals (Sweden)

Gennady Bisnovatyi-Kogan

2013-12-01

Full Text Available We consider accretion disks around black holes at high luminosity, and the problem of the formation of a large-scale magnetic field in such disks, taking into account the non-uniform vertical structure of the disk. The structure of advective accretion disks is investigated, and conditions for the formation of optically thin regions in central parts of the accretion disk are found. The high electrical conductivity of the outer layers of the disk prevents outward diffusion of the magnetic field. This implies a stationary state with a strong magnetic field in the inner parts of the accretion disk close to the black hole, and zero radial velocity at the surface of the disk. The problem of jet collimation by magneto-torsion oscillations is investigated.

Accretion disks in active galactic nuclei

International Nuclear Information System (INIS)

Begelman, M.C.

1985-01-01

The innermost regions of the central engines in active galactic nuclei are examined, and it is shown how different modes of accretion with angular momentum may account for the diverse manifestations of activity in the nuclei of galaxies. These modes are subsequently compared with the observed properties of quasars, Type I Seyferts, and radio galaxies. It was found that the qualitative features of an accretion flow orbiting a massive black hole depend principally on the ratio of the actual accretion rate to the Eddington accretion rate. For a value of this ratio much less than one, the flow may become an ion torus supported by gas pressure; for a value much greater than one, the flow traps its radiative output and becomes an inefficient radiation torus. At intermediate values, the flow may settle into a thin accretion disk. 62 references

Chaotic advection near a three-vortex collapse

International Nuclear Information System (INIS)

Leoncini, X.; Kuznetsov, L.; Zaslavsky, G. M.

2001-01-01

Dynamical and statistical properties of tracer advection are studied in a family of flows produced by three point-vortices of different signs. Tracer dynamics is analyzed by numerical construction of Poincare sections, and is found to be strongly chaotic: advection pattern in the region around the center of vorticity is dominated by a well developed stochastic sea, which grows as the vortex system's initial conditions are set closer to those leading to the collapse of the vortices; at the same time, the islands of regular motion around vortices, known as vortex cores, shrink. An estimation of the core's radii from the minimum distance of vortex approach to each other is obtained. Tracer transport was found to be anomalous: for all of the three numerically investigated cases, the variance of the tracer distribution grows faster than a linear function of time, corresponding to a superdiffusive regime. The transport exponent varies with time decades, implying the presence of multi-fractal transport features. Yet, its value is never too far from 3/2, indicating some kind of universality. Statistics of Poincare recurrences is non-Poissonian: distributions have long power-law tails. The anomalous properties of tracer statistics are the result of the complex structure of the advection phase space, in particular, of strong stickiness on the boundaries between the regions of chaotic and regular motion. The role of the different phase space structures involved in this phenomenon is analyzed. Based on this analysis, a kinetic description is constructed, which takes into account different time and space scalings by using a fractional equation

Low-wave-number statistics of randomly advected passive scalars

International Nuclear Information System (INIS)

Kerstein, A.R.; McMurtry, P.A.

1994-01-01

A heuristic analysis of the decay of a passive scalar field subject to statistically steady random advection, predicts two low-wave-number spectral scaling regimes analogous to the similarity states previously identified by Chasnov [Phys. Fluids 6, 1036 (1994)]. Consequences of their predicted coexistence in a single flow are examined. The analysis is limited to the idealized case of narrow band advection. To complement the analysis, and to extend the predictions to physically more realistic advection processes, advection diffusion is simulated using a one-dimensional stochastic model. An experimental test of the predictions is proposed

The Possible Submillimeter Bump and Accretion-jet in the Central Supermassive Black Hole of NGC 4993

Science.gov (United States)

Wu, Qingwen; Feng, Jianchao; Fan, Xuliang

2018-03-01

NGC 4993, as a host galaxy of the electromagnetic counterpart of the first gravitational-wave detection of a binary neutron-star merger, was observed by many powerful telescopes from radio to γ-ray wavebands. The weak nuclear activities of NGC 4993 suggest that it is a low-luminosity active galactic nuclei (LLAGNs). We build the multiwaveband spectral energy distributions (SEDs) of NGC 4993 from the literature. We find that the radio spectrum at ∼100–300 GHz is much steeper than that of the low-frequency waveband (e.g., 6–100 GHz), where this break was also found in the supermassive black holes (SMBHs) in our galaxy center (Sgr A*), and in some other nearby AGNs. The radio emission above and below this break may have different physical origins, which provide an opportunity to probe the accretion and jet properties. We model the multiwaveband SEDs of NGC 4993 with an advection-dominated accretion flow (ADAF) jet model. We find that the high-frequency steep radio emission at the millimeter waveband is consistent with the prediction of the ADAF, while the low-frequency flat radio spectrum is better fitted by the jet. Furthermore, the X-ray emission can also be simultaneously explained by the ADAF model. From the model fits, we estimate important parameters of the central engine (e.g., the accretion rate near the horizon of the black hole and the mass-loss rate in the jet) for NGC 4993. This result strengthens the theory that the millimeter, submillimeter, and deep X-ray observations are crucial to understanding the weak or quiescent activities in SMBH systems. Further simultaneous millimeter and X-ray monitoring of this kind of LLAGN will help us to better understand the physical origin of multiwaveband emission.

Diffusion-advection within dynamic biological gaps driven by structural motion

Science.gov (United States)

Asaro, Robert J.; Zhu, Qiang; Lin, Kuanpo

2018-04-01

To study the significance of advection in the transport of solutes, or particles, within thin biological gaps (channels), we examine theoretically the process driven by stochastic fluid flow caused by random thermal structural motion, and we compare it with transport via diffusion. The model geometry chosen resembles the synaptic cleft; this choice is motivated by the cleft's readily modeled structure, which allows for well-defined mechanical and physical features that control the advection process. Our analysis defines a Péclet-like number, AD, that quantifies the ratio of time scales of advection versus diffusion. Another parameter, AM, is also defined by the analysis that quantifies the full potential extent of advection in the absence of diffusion. These parameters provide a clear and compact description of the interplay among the well-defined structural, geometric, and physical properties vis-a ̀-vis the advection versus diffusion process. For example, it is found that AD˜1 /R2 , where R is the cleft diameter and hence diffusion distance. This curious, and perhaps unexpected, result follows from the dependence of structural motion that drives fluid flow on R . AM, on the other hand, is directly related (essentially proportional to) the energetic input into structural motion, and thereby to fluid flow, as well as to the mechanical stiffness of the cleftlike structure. Our model analysis thus provides unambiguous insight into the prospect of competition of advection versus diffusion within biological gaplike structures. The importance of the random, versus a regular, nature of structural motion and of the resulting transient nature of advection under random motion is made clear in our analysis. Further, by quantifying the effects of geometric and physical properties on the competition between advection and diffusion, our results clearly demonstrate the important role that metabolic energy (ATP) plays in this competitive process.

Linear analysis on the growth of non-spherical perturbations in supersonic accretion flows

Energy Technology Data Exchange (ETDEWEB)

Takahashi, Kazuya; Yamada, Shoichi, E-mail: ktakahashi@heap.phys.waseda.ac.jp [Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku 169-8555 (Japan)

2014-10-20

We analyzed the growth of non-spherical perturbations in supersonic accretion flows. We have in mind an application to the post-bounce phase of core-collapse supernovae (CCSNe). Such non-spherical perturbations have been suggested by a series of papers by Arnett, who has numerically investigated violent convections in the outer layers of pre-collapse stars. Moreover, Couch and Ott demonstrated in their numerical simulations that such perturbations may lead to a successful supernova even for a progenitor that fails to explode without fluctuations. This study investigated the linear growth of perturbations during the infall onto a stalled shock wave. The linearized equations are solved as an initial and boundary value problem with the use of a Laplace transform. The background is a Bondi accretion flow whose parameters are chosen to mimic the 15 M {sub ☉} progenitor model by Woosley and Heger, which is supposed to be a typical progenitor of CCSNe. We found that the perturbations that are given at a large radius grow as they flow down to the shock radius; the density perturbations can be amplified by a factor of 30, for example. We analytically show that the growth rate is proportional to l, the index of the spherical harmonics. We also found that the perturbations oscillate in time with frequencies that are similar to those of the standing accretion shock instability. This may have an implication for shock revival in CCSNe, which will be investigated in our forthcoming paper in more detail.

Hot Accretion onto Black Holes with Outflow

Directory of Open Access Journals (Sweden)

Park Myeong-Gu

2018-01-01

Full Text Available Classic Bondi accretion flow can be generalized to rotating viscous accretion flow. Study of hot accretion flow onto black holes show that its physical charateristics change from Bondi-like for small gas angular momentum to disk-like for Keperian gas angular momentum. Especially, the mass accretion rate divided by the Bondi accretion rate is proportional to the viscosity parameter alpha and inversely proportional to the gas angular momentum divided by the Keplerian angular momentum at the Bondi radius for gas angular momentum comparable to the Keplerian value. The possible presence of outflow will increase the mass inflow rate at the Bondi radius but decrease the mass accretion rate across the black hole horizon by many orders of magnitude. This implies that the growth history of supermassive black holes and their coevolution with host galaxies will be dramatically changed when the accreted gas has angular momentum or develops an outflow.

Synchrotron radiation from spherically accreting black holes

International Nuclear Information System (INIS)

Ipser, J.R.; Price, R.H.

1982-01-01

Spherical accretion onto a Schwartzchild black hole, of gas with frozen-in magnetic field, is studied numerically and analytically for a range of hole masses and accretion rates in which synchrotron emission is the dominant radiative mechanism. At small radii the equipartition of magnetic, kinetic, and gravitational energy is assumed to apply, and the gas is heated by dissipation of infalling magnetic energy, turbulent energy, etc. The models can be classified into three types: (a) synchrotron cooling negligible, (b) synchrotron cooling important but synchrotron self-absorption negligible, (c) synchrotron cooling and self-absorption important. In the first case gas temperatures become very high near the horizon but luminosity efficiencies (luminosity/mass-energy accretion rate) are low. In cases (b) and (c) the gas flow near the horizon is essentially isothermal and luminosity efficiencies are fairly high. The analysis and results for the isothermal cases (b) and (c) are valid only for moderate dissipative heating and synchrotron self-absorption. If self-absorption is very strong or if dissipated energy is comparable to infall energy, Comptonization effects, not included in the analysis, become important

Criteria for retrograde rotation of accreting black holes

Science.gov (United States)

Mikhailov, A. G.; Piotrovich, M. Yu; Gnedin, Yu N.; Natsvlishvili, T. M.; Buliga, S. D.

2018-06-01

Rotating supermassive black holes produce jets and their origin is connected to the magnetic field that is generated by accreting matter flow. There is a point of view that electromagnetic fields around rotating black holes are brought to the hole by accretion. In this situation the prograde accreting discs produce weaker large-scale black hole threading magnetic fields, implying weaker jets than in retrograde regimes. The basic goal of this paper is to find the best candidates for retrograde accreting systems in observed active galactic nuclei. We show that active galactic nuclei with low Eddington ratio are really the best candidates for retrograde systems. This conclusion is obtained for kinetically dominated Fanaroff-Riley class II radio galaxies, flat-spectrum radio-loud narrow-line Seyfert I galaxies and a number of nearby galaxies. Our conclusion is that the best candidates for retrograde systems are the noticeable population of active galactic nuclei in the Universe. This result corresponds to the conclusion that in the merging process the interaction of merging black holes with a retrograde circumbinary disc is considerably more effective for shrinking the binary system.

ESTIMATING THE PARAMETERS OF SAGITTARIUS A*'s ACCRETION FLOW VIA MILLIMETER VLBI

Energy Technology Data Exchange (ETDEWEB)

Broderick, Avery E [Canadian Institute for Theoretical Astrophysics, 60 St. George St., Toronto, ON M5S 3H8 (Canada); Fish, Vincent L; Doeleman, Sheperd S [Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886 (United States); Loeb, Abraham [Institute for Theory and Computation, Harvard University, Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 (United States)

2009-05-20

Recent millimeter-VLBI observations of Sagittarius A* (Sgr A*) have, for the first time, directly probed distances comparable to the horizon scale of a black hole. This provides unprecedented access to the environment immediately around the horizon of an accreting black hole. We leverage both existing spectral and polarization measurements and our present understanding of accretion theory to produce a suite of generic radiatively inefficient accretion flow (RIAF) models of Sgr A*, which we then fit to these recent millimeter-VLBI observations. We find that if the accretion flow onto Sgr A* is well described by an RIAF model, the orientation and magnitude of the black hole's spin are constrained to a two-dimensional surface in the spin, inclination, position angle parameter space. For each of these, we find the likeliest values and their 1{sigma} and 2{sigma} errors to be a = 0{sup +0.4+0.7}, {theta}=50{sup o+10{sup o}}{sup +30{sup o}}{sub -10{sup o}}{sub -10{sup o}}, and {xi}=-20{sup o+31{sup o}}{sup +107{sup o}}{sub -16{sup o}}{sub -29{sup o}}, when the resulting probability distribution is marginalized over the others. The most probable combination is a = 0{sup +0.2+0.4}, {theta}=90{sup o}{sub -40{sup o}}{sub -50{sup o}}, and {xi}=-14{sup o+7{sup o}}{sup +11{sup o}}{sub -7{sup o}}{sub -11{sup o}}, though the uncertainties on these are very strongly correlated, and high probability configurations exist for a variety of inclination angles above 30 deg. and spins below 0.99. Nevertheless, this demonstrates the ability millimeter-VLBI observations, even with only a few stations, to significantly constrain the properties of Sgr A*.

Enhanced separation of diffusing particles by chaotic advection

International Nuclear Information System (INIS)

Aref, H.; Jones, S.W.

1989-01-01

Combining the reversibility of advection by a Stokes flow with the irreversibility of diffusion leads to a separation strategy for diffusing substances. This basic idea goes back to Taylor and Heller. It is shown here that the sensitivity of the method can be greatly enhanced by making the advection chaotic. The separation is particularly efficient when the thinnest structures resulting from advection are made comparable in size to a diffusion length. Simple heuristic estimates based on an understanding of chaotic motion and diffusion lead to a certain scaling that is seen in numerical experiments on this separation method

Advection endash diffusion around a curved obstacle

International Nuclear Information System (INIS)

Ahluwalia, D.S.; Keller, J.B.; Knessl, C.

1998-01-01

Advection and diffusion of a substance around a curved obstacle is analyzed when the advection velocity is large compared to the diffusion velocity, i.e., when the Peclet number is large. Asymptotic expressions for the concentration are obtained by the use of boundary layer theory, matched asymptotic expansions, etc. The results supplement and extend previous ones for straight obstacles. They apply to electrophoresis, the flow of ground water, chromatography, sedimentation, etc. copyright 1998 American Institute of Physics

Advection within side-by-side liquid micro-cylinders in a cross-flow

Science.gov (United States)

Dong, Qingming; Sau, Amalendu

2017-11-01

The gaseous SO2 entrainment from outer air stream and dispersion in binary and ternary liquid micro-cylinders appearing side-by-side are examined hereby. The separation/attachment regulated non-uniform interfacial momentum exchange creates main stream driven "primary" and shear reversed "secondary" vortices in the liquid cylinders. At separation points, the sense of rotation of the generated "primary-secondary" vortex pair remains inward directed. We define such a vortex pair as the "inflow" type. However, at stagnation or attachment points, the sense of rotation of a "primary-primary" or "secondary-secondary" vortex pair remains outward directed, and such a vortex pair is defined as the "outflow" type. For the coupled water cylinders facing an oncoming stream contaminated by gaseous SO2, its absorption and internal transport are effectively controlled by dominant "inflow" and "outflow" natured dynamics of the said vortex pairs, besides by diffusion. The evolving "inflow" natured "primary-secondary" vortex pairs at separation points actively entrain the outer SO2, whereas the "outflow" natured vortex-pairs oppose SO2 entry through the stagnation regions. Moreover, the blockage induced steady-symmetric, steady-deflected, and flip-flopping air-jets through gaps, for varied gap-ratio (1 ≤ G/R ≤ 4) and Reynolds number (30 ≤ Re ≤ 160), create distinctive impact both on quantitative SO2 absorption (mso2 ') and convective nature of the SO2 transport in upper, lower, and middle cylinders, by virtue of modified strength and size of the inflow and outflow paired vortices. The present study shows that the tiny "secondary vortices" play important roles in SO2 entrainment and in effectively controlling the local absorption rate Rs o2. The sudden acceleration and upward/downward deflection of gap-flows enhanced near-neck advective SO2 entrainment by suitably strengthening the "inflow" natured local vortex dynamics. Conversely, for the reduced size of secondary vortices

Analytical solutions for benchmarking cold regions subsurface water flow and energy transport models: one-dimensional soil thaw with conduction and advection

Science.gov (United States)

Kurylyk, Barret L.; McKenzie, Jeffrey M; MacQuarrie, Kerry T. B.; Voss, Clifford I.

2014-01-01

Numerous cold regions water flow and energy transport models have emerged in recent years. Dissimilarities often exist in their mathematical formulations and/or numerical solution techniques, but few analytical solutions exist for benchmarking flow and energy transport models that include pore water phase change. This paper presents a detailed derivation of the Lunardini solution, an approximate analytical solution for predicting soil thawing subject to conduction, advection, and phase change. Fifteen thawing scenarios are examined by considering differences in porosity, surface temperature, Darcy velocity, and initial temperature. The accuracy of the Lunardini solution is shown to be proportional to the Stefan number. The analytical solution results obtained for soil thawing scenarios with water flow and advection are compared to those obtained from the finite element model SUTRA. Three problems, two involving the Lunardini solution and one involving the classic Neumann solution, are recommended as standard benchmarks for future model development and testing.

Measuring groundwater transport through lake sediments by advection and diffusion

International Nuclear Information System (INIS)

Cornett, R.J.; Risto, B.A.; Lee, D.R.

1989-08-01

A method for estimating low rates of groundwater inflow and outflow through the bottom sediments of surface waters was developed and tested. A one-dimensional advection-diffusion model was fitted to measured pore water profiles of two nonreactive solutes, tritiated water and chloride, and the advection rate was calculated by a nonlinear least squares technique. Using 3 H profiles measured 0-0.5 m below the sediment-water interface, rates of groundwater advection into a lake through interbedded sands and gyttja were estimated to be about 1.0 m/year. In midlake locations underlain by soft organic gyttja, rates of advection were much lower (<0.1 m/year). Knowledge of the rate and direction of groundwater flow substantially altered the interpretation of pore water profiles within the sediments and the fluxes of solutes. This technique can be used to estimate flow rates less than 2 m/annum with minimal disturbance, without enclosing the sediments in a container, in a diversity of systems. (author)

On accretion from an inhomogeneous medium

International Nuclear Information System (INIS)

Davies, R.E.; Pringle, J.E.

1980-01-01

Hypersonic accretion flow in two dimensions from an infinite medium which contains a small density and/or velocity gradient is considered. To first order in rsub(a)/h, where rsub(a) is the accretion radius and h the scale of the gradient, the accretion rate is unaffected and the accreted angular momentum is zero. Thus previous estimates of the amount of angular momentum accreted may severely overestimate the actual value. (author)

Accretion and evaporation of modified Hayward black hole

International Nuclear Information System (INIS)

Debnath, Ujjal

2015-01-01

We assume the most general static spherically symmetric black hole metric. The accretion of any general kind of fluid flow around the black hole is investigated. The accretion of the fluid flow around the modified Hayward black hole is analyzed, and we then calculate the critical point, the fluid's four-velocity, and the velocity of sound during the accretion process. Also the nature of the dynamical mass of the black hole during accretion of the fluid flow, taking into consideration Hawking radiation from the black hole, i.e., evaporation of the black hole, is analyzed. (orig.)

The outflows accelerated by the magnetic fields and radiation force of accretion disks

Energy Technology Data Exchange (ETDEWEB)

Cao, Xinwu, E-mail: cxw@shao.ac.cn [Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030 (China)

2014-03-01

The inner region of a luminous accretion disk is radiation-pressure-dominated. We estimate the surface temperature of a radiation-pressure-dominated accretion disk, Θ=c{sub s}{sup 2}/r{sup 2}Ω{sub K}{sup 2}≪(H/r){sup 2}, which is significantly lower than that of a gas-pressure-dominated disk, Θ ∼ (H/r){sup 2}. This means that the outflow can be launched magnetically from the photosphere of the radiation-pressure-dominated disk only if the effective potential barrier along the magnetic field line is extremely shallow or no potential barrier is present. For the latter case, the slow sonic point in the outflow will probably be in the disk, which leads to a slow circular dense flow above the disk. This implies that hot gas (probably in the corona) is necessary for launching an outflow from the radiation-pressure-dominated disk, which provides a natural explanation for the observational evidence that the relativistic jets are related to hot plasma in some X-ray binaries and active galactic nuclei. We investigate the outflows accelerated from the hot corona above the disk by the magnetic field and radiation force of the accretion disk. We find that with the help of the radiation force, the mass loss rate in the outflow is high, which leads to a slow outflow. This may be why the jets in radio-loud narrow-line Seyfert galaxies are in general mildly relativistic compared with those in blazars.

Fermi bubbles inflated by winds launched from the hot accretion flow in Sgr A*

International Nuclear Information System (INIS)

Mou, Guobin; Yuan, Feng; Bu, Defu; Sun, Mouyuan; Su, Meng

2014-01-01

A pair of giant gamma-ray Bubbles has been revealed by Fermi-LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than at the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be 10 3 -10 4 times higher during the past ∼10 7 yr. On the other hand, recent magnetohydrodynamic numerical simulations of hot accretion flows have unambiguously shown the existence and obtained the properties of strong winds. Based on this knowledge, by performing three-dimensional hydrodynamical simulations, we show in this paper that the Fermi Bubbles could be inflated by winds launched from the 'past' hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The central molecular zone (CMZ) is included and it collimates the wind orientation toward the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and Kelvin-Helmholtz instabilities and results in the smoothness of the Bubbles edge. The main observational features of the Bubbles can be well explained. Specifically, the ROSAT X-ray features are interpreted by the shocked interstellar medium and the interaction region between the wind and CMZ gas. The thermal pressure and temperature obtained in our model are consistent with recent Suzaku observations.

Hydrodynamic ejection of bipolar flows from objects undergoing disk accretion: T Tauri stars, massive pre-main-sequence objects, and cataclysmic variables

International Nuclear Information System (INIS)

Torbett, M.V.

1984-01-01

A general mechanism is presented for generating pressure-driven winds that are intrinsically bipolar from objects undergoing disk accretion. The energy librated in a boundary layer shock as the disk matter impacts the central object is shown to be sufficient to eject a fraction βapprox.10 -2 to 10 -3 of the accreted mass. These winds are driven by a mechanism that accelerates the flow perpendicular to the plane of the disk and can therefore account for the bipolar geometry of the mass loss observed near young stars. The mass loss contained in these winds is comparable to that inferred for young stars. Thus, disk accretion-driven winds may constitute the T Tauri phase of stellar evolution. This mechanism is generally applicable, and thus massive pre-main-sequence objects as well as cataclysmic variables at times of enhanced accretion are predicted to eject bipolar outflows as well. Unmagnetized accreting neutron stas are also expected to eject bipolar flows. Since this mechanism requires stellar surfaces, however, it will not operate in disk accretion onto black holes

Outflow and Accretion Physics in Active Galactic Nuclei

Science.gov (United States)

McGraw, Sean Michael

2016-09-01

intervals are associated with high-ionization species such as C IV and N V, low-ionization lines including Mg II and Al III, and ground and excited states from Fe II multiplets. The detected BAL and mini-BAL variations in a subset of sources provide evidence supporting scenarios involving either transverse motions of gas or ionization changes within the absorbers. We conclude that some outflows in our samples likely exist on the order of 0.01-1 pc from the SMBH, and the possibility remains that we are tracing outflowing gas on larger scales within limits ranging from ≤10 pc to ≤1 kpc from the central source. We estimate outflow kinetic luminosities between ˜10 6 and 1 times the bolometric luminosity of the quasar, indicating that the BAL outflows we probe likely possess a range of energies and only some absorber energies are likely sufficient for AGN feedback processes. We estimate the SMBH mass in the LLAGN in NGC 4203 to be ˜1.1x10 7 solar masses within a factor of ˜2. This mass estimate in conjunction with theoretical predictions is consistent with the existence of a two-component accretion flow in the nucleus of NGC 4203, consisting of a hot, advection-dominated torus at small radii connected with a thin, radiatively efficient disk at larger scales. These results provide a significant increase in the information available for quasar outflow properties and the conditions in low-luminosity accretion disks, and will inform future observational and theoretical studies that attempt to construct a more complete picture of AGN and their effects on the surrounding environments.

Correlation Networks from Flows. The Case of Forced and Time-Dependent Advection-Diffusion Dynamics.

Directory of Open Access Journals (Sweden)

Liubov Tupikina

Full Text Available Complex network theory provides an elegant and powerful framework to statistically investigate different types of systems such as society, brain or the structure of local and long-range dynamical interrelationships in the climate system. Network links in climate networks typically imply information, mass or energy exchange. However, the specific connection between oceanic or atmospheric flows and the climate network's structure is still unclear. We propose a theoretical approach for verifying relations between the correlation matrix and the climate network measures, generalizing previous studies and overcoming the restriction to stationary flows. Our methods are developed for correlations of a scalar quantity (temperature, for example which satisfies an advection-diffusion dynamics in the presence of forcing and dissipation. Our approach reveals that correlation networks are not sensitive to steady sources and sinks and the profound impact of the signal decay rate on the network topology. We illustrate our results with calculations of degree and clustering for a meandering flow resembling a geophysical ocean jet.

Diffusion of a passive scalar with random advection

International Nuclear Information System (INIS)

Molyneux, J.E.; Witten, A.J.

1980-01-01

To investigate the instantaneous release of a passive additive into a flow, we assume that the concentration of the additive is governed by the one-dimensional advective diffusion equation in which the advecting flow velocity is a given time-dependent stochastic process. We determine both the one- and two-space-time point probability distributions of the random concentration field. This problem, or more elaborate variations of it, is a rather idealized model for a variety of environmentally important flow situations, for example, the accidental or planned release of a contaminant into a river by a power station, and, as such, it has been investigated by a number of authors. Previous treatments, however, have concentrated on deriving information about the statistical moments of the concentration. Although such information is important, it may be inadequate for accessing the true effects of a flow additive on the environment. Our investigation demonstrates the possibility of obtaining a more complete statistical description

Dense-gas dispersion advection-diffusion model

International Nuclear Information System (INIS)

Ermak, D.L.

1992-07-01

A dense-gas version of the ADPIC particle-in-cell, advection- diffusion model was developed to simulate the atmospheric dispersion of denser-than-air releases. In developing the model, it was assumed that the dense-gas effects could be described in terms of the vertically-averaged thermodynamic properties and the local height of the cloud. The dense-gas effects were treated as a perturbation to the ambient thermodynamic properties (density and temperature), ground level heat flux, turbulence level (diffusivity), and windfield (gravity flow) within the local region of the dense-gas cloud. These perturbations were calculated from conservation of energy and conservation of momentum principles along with the ideal gas law equation of state for a mixture of gases. ADPIC, which is generally run in conjunction with a mass-conserving wind flow model to provide the advection field, contains all the dense-gas modifications within it. This feature provides the versatility of coupling the new dense-gas ADPIC with alternative wind flow models. The new dense-gas ADPIC has been used to simulate the atmospheric dispersion of ground-level, colder-than-ambient, denser-than-air releases and has compared favorably with the results of field-scale experiments

A MODEL FOR THE CORRELATION OF HARD X-RAY INDEX WITH EDDINGTON RATIO IN BLACK HOLE X-RAY BINARIES

International Nuclear Information System (INIS)

Qiao, Erlin; Liu, B. F.

2013-01-01

Observations show that there is a positive correlation between the Eddington ratio λ and hard X-ray index Γ for λ ∼> 0.01, and there is an anti-correlation between λ and Γ for λ ∼ bol /L Edd ). In this work, we theoretically investigate the correlation between Γ and λ within the framework of a disk-corona model. We improve the model by taking into account all cooling processes, including synchrotron and self-Compton radiations in the corona, Comptonization of the soft photons from the underlying accretion disk, and the bremsstrahlung radiations. Presuming that the coronal flow above the disk can reach up to the 0.1 Eddington rate at the outer region, we calculate the structure of the two-phase accretion flows and the emergent spectra for accretion rates from 0.003 to 0.1. We find that at accretion rates larger than bsim0.01 Eddington rate, a fraction of coronal gas condenses into the disk and an inner disk can be sustained by condensation. In this case, the X-ray emission is dominated by the scattering of the soft photon from the underlying disk in the corona. The emission from the inner disk and corona can produce the positive correlation between λ and Γ. While at accretion rates lower than bsim0.01 Eddington accretion rate, the inner disk vanishes completely by evaporation, and the accretion is dominated by advection-dominated accretion flows (ADAFs), in which the X-ray emission is produced by the Comptonization of the synchrotron and bremsstrahlung photons of ADAF itself. The emission from ADAFs can produce the anti-correlation between λ and Γ. We show that our model can roughly explain the observed evolution of Γ 3-25keV with L 0.5-25keV /L Edd for the black hole X-ray transient H1743–322 in the decay of 2003 from the thermal-dominated state to low/hard state.

Super-Eddington Accretion in Tidal Disruption Events: the Impact of Realistic Fallback Rates on Accretion Rates

Science.gov (United States)

Wu, Samantha; Coughlin, Eric R.; Nixon, Chris

2018-04-01

After the tidal disruption of a star by a massive black hole, disrupted stellar debris can fall back to the hole at a rate significantly exceeding its Eddington limit. To understand how black hole mass affects the duration of super-Eddington accretion in tidal disruption events, we first run a suite of simulations of the disruption of a Solar-like star by a supermassive black hole of varying mass to directly measure the fallback rate onto the hole, and we compare these fallback rates to the analytic predictions of the "frozen-in" model. Then, adopting a Zero-Bernoulli Accretion flow as an analytic prescription for the accretion flow around the hole, we investigate how the accretion rate onto the black hole evolves with the more accurate fallback rates calculated from the simulations. We find that numerically-simulated fallback rates yield accretion rates onto the hole that can, depending on the black hole mass, be nearly an order of magnitude larger than those predicted by the frozen-in approximation. Our results place new limits on the maximum black hole mass for which super-Eddington accretion occurs in tidal disruption events.

Modeling Polarized Emission from Black Hole Jets: Application to M87 Core Jet

Directory of Open Access Journals (Sweden)

Monika Mościbrodzka

2017-09-01

Full Text Available We combine three-dimensional general-relativistic numerical models of hot, magnetized Advection Dominated Accretion Flows around a supermassive black hole and the corresponding outflows from them with a general relativistic polarized radiative transfer model to produce synthetic radio images and spectra of jet outflows. We apply the model to the underluminous core of M87 galaxy. The assumptions and results of the calculations are discussed in context of millimeter observations of the M87 jet launching zone. Our ab initio polarized emission and rotation measure models allow us to address the constrains on the mass accretion rate onto the M87 supermassive black hole.

Differentiating transpiration from evaporation in seasonal agricultural wetlands and the link to advective fluxes in the root zone

Science.gov (United States)

Bachand, P.A.M.; S. Bachand,; Fleck, Jacob A.; Anderson, Frank E.; Windham-Myers, Lisamarie

2014-01-01

The current state of science and engineering related to analyzing wetlands overlooks the importance of transpiration and risks data misinterpretation. In response, we developed hydrologic and mass budgets for agricultural wetlands using electrical conductivity (EC) as a natural conservative tracer. We developed simple differential equations that quantify evaporation and transpiration rates using flowrates and tracer concentrations atwetland inflows and outflows. We used two ideal reactormodel solutions, a continuous flowstirred tank reactor (CFSTR) and a plug flow reactor (PFR), to bracket real non-ideal systems. From those models, estimated transpiration ranged from 55% (CFSTR) to 74% (PFR) of total evapotranspiration (ET) rates, consistent with published values using standard methods and direct measurements. The PFR model more appropriately represents these nonideal agricultural wetlands in which check ponds are in series. Using a fluxmodel, we also developed an equation delineating the root zone depth at which diffusive dominated fluxes transition to advective dominated fluxes. This relationship is similar to the Peclet number that identifies the dominance of advective or diffusive fluxes in surface and groundwater transport. Using diffusion coefficients for inorganic mercury (Hg) and methylmercury (MeHg) we calculated that during high ET periods typical of summer, advective fluxes dominate root zone transport except in the top millimeters below the sediment–water interface. The transition depth has diel and seasonal trends, tracking those of ET. Neglecting this pathway has profound implications: misallocating loads along different hydrologic pathways; misinterpreting seasonal and diel water quality trends; confounding Fick's First Law calculations when determining diffusion fluxes using pore water concentration data; and misinterpreting biogeochemicalmechanisms affecting dissolved constituent cycling in the root zone. In addition,our understanding of internal

Verification of Advective Bar Elements Implemented in the Aria Thermal Response Code.

Energy Technology Data Exchange (ETDEWEB)

Mills, Brantley [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

2016-01-01

A verification effort was undertaken to evaluate the implementation of the new advective bar capability in the Aria thermal response code. Several approaches to the verification process were taken : a mesh refinement study to demonstrate solution convergence in the fluid and the solid, visually examining the mapping of the advective bar element nodes to the surrounding surfaces, and a comparison of solutions produced using the advective bars for simple geometries with solutions from commercial CFD software . The mesh refinement study has shown solution convergence for simple pipe flow in both temperature and velocity . Guidelines were provided to achieve appropriate meshes between the advective bar elements and the surrounding volume. Simulations of pipe flow using advective bars elements in Aria have been compared to simulations using the commercial CFD software ANSYS Fluent (r) and provided comparable solutions in temperature and velocity supporting proper implementation of the new capability. Verification of Advective Bar Elements iv Acknowledgements A special thanks goes to Dean Dobranich for his guidance and expertise through all stages of this effort . His advice and feedback was instrumental to its completion. Thanks also goes to Sam Subia and Tolu Okusanya for helping to plan many of the verification activities performed in this document. Thank you to Sam, Justin Lamb and Victor Brunini for their assistance in resolving issues encountered with running the advective bar element model. Finally, thanks goes to Dean, Sam, and Adam Hetzler for reviewing the document and providing very valuable comments.

A numerical study of the stability of radiative shocks. [in accretion flows onto white dwarf stars

Science.gov (United States)

Imamura, J. N.; Wolff, M. T.; Durisen, R. H.

1984-01-01

Attention is given to the oscillatory instability of optically thin radiative shocks in time-dependent numerical calculations of accretion flows onto degenerate dwarfs. The present nonlinear calculations yield good quantitative agreement with the linear results obtained for oscillation frequencies, damping rates, and critical alpha-values. The fundamental mode and the first overtone in the shock radius and luminosity variations can be clearly identified, and evidence is sometimes seen for the second overtone. Time-dependent calculations are also performed which include additional physics relevant to degenerate dwarf accretion, such as electron thermal conduction, unequal electron and ion temperatures, Compton cooling, and relativistic corrections to the bremsstrahlung cooling law. All oscillatory modes are found to be damped, and hence stable, in the case of a 1-solar mass white dwarf accreting in spherical symmetry.

High Order Semi-Lagrangian Advection Scheme

Science.gov (United States)

Malaga, Carlos; Mandujano, Francisco; Becerra, Julian

2014-11-01

In most fluid phenomena, advection plays an important roll. A numerical scheme capable of making quantitative predictions and simulations must compute correctly the advection terms appearing in the equations governing fluid flow. Here we present a high order forward semi-Lagrangian numerical scheme specifically tailored to compute material derivatives. The scheme relies on the geometrical interpretation of material derivatives to compute the time evolution of fields on grids that deform with the material fluid domain, an interpolating procedure of arbitrary order that preserves the moments of the interpolated distributions, and a nonlinear mapping strategy to perform interpolations between undeformed and deformed grids. Additionally, a discontinuity criterion was implemented to deal with discontinuous fields and shocks. Tests of pure advection, shock formation and nonlinear phenomena are presented to show performance and convergence of the scheme. The high computational cost is considerably reduced when implemented on massively parallel architectures found in graphic cards. The authors acknowledge funding from Fondo Sectorial CONACYT-SENER Grant Number 42536 (DGAJ-SPI-34-170412-217).

Numerical Simulations of Wind Accretion in Symbiotic Binaries

Science.gov (United States)

de Val-Borro, M.; Karovska, M.; Sasselov, D.

2009-08-01

About half of the binary systems are close enough to each other for mass to be exchanged between them at some point in their evolution, yet the accretion mechanism in wind accreting binaries is not well understood. We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an asymptotic giant branch (AGB) mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass-loss rate, wind temperature, and binary orbital parameters. A two-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of state and a modified version of the isothermal approximation, where the temperature depends on the distance from the mass losing star and its companion. The code uses a block-structured adaptive mesh refinement method that allows us to have high resolution at the position of the secondary and resolve the formation of bow shocks and accretion disks. We explore the accretion flow between the components and formation of accretion disks for a range of orbital separations and wind parameters. Our results show the formation of stream flow between the stars and accretion disks of various sizes for certain orbital configurations. For a typical slow and massive wind from an AGB star the flow pattern is similar to a Roche lobe overflow with accretion rates of 10% of the mass loss from the primary. Stable disks with exponentially decreasing density profiles and masses of the order 10-4 solar masses are formed when wind acceleration occurs at several stellar radii. The disks are geometrically thin with eccentric streamlines and close to Keplerian velocity profiles. The formation of tidal streams and accretion disks is found to be weakly dependent on

NUMERICAL SIMULATIONS OF WIND ACCRETION IN SYMBIOTIC BINARIES

International Nuclear Information System (INIS)

De Val-Borro, M.; Karovska, M.; Sasselov, D.

2009-01-01

About half of the binary systems are close enough to each other for mass to be exchanged between them at some point in their evolution, yet the accretion mechanism in wind accreting binaries is not well understood. We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an asymptotic giant branch (AGB) mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass-loss rate, wind temperature, and binary orbital parameters. A two-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of state and a modified version of the isothermal approximation, where the temperature depends on the distance from the mass losing star and its companion. The code uses a block-structured adaptive mesh refinement method that allows us to have high resolution at the position of the secondary and resolve the formation of bow shocks and accretion disks. We explore the accretion flow between the components and formation of accretion disks for a range of orbital separations and wind parameters. Our results show the formation of stream flow between the stars and accretion disks of various sizes for certain orbital configurations. For a typical slow and massive wind from an AGB star the flow pattern is similar to a Roche lobe overflow with accretion rates of 10% of the mass loss from the primary. Stable disks with exponentially decreasing density profiles and masses of the order 10 -4 solar masses are formed when wind acceleration occurs at several stellar radii. The disks are geometrically thin with eccentric streamlines and close to Keplerian velocity profiles. The formation of tidal streams and accretion disks is found to be weakly dependent

Accretion of Ghost Condensate by Black Holes

Energy Technology Data Exchange (ETDEWEB)

Frolov, A

2004-06-02

The intent of this letter is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

Electron-positron pair production in a hot accretion plasma around a massive black hole

International Nuclear Information System (INIS)

Takahara, Fumio; Kusunose, Masaaki.

1985-01-01

We investigate the electron-positron pair production in a hot accretion plasma around a supermassive black hole in connection with active galactic nuclei. Assuming that an optically thin two-temperature plasma is produced in the vicinity of the central black hole, we examine the condition for the significant pair production by comparing relevant time scales. Since the pair production is dominated by collisions between hard photons, the conditions for significant pair production depend on the production rate of hard photons. We examine the case where the unsaturated Comptonization of soft photons produces hard photons as well as that of bremsstrahlung. We show that significant pair production occurs for a moderately high accretion rate with relatively slow accretion flow as compared to the free fall velocity in both cases. Possible consequences of pair production are briefly discussed. (author)

Period variations in pulsating X-ray sources. I. Accretion flow parameters and neutron star structure from timing observations

International Nuclear Information System (INIS)

Lamb, F.K.; Pines, D.; Shaham, J.

1978-01-01

We show that valuable information about both accretion flows and neutron star structure can be obtained from X-ray timing observations of period variations in pulsating sources. Such variations can result from variations in the accretion flow, or from internal torque variations, associated with oscillations of the fluid core or the unpinning of vortices in the inner crust. We develop a statistical description of torque variations in terms of noise processes, indicate how the applicability of such a description may be tested observationally, and show how it may be used to determine from observation both the properties of accretion flows and the internal structure of neutron stars, including the relative inertial moments of the crust and superfluid neutron core, the crust-core coupling time, and the frequencies of any low-frequency internal collective modes. Particular attention is paid to the physical origin of spin-down episodes; it is shown that usyc episodes may result either from external torque reversals or from internal torque variations.With the aid of the statistical description, the response of the star to torque fluctuations is calculated for three stellar models: (i) a completely rigid star; (ii) a two-component star; and (iii) a two-component star with a finite-frequency internal mode, such as the Tkachenko mode of a rotating neutron superfluid. Our calculations show that fluctuating torques could account for the period the period variations and spin-down episodes observed in Her X-1 and Cen X-3, including the large spin-down event observed in the latter source during 1972 September-October. The torque noise strengths inferred from current timing observations using the simple two-component models are shown to be consistent with those to be expected from fluctuations in accretion flows onto magnetic neutron stars

Emergent structures in reaction-advection-diffusion systems on a sphere

Science.gov (United States)

Krause, Andrew L.; Burton, Abigail M.; Fadai, Nabil T.; Van Gorder, Robert A.

2018-04-01

We demonstrate unusual effects due to the addition of advection into a two-species reaction-diffusion system on the sphere. We find that advection introduces emergent behavior due to an interplay of the traditional Turing patterning mechanisms with the compact geometry of the sphere. Unidirectional advection within the Turing space of the reaction-diffusion system causes patterns to be generated at one point of the sphere, and transported to the antipodal point where they are destroyed. We illustrate these effects numerically and deduce conditions for Turing instabilities on local projections to understand the mechanisms behind these behaviors. We compare this behavior to planar advection which is shown to only transport patterns across the domain. Analogous transport results seem to hold for the sphere under azimuthal transport or away from the antipodal points in unidirectional flow regimes.

Super-Eddington accreting massive black holes as long-lived cosmological standards.

Science.gov (United States)

Wang, Jian-Min; Du, Pu; Valls-Gabaud, David; Hu, Chen; Netzer, Hagai

2013-02-22

Super-Eddington accreting massive black holes (SEAMBHs) reach saturated luminosities above a certain accretion rate due to photon trapping and advection in slim accretion disks. We show that these SEAMBHs could provide a new tool for estimating cosmological distances if they are properly identified by hard x-ray observations, in particular by the slope of their 2-10 keV continuum. To verify this idea we obtained black hole mass estimates and x-ray data for a sample of 60 narrow line Seyfert 1 galaxies that we consider to be the most promising SEAMBH candidates. We demonstrate that the distances derived by the new method for the objects in the sample get closer to the standard luminosity distances as the hard x-ray continuum gets steeper. The results allow us to analyze the requirements for using the method in future samples of active black holes and to demonstrate that the expected uncertainty, given large enough samples, can make them into a useful, new cosmological ruler.

Analytical simulation of two dimensional advection dispersion ...

African Journals Online (AJOL)

The study was designed to investigate the analytical simulation of two dimensional advection dispersion equation of contaminant transport. The steady state flow condition of the contaminant transport where inorganic contaminants in aqueous waste solutions are disposed of at the land surface where it would migrate ...

Analytical Simulation of Two Dimensional Advection Dispersion ...

African Journals Online (AJOL)

ADOWIE PERE

ABSTRACT: The study was designed to investigate the analytical simulation of two dimensional advection dispersion equation of contaminant transport. The steady state flow condition of the contaminant transport where inorganic contaminants in aqueous waste solutions are disposed of at the land surface where it would ...

Development of Multigrid Methods for diffusion, Advection, and the incompressible Navier-Stokes Equations

Energy Technology Data Exchange (ETDEWEB)

Gjesdal, Thor

1997-12-31

This thesis discusses the development and application of efficient numerical methods for the simulation of fluid flows, in particular the flow of incompressible fluids. The emphasis is on practical aspects of algorithm development and on application of the methods either to linear scalar model equations or to the non-linear incompressible Navier-Stokes equations. The first part deals with cell centred multigrid methods and linear correction scheme and presents papers on (1) generalization of the method to arbitrary sized grids for diffusion problems, (2) low order method for advection-diffusion problems, (3) attempt to extend the basic method to advection-diffusion problems, (4) Fourier smoothing analysis of multicolour relaxation schemes, and (5) analysis of high-order discretizations for advection terms. The second part discusses a multigrid based on pressure correction methods, non-linear full approximation scheme, and papers on (1) systematic comparison of the performance of different pressure correction smoothers and some other algorithmic variants, low to moderate Reynolds numbers, and (2) systematic study of implementation strategies for high order advection schemes, high-Re flow. An appendix contains Fortran 90 data structures for multigrid development. 160 refs., 26 figs., 22 tabs.

On the thermal stability of radiation-dominated accretion disks

Energy Technology Data Exchange (ETDEWEB)

Jiang, Yan-Fei; Stone, James M. [Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States); Davis, Shane W. [Canadian Institute for Theoretical Astrophysics, Toronto, ON M5S3H4 (Canada)

2013-11-20

We study the long-term thermal stability of radiation-dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to the dissipation of MHD turbulence driven by magneto-rotational instability (MRI) and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions are at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway that causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard α disk model; for example, the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

Spinning up black holes with super-critical accretion flows

Czech Academy of Sciences Publication Activity Database

Sądowski, A.; Bursa, Michal; Abramowicz, M. A.; Kluzniak, W.; Lasota, J.-P.; Moderski, R.; Safarzadeh, M.

2011-01-01

Roč. 532, August (2011), A41/1-A41/11 ISSN 0004-6361 Institutional research plan: CEZ:AV0Z10030501 Keywords : black hole physics * accretion * accretion disks Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.587, year: 2011

Accretion onto a noncommutative geometry inspired black hole

Energy Technology Data Exchange (ETDEWEB)

Kumar, Rahul [Jamia Millia Islamia, Centre for Theoretical Physics, New Delhi (India); Ghosh, Sushant G. [Jamia Millia Islamia, Centre for Theoretical Physics, New Delhi (India); Jamia Millia Islamia, Multidisciplinary Centre for Advanced Research and Studies (MCARS), New Delhi (India); University of KwaZulu-Natal, Astrophysics and Cosmology Research Unit, School of Mathematics, Statistics and Computer Science, Durban (South Africa)

2017-09-15

The spherically symmetric accretion onto a noncommutative (NC) inspired Schwarzschild black hole is treated for a polytropic fluid. The critical accretion rate M, sonic speed a{sub s} and other flow parameters are generalized for the NC inspired static black hole and compared with the results obtained for the standard Schwarzschild black holes. Also explicit expressions for gas compression ratios and temperature profiles below the accretion radius and at the event horizon are derived. This analysis is a generalization of Michel's solution to the NC geometry. Owing to the NC corrected black hole, the accretion flow parameters also have been modified. It turns out that M ∼ M{sup 2} is still achievable but r{sub s} seems to be substantially decreased due to the NC effects. They in turn do affect the accretion process. (orig.)

Cold, clumpy accretion onto an active supermassive black hole.

Science.gov (United States)

Tremblay, Grant R; Oonk, J B Raymond; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P; Baum, Stefi A; Voit, G Mark; Donahue, Megan; McNamara, Brian R; Davis, Timothy A; McDonald, Michael A; Edge, Alastair C; Clarke, Tracy E; Galván-Madrid, Roberto; Bremer, Malcolm N; Edwards, Louise O V; Fabian, Andrew C; Hamer, Stephen; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael W

2016-06-09

Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds--a departure from the 'hot mode' accretion model--although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

Fractional vector calculus for fractional advection dispersion

Science.gov (United States)

Meerschaert, Mark M.; Mortensen, Jeff; Wheatcraft, Stephen W.

2006-07-01

We develop the basic tools of fractional vector calculus including a fractional derivative version of the gradient, divergence, and curl, and a fractional divergence theorem and Stokes theorem. These basic tools are then applied to provide a physical explanation for the fractional advection-dispersion equation for flow in heterogeneous porous media.

Accretion of a ghost condensate by black holes

International Nuclear Information System (INIS)

Frolov, Andrei V.

2004-01-01

The intent of this paper is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as a tenth of a solar mass per second for 10 MeV scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model

Solar neutrinos and solar accretion of interstellar matter

International Nuclear Information System (INIS)

Newman, M.J.; Talbot, R.J. Jr.

1976-01-01

It is argued that if the Hoyle-Lyttleton mass accretion rate applies (Proc. Camb. Phil. Soc., Math. Phys. Sci. 35: 405 (1939)) the accretion of interstellar matter by the Sun is sufficient to enhance the surface heavy element abundances. This will also apply to other solar-type stars. The enhancement may be sufficient to allow the construction of consistent solar models with an interior heavy element abundance significantly lower than the observed surface abundance. This state of affairs lowers the predicted solar neutrino flux. It has been suggested that a similar enhancement of surface abundances might occur due to accretion of 'planetesimals' left over after formation of the solar system, and both processes may occur, thereby increasing the effect. The simple accretion model of Hoyle and Lyttleton is discussed mathematically. A crucial question to be answered by future research, however, is whether or not accretion on to the solar surface actually occurs. One of the most obvious obstacles is the outward flowing solar wind, and this is discussed. It appears that the outward flow can be reversed to an inward flow for certain interstellar cloud densities. (U.K.)

Stokes flow heat transfer in an annular, rotating heat exchanger

International Nuclear Information System (INIS)

Saatdjian, E.; Rodrigo, A.J.S.; Mota, J.P.B.

2011-01-01

The heat transfer rate into highly viscous, low thermal-conductivity fluids can be enhanced significantly by chaotic advection in three-dimensional flows dominated by viscous forces. The physical effect of chaotic advection is to render the cross-sectional temperature field uniform, thus increasing both the wall temperature gradient and the heat flux into the fluid. A method of analysis for one such flow-the flow in the eccentric, annular, rotating heat exchanger-and a procedure to determine the best heat transfer conditions, namely the optimal values of the eccentricity ratio and time-periodic rotating protocol, are discussed. It is shown that in continuous flows, such as the one under consideration, there exists an optimum frequency of the rotation protocol for which the heat transfer rate is a maximum. - Highlights: → The eccentric, annular, rotating heat exchanger is studied for periodic Stokes flow. → Counter-rotating the inner tube with a periodic velocity enhances the heat transfer. → The heat-transfer enhancement under such conditions is due to chaotic advection. → For a given axial flow rate there is a frequency that maximizes the heat transfer. → There is also an optimum value of the eccentricity ratio.

Local and nonlocal advected invariants and helicities in magnetohydrodynamics and gas dynamics I: Lie dragging approach

International Nuclear Information System (INIS)

Webb, G M; Dasgupta, B; McKenzie, J F; Hu, Q; Zank, G P

2014-01-01

In this paper advected invariants and conservation laws in ideal magnetohydrodynamics (MHD) and gas dynamics are obtained using Lie dragging techniques. There are different classes of invariants that are advected or Lie dragged with the flow. Simple examples are the advection of the entropy S (a 0-form), and the conservation of magnetic flux (an invariant 2-form advected with the flow). The magnetic flux conservation law is equivalent to Faraday's equation. The gauge condition for the magnetic helicity to be advected with the flow is determined. Different variants of the helicity in ideal fluid dynamics and MHD including: fluid helicity, cross helicity and magnetic helicity are investigated. The fluid helicity conservation law and the cross-helicity conservation law in MHD are derived for the case of a barotropic gas. If the magnetic field lies in the constant entropy surface, then the gas pressure can depend on both the entropy and the density. In these cases the conservation laws are local conservation laws. For non-barotropic gases, we obtain nonlocal conservation laws for fluid helicity and cross helicity by using Clebsch variables. These nonlocal conservation laws are the main new results of the paper. Ertel's theorem and potential vorticity, the Hollman invariant, and the Godbillon–Vey invariant for special flows for which the magnetic helicity is zero are also discussed. (paper)

Nearly collisionless spherical accretion

International Nuclear Information System (INIS)

Begelman, M.C.

1977-01-01

A fluid-like gas accretes much more efficiently than a collisionless gas. The ability of an accreting gas to behave like a fluid depends on the relationship of the mean free path of a gas particle at r → infinity lambdasub(infinity), to the typical length scales associated with the star-gas system. This relationship is examined in detail. For constant collision cross-section evidence is found for a rapid changeover from collisionless to fluid-like accretion flow when lambdasub(infinity) drops below a certain value, but for hard Coulomb collisions, the transition is more gradual, and is sensitive to the adiabatic index of the gas at r→ infinity. To these results must be added the effects of the substantial cusp of bound particles, which always develops in a system with arbitrarily small but non-zero cross-section. The density run in such a cusp depends on the collision properties of the particles. 'Loss-cone' accretion from the cusp may in some cases exceed the predicted accretion rate. (author)

Accretion Disk Spectra of the Ultra-Luminous X-Ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Energy Technology Data Exchange (ETDEWEB)

Mizuno, T

2003-12-11

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (> 300 M{sub solar}). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super-Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and that their X-ray emission is from the slim disk shining at super-Eddington luminosities.

A novel finite volume discretization method for advection-diffusion systems on stretched meshes

Science.gov (United States)

Merrick, D. G.; Malan, A. G.; van Rooyen, J. A.

2018-06-01

This work is concerned with spatial advection and diffusion discretization technology within the field of Computational Fluid Dynamics (CFD). In this context, a novel method is proposed, which is dubbed the Enhanced Taylor Advection-Diffusion (ETAD) scheme. The model equation employed for design of the scheme is the scalar advection-diffusion equation, the industrial application being incompressible laminar and turbulent flow. Developed to be implementable into finite volume codes, ETAD places specific emphasis on improving accuracy on stretched structured and unstructured meshes while considering both advection and diffusion aspects in a holistic manner. A vertex-centered structured and unstructured finite volume scheme is used, and only data available on either side of the volume face is employed. This includes the addition of a so-called mesh stretching metric. Additionally, non-linear blending with the existing NVSF scheme was performed in the interest of robustness and stability, particularly on equispaced meshes. The developed scheme is assessed in terms of accuracy - this is done analytically and numerically, via comparison to upwind methods which include the popular QUICK and CUI techniques. Numerical tests involved the 1D scalar advection-diffusion equation, a 2D lid driven cavity and turbulent flow case. Significant improvements in accuracy were achieved, with L2 error reductions of up to 75%.

Topics in the physics of accretion onto black holes

International Nuclear Information System (INIS)

Stoeger, W.R.

1977-06-01

The subject is covered in chapters, entitled: introduction and overview; boundary-condition modification of accretion-disk models; standard assumptions and nonkeplerian inner-disk models; the 'inner edge' of accretion disks and spiral orbits; a review of comptonization in accretion disks and a criterion for Lightman-Eardley stability; the thickening of accretion disks and flows; radial pressure gradients and low-angular-momentum accretion; accretion-disk scenarios for X-ray transient and burst sources; photon pair-creation processes in transrelativistic plasmas; and the astrophysical consequences of Rosen's bi-metric theory of gravity. (U.K.)

Radiation-driven Turbulent Accretion onto Massive Black Holes

Energy Technology Data Exchange (ETDEWEB)

Park, KwangHo; Wise, John H.; Bogdanović, Tamara, E-mail: kwangho.park@physics.gatech.edu [Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 (United States)

2017-09-20

Accretion of gas and interaction of matter and radiation are at the heart of many questions pertaining to black hole (BH) growth and coevolution of massive BHs and their host galaxies. To answer them, it is critical to quantify how the ionizing radiation that emanates from the innermost regions of the BH accretion flow couples to the surrounding medium and how it regulates the BH fueling. In this work, we use high-resolution three-dimensional (3D) radiation-hydrodynamic simulations with the code Enzo , equipped with adaptive ray-tracing module Moray , to investigate radiation-regulated BH accretion of cold gas. Our simulations reproduce findings from an earlier generation of 1D/2D simulations: the accretion-powered UV and X-ray radiation forms a highly ionized bubble, which leads to suppression of BH accretion rate characterized by quasi-periodic outbursts. A new feature revealed by the 3D simulations is the highly turbulent nature of the gas flow in vicinity of the ionization front. During quiescent periods between accretion outbursts, the ionized bubble shrinks in size and the gas density that precedes the ionization front increases. Consequently, the 3D simulations show oscillations in the accretion rate of only ∼2–3 orders of magnitude, significantly smaller than 1D/2D models. We calculate the energy budget of the gas flow and find that turbulence is the main contributor to the kinetic energy of the gas but corresponds to less than 10% of its thermal energy and thus does not contribute significantly to the pressure support of the gas.

On the potential importance of transient air flow in advective radon entry into buildings

International Nuclear Information System (INIS)

Narasimhan, T.N.; Tsang, Y.W.; Holman, H.Y.

1990-01-01

The authors have investigated, using a mathematical model, the temporal variations of air flux within the soil mass surrounding a basement in the presence of time dependent periodic variations of barometric pressure and a persistent under-pressure at the basement. The results of transient air flow show that for a homogeneous soil medium, the effects of barometric fluctuations are most significant in the cases where soil permeability to air is low and the fluctuation frequency is high. In these cases, the barometric fluctuation can greatly enhance the magnitude of fluxes as well as introduce flow direction reversals from surrounding soil into the basement. These large fluxes with direction reversals have strong implications in regard to advective transport of radon. The results suggest that the transient oscillations have to be accounted for in quantifying radon entry into buildings. In the actual field set up, the transient behavior will be further influenced by soil permeability heterogeneity, by soil moisture variations, and by the effects of multiple periodic components in the barometric pressure fluctuations

A spin-down mechanism for accreting neutron stars

International Nuclear Information System (INIS)

Illarionov, A.F.; AN SSSR, Moscow. Fizicheskij Inst.); Kompaneets, D.A.

1990-01-01

We propose a new spin-down mechanism for accreting neutron stars that explains the existence of a number of long-period (p≅100-1000 s) X-ray pulsars in wide binaries with OB-stars. The spin-down is a result of efficient angular momentum transfer from the rotating magnetosphere of the accreting star to an outflowing stream of magnetized matter. The outflow is formed within a limited solid angle, and the outflow rate is less than the accretion rate. The outflow formation is connected with the anisotropy and intensity of the hard X-ray emission of the neutron star. X-rays from the pulsar heat through Compton scattering the accreting matter anisotropically. The heated matter has a lower density than the surrounding accreting matter and flows up by the action of the buoyancy force. We find the criterion for the outflow to form deep in the accretion flow (i.e., close to the neutron star magnetosphere). The neutron star loses angular momentum when the outflow forms so deep as to capture the magnetic field lines from the rotating magnetosphere. The balance between angular momentum gain by accreting gas and loss by outflowing matter takes place at a particular value of the period of the spinning neutron star. (orig.)

On the spin period distribution in Be/X-ray binaries

Energy Technology Data Exchange (ETDEWEB)

Cheng, Z.-Q.; Shao, Y.; Li, X.-D., E-mail: lixd@nju.edu.cn [Department of Astronomy, Nanjing University, Nanjing 210093 (China)

2014-05-10

There is a remarkable correlation between the spin periods of the accreting neutron stars (NSs) in Be/X-ray binaries (BeXBs) and their orbital periods. Recently, Knigge et al. showed that the distribution of the spin periods contains two distinct subpopulations peaked at ∼10 s and ∼200 s, respectively, and suggested that they may be related to two types of supernovae for the formation of the NSs, i.e., core-collapse and electron-capture supernovae. Here we propose that the bimodal spin period distribution is likely to be ascribed to different accretion modes of the NSs in BeXBs. When the NS tends to capture material from the warped, outer part of the Be star disk and experiences giant outbursts, a radiatively cooling dominated disk is formed around the NS, which spins up the NS and is responsible for the short-period subpopulation. In BeXBs that are dominated by normal outbursts or are persistent, the accretion flow is advection-dominated or quasi-spherical. The spin-up process is accordingly inefficient, leading to longer periods of the neuron stars. The potential relation between the subpopulations and the supernova mechanism is also discussed.

DEVELOPMENT AND DEMONSTRATION OF A BIDIRECTIONAL ADVECTIVE FLUX METER FOR SEDIMENT-WATER INTERFACE

Science.gov (United States)

A bidirectional advective flux meter for measuring water transport across the sediment-water interface has been successfully developed and field tested. The flow sensor employs a heat-pulse technique combined with a flow collection funnel for the flow measurement. Because the dir...

Theory of advection-driven long range biotic transport

Science.gov (United States)

We propose a simple mechanistic model to examine the effects of advective flow on the spread of fungal diseases spread by wind-blown spores. The model is defined by a set of two coupled non-linear partial differential equations for spore densities. One equation describes the long-distance advectiv...

SPREADING LAYERS IN ACCRETING OBJECTS: ROLE OF ACOUSTIC WAVES FOR ANGULAR MOMENTUM TRANSPORT, MIXING, AND THERMODYNAMICS

Energy Technology Data Exchange (ETDEWEB)

Philippov, Alexander A.; Rafikov, Roman R.; Stone, James M., E-mail: sashaph@princeton.edu [Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540 (United States)

2016-01-20

Disk accretion at a high rate onto a white dwarf (WD) or a neutron star has been suggested to result in the formation of a spreading layer (SL)—a belt-like structure on the object's surface, in which the accreted matter steadily spreads in the poleward (meridional) direction while spinning down. To assess its basic characteristics, we perform two-dimensional hydrodynamic simulations of supersonic SLs in the relevant morphology with a simple prescription for cooling. We demonstrate that supersonic shear naturally present at the base of the SL inevitably drives sonic instability that gives rise to large-scale acoustic modes governing the evolution of the SL. These modes dominate the transport of momentum and energy, which is intrinsically global and cannot be characterized via some form of local effective viscosity (e.g., α-viscosity). The global nature of the wave-driven transport should have important implications for triggering Type I X-ray bursts in low-mass X-ray binaries. The nonlinear evolution of waves into a system of shocks drives effective rearrangement (sensitively depending on thermodynamical properties of the flow) and deceleration of the SL, which ultimately becomes transonic and susceptible to regular Kelvin–Helmholtz instability. We interpret this evolution in terms of the global structure of the SL and suggest that mixing of the SL material with the underlying stellar fluid should become effective only at intermediate latitudes on the accreting object's surface, where the flow has decelerated appreciably. In the near-equatorial regions the transport is dominated by acoustic waves and mixing is less efficient. We speculate that this latitudinal nonuniformity of mixing in accreting WDs may be linked to the observed bipolar morphology of classical nova ejecta.

SIMULATIONS OF VISCOUS ACCRETION FLOW AROUND BLACK HOLES IN A TWO-DIMENSIONAL CYLINDRICAL GEOMETRY

Energy Technology Data Exchange (ETDEWEB)

Lee, Seong-Jae; Hyung, Siek [School of Science Education (Astronomy), Chungbuk National University, Chungbuk 28644 (Korea, Republic of); Chattopadhyay, Indranil; Kumar, Rajiv [ARIES, Manora Peak, Nainital-263002, Uttarakhand (India); Ryu, Dongsu, E-mail: seong@chungbuk.ac.kr [Department of Physics, School of Natural Sciences UNIST, Ulsan 44919 (Korea, Republic of)

2016-11-01

We simulate shock-free and shocked viscous accretion flows onto a black hole in a two-dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian total variation diminishing plus remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. The inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any quasi-periodic oscillation (QPO)-like activity developed. The steady-state shocked solution in the inviscid as well as in the viscous regime matched theoretical predictions well. However, increasing viscosity renders the accretion shock unstable. Large-amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. This oscillation of the inner part of the disk is interpreted as the source of QPO in hard X-rays observed in micro-quasars. Strong shock oscillation induces strong episodic jet emission. The jets also show the existence of shocks, which are produced as one shell hits the preceding one. The periodicities of the jets and shock oscillation are similar; the jets for the higher viscosity parameter appear to be stronger and faster.

WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. II. RADIAL DEPENDENCE AND GLOBAL PICTURE

Energy Technology Data Exchange (ETDEWEB)

Bai Xuening, E-mail: xbai@cfa.harvard.edu [Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138 (United States)

2013-08-01

Non-ideal magnetohydrodynamical effects play a crucial role in determining the mechanism and efficiency of angular momentum transport as well as the level of turbulence in protoplanetary disks (PPDs), which are the key to understanding PPD evolution and planet formation. It was shown in our previous work that at 1 AU, the magnetorotational instability (MRI) is completely suppressed when both ohmic resistivity and ambipolar diffusion (AD) are taken into account, resulting in a laminar flow with accretion driven by magnetocentrifugal wind. In this work, we study the radial dependence of the laminar wind solution using local shearing-box simulations. The scaling relation on the angular momentum transport for the laminar wind is obtained, and we find that the wind-driven accretion rate can be approximated as M-dot approx. 0.91 x 10{sup -8}R{sub AU}{sup 1.21}(B{sub p}/10 mG){sup 0.93} M{sub Sun} yr{sup -1}, where B{sub p} is the strength of the large-scale poloidal magnetic field threading the disk. The result is independent of disk surface density. Four criteria are outlined for the existence of the laminar wind solution: (1) ohmic resistivity dominated the midplane region, (2) the AD-dominated disk upper layer, (3) the presence of a (not too weak) net vertical magnetic flux, and (4) sufficiently well-ionized gas beyond the disk surface. All these criteria are likely to be met in the inner region of the disk from {approx}0.3 AU to about 5-10 AU for typical PPD accretion rates. Beyond this radius, the angular momentum transport is likely to proceed due to a combination of the MRI and disk wind, and eventually completely dominated by the MRI (in the presence of strong AD) in the outer disk. Our simulation results provide key ingredients for a new paradigm on the accretion processes in PPDs.

Neutron star accretion and the neutrino fireball

International Nuclear Information System (INIS)

Colgate, S.A.; Herant, M.E.; Benz, W.

1991-01-01

The mixing necessary to explain the ''Fe'' line widths and possibly the observed red shifts of 1987A is explained in terms of large scale, entropy conserving, up and down flows (calculated with a smooth particle 2-D code) taking place between the neutron star and the explosion shock wave due to the gravity and neutrino deposition. Depending upon conditions of entropy and mass flux further accretion takes place in single events, similar to relaxation oscillator, fed by the downward flows of low entropy matter. The shock, in turn, is driven by the upflow of the buoyant high entropy bubbles. Some accretion events will reach a temperature high enough to create a neutrino ''fireball,'' a region hot enough, 11 Mev, so as to be partially opaque to its own (neutrino) radiation. The continuing neutrino deposition drives the explosion shock until the entropy of matter flowing downwards onto the neutron star is high enough to prevent further accretion. This process should result in a robust supernova explosion

Probing the accretion flow and emission-line regions of M81, the nearest broad-lined low-luminosity AGN

Science.gov (United States)

Barth, Aaron

2017-08-01

The nucleus of M81 is an object of singular importance as a template for low-luminosity accretion flows onto supermassive black holes. We propose to obtain a complete, small-aperture, high S/N STIS UV/optical spectrum of the M81 nucleus and multi-filter WFC3 imaging covering the UV through near-IR. Such data have never previously been obtained with HST; the only prior archival UV/optical spectra of M81 have low S/N, incomplete wavelength coverage, and are strongly contaminated by starlight. Combined with new Chandra X-ray data, our proposed observations will comprise the definitive reference dataset on the spectral energy distribution of this benchmark low-luminosity AGN. These data will provide unique new constraints on the possible contribution of a truncated thin accretion disk to the AGN emission spectrum, clarifying a fundamental property of low-luminosity accretion flows. The data will additionally provide new insights into broad-line region structure and black hole mass scaling relationships at the lowest AGN luminosities, and spatially resolved diagnostics of narrow-line region excitation conditions at unprecedented spatial resolution to assess the impact of the AGN on the ionization state of the gas in the host galaxy bulge.

MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

Directory of Open Access Journals (Sweden)

Lii Patrick

2014-01-01

Full Text Available Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

Modeling the response of a standard accretion disc to stochastic viscous fluctuations

Science.gov (United States)

Ahmad, Naveel; Misra, Ranjeev; Iqbal, Naseer; Maqbool, Bari; Hamid, Mubashir

2018-01-01

The observed variability of X-ray binaries over a wide range of time-scales can be understood in the framework of a stochastic propagation model, where viscous fluctuations at different radii induce accretion rate variability that propagate inwards to the X-ray producing region. The scenario successfully explains the power spectra, the linear rms-flux relation as well as the time-lag between different energy photons. The predictions of this model have been obtained using approximate analytical solutions or empirically motivated models which take into account the effect of these propagating variability on the radiative process of complex accretion flows. Here, we study the variation of the accretion rate due to such viscous fluctuations using a hydro-dynamical code for the standard geometrically thin, gas pressure dominated α-disc with a zero torque boundary condition. Our results confirm earlier findings that the time-lag between a perturbation and the resultant inner accretion rate variation depends on the frequency (or time-period) of the perturbation. Here we have quantified that the time-lag tlag ∝f-0.54 , for time-periods less than the viscous time-scale of the perturbation radius and is nearly constant otherwise. This, coupled with radiative process would produce the observed frequency dependent time-lag between different energy bands. We also confirm that if there are random Gaussian fluctuations of the α-parameter at different radii, the resultant inner accretion rate has a power spectrum which is a power-law.

The effect of coherent stirring on the advection?condensation of water vapour

OpenAIRE

Tsang, Yue-Kin; Vanneste, Jacques

2017-01-01

Atmospheric water vapour is an essential ingredient of weather and climate. Key features of its distribution can be represented by kinematic models which treat it as a passive scalar advected by a prescribed flow and reacting through condensation. Condensation acts as a sink that maintains specific humidity below a prescribed, space-dependent saturation value. In order to investigate how the interplay between large-scale advection, small-scale turbulence and condensation controls the moisture...

Some numerical studies of interface advection properties of level set ...

Indian Academy of Sciences (India)

explicit computational elements moving through an Eulerian grid. ... location. The interface is implicitly defined (captured) as the location of the discontinuity in the ... This level set function is advected with the background flow field and thus ...

Analysis Of First Fall And Last Spring Advection and Radiation-Advection Frosts In Azerbaijan Provinces

International Nuclear Information System (INIS)

Noohi, K.; Pedram, M.; Sahraian, F.; Kamali, G. A.

2007-01-01

Atmospheric Science and Meteorological Research Center (ASMERC)Dates of first fall and last spring frosts on the basis of minimum shelter temperature equal or less than 0°C were determined for 12 synoptic stations for period 1986-2000 in Azerbaijan region. The advection frost was determined based on using of synoptic maps and studying of meteorological elements in different hours. In this work, we found that series of first fall and last spring advection and radiation-advection frosts are random and normally distributed. This study shows that on the average advection frosts start from 6 to 40 days later than radiation-advection frosts in fall and ends 2 to 25 days earlier in spring. Potential growing season that is interval between last spring and first fall advection frost is found to be from 5 to 65 days longer than the growing season defined by the interval from last spring to first fall occurrences of minimum temperature equal or less than 0°C. Crop protection against radiation frosts can bring about too much benefit. To assess whether practical protection of some special crops against radiation frosts is done or not, the number of radiation frosts before first advection frost in fall and after last advection frost in spring, were determined

Testing General Relativity with Accretion-Flow Imaging of Sgr A^{*}.

Science.gov (United States)

Johannsen, Tim; Wang, Carlos; Broderick, Avery E; Doeleman, Sheperd S; Fish, Vincent L; Loeb, Abraham; Psaltis, Dimitrios

2016-08-26

The Event Horizon Telescope is a global, very long baseline interferometer capable of probing potential deviations from the Kerr metric, which is believed to provide the unique description of astrophysical black holes. Here, we report an updated constraint on the quadrupolar deviation of Sagittarius A^{*} within the context of a radiatively inefficient accretion flow model in a quasi-Kerr background. We also simulate near-future constraints obtainable by the forthcoming eight-station array and show that in this model already a one-day observation can measure the spin magnitude to within 0.005, the inclination to within 0.09°, the position angle to within 0.04°, and the quadrupolar deviation to within 0.005 at 3σ confidence. Thus, we are entering an era of high-precision strong gravity measurements.

The role of horizontal thermal advection in regulating wintertime mean and extreme temperatures over the central United States during the past and future

Science.gov (United States)

Wang, F.; Vavrus, S. J.

2017-12-01

Horizontal temperature advection plays an especially prominent role in affecting winter climate over continental interiors, where both climatological conditions and extreme weather are strongly regulated by transport of remote air masses. Central North America is one such region, and it experienced a major cold-air outbreak (CAO) a few years ago that some have related to amplified Arctic warming. Despite the known importance of dynamics in shaping the winter climate of this sector and the potential for climate change to modify heat transport, limited attention has been paid to the regional impact of thermal advection. Here, we use a reanalysis product and output from the Community Earth System Model's Large Ensemble to quantify the roles of zonal and meridional temperature advection over the central U. S. during winter, both in the late 20th and 21st centuries. We frame our findings as a "tug of war" between opposing influences of the two advection components and between these dynamical forcings vs. thermodynamic changes under greenhouse warming. For example, Arctic amplification leads to much warmer polar air masses, causing a moderation of cold-air advection into the central U. S., yet the model also simulates a wavier mean circulation and stronger northerly flow during CAOs, favoring lower regional temperatures. We also compare the predominant warming effect of zonal advection and overall cooling effect of meridional temperature advection as an additional tug of war. During both historical and future periods, zonal temperature advection is stronger than meridional advection over the Central U. S. The model simulates a future weakening of both zonal and meridional temperature advection, such that westerly flow provides less warming and northerly flow less cooling. On the most extreme warm days in the past and future, both zonal and meridional temperature advection have positive (warming) contributions. On the most extreme cold days, meridional cold air advection

Benthic solute exchange and carbon mineralization in two shallow subtidal sandy sediments: Effect of advective pore-water exchange

DEFF Research Database (Denmark)

Cook, Perran L. M.; Wenzhofer, Frank; Glud, Ronnie N.

2007-01-01

within the range measured in the chambers. The contribution of advection to solute exchange was highly variable and dependent on sediment topography. Advective processes also had a pronounced influence on the in situ distribution of O-2 within the sediment, with characteristic two-dimensional patterns...... of O-2 distribution across ripples, and also deep subsurface O-2 pools, being observed. Mineralization pathways were predominantly aerobic when benthic mineralization rates were low and advective pore-water flow high as a result of well-developed sediment topography. By contrast, mineralization...... proceeded predominantly through sulfate reduction when benthic mineralization rates were high and advective pore-water flow low as a result of poorly developed topography. Previous studies of benthic mineralization in shallow sandy sediments have generally ignored these dynamics and, hence, have overlooked...

The effect of coherent stirring on the advection-condensation of water vapour

Science.gov (United States)

Tsang, Yue-Kin; Vanneste, Jacques

2017-06-01

Atmospheric water vapour is an essential ingredient of weather and climate. The key features of its distribution can be represented by kinematic models which treat it as a passive scalar advected by a prescribed flow and reacting through condensation. Condensation acts as a sink that maintains specific humidity below a prescribed, space-dependent saturation value. To investigate how the interplay between large-scale advection, small-scale turbulence and condensation controls moisture distribution, we develop simple kinematic models which combine a single circulating flow with a Brownian-motion representation of turbulence. We first study the drying mechanism of a water-vapour anomaly released inside a vortex at an initial time. Next, we consider a cellular flow with a moisture source at a boundary. The statistically steady state attained shows features reminiscent of the Hadley cell such as boundary layers, a region of intense precipitation and a relative humidity minimum. Explicit results provide a detailed characterization of these features in the limit of strong flow.

Theories of magnetospheres around accreting compact objects

International Nuclear Information System (INIS)

Vasyliunas, V.M.

1979-01-01

A wide class of galactic X-ray sources are believed to be binary systems where mass is flowing from a normal star to a companion that is a compact object, such as a neutron star. The strong magnetic fields of the compact object create a magnetosphere around it. We review the theoretical models developed to describe the properties of magnetospheres in such accreting binary systems. The size of the magnetosphere can be estimated from pressure balance arguments and is found to be small compared to the over-all size of the accretion region but large compared object if the latter is a neutron star. In the early models the magnetosphere was assumed to have open funnels in the polar regions, through which accreting plasma could pour in. Later, magnetically closed models were developed, with plasma entry made possible by instabilities at the magnetosphere boundary. The theory of plasma flow inside the magnetosphere has been formulated in analogy to a stellar wind with reversed flow; a complicating factor is the instability of the Alfven critical point for inflow. In the case of accretion via a well-defined disk, new problems if magnetospheric structure appear, in particular the question to what extent and by what process the magnetic fields from the compact object can penetrate into the acretion disk. Since the X-ray emission is powered by the gravitational energy released in the accretion process, mass transfer into the magnetosphere is of fundamental importance; the various proposed mechanisms are critically examined. (orig.)

Probing the Accretion Geometry of Black Holes with X-Ray Polarization

Science.gov (United States)

Schnitman, Jeremy D.

2011-01-01

In the coming years, new space missions will be able to measure X-ray polarization at levels of 1% or better in the approx.1-10 keV energy band. In particular, X-ray polarization is an ideal tool for determining the nature of black hole (BH) accretion disks surrounded by hot coronae. Using a Monte Carlo radiation transport code in full general relativity, we calculate the spectra and polarization features of these BH systems. At low energies, the signal is dominated by the thermal flux coming directly from the optically thick disk. At higher energies, the thermal seed photons have been inverse-Compton scattered by the corona, often reflecting back off the disk before reaching the observer, giving a distinctive polarization signature. By measuring the degree and angle of this X-ray polarization, we can infer the BH inclination, the emission geometry of the accretion flow, and also determine the spin of the black hole.

Analytical solutions of accreting black holes immersed in a {Lambda}CDM model

Energy Technology Data Exchange (ETDEWEB)

Lima, J.A.S., E-mail: limajas@astro.iag.usp.b [Universidade de Sao Paulo - Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Rua do Matao, 1226, 05508-090 Cidade Universitaria, Sao Paulo - SP (Brazil); Guariento, Daniel C., E-mail: carrasco@fma.if.usp.b [Universidade de Sao Paulo - Instituto de Fisica, Rua do Matao, Travessa R, 187, 05508-090 Cidade Universitaria, Sao Paulo - SP (Brazil); Horvath, J.E., E-mail: foton@astro.iag.usp.b [Universidade de Sao Paulo - Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Rua do Matao, 1226, 05508-090 Cidade Universitaria, Sao Paulo - SP (Brazil)

2010-10-04

The evolution of the mass of a black hole embedded in a universe filled with dark energy and cold dark matter is calculated in a closed form within a test fluid model in a Schwarzschild metric, taking into account the cosmological evolution of both fluids. The result describes exactly how accretion asymptotically switches from the matter-dominated to the {Lambda}-dominated regime. For early epochs, the black hole mass increases due to dark matter accretion, and on later epochs the increase in mass stops as dark energy accretion takes over. Thus, the unphysical behaviour of previous analyses is improved in this simple exact model.

The Properties of Reconnection Current Sheets in GRMHD Simulations of Radiatively Inefficient Accretion Flows

Science.gov (United States)

Ball, David; Özel, Feryal; Psaltis, Dimitrios; Chan, Chi-Kwan; Sironi, Lorenzo

2018-02-01

Non-ideal magnetohydrodynamic (MHD) effects may play a significant role in determining the dynamics, thermal properties, and observational signatures of radiatively inefficient accretion flows onto black holes. In particular, particle acceleration during magnetic reconnection events may influence black hole spectra and flaring properties. We use representative general relativistic magnetohydrodynamic (GRMHD) simulations of black hole accretion flows to identify and explore the structures and properties of current sheets as potential sites of magnetic reconnection. In the case of standard and normal evolution (SANE) disks, we find that in the reconnection sites, the plasma beta ranges from 0.1 to 1000, the magnetization ranges from 10‑4 to 1, and the guide fields are weak compared with the reconnecting fields. In magnetically arrested (MAD) disks, we find typical values for plasma beta from 10‑2 to 103, magnetizations from 10‑3 to 10, and typically stronger guide fields, with strengths comparable to or greater than the reconnecting fields. These are critical parameters that govern the electron energy distribution resulting from magnetic reconnection and can be used in the context of plasma simulations to provide microphysics inputs to global simulations. We also find that ample magnetic energy is available in the reconnection regions to power the fluence of bright X-ray flares observed from the black hole in the center of the Milky Way.

Distinguishing advective and powered motion in self-propelled colloids

Science.gov (United States)

Byun, Young-Moo; Lammert, Paul E.; Hong, Yiying; Sen, Ayusman; Crespi, Vincent H.

2017-11-01

Self-powered motion in catalytic colloidal particles provides a compelling example of active matter, i.e. systems that engage in single-particle and collective behavior far from equilibrium. The long-time, long-distance behavior of such systems is of particular interest, since it connects their individual micro-scale behavior to macro-scale phenomena. In such analyses, it is important to distinguish motion due to subtle advective effects—which also has long time scales and length scales—from long-timescale phenomena that derive from intrinsically powered motion. Here, we develop a methodology to analyze the statistical properties of the translational and rotational motions of powered colloids to distinguish, for example, active chemotaxis from passive advection by bulk flow.

Wind accretion and formation of disk structures in symbiotic binary systems

Science.gov (United States)

de Val-Borro, M.; Karovska, M.; Sasselov, D. D.; Stone, J. M.

2015-05-01

We investigate gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion. We study the mass accretion and formation of an accretion disk around the secondary caused by the strong wind from the primary late-type component using global 2D and 3D hydrodynamic numerical simulations. In particular, the dependence of the mass accretion rate on the mass loss rate, wind temperature and orbital parameters of the system is considered. For a typical slow and massive wind from an evolved star the mass transfer through a focused wind results in rapid infall onto the secondary. A stream flow is created between the stars with accretion rates of a 2--10% percent of the mass loss from the primary. This mechanism could be an important method for explaining periodic modulations in the accretion rates for a broad range of interacting binary systems and fueling of a large population of X-ray binary systems. We test the plausibility of these accretion flows indicated by the simulations by comparing with observations of the symbiotic variable system CH Cyg.

ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU

International Nuclear Information System (INIS)

Tofflemire, Benjamin M.; Mathieu, Robert D.; Ardila, David R.; Akeson, Rachel L.; Ciardi, David R.; Johns-Krull, Christopher; Herczeg, Gregory J.; Quijano-Vodniza, Alberto

2017-01-01

The theory of binary star formation predicts that close binaries ( a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU

Energy Technology Data Exchange (ETDEWEB)

Tofflemire, Benjamin M.; Mathieu, Robert D. [Department of Astronomy, University of Wisconsin–Madison, 475 North Charter Street, Madison, WI 53706 (United States); Ardila, David R. [The Aerospace Corporation, M2-266, El Segundo, CA 90245 (United States); Akeson, Rachel L.; Ciardi, David R. [NASA Exoplanet Science Institute, IPAC/Caltech, Pasadena, CA 91125 (United States); Johns-Krull, Christopher [Department of Physics and Astronomy, Rice University, Houston, TX 77005 (United States); Herczeg, Gregory J. [The Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871 (China); Quijano-Vodniza, Alberto [University of Nariño Observatory, Pasto, Nariño (Colombia)

2017-01-20

The theory of binary star formation predicts that close binaries ( a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

Probability and Cumulative Density Function Methods for the Stochastic Advection-Reaction Equation

Energy Technology Data Exchange (ETDEWEB)

Barajas-Solano, David A.; Tartakovsky, Alexandre M.

2018-01-01

We present a cumulative density function (CDF) method for the probabilistic analysis of $d$-dimensional advection-dominated reactive transport in heterogeneous media. We employ a probabilistic approach in which epistemic uncertainty on the spatial heterogeneity of Darcy-scale transport coefficients is modeled in terms of random fields with given correlation structures. Our proposed CDF method employs a modified Large-Eddy-Diffusivity (LED) approach to close and localize the nonlocal equations governing the one-point PDF and CDF of the concentration field, resulting in a $(d + 1)$ dimensional PDE. Compared to the classsical LED localization, the proposed modified LED localization explicitly accounts for the mean-field advective dynamics over the phase space of the PDF and CDF. To illustrate the accuracy of the proposed closure, we apply our CDF method to one-dimensional single-species reactive transport with uncertain, heterogeneous advection velocities and reaction rates modeled as random fields.

Dynamical and Radiative Properties of X-Ray Pulsar Accretion Columns: Phase-averaged Spectra

Energy Technology Data Exchange (ETDEWEB)

West, Brent F. [Department of Electrical and Computer Engineering, United States Naval Academy, Annapolis, MD (United States); Wolfram, Kenneth D. [Naval Research Laboratory (retired), Washington, DC (United States); Becker, Peter A., E-mail: bwest@usna.edu, E-mail: kswolfram@gmail.com, E-mail: pbecker@gmu.edu [Department of Physics and Astronomy, George Mason University, Fairfax, VA (United States)

2017-02-01

The availability of the unprecedented spectral resolution provided by modern X-ray observatories is opening up new areas for study involving the coupled formation of the continuum emission and the cyclotron absorption features in accretion-powered X-ray pulsar spectra. Previous research focusing on the dynamics and the associated formation of the observed spectra has largely been confined to the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface, while the dynamical effect of gas pressure is ignored. In a companion paper, we have presented a detailed analysis of the hydrodynamic and thermodynamic structure of the accretion column obtained using a new self-consistent model that includes the effects of both gas and radiation pressures. In this paper, we explore the formation of the associated X-ray spectra using a rigorous photon transport equation that is consistent with the hydrodynamic and thermodynamic structure of the column. We use the new model to obtain phase-averaged spectra and partially occulted spectra for Her X-1, Cen X-3, and LMC X-4. We also use the new model to constrain the emission geometry, and compare the resulting parameters with those obtained using previously published models. Our model sheds new light on the structure of the column, the relationship between the ionized gas and the photons, the competition between diffusive and advective transport, and the magnitude of the energy-averaged cyclotron scattering cross-section.

WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

International Nuclear Information System (INIS)

Perets, Hagai B.; Kenyon, Scott J.

2013-01-01

Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10 –5 -10 –3 M ☉ , with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M ☉ . When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

Flow Patterns in an Open Channel Confluence with Increasingly Dominant Tributary Inflow

Directory of Open Access Journals (Sweden)

Laurent Schindfessel

2015-08-01

Full Text Available Despite the ratio of incoming discharges being recognized as a key parameter in open-channel confluence hydrodynamics, little is known about the flow patterns when the tributary provides more than 90% of the total discharge. This paper offers a systematic study of flow features when the tributary becomes increasingly dominant in a 90° confluence with a fixed concordant bed. Large-eddy simulations are used to investigate the three-dimensional complex flow patterns for three different discharge ratios. It is found that the tributary flow impinges on the opposing bank when the tributary flow becomes sufficiently dominant, causing a recirculating eddy in the upstream channel of the confluence, which induces significant changes in the incoming velocity distribution. Moreover, it results in stronger helicoidal cells in the downstream channel, along with zones of upwelling flow. In turn, the changed flow patterns also influence the mixing layer and the flow recovery. Finally, intermittent events of stronger upwelling flow are discerned. Improved understanding of flow patterns at confluences where the tributary is dominant is applicable to both engineering and earth sciences.

Accretion onto a charged higher-dimensional black hole

International Nuclear Information System (INIS)

Sharif, M.; Iftikhar, Sehrish

2016-01-01

This paper deals with the steady-state polytropic fluid accretion onto a higher-dimensional Reissner-Nordstroem black hole. We formulate the generalized mass flux conservation equation, energy flux conservation and relativistic Bernoulli equation to discuss the accretion process. The critical accretion is investigated by finding the critical radius, the critical sound velocity, and the critical flow velocity. We also explore gas compression and temperature profiles to analyze the asymptotic behavior. It is found that the results for the Schwarzschild black hole are recovered when q = 0 in four dimensions. We conclude that the accretion process in higher dimensions becomes slower in the presence of charge. (orig.)

Accretion onto a charged higher-dimensional black hole

Energy Technology Data Exchange (ETDEWEB)

Sharif, M.; Iftikhar, Sehrish [University of the Punjab, Department of Mathematics, Lahore (Pakistan)

2016-03-15

This paper deals with the steady-state polytropic fluid accretion onto a higher-dimensional Reissner-Nordstroem black hole. We formulate the generalized mass flux conservation equation, energy flux conservation and relativistic Bernoulli equation to discuss the accretion process. The critical accretion is investigated by finding the critical radius, the critical sound velocity, and the critical flow velocity. We also explore gas compression and temperature profiles to analyze the asymptotic behavior. It is found that the results for the Schwarzschild black hole are recovered when q = 0 in four dimensions. We conclude that the accretion process in higher dimensions becomes slower in the presence of charge. (orig.)

Topography significantly influencing low flows in snow-dominated watersheds

Science.gov (United States)

Li, Qiang; Wei, Xiaohua; Yang, Xin; Giles-Hansen, Krysta; Zhang, Mingfang; Liu, Wenfei

2018-03-01

Watershed topography plays an important role in determining the spatial heterogeneity of ecological, geomorphological, and hydrological processes. Few studies have quantified the role of topography in various flow variables. In this study, 28 watersheds with snow-dominated hydrological regimes were selected with daily flow records from 1989 to 1996. These watersheds are located in the Southern Interior of British Columbia, Canada, and range in size from 2.6 to 1780 km2. For each watershed, 22 topographic indices (TIs) were derived, including those commonly used in hydrology and other environmental fields. Flow variables include annual mean flow (Qmean), Q10 %, Q25 %, Q50 %, Q75 %, Q90 %, and annual minimum flow (Qmin), where Qx % is defined as the daily flow that occurred each year at a given percentage (x). Factor analysis (FA) was first adopted to exclude some redundant or repetitive TIs. Then, multiple linear regression models were employed to quantify the relative contributions of TIs to each flow variable in each year. Our results show that topography plays a more important role in low flows (flow magnitudes ≤ Q75 %) than high flows. However, the effects of TIs on different flow magnitudes are not consistent. Our analysis also determined five significant TIs: perimeter, slope length factor, surface area, openness, and terrain characterization index. These can be used to compare watersheds when low flow assessments are conducted, specifically in snow-dominated regions with the watershed size less than several thousand square kilometres.

Stability of black hole accretion disks

Directory of Open Access Journals (Sweden)

Czerny B.

2012-12-01

Full Text Available We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.

EVOLUTION OF MASSIVE PROTOSTARS VIA DISK ACCRETION

International Nuclear Information System (INIS)

Hosokawa, Takashi; Omukai, Kazuyuki; Yorke, Harold W.

2010-01-01

Mass accretion onto (proto-)stars at high accretion rates M-dot * > 10 -4 M sun yr -1 is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper, we examine the evolution via disk accretion. We consider a limiting case of 'cold' disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10 -3 M sun yr -1 , the radius of a protostar is initially small, R * ≅ a few R sun . After several solar masses have accreted, the protostar begins to bloat up and for M * ≅ 10 M sun the stellar radius attains its maximum of 30-400 R sun . The large radius ∼100 R sun is also a feature of spherically symmetric accretion at the same accreted mass and accretion rate. Hence, expansion to a large radius is a robust feature of accreting massive protostars. At later times, the protostar eventually begins to contract and reaches the zero-age main sequence (ZAMS) for M * ≅ 30 M sun , independent of the accretion geometry. For accretion rates exceeding several 10 -3 M sun yr -1 , the protostar never contracts to the ZAMS. The very large radius of several hundreds R sun results in the low effective temperature and low UV luminosity of the protostar. Such bloated protostars could well explain the existence of bright high-mass protostellar objects, which lack detectable H II regions.

A Global Three-Dimensional Radiation Hydrodynamic Simulation of a Self-Gravitating Accretion Disk

Science.gov (United States)

Phillipson, Rebecca; Vogeley, Michael S.; McMillan, Stephen; Boyd, Patricia

2018-01-01

We present three-dimensional, radiation hydrodynamic simulations of initially thin accretion disks with self-gravity using the grid-based code PLUTO. We produce simulated light curves and spectral energy distributions and compare to observational data of X-ray binary (XRB) and active galactic nuclei (AGN) variability. These simulations are of interest for modeling the role of radiation in accretion physics across decades of mass and frequency. In particular, the characteristics of the time variability in various bandwidths can probe the timescales over which different physical processes dominate the accretion flow. For example, in the case of some XRBs, superorbital periods much longer than the companion orbital period have been observed. Smoothed particle hydrodynamics (SPH) calculations have shown that irradiation-driven warping could be the mechanism underlying these long periods. In the case of AGN, irradiation-driven warping is also predicted to occur in addition to strong outflows originating from thermal and radiation pressure driving forces, which are important processes in understanding feedback and star formation in active galaxies. We compare our simulations to various toy models via traditional time series analysis of our synthetic and observed light curves.

Relativistic jets from accreting black holes

International Nuclear Information System (INIS)

Coriat, Mickael

2010-01-01

Matter ejection processes, more commonly called jets, are among the most ubiquitous phenomena of the universe at ail scales of size and energy and are inseparable from accretion process. This intimate link, still poorly understood, is the main focus of this thesis. Through multi-wavelength observations of X-ray binary Systems hosting a black hole, I will try to bring new constraints on the physics of relativistic jets and the accretion - ejection coupling. We strive first to compare the simultaneous infrared, optical and X-ray emissions of the binary GX 339-4 over a period of five years. We study the nature of the central accretion flow, one of the least understood emission components of X-ray binaries, both in its geometry and in term of the physical processes that take place. This component is fundamental since it is could be the jets launching area or be highly connected to it. Then we focus on the infrared emission of the jets to investigate the physical conditions close to the jets base. We finally study the influence of irradiation of the outer accretion disc by the central X-ray source. Then, we present the results of a long-term radio and X-ray study of the micro-quasar H1743- 322. This System belongs to a population of accreting black holes that display, for a given X-ray luminosity, a radio emission fainter than expected. We make several assumptions about the physical origin of this phenomenon and show in particular that these sources could have a radiatively efficient central accretion flow. We finally explore the phases of return to the hard state of GX 339-4. We follow the re-emergence of the compact jets emission and try to bring new constraints on the physics of jet formation. (author) [fr

Inefficient Angular Momentum Transport in Accretion Disk Boundary Layers: Angular Momentum Belt in the Boundary Layer

Science.gov (United States)

Belyaev, Mikhail A.; Quataert, Eliot

2018-04-01

We present unstratified 3D MHD simulations of an accretion disk with a boundary layer (BL) that have a duration ˜1000 orbital periods at the inner radius of the accretion disk. We find the surprising result that angular momentum piles up in the boundary layer, which results in a rapidly rotating belt of accreted material at the surface of the star. The angular momentum stored in this belt increases monotonically in time, which implies that angular momentum transport mechanisms in the BL are inefficient and do not couple the accretion disk to the star. This is in spite of the fact that magnetic fields are advected into the BL from the disk and supersonic shear instabilities in the BL excite acoustic waves. In our simulations, these waves only carry a small fraction (˜10%) of the angular momentum required for steady state accretion. Using analytical theory and 2D viscous simulations in the R - ϕ plane, we derive an analytical criterion for belt formation to occur in the BL in terms of the ratio of the viscosity in the accretion disk to the viscosity in the BL. Our MHD simulations have a dimensionless viscosity (α) in the BL that is at least a factor of ˜100 smaller than that in the disk. We discuss the implications of these results for BL dynamics and emission.

Analogue Hawking radiation from astrophysical black-hole accretion

International Nuclear Information System (INIS)

Das, Tapas K

2004-01-01

We show that spherical accretion onto astrophysical black holes can be considered as a natural example of an analogue system. We provide, for the first time, an exact analytical scheme for calculating the analogue Hawking temperature and surface gravity for general relativistic accretion onto astrophysical black holes. Our calculation may bridge the gap between the theory of transonic astrophysical accretion and the theory of analogue Hawking radiation. We show that the domination of the analogue Hawking temperature over the actual Hawking temperature may be a real astrophysical phenomenon, though observational tests of this fact will at best be difficult and at worst might prove to be impossible. We also discuss the possibilities of the emergence of analogue white holes around astrophysical black holes. Our calculation is general enough to accommodate accreting black holes with any mass

Circum-Pacific accretion of oceanic terranes to continental blocks: accretion of the Early Permian Dun Mountain ophiolite to the E Gondwana continental margin, South Island, New Zealand

Science.gov (United States)

Robertson, Alastair

2016-04-01

also resulted in the formation of the overlying Maitai continental margin fore-arc basin (possibly related to rollback or a decrease in dip of the remaining subduction zone).Very coarse clastic material (up to ca. 700 m thick) including detached blocks of basaltic and gabbroic rocks, up to tens or metres in size (or more), was shed down fault scarps from relatively shallow water into a deeper water setting by gravity flow processes, ranging from rock fall, to debris flow, to turbidity currents. In addition, relatively fine-grained volcaniclastic-terrigenous sediment was input from an E Gondwana continental margin arc in the form of distal gravity flows, as indicated by geochemical data (e.g. Rare Earth Element analysis of sandstones and shales). The lowest part of the overlying Maitai fore-arc sequence in some areas is represented by hundreds of metres-thick sequences of mixed carbonate-volcaniclastic-terrigenous gravity flows (Wooded Peak Fm.), which are interpreted to have been derived from the E Gondwana continental margin and which finally accumulated in fault-controlled depocentres. Input of shallow-water carbonate material later waned and the Late Permian-Triassic Maitai fore-arc basin was dominated by gravity flows that were largely derived from a contemporaneous continental margin arc (partially preserved in present SE Australia). Subsequent tectonic deformation included on-going subduction, strike-slip and terrane accretion. The sedimentary covers of comparable accreted ophiolites elsewhere (e.g. Coast Range ophiolite, California) may reveal complementary evidence of fundamental terrane accretion processes. Acknowledgements: Hamish Campbell, Dave Craw, Mike Johnson, Chuck Landis, Nick Mortimer, Dhana Pillai and other members of the South Island geological research community

Numerical study of nonspherical black hole accretion

International Nuclear Information System (INIS)

Hawley, J.F.

1984-01-01

This thesis describes in detail a two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. The aim is to study fully dynamic inviscid fluid accretion onto black holes, as well as to study the evolution and development of nonlinear instabilities in pressure supported accretion disks. In order to fully calibrate and document the code, certain analytic solutions for shock tubes and special accretion flows are derived; these solutions form the basis for code testing. The numerical techniques used are developed and discussed. A variety of alternate differencing schemes are compared on an analytic test bed. Some discussion is devoted to general issues in finite differencing. The working code is calibrated using analytically solvable accretion problems, including the radial accretion of dust and of fluid with pressure (Bondi accretion). Two dimensional test problems include the spiraling infall of low angular momentum fluid, the formation of a pressure supported torus, and the stable evolution of a torus. A series of numerical models are discussed and illustrated with selected plots

MIGRATION OF EXTRASOLAR PLANETS: EFFECTS FROM X-WIND ACCRETION DISKS

International Nuclear Information System (INIS)

Adams, Fred C.; Cai, Mike J.; Lizano, Susana

2009-01-01

Magnetic fields are dragged in from the interstellar medium during the gravitational collapse that forms star/disk systems. Consideration of mean field magnetohydrodynamics in these disks shows that magnetic effects produce sub-Keplerian rotation curves and truncate the inner disk. This Letter explores the ramifications of these predicted disk properties for the migration of extrasolar planets. Sub-Keplerian flow in gaseous disks drives a new migration mechanism for embedded planets and modifies the gap-opening processes for larger planets. This sub-Keplerian migration mechanism dominates over Type I migration for sufficiently small planets (m P ∼ + ) and/or close orbits (r ∼< 1 AU). Although the inclusion of sub-Keplerian torques shortens the total migration time by only a moderate amount, the mass accreted by migrating planetary cores is significantly reduced. Truncation of the inner disk edge (for typical system parameters) naturally explains final planetary orbits with periods P ∼ 4 days. Planets with shorter periods, P ∼ 2 days, can be explained by migration during FU-Orionis outbursts, when the mass accretion rate is high and the disk edge moves inward. Finally, the midplane density is greatly increased at the inner truncation point of the disk (the X-point); this enhancement, in conjunction with continuing flow of gas and solids through the region, supports the in situ formation of giant planets.

Conservative and bounded volume-of-fluid advection on unstructured grids

Science.gov (United States)

Ivey, Christopher B.; Moin, Parviz

2017-12-01

This paper presents a novel Eulerian-Lagrangian piecewise-linear interface calculation (PLIC) volume-of-fluid (VOF) advection method, which is three-dimensional, unsplit, and discretely conservative and bounded. The approach is developed with reference to a collocated node-based finite-volume two-phase flow solver that utilizes the median-dual mesh constructed from non-convex polyhedra. The proposed advection algorithm satisfies conservation and boundedness of the liquid volume fraction irrespective of the underlying flux polyhedron geometry, which differs from contemporary unsplit VOF schemes that prescribe topologically complicated flux polyhedron geometries in efforts to satisfy conservation. Instead of prescribing complicated flux-polyhedron geometries, which are prone to topological failures, our VOF advection scheme, the non-intersecting flux polyhedron advection (NIFPA) method, builds the flux polyhedron iteratively such that its intersection with neighboring flux polyhedra, and any other unavailable volume, is empty and its total volume matches the calculated flux volume. During each iteration, a candidate nominal flux polyhedron is extruded using an iteration dependent scalar. The candidate is subsequently intersected with the volume guaranteed available to it at the time of the flux calculation to generate the candidate flux polyhedron. The difference in the volume of the candidate flux polyhedron and the actual flux volume is used to calculate extrusion during the next iteration. The choice in nominal flux polyhedron impacts the cost and accuracy of the scheme; however, it does not impact the methods underlying conservation and boundedness. As such, various robust nominal flux polyhedron are proposed and tested using canonical periodic kinematic test cases: Zalesak's disk and two- and three-dimensional deformation. The tests are conducted on the median duals of a quadrilateral and triangular primal mesh, in two-dimensions, and on the median duals of a

WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

Energy Technology Data Exchange (ETDEWEB)

Perets, Hagai B. [Technion-Israel Institute of Technology, Haifa (Israel); Kenyon, Scott J., E-mail: hperets@physics.technion.ac.il [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

2013-02-20

Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10{sup -5}-10{sup -3} M {sub Sun }, with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M {sub Sun }. When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

Compact binary merger and kilonova: outflows from remnant disc

Science.gov (United States)

Yi, Tuan; Gu, Wei-Min; Liu, Tong; Kumar, Rajiv; Mu, Hui-Jun; Song, Cui-Ying

2018-05-01

Outflows launched from a remnant disc of compact binary merger may have essential contribution to the kilonova emission. Numerical calculations are conducted in this work to study the structure of accretion flows and outflows. By the incorporation of limited-energy advection in the hyper-accretion discs, outflows occur naturally from accretion flows due to imbalance between the viscous heating and the sum of the advective and radiative cooling. Following this spirit, we revisit the properties of the merger outflow ejecta. Our results show that around 10-3 ˜ 10-1 M⊙ of the disc mass can be launched as powerful outflows. The amount of unbound mass varies with the disc mass and the viscosity. The outflow-contributed peak luminosity is around 1040 ˜ 1041 erg s-1. Such a scenario can account for the observed kilonovae associated with short gamma-ray bursts, including the recent event AT2017gfo (GW170817).

THE EXTREME ULTRAVIOLET DEFICIT AND MAGNETICALLY ARRESTED ACCRETION IN RADIO-LOUD QUASARS

Energy Technology Data Exchange (ETDEWEB)

Punsly, Brian, E-mail: brian.punsly1@verizon.net [1415 Granvia Altamira, Palos Verdes Estates, CA 90274 (United States); ICRANet, Piazza della Repubblica, I-65100 10 Pescara (Italy)

2014-12-20

The Hubble Space Telescope composite quasar spectra presented in Telfer et al. show a significant deficit of emission in the extreme ultraviolet for the radio-loud component of the quasar population (RLQs) compared to the radio-quiet component of the quasar population. The composite quasar continuum emission between 1100 Å and ∼580 Å is generally considered to be associated with the innermost regions of the accretion flow onto the central black hole. The deficit between 1100 Å and 580 Å in RLQs has a straightforward interpretation as a missing or a suppressed innermost region of local energy dissipation in the accretion flow. It is proposed that this can be the result of islands of large-scale magnetic flux in RLQs that are located close to the central black hole that remove energy from the accretion flow as Poynting flux (sometimes called magnetically arrested accretion). These magnetic islands are natural sites for launching relativistic jets. Based on the Telfer et al. data and the numerical simulations of accretion flows in Penna et al., the magnetic islands are concentrated between the event horizon and an outer boundary of <2.8 M (in geometrized units) for rapidly rotating black holes and <5.5 M for modestly rotating black holes.

Thermal structure of the accreting earth

International Nuclear Information System (INIS)

Turcotte, D.L.; Pflugrath, J.C.

1985-01-01

The energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth. In order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms. In this paper we postulate the existence of a global magma ocean and carry out calculations of the heat flux through it in order to determine its depth. In the solid mantle heat is transferred by the upward migration of magma. This magma supplies the magma ocean. The increase in the mantle liquidus with depth (pressure) is the dominant effect influencing heat transfer through the magma ocean. We find that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted. We conclude that the core segregated and an atmosphere was formed during accretion

Comparing CO2 Storage and Advection Conditions at Night at Different Carboeuroflux Sites

Science.gov (United States)

Aubinet, M.; Berbigier, P.; Bernhofer, Ch.; et al.

Anemometer and CO2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While storage was observed to be significant only during periods of both low turbulence and low advection, the advective fluxes strongly influence the nocturnal CO2 balance, with the exception of almost flat and highly homogeneous sites. On the basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’, which takes net radiation and local topography into account, was introduced as a criterion to characterise the conditions of storage enrichment/depletion. Comparative analyses of the six sites showed several common features of the advective fluxes but also some substantial differences. In particular, all sites where advection occurs show the onset of a boundary layer characterised by a downslope flow, negative vertical velocities and negative vertical CO2 concentration gradients during nighttime. As a consequence, vertical advection was observed to be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. The main differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO2 concentration gradients, which is probably determined by the source/sink distribution. As a consequence, both positive and negative horizontal advective fluxes (corresponding respectively to CO2 removal from the ecosystem and to CO2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal

Stratified Simulations of Collisionless Accretion Disks

Energy Technology Data Exchange (ETDEWEB)

Hirabayashi, Kota; Hoshino, Masahiro, E-mail: hirabayashi-k@eps.s.u-tokyo.ac.jp [Department of Earth and Planetary Science, The University of Tokyo, Tokyo, 113-0033 (Japan)

2017-06-10

This paper presents a series of stratified-shearing-box simulations of collisionless accretion disks in the recently developed framework of kinetic magnetohydrodynamics (MHD), which can handle finite non-gyrotropy of a pressure tensor. Although a fully kinetic simulation predicted a more efficient angular-momentum transport in collisionless disks than in the standard MHD regime, the enhanced transport has not been observed in past kinetic-MHD approaches to gyrotropic pressure anisotropy. For the purpose of investigating this missing link between the fully kinetic and MHD treatments, this paper explores the role of non-gyrotropic pressure and makes the first attempt to incorporate certain collisionless effects into disk-scale, stratified disk simulations. When the timescale of gyrotropization was longer than, or comparable to, the disk-rotation frequency of the orbit, we found that the finite non-gyrotropy selectively remaining in the vicinity of current sheets contributes to suppressing magnetic reconnection in the shearing-box system. This leads to increases both in the saturated amplitude of the MHD turbulence driven by magnetorotational instabilities and in the resultant efficiency of angular-momentum transport. Our results seem to favor the fast advection of magnetic fields toward the rotation axis of a central object, which is required to launch an ultra-relativistic jet from a black hole accretion system in, for example, a magnetically arrested disk state.

A MECHANISM FOR HYSTERESIS IN BLACK HOLE BINARY STATE TRANSITIONS

International Nuclear Information System (INIS)

Begelman, Mitchell C.; Armitage, Philip J.

2014-01-01

We suggest that the hysteretic cycle of black hole state transitions arises from two established properties of accretion disks: the increase in turbulent stress in disks threaded by a net magnetic field and the ability of thick (but not thin) disks to advect such a field radially. During quiescence, magnetic field loops are generated by the magnetorotational instability at the interface between the inner hot flow and outer thin disk. Vertical flux is advected into and accumulates stochastically within the inner flow, where it stimulates the turbulence so that α ∼ 1. The transition to a geometrically thin inner disk occurs when L ∼ α 2 L Edd ∼ L Edd , and the first ''thin'' disk to form is itself moderately thick, strongly magnetized, and able to advect field inward. These properties favor episodic jet production. As the accretion rate declines magnetic flux escapes, α decreases to α ∼ 0.01-0.1, and a hot inner flow is not re-established until L << L Edd . We discuss possible observational consequences of our scenario

ON THE ROLE AND ORIGIN OF NONTHERMAL ELECTRONS IN HOT ACCRETION FLOWS

Energy Technology Data Exchange (ETDEWEB)

Niedźwiecki, Andrzej; Stȩpnik, Agnieszka [Department of Astrophysics, University of Łódź, Pomorska 149/153, 90-236 Łódź (Poland); Xie, Fu-Guo, E-mail: niedzwiecki@uni.lodz.pl, E-mail: agajer@o2.pl, E-mail: fgxie@shao.ac.cn [Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030 (China)

2015-02-01

We study the X-ray spectra of tenuous, two-temperature accretion flows using a model involving an exact, Monte Carlo computation of the global Comptonization effect as well as a general relativistic description of both the flow structure and radiative processes. In our previous work, we found that in flows surrounding supermassive black holes, thermal synchrotron radiation is not capable of providing a sufficient seed photon flux to explain the X-ray spectral indices as well as the cut-off energies measured in several best-studied active galactic nuclei (AGNs). In this work, we complete the model by including seed photons provided by nonthermal synchrotron radiation and we find that it allows us to reconcile the hot flow model with the AGN data. We take into account two possible sources of nonthermal electrons. First, we consider e {sup ±} produced by charged-pion decay, which should always be present in the innermost part of a two-temperature flow due to proton-proton interactions. We find that for a weak heating of thermal electrons (small δ) the synchrotron emission of pion-decay e {sup ±} is much stronger than the thermal synchrotron emission in the considered range of bolometric luminosities, L ∼ (10{sup –4}-10{sup –2}) L {sub Edd}. The small-δ model including hadronic effects, in general, agrees with the AGN data, except for the case of a slowly rotating black hole and a thermal distribution of protons. For large δ, the pion-decay e {sup ±} have a negligible effect and, in this model, we consider nonthermal electrons produced by direct acceleration. We find an approximate agreement with the AGN data for the fraction of the heating power of electrons, which is used for the nonthermal acceleration η ∼ 0.1. However, for constant η and δ, the model predicts a positive correlation of the X-ray spectral index with the Eddington ratio, and hence a fine tuning of η and/or δ with the accretion rate is required to explain the negative correlation

MODERATE-LUMINOSITY GROWING BLACK HOLES FROM 1.25 < z < 2.7: VARIED ACCRETION IN DISK-DOMINATED HOSTS

Energy Technology Data Exchange (ETDEWEB)

Simmons, B. D.; Glikman, E. [Astronomy Department, Yale University, New Haven, CT 06511 (United States); Urry, C. M.; Schawinski, K. [Yale Center for Astronomy and Astrophysics, Physics Department, Yale University, New Haven, CT 06511 (United States); Cardamone, C., E-mail: brooke.simmons@astro.ox.ac.uk [Sheridan Center for Teaching and Learning, Brown University, 96 Waterman St., Providence RI 02912 (United States)

2012-12-10

We compute black hole masses and bolometric luminosities for 57 active galactic nuclei (AGNs) in the redshift range 1.25 {<=} z {<=} 2.67, selected from the GOODS-South deep multi-wavelength survey field via their X-ray emission. We determine host galaxy morphological parameters by separating the galaxies from their central point sources in deep Hubble Space Telescope images, and host stellar masses and colors by multi-wavelength spectral energy distribution fitting. Of GOODS AGNs at these redshifts, 90% have detected rest-frame optical nuclear point sources; bolometric luminosities range from 2 Multiplication-Sign 10{sup 43} to 2 Multiplication-Sign 10{sup 46} erg s{sup -1}. The black holes are growing at a range of accretion rates, with {approx}> 50% of the sample having L/L{sub Edd} < 0.1. Of the host galaxies, 70% have stellar masses M{sub *} > 10{sup 10} M{sub Sun }, with a range of colors suggesting a complex star formation history. We find no evolution of AGN bolometric luminosity within the sample, and no correlation between AGN bolometric luminosity and host stellar mass, color, or morphology. Fully half the sample of host galaxies are disk-dominated, with another 25% having strong disk components. Fewer than 15% of the systems appear to be at some stage of a major merger. These moderate-luminosity AGN hosts are therefore inconsistent with a dynamical history dominated by mergers strong enough to destroy disks, indicating that minor mergers or secular processes dominate the coevolution of galaxies and their central black holes at z {approx} 2.

Standing non-dissipative shocks in black hole accretion and winds

International Nuclear Information System (INIS)

Chakrabarti, S.K.

1988-07-01

We present all non-dissipative shock solutions for stationary, axially symmetric and rotating adiabatic flows of small transverse thickness in black hole potential. We show that for a given initial and final states of the flow, there can be as many as four formal shock locations in both the accretion and the winds. Only two (three) of these locations are acceptable for accretion onto black holes (neutron stars) and three of these locations are acceptable for winds. We prove that the shock strength and the temperature jump have a lower limit which does not depend upon the parameters of the flow or the force field in which the flow moves and is only a function of the adiabatic index of flow. (author). 14 refs, 12 figs

Magnetic flux and heat losses by diffusive, advective, and Nernst effects in MagLIF-like plasma

International Nuclear Information System (INIS)

Velikovich, A. L.; Giuliani, J. L.; Zalesak, S. T.

2014-01-01

The MagLIF approach to inertial confinement fusion involves subsonic/isobaric compression and heating of a DT plasma with frozen-in magnetic flux by a heavy cylindrical liner. The losses of heat and magnetic flux from the plasma to the liner are thereby determined by plasma advection and gradient-driven transport processes, such as thermal conductivity, magnetic field diffusion and thermomagnetic effects. Theoretical analysis based on obtaining exact self-similar solutions of the classical collisional Braginskii's plasma transport equations in one dimension demonstrates that the heat loss from the hot plasma to the cold liner is dominated by the transverse heat conduction and advection, and the corresponding loss of magnetic flux is dominated by advection and the Nernst effect. For a large electron Hall parameter ω e τ e effective diffusion coefficients determining the losses of heat and magnetic flux are both shown to decrease with ω e τ e as does the Bohm diffusion coefficient, which is commonly associated with low collisionality and two-dimensional transport. This family of exact solutions can be used for verification of codes that model the MagLIF plasma dynamics

Spectral and Timing Diagnostics of Accretion in XRBs

Science.gov (United States)

Nowak, M. N.

One of the truly great advantages of the Rossi X-ray Timing Explorer has been its flexible scheduling coupled with the presence of the All Sky Monitor. This has allowed mutliple observations of given objects over a wide range of luminosities that, thanks to the ASM, can be placed within the context of the overall behavior of the source. This has begun to allow us to develop theories of how the accretion flow in black hole candidates changes as a function of state and accretion rate. A number of spectral and temporal correlations have been seen, others have merely been suggested as being probably or possible. In this talk I will review some of these suggestions, and outline those correlations that I think are firm and contrast them to those that I believe are still very speculative. I will discuss these observations in the context of suggested models for the structure, size scale, and dynamics of the accretion flow.

Symbiotic stars - a binary model with super-critical accretion

Energy Technology Data Exchange (ETDEWEB)

Bath, G T [National Radio Astronomy Observatory, Charlottesville, Va. (USA)

1977-01-01

The structure of symbiotic variables is discussed in terms of a binary model. Disc accretion by a main sequence star or white dwarf at rates close to the Eddington limit produces an ultraviolet continuum source near the accreting star surface. This generates a variable, radiatively-driven, out-flowing wind. The wind is optically thick and the disc luminosity is absorbed and scattered and thus degraded into the optical region. Variations in the rate of mass loss in the wind lead to optical eruptions through shifts in the position of, and conditions in, the last scattering surface. The behaviour of Z And determined by Boyarchuk is shown to be in agreement with such a model. The conditions in the out-flowing wind are discussed. Limits on the mass loss rate are derived from conditions at the surface of the accreting star. It is suggested that variable out-flow in the wind is generated by fluctuations in disc luminosity produced by changes in the giant companions rate of mass transfer. The relation between symbiotic variables and classical and dwarf novae is discussed.

Double Relics in the Outskirts of A3376: Accretion Flows Meet ...

Indian Academy of Sciences (India)

scale structures form first and these merge to form large structures such as galaxy clus- ters. In the process of structure formation, the gravity-driven supersonic flows of intergalactic matter on to dark matter-dominated collapsing structures such as ...

Advective isotope transport by mixing cell and particle tracking algorithms

International Nuclear Information System (INIS)

Tezcan, L.; Meric, T.

1999-01-01

The 'mixing cell' algorithm of the environmental isotope data evaluation is integrated with the three dimensional finite difference ground water flow model (MODFLOW) to simulate the advective isotope transport and the approach is compared with the 'particle tracking' algorithm of the MOC3D, that simulates three-dimensional solute transport with the method of characteristics technique

Two-level schemes for the advection equation

Science.gov (United States)

Vabishchevich, Petr N.

2018-06-01

The advection equation is the basis for mathematical models of continuum mechanics. In the approximate solution of nonstationary problems it is necessary to inherit main properties of the conservatism and monotonicity of the solution. In this paper, the advection equation is written in the symmetric form, where the advection operator is the half-sum of advection operators in conservative (divergent) and non-conservative (characteristic) forms. The advection operator is skew-symmetric. Standard finite element approximations in space are used. The standard explicit two-level scheme for the advection equation is absolutely unstable. New conditionally stable regularized schemes are constructed, on the basis of the general theory of stability (well-posedness) of operator-difference schemes, the stability conditions of the explicit Lax-Wendroff scheme are established. Unconditionally stable and conservative schemes are implicit schemes of the second (Crank-Nicolson scheme) and fourth order. The conditionally stable implicit Lax-Wendroff scheme is constructed. The accuracy of the investigated explicit and implicit two-level schemes for an approximate solution of the advection equation is illustrated by the numerical results of a model two-dimensional problem.

Undergoing spherically symmetric steady-state accretion stability of white dwarfs

Energy Technology Data Exchange (ETDEWEB)

Sienkiewicz, R [Polska Akademia Nauk, Warsaw. N. Copernicus Astronomical Center

1980-01-01

Thermal and vibrational stabilities of accreting white dwarfs with steady-state nuclear burning were considered, assuming spherically symmetric accretion of the hydrogen-rich matter and using linear stability analysis. Almost all models with masses 0.2 M(sun) - 1.39 M(sun) were found to be unstable in some way. The type of instability expected to dominate is given as a function of the accretion rate. For most accretion rates it is the thermal instability. Oscillation periods are given for the models in which the vibrational instability is the most violent one. These periods are of the order of seconds or minutes. We expect that our stability analysis may suggest the cause of the variabilities of the hot components of some symbiotic stars, for a wide range of the accretion rates. In this case our models may serve as the initial conditions for evolutionary computations. The results predict that short-period oscillations should be observed in some hot nuclei of planetary nebulae.

Fluid Flow and Mixing Induced by AC Continuous Electrowetting of Liquid Metal Droplet

Directory of Open Access Journals (Sweden)

Qingming Hu

2017-04-01

Full Text Available In this work, we proposed a novel design of a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity at small or large voltages, respectively. Unlike previous researchers exploiting the unidirectional surface stress with direct current (DC bias at droplet/medium interface for pumping of electrolytes where the resulting flow rate is linearly proportional to the field intensity, dominance of another kind of dipolar flow pattern caused by local Marangoni stress at the drop surface in a sufficiently intense AC electric field is demonstrated by both theoretical analysis and experimental observation, which exhibits a quadratic growth trend as a function of the applied voltage. The dipolar shear stress merely appears at larger voltages and greatly enhances the mixing performance by inducing chaotic advection between the neighboring laminar flow. The mixer design developed herein, on the basis of amplified Marangoni chaotic advection around a liquid metal droplet at larger AC voltages, has great potential for chemical reaction and microelectromechanical systems (MEMS actuator applications because of generating high-throughput and excellent mixing performance at the same time.

WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. I. SUPPRESSION OF THE MAGNETOROTATIONAL INSTABILITY AND LAUNCHING OF THE MAGNETOCENTRIFUGAL WIND

International Nuclear Information System (INIS)

Bai Xuening; Stone, James M.

2013-01-01

We perform local, vertically stratified shearing-box MHD simulations of protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account the effects of both Ohmic resistivity and ambipolar diffusion (AD). The magnetic diffusion coefficients are evaluated self-consistently from a look-up table based on equilibrium chemistry. We first show that the inclusion of AD dramatically changes the conventional picture of layered accretion. Without net vertical magnetic field, the system evolves into a toroidal field dominated configuration with extremely weak turbulence in the far-UV ionization layer that is far too inefficient to drive rapid accretion. In the presence of a weak net vertical field (plasma β ∼ 10 5 at midplane), we find that the magnetorotational instability (MRI) is completely suppressed, resulting in a fully laminar flow throughout the vertical extent of the disk. A strong magnetocentrifugal wind is launched that efficiently carries away disk angular momentum and easily accounts for the observed accretion rate in PPDs. Moreover, under a physical disk wind geometry, all the accretion flow proceeds through a strong current layer with a thickness of ∼0.3H that is offset from disk midplane with radial velocity of up to 0.4 times the sound speed. Both Ohmic resistivity and AD are essential for the suppression of the MRI and wind launching. The efficiency of wind transport increases with increasing net vertical magnetic flux and the penetration depth of the FUV ionization. Our laminar wind solution has important implications on planet formation and global evolution of PPDs.

High-order finite volume advection

OpenAIRE

Shaw, James

2018-01-01

The cubicFit advection scheme is limited to second-order convergence because it uses a polynomial reconstruction fitted to point values at cell centres. The highOrderFit advection scheme achieves higher than second order by calculating high-order moments over the mesh geometry.

Collimation of particle beams from thick accretion discs

Energy Technology Data Exchange (ETDEWEB)

Sikora, M [N. Copernicus Astronomical Center, Warszawa (Poland); Wilson, D B [Cambridge Univ. (UK). Inst. of Astronomy

1981-11-01

The acceleration and collimation of particle beams in the funnel of thick accretion discs is studied in the approximation that the flow is optically thin. Such flows can be collimated to within approximately 0.1 radians by sufficiently thick discs. The flow cannot convert more than a small fraction of the disc's (super-Eddington) luminosity into the energy flow of a narrow beam without being optically thick.

Influence of nuclear burning of the stability of degenerate and nondegenerate accretion disks

International Nuclear Information System (INIS)

Taam, R.E.; Fryxell, B.A.

1985-01-01

The structure and stability of accretion disks composed of hydrogen-rich matter rotating about a central neutron star have been investigated for known sources of viscosity. Two general classes of solutions have been found. For one class the energy generated in the disk is provided by hydrogen burning, whereas for the other class the gravitational binding energy released by viscous dissipation dominates. The former solutions are thermally unstable (stable) whenever hydrogen burns via the normal CNO cycle ( pp chain) in a partially or fully degenerate region of the disk. Solutions characterized by nuclear burning via the β-limited CNO cycle or by viscous dissipation only are always stable. On the basis of a local analysis it is shown that modulations of the mass flow in the disk are possible for a range of mass inflow rates into the disk. In such circumstances the disk can undergo a phase transition from a cold, low-viscosity state to a hot, high-viscosity state as a result of the thermonuclear flash instability. Phase transitions from the hot state to the cold state also occur whenever the mass input rate into the disk is less than the equilibrium mass flow rate corresponding to the hot state. It is also shown that for sufficiently high mass flow rates all the hydrogen-rich matter can be processed to helium in the inner regions of the disk before it can be accreted by a neutron star

HEROIC: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs

Science.gov (United States)

Narayan, Ramesh; Zhu, Yucong; Psaltis, Dimitrios; Saḑowski, Aleksander

2016-03-01

We describe Hybrid Evaluator for Radiative Objects Including Comptonization (HEROIC), an upgraded version of the relativistic radiative post-processor code HERO described in a previous paper, but which now Includes Comptonization. HEROIC models Comptonization via the Kompaneets equation, using a quadratic approximation for the source function in a short characteristics radiation solver. It employs a simple form of accelerated lambda iteration to handle regions of high scattering opacity. In addition to solving for the radiation field, HEROIC also solves for the gas temperature by applying the condition of radiative equilibrium. We present benchmarks and tests of the Comptonization module in HEROIC with simple 1D and 3D scattering problems. We also test the ability of the code to handle various relativistic effects using model atmospheres and accretion flows in a black hole space-time. We present two applications of HEROIC to general relativistic magnetohydrodynamics simulations of accretion discs. One application is to a thin accretion disc around a black hole. We find that the gas below the photosphere in the multidimensional HEROIC solution is nearly isothermal, quite different from previous solutions based on 1D plane parallel atmospheres. The second application is to a geometrically thick radiation-dominated accretion disc accreting at 11 times the Eddington rate. Here, the multidimensional HEROIC solution shows that, for observers who are on axis and look down the polar funnel, the isotropic equivalent luminosity could be more than 10 times the Eddington limit, even though the spectrum might still look thermal and show no signs of relativistic beaming.

Effect of accretion on primordial black holes in Brans-Dicke theory

International Nuclear Information System (INIS)

Nayak, B.; Singh, L. P.; Majumdar, A. S.

2009-01-01

We consider the effect of accretion of radiation in the early Universe on primordial black holes in Brans-Dicke theory. The rate of growth of a primordial black hole due to accretion of radiation in Brans-Dicke theory is considerably smaller than the rate of growth of the cosmological horizon, thus making available sufficient radiation density for the black hole to accrete causally. We show that accretion of radiation by Brans-Dicke black holes overrides the effect of Hawking evaporation during the radiation dominated era. The subsequent evaporation of the black holes in later eras is further modified due to the variable gravitational 'constant', and they could survive up to longer times compared to the case of standard cosmology. We estimate the impact of accretion on modification of the constraint on their initial mass fraction obtained from the γ-ray background limit from presently evaporating primordial black holes.

Shocks in the relativistic transonic accretion with low angular momentum

Science.gov (United States)

Suková, P.; Charzyński, S.; Janiuk, A.

2017-12-01

We perform 1D/2D/3D relativistic hydrodynamical simulations of accretion flows with low angular momentum, filling the gap between spherically symmetric Bondi accretion and disc-like accretion flows. Scenarios with different directional distributions of angular momentum of falling matter and varying values of key parameters such as spin of central black hole, energy and angular momentum of matter are considered. In some of the scenarios the shock front is formed. We identify ranges of parameters for which the shock after formation moves towards or outwards the central black hole or the long-lasting oscillating shock is observed. The frequencies of oscillations of shock positions which can cause flaring in mass accretion rate are extracted. The results are scalable with mass of central black hole and can be compared to the quasi-periodic oscillations of selected microquasars (such as GRS 1915+105, XTE J1550-564 or IGR J17091-3624), as well as to the supermassive black holes in the centres of weakly active galaxies, such as Sgr A*.

Fate of an accretion disc around a black hole when both the viscosity and dark energy is in effect

Energy Technology Data Exchange (ETDEWEB)

Dutta, Sandip; Biswas, Ritabrata [The University of Burdwan, Department of Mathematics, Burdwan, West Bengal (India)

2017-10-15

This paper deals with the viscous accretion flow of a modified Chaplygin gas towards a black hole as the central gravitating object. A modified Chaplygin gas is a particular type of dark energy model which mimics of radiation era to phantom era depending on the different values of its parameters. We compare the dark energy accretion with the flow of adiabatic gas. An accretion disc flowing around a black hole is an example of a transonic flow. To construct the model, we consider three components of the Navier-Stokes equation, the equation of continuity and the modified Chaplygin gas equation of state. As a transonic flow passes through the sonic point, the velocity gradient being apparently singular there, it gives rise to two flow branches: one in-falling, the accretion and the other outgoing, the wind. We show that the wind curve is stronger and the wind speed reaches that of light at a finite distance from the black hole when dark energy is considered. Besides, if we increase the viscosity, the accretion disc is shortened in radius. These two processes acting together make the system deviate much from the adiabatic accretion case. It shows a weakening process for the accretion procedure by the work of the viscous system influencing both the angular momentum transport and the repulsive force of the modified Chaplygin gas. (orig.)

Report on Hydrologic Flow in Low-Permeability Media

Energy Technology Data Exchange (ETDEWEB)

Liu, Hui-Hai; Birkholzer, Jens

2013-11-13

We demonstrate that under normal conditions (under which there are no intersections between tunnels/drifts and conductive geological structures, such as faults), the water flow velocity in the damage zone, as a result of non-Darcian flow behavior, is extremely small such that solute transport is dominated by diffusion, rather than advection. We show that unless non-Darcian flow behavior is considered, significant errors can occur in the “measured” relative-permeability values. We propose a hypothesis to consider the temperature impact based on limited test results from the petroleum literature. To consider the bedding effects, we present an empirical relationship between water flux and hydraulic gradient for non-Darcian water flow in anisotropic cases.

Accreting fluids onto regular black holes via Hamiltonian approach

Energy Technology Data Exchange (ETDEWEB)

Jawad, Abdul [COMSATS Institute of Information Technology, Department of Mathematics, Lahore (Pakistan); Shahzad, M.U. [COMSATS Institute of Information Technology, Department of Mathematics, Lahore (Pakistan); University of Central Punjab, CAMS, UCP Business School, Lahore (Pakistan)

2017-08-15

We investigate the accretion of test fluids onto regular black holes such as Kehagias-Sfetsos black holes and regular black holes with Dagum distribution function. We analyze the accretion process when different test fluids are falling onto these regular black holes. The accreting fluid is being classified through the equation of state according to the features of regular black holes. The behavior of fluid flow and the existence of sonic points is being checked for these regular black holes. It is noted that the three-velocity depends on critical points and the equation of state parameter on phase space. (orig.)

Theoretical modeling of Comptonized X-ray spectra of super-Eddington accretion flow: Origin of hard excess in ultraluminous X-ray sources

Science.gov (United States)

Kitaki, Takaaki; Mineshige, Shin; Ohsuga, Ken; Kawashima, Tomohisa

2017-12-01

X-ray continuum spectra of super-Eddington accretion flow are studied by means of Monte Carlo radiative transfer simulations based on the radiation hydrodynamic simulation data, in which both thermal- and bulk-Compton scatterings are taken into account. We compare the calculated spectra of accretion flow around black holes with masses of MBH = 10, 102, 103, and 104 M⊙ for a fixed mass injection rate (from the computational boundary at 103 rs) of 103 LEdd/c2 (with rs, LEdd, and c being the Schwarzschild radius, the Eddington luminosity, and the speed of light, respectively). The soft X-ray spectra exhibit mass dependence in accordance with the standard-disk relation; the maximum surface temperature is scaled as T ∝ M_{ BH}^{ -1/4}. The spectra in the hard X-ray band, by contrast with soft X-ray, look to be quite similar among different models, if we normalize the radiation luminosity by MBH. This reflects that the hard component is created by thermal- and bulk-Compton scatterings of soft photons originating from an accretion flow in the overheated and/or funnel regions, the temperatures of which have no dependence on mass. The hard X-ray spectra can be reproduced by a Wien spectrum with the temperature of T ˜ 3 keV accompanied by a hard excess at photon energy above several keV. The excess spectrum can be fitted well with a power law with a photon index of Γ ˜ 3. This feature is in good agreement with that of the recent NuSTAR observations of ULXs (ultra-luminous X-ray sources).

SWIFT X-RAY TELESCOPE STUDY OF THE BLACK HOLE BINARY MAXI J1659–152: VARIABILITY FROM A TWO COMPONENT ACCRETION FLOW

Energy Technology Data Exchange (ETDEWEB)

Kalamkar, M.; Klis, M. van der; Heil, L. [Astronomical Institute, “Anton Pannekoek,” University of Amsterdam, Science Park 904, 1098 XH, Amsterdam (Netherlands); Homan, J., E-mail: maithili@oa-roma.inaf.it [MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139 (United States)

2015-08-01

We present an energy dependent X-ray variability study of the 2010 outburst of the black hole X-ray binary MAXI J1659–152 with the Swift X-ray Telescope (XRT). The broadband noise components and the quasi-periodic oscillations (QPO) observed in the power spectra show a strong and varied energy dependence. Combining Swift XRT data with data from the Rossi X-ray Timing Explorer, we report, for the first time, an rms spectrum (fractional rms amplitude as a function of energy) of these components in the 0.5–30 keV energy range. We find that the strength of the low-frequency component (<0.1 Hz) decreases with energy, contrary to the higher frequency components (>0.1 Hz) whose strengths increase with energy. In the context of the propagating fluctuations model for X-ray variability, we suggest that the low-frequency component originates in the accretion disk (which dominates emission below ∼2 keV) and the higher frequency components are formed in the hot flow (which dominates emission above ∼2 keV). As the properties of the QPO suggest that it may have a different driving mechanism, we investigate the Lense–Thirring precession of the hot flow as a candidate model. We also report on the QPO coherence evolution for the first time in the energy band below 2 keV. While there are strong indications that the QPO is less coherent at energies below 2 keV than above 2 keV, the coherence increases with intensity similar to what is observed at energies above 2 keV in other black hole X-ray binaries.

Accretion Disks and Coronae in the X-Ray Flashlight

Science.gov (United States)

Degenaar, Nathalie; Ballantyne, David R.; Belloni, Tomaso; Chakraborty, Manoneeta; Chen, Yu-Peng; Ji, Long; Kretschmar, Peter; Kuulkers, Erik; Li, Jian; Maccarone, Thomas J.; Malzac, Julien; Zhang, Shu; Zhang, Shuang-Nan

2018-02-01

Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a hundred accreting neutron stars. The intense, often Eddington-limited, radiation generated in these thermonuclear explosions can have a discernible effect on the surrounding accretion flow that consists of an accretion disk and a hot electron corona. Type-I X-ray bursts can therefore serve as direct, repeating probes of the internal dynamics of the accretion process. In this work we review and interpret the observational evidence for the impact that Type-I X-ray bursts have on accretion disks and coronae. We also provide an outlook of how to make further progress in this research field with prospective experiments and analysis techniques, and by exploiting the technical capabilities of the new and concept X-ray missions ASTROSAT, NICER, Insight-HXMT, eXTP, and STROBE-X.

Subsurface barrier design alternatives for confinement and controlled advection flow

International Nuclear Information System (INIS)

Phillips, S.J.; Stewart, W.E.; Alexander, R.G.; Cantrell, K.J.; McLaughlin, T.J.

1994-02-01

Various technologies and designs are being considered to serve as subsurface barriers to confine or control contaminant migration from underground waste storage or disposal structures containing radioactive and hazardous wastes. Alternatives including direct-coupled flood and controlled advection designs are described as preconceptual examples. Prototype geotechnical equipment for testing and demonstration of these alternative designs tested at the Hanford Geotechnical Development and Test Facility and the Hanford Small-Tube Lysimeter Facility include mobile high-pressure injectors and pumps, mobile transport and pumping units, vibratory and impact pile drivers, and mobile batching systems. Preliminary laboratory testing of barrier materials and additive sequestering agents have been completed and are described

Bondi-Hoyle-Lyttleton Accretion onto Binaries

Science.gov (United States)

Antoni, Andrea; MacLeod, Morgan; Ramírez-Ruiz, Enrico

2018-01-01

Binary stars are not rare. While only close binary stars will eventually interact with one another, even the widest binary systems interact with their gaseous surroundings. The rates of accretion and the gaseous drag forces arising in these interactions are the key to understanding how these systems evolve. This poster examines accretion flows around a binary system moving supersonically through a background gas. We perform three-dimensional hydrodynamic simulations of Bondi-Hoyle-Lyttleton accretion using the adaptive mesh refinement code FLASH. We simulate a range of values of semi-major axis of the orbit relative to the gravitational focusing impact parameter of the pair. On large scales, gas is gravitationally focused by the center-of-mass of the binary, leading to dynamical friction drag and to the accretion of mass and momentum. On smaller scales, the orbital motion imprints itself on the gas. Notably, the magnitude and direction of the forces acting on the binary inherit this orbital dependence. The long-term evolution of the binary is determined by the timescales for accretion, slow down of the center-of-mass, and decay of the orbit. We use our simulations to measure these timescales and to establish a hierarchy between them. In general, our simulations indicate that binaries moving through gaseous media will slow down before the orbit decays.

The formation of stars by gravitational collapse rather than competitive accretion

Science.gov (United States)

Krumholz, Mark R.; McKee, Christopher F.; Klein, Richard I.

2005-11-01

There are two dominant models of how stars form. Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (Msolar), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (~0.5Msolar), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump. Competitive accretion models interpret brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before they have accreted much mass; key predictions of this model are that such objects should lack disks, have high velocity dispersions, form more frequently in denser clumps, and that the mean stellar mass should vary within the Galaxy. Here we derive the rate of competitive accretion as a function of the star-forming environment, based partly on simulation, and determine in what types of environments competitive accretion can occur. We show that no observed star-forming region can undergo significant competitive accretion, and that the simulations that show competitive accretion do so because the assumed properties differ from those determined by observation. Our result shows that stars form by gravitational collapse, and explains why observations have failed to confirm predictions of the competitive accretion model.

Orbital Advection with Magnetohydrodynamics and Vector Potential

Energy Technology Data Exchange (ETDEWEB)

Lyra, Wladimir [Department of Physics and Astronomy, California State University Northrige, 18111 Nordhoff Street, Northridge CA 91130 (United States); McNally, Colin P. [Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS (United Kingdom); Heinemann, Tobias [Niels Bohr International Academy, The Niels Bohr Institute, Blegdamsvej 17, DK-2100, Copenhagen Ø (Denmark); Masset, Frédéric, E-mail: wlyra@csun.edu [Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Av. Universidad s/n, 62210 Cuernavaca, Mor. (Mexico)

2017-10-01

Orbital advection is a significant bottleneck in disk simulations, and a particularly tricky one when used in connection with magnetohydrodynamics. We have developed an orbital advection algorithm suitable for the induction equation with magnetic potential. The electromotive force is split into advection and shear terms, and we find that we do not need an advective gauge since solving the orbital advection implicitly precludes the shear term from canceling the advection term. We prove and demonstrate the third order in time accuracy of the scheme. The algorithm is also suited to non-magnetic problems. Benchmarked results of (hydrodynamical) planet–disk interaction and of the magnetorotational instability are reproduced. We include detailed descriptions of the construction and selection of stabilizing dissipations (or high-frequency filters) needed to generate practical results. The scheme is self-consistent, accurate, and elegant in its simplicity, making it particularly efficient for straightforward finite-difference methods. As a result of the work, the algorithm is incorporated in the public version of the Pencil Code, where it can be used by the community.

Orbital Advection with Magnetohydrodynamics and Vector Potential

International Nuclear Information System (INIS)

Lyra, Wladimir; McNally, Colin P.; Heinemann, Tobias; Masset, Frédéric

2017-01-01

Orbital advection is a significant bottleneck in disk simulations, and a particularly tricky one when used in connection with magnetohydrodynamics. We have developed an orbital advection algorithm suitable for the induction equation with magnetic potential. The electromotive force is split into advection and shear terms, and we find that we do not need an advective gauge since solving the orbital advection implicitly precludes the shear term from canceling the advection term. We prove and demonstrate the third order in time accuracy of the scheme. The algorithm is also suited to non-magnetic problems. Benchmarked results of (hydrodynamical) planet–disk interaction and of the magnetorotational instability are reproduced. We include detailed descriptions of the construction and selection of stabilizing dissipations (or high-frequency filters) needed to generate practical results. The scheme is self-consistent, accurate, and elegant in its simplicity, making it particularly efficient for straightforward finite-difference methods. As a result of the work, the algorithm is incorporated in the public version of the Pencil Code, where it can be used by the community.

Magnetospheres of accreting compact objects in binary systems

International Nuclear Information System (INIS)

Aly, J.J.

1985-09-01

Bright pulsating X-ray sources (X-ray pulsars, AM Her stars,...) have been identified as strongly magnetized compact objects accreting matter from a binary companion. We give here a summary of some of the work which has been recently done to try to understand the interaction between the magnetic field of the compact object and the matter around. We examine in turn the models describing the interaction of the field with: i) a spherically symmetric accretion flow; ii) a thin keplerian accretion disk; iii) the companion itself. In all these cases, we pay particular attention to the following problems: i) how the external plasma interacting with the magnetosphere can get mixed with the field; ii) by which mechanism the magnetic field controls the mass-momentum-energy exchanges between the two stars. In conclusion, we compare the magnetosphere of an accreting compact object with that one of a planet [fr

Temporal signatures of advective versus diffusive radon transport at a geothermal zone in Central Nepal

International Nuclear Information System (INIS)

Richon, Patrick; Perrier, Frederic; Koirala, Bharat Prasad; Girault, Frederic; Bhattarai, Mukunda; Sapkota, Soma Nath

2011-01-01

Temporal variation of radon-222 concentration was studied at the Syabru-Bensi hot springs, located on the Main Central Thrust zone in Central Nepal. This site is characterized by several carbon dioxide discharges having maximum fluxes larger than 10 kg m -2 d -1 . Radon concentration was monitored with autonomous Barasol TM probes between January 2008 and November 2009 in two small natural cavities with high CO 2 concentration and at six locations in the soil: four points having a high flux, and two background reference points. At the reference points, dominated by radon diffusion, radon concentration was stable from January to May, with mean values of 22 ± 6.9 and 37 ± 5.5 kBq m -3 , but was affected by a large increase, of about a factor of 2 and 1.6, respectively, during the monsoon season from June to September. At the points dominated by CO 2 advection, by contrast, radon concentration showed higher mean values 39.0 ± 2.6 to 78 ± 1.4 kBq m -3 , remarkably stable throughout the year with small long-term variation, including a possible modulation of period around 6 months. A significant difference between the diffusion dominated reference points and the advection-dominated points also emerged when studying the diurnal S 1 and semi-diurnal S 2 periodic components. At the advection-dominated points, radon concentration did not exhibit S 1 or S 2 components. At the reference points, however, the S 2 component, associated with barometric tide, could be identified during the dry season, but only when the probe was installed at shallow depth. The S 1 component, associated with thermal and possibly barometric diurnal forcing, was systematically observed, especially during monsoon season. The remarkable short-term and long-term temporal stability of the radon concentration at the advection-dominated points, which suggests a strong pressure source at depth, may be an important asset to detect possible temporal variations associated with the seismic cycle. - Graphical

COMPARING THE ACCRETION DISK EVOLUTION OF BLACK HOLE AND NEUTRON STAR X-RAY BINARIES FROM LOW TO SUPER-EDDINGTON LUMINOSITY

International Nuclear Information System (INIS)

Weng Shanshan; Zhang Shuangnan

2011-01-01

Low-mass X-ray binaries (LMXBs) are systems in which a low-mass companion transfers mass via Roche-lobe overflow onto a black hole (BH) or a weakly magnetized neutron star (NS). It is believed that both the solid surface and the magnetic field of an NS can affect the accretion flow and show some observable effects. Using the disk emission dominant data, we compare the disk evolution of the two types of systems from low luminosity to super-Eddington luminosity. As the luminosity decreases the disk in the NS LMXB 4U1608-522 begins to leave the innermost stable circular orbit (ISCO) at much higher luminosity (∼0.1 L Edd ), compared with BH LMXBs at much lower luminosity (∼0.03 L Edd ), due to the interaction between the NS magnetosphere and accretion flow. However, as the luminosity increases above a critical luminosity, the disks in BH and NS LMXBs trace the same evolutionary pattern, because the magnetosphere is restricted inside ISCO, and then both the NS surface emission and (dipole) magnetic field do not significantly affect the secular evolution of the accretion disk, which is driven by the increased radiation pressure in the inner region. We further suggest that the NS surface emission provides additional information about the accretion disk not available in BH systems. Through the observed NS surface emission, we argue that the disk thickness H/R is less than 0.3-0.4, and that the significant outflow from the inner disk edge exists at a luminosity close to Eddington luminosity.

Minidisks in Binary Black Hole Accretion

Energy Technology Data Exchange (ETDEWEB)

Ryan, Geoffrey; MacFadyen, Andrew, E-mail: gsr257@nyu.edu [Center for Cosmology and Particle Physics, Physics Department, New York University, New York, NY 10003 (United States)

2017-02-01

Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress that causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov–Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing-angle-dependent images for comparison with observations.

Preliminary evaluation of the importance of existing hydraulic-head observation locations to advective-transport predictions, Death Valley regional flow system, California and Nevada

International Nuclear Information System (INIS)

Hill, M.C.; Ely, D.M.; Tiedeman, C.R.; O'Brien, G.M.; D'Agnese, F.A.; Faunt, C.C.

2001-01-01

When a model is calibrated by nonlinear regression, calculated diagnostic statistics and measures of uncertainty provide a wealth of information about many aspects of the system. This report presents a method of ranking the likely importance of existing observation locations using measures of prediction uncertainty. It is suggested that continued monitoring is warranted at more important locations, and unwarranted or less warranted at less important locations. The report develops the methodology and then demonstrates it using the hydraulic-head observation locations of a three-layer model of the Death Valley regional flow system (DVRFS). The predictions of interest are subsurface transport from beneath Yucca Mountain and 14 underground Test Area (UGTA) sites. The advective component of transport is considered because it is the component most affected by the system dynamics represented by the regional-scale model being used. The problem is addressed using the capabilities of the U.S. Geological Survey computer program MODFLOW-2000, with its ADVective-Travel Observation (ADV) Package, and an additional computer program developed for this work

A Fast Implicit Finite Difference Method for Fractional Advection-Dispersion Equations with Fractional Derivative Boundary Conditions

Directory of Open Access Journals (Sweden)

Taohua Liu

2017-01-01

Full Text Available Fractional advection-dispersion equations, as generalizations of classical integer-order advection-dispersion equations, are used to model the transport of passive tracers carried by fluid flow in a porous medium. In this paper, we develop an implicit finite difference method for fractional advection-dispersion equations with fractional derivative boundary conditions. First-order consistency, solvability, unconditional stability, and first-order convergence of the method are proven. Then, we present a fast iterative method for the implicit finite difference scheme, which only requires storage of O(K and computational cost of O(Klog⁡K. Traditionally, the Gaussian elimination method requires storage of O(K2 and computational cost of O(K3. Finally, the accuracy and efficiency of the method are checked with a numerical example.

The Accretion-Ejection Mechanisms in X-ray Binaries: an Unified View

International Nuclear Information System (INIS)

Petrucci, P. O.; Foellmi, C.; Ferreira, J.; Henri, G.; Cabanac, C.; Belmont, R.; Malzac, J.

2009-01-01

We present a new keplerian accretion disc solution, the so-called Jet Emitting Disc (JED hereafter), which is part of global self-consistent disc-jet MHD structure. In our framework, a large scale, organized vertical magnetic field is threading the JED giving birth, when conditions are met, to stationnary self-collimated non relativistic jets. The main condition is that the magnetic pressure P mag must be of the order of the total pressure P tot in the JED and a direct consequence is a jet torque largely dominating the viscuous torque. This in turn implies an accretion velocity of the order of the sound speed and then a density much lower than a standard accretion disc. Moreover, most of the accretion power P acc being extracted by the jet, only part of it (<50%) is liberated in the JED as heating power.

Numerical modelling of hydro-morphological processes dominated by fine suspended sediment in a stormwater pond

Science.gov (United States)

Guan, Mingfu; Ahilan, Sangaralingam; Yu, Dapeng; Peng, Yong; Wright, Nigel

2018-01-01

Fine sediment plays crucial and multiple roles in the hydrological, ecological and geomorphological functioning of river systems. This study employs a two-dimensional (2D) numerical model to track the hydro-morphological processes dominated by fine suspended sediment, including the prediction of sediment concentration in flow bodies, and erosion and deposition caused by sediment transport. The model is governed by 2D full shallow water equations with which an advection-diffusion equation for fine sediment is coupled. Bed erosion and sedimentation are updated by a bed deformation model based on local sediment entrainment and settling flux in flow bodies. The model is initially validated with the three laboratory-scale experimental events where suspended load plays a dominant role. Satisfactory simulation results confirm the model's capability in capturing hydro-morphodynamic processes dominated by fine suspended sediment at laboratory-scale. Applications to sedimentation in a stormwater pond are conducted to develop the process-based understanding of fine sediment dynamics over a variety of flow conditions. Urban flows with 5-year, 30-year and 100-year return period and the extreme flood event in 2012 are simulated. The modelled results deliver a step change in understanding fine sediment dynamics in stormwater ponds. The model is capable of quantitatively simulating and qualitatively assessing the performance of a stormwater pond in managing urban water quantity and quality.

ON THE ROLE OF FAST MAGNETIC RECONNECTION IN ACCRETING BLACK HOLE SOURCES

International Nuclear Information System (INIS)

Singh, C. B.; De Gouveia Dal Pino, E. M.; Kadowaki, L. H. S.

2015-01-01

We attempt to explain the observed radio and gamma-ray emission produced in the surroundings of black holes by employing a magnetically dominated accretion flow model and fast magnetic reconnection triggered by turbulence. In earlier work, a standard disk model was used and we refine the model by focusing on the sub-Eddington regime to address the fundamental plane of black hole activity. The results do not change substantially with regard to previous work, ensuring that the details of accretion physics are not relevant in the magnetic reconnection process occurring in the corona. Rather, our work puts fast magnetic reconnection events as a powerful mechanism operating in the core region near the jet base of black hole sources on more solid ground. For microquasars and low-luminosity active galactic nuclei, the observed correlation between radio emission and the mass of the sources can be explained by this process. The corresponding gamma-ray emission also seems to be produced in the same core region. On the other hand, emission from blazars and gamma-ray bursts cannot be correlated to core emission based on fast reconnection

Accretion-induced quasinormal mode excitation of a Schwarzschild black hole

International Nuclear Information System (INIS)

Nagar, Alessandro; Zanotti, Olindo; Font, Jose A.; Rezzolla, Luciano

2007-01-01

By combining the numerical solution of the nonlinear hydrodynamics equations with the solution of the linear inhomogeneous Zerilli-Moncrief and Regge-Wheeler equations, we investigate the properties of the gravitational radiation emitted during the axisymmetric accretion of matter onto a Schwarzschild black hole. The matter models considered include quadrupolar dust shells and thick accretion disks, permitting us to simulate situations which may be encountered at the end stages of stellar gravitational collapse or binary neutron star merger. We focus on the interference pattern appearing in the energy spectra of the emitted gravitational waves and on the amount of excitation of the quasinormal modes of the accreting black hole. We show that, quite generically in the presence of accretion, the black-hole ringdown is not a simple superposition of quasinormal modes, although the fundamental mode is usually present and often dominates the gravitational-wave signal. We interpret this as due to backscattering of waves off the nonexponentially decaying part of the black-hole potential and to the finite spatial extension of the accreting matter. Our results suggest that the black-hole QNM contributions to the full gravitational-wave signal should be extremely small and possibly not detectable in generic astrophysical scenarios involving the accretion of extended distributions of matter

Gas-rich dwarfs and accretion phenomena in early-type galaxies

Science.gov (United States)

Silk, J.; Norman, C.

1979-01-01

An analysis is presented of the combined effects of cloud accretion and galactic winds and coronae. An accretion model is developed wherein gas-rich dwarf galaxies are accreted into galactic halos, which provides an adequate source of H I to account for observations of neutral gas in early-type galaxies. Accretion is found to fuel the wind, thereby regulating the accretion flow and yielding a time-dependent model for star formation, enrichment, and nuclear activity. The permissible parameter range for intergalactic gas clouds and galaxy groups is discussed, along with the frequency of gas-rich dwarfs and their large ratios of gas mass to luminosity. Also considered is the occurrence of gas stripping and the consequent formation of dwarf spheroidal systems that remain in the halo, and gas clouds that dissipate and suffer further infall. A cosmological implication of the model is that, because the characteristic time scale of a gas-rich dwarf galaxy to be accreted and lose its gas is comparable to a Hubble time, there may have been a far more extensive primordial distribution of such systems at earlier epochs.

Waste dissolution with chemical reaction, diffusion and advection

International Nuclear Information System (INIS)

Chambre, P.L.; Kang, C.H.; Lee, W.W.L.; Pigford, T.H.

1987-06-01

This paper extends the mass-transfer analysis to include the effect of advective transport in predicting the steady-state dissolution rate, with a chemical-reaction-rate boundary condition at the surface of a waste form of arbitrary shape. This new theory provides an analytic means of predicting the ground-water velocities at which dissolution rate in a geologic environment will be governed entirely to the chemical reaction rate. As an illustration, we consider the steady-state potential flow of ground water in porous rock surrounding a spherical waste solid. 3 refs., 2 figs

Probing the Jet Turnover Frequency Dependence on Mass and Mass Accretion Rate

Science.gov (United States)

Hammerstein, Erica; Gültekin, Kayhan; King, Ashley

2018-01-01

We have examined a sample of 15 sub-Eddington supermassive black holes (SMBHs) in a variety of galaxy classifications to further understand the proposed fundamental plane of black hole activity and scaling relations between black hole masses and their radio and X-ray luminosities. This plane describes black holes from stellar-mass to supermassive. The physics probed by these sub-Eddington systems is thought to be a radiatively inefficient, jet-dominated accretion flow. By studying black holes in this regime, we can learn important information on the disk-jet connection for accreting black holes.A key factor in studying the fundamental plane is the turnover frequency — the frequency at which emission transitions from optically thick at lower frequencies to optically thin at higher frequencies. This turnover point can be measured by observing the source in both radio and X-ray. Our project aims to test the dependence of the turnover frequency on mass and mass accretion rate.Radio observations of the sample were obtained using the Karl G. Jansky Very Large Array (VLA) in the range of 5-40 GHz across four different frequency bands in A configuration to give the highest spatial resolution to focus on the core emission. Our carefully chosen sample of SMBHs with dynamically measured masses consists of two sub-samples: those with approximately constant mass accretion rate (LX/LEdd ~ 10‑7) and those with approximately constant mass (MBH ~ 108 Msun). X-ray data were obtained from archival Chandra observations. To find the turnover frequency, we used Markov Chain Monte Carlo methods to fit two power laws to the radio data and the archival X-ray data. The intersection of the radio and X-ray fits is the turnover frequency.We present the results for both subsamples of SMBHs and their relationship between the turnover frequency and X-ray luminosity, which we take to scale with mass accretion rate, and jet power derived from both radio and X-ray properties.

Estimating Advective Near-surface Currents from Ocean Color Satellite Images

Science.gov (United States)

2015-01-01

on the SuomiNational Polar-Orbiting Partner- ship (S- NPP ) satellite. The GOCI is the world’s first geostationary orbit satellite sensor over the...radiance Lwn at several wave - lengths. These spectral Lwn channels are used to derive several in- water bio-optical properties (Lee, Carder, & Arnone...the same surface flow, it is the inter-product similarities, instead of the differences, that are more likely to stand for the surface advection. If

Progress towards experimental realization of extreme-velocity flow-dominated magnetized plasmas

Science.gov (United States)

Weber, T. E.; Adams, C. S.; Welch, D. R.; Kagan, G.; Bean, I. A.; Henderson, B. R.; Klim, A. J.

2017-10-01

Interactions of flow-dominated plasmas with other plasmas, neutral gases, magnetic fields, solids etc., take place with sufficient velocity that kinetic energy dominates the dynamics of the interaction (as opposed to magnetic or thermal energy, which dominates in most laboratory plasma experiments). Building upon progress made by the Magnetized Shock Experiment (MSX) at LANL, we are developing the experimental and modeling capability to increase our ultimate attainable plasma velocities well in excess of 1000 km/s. Ongoing work includes designing new pulsed power switches, triggering, and inductive adder topologies; development of novel high-speed optical diagnostics; and exploration of new numerical techniques to specifically model the unique physics of translating/stagnating flow-dominated plasmas. Furthering our understanding of the physical mechanisms of energy conversion from kinetic to other forms, such as thermal energy, non-thermal tails/accelerated populations, enhanced magnetic fields, and radiation (both continuum and line), has wide-ranging significance in basic plasma science, astrophysics, and plasma technology applications such as inertial confinement fusion and intense radiation sources. This work is supported by the U.S. Department of Energy, National Nuclear Security Administration. LA-UR-17-25786.

Influence of porewater advection on denitrification in carbonate sands: Evidence from repacked sediment column experiments

DEFF Research Database (Denmark)

Santos, Isaac R.; Eyre, Bradley D.; Glud, Ronnie N.

2012-01-01

Porewater flow enhances mineralization rates in organic-poor permeable sands. Here, a series of sediment column experiments were undertaken to assess the potential effect of advective porewater transport on denitrification in permeable carbonate sands collected from Heron Island (Great Barrier Re...

The accretion of migrating giant planets

Science.gov (United States)

Dürmann, Christoph; Kley, Wilhelm

2017-02-01

Aims: Most studies concerning the growth and evolution of massive planets focus either on their accretion or their migration only. In this work we study both processes concurrently to investigate how they might mutually affect one another. Methods: We modeled a two-dimensional disk with a steady accretion flow onto the central star and embedded a Jupiter mass planet at 5.2 au. The disk is locally isothermal and viscosity is modeled using a constant α. The planet is held on a fixed orbit for a few hundred orbits to allow the disk to adapt and carve a gap. After this period, the planet is released and free to move according to the gravitational interaction with the gas disk. The mass accretion onto the planet is modeled by removing a fraction of gas from the inner Hill sphere, and the removed mass and momentum can be added to the planet. Results: Our results show that a fast migrating planet is able to accrete more gas than a slower migrating planet. Utilizing a tracer fluid we analyzed the origin of the accreted gas originating predominantly from the inner disk for a fast migrating planet. In the case of slower migration, the fraction of gas from the outer disk increases. We also found that even for very high accretion rates, in some cases gas crosses the planetary gap from the inner to the outer disk. Our simulations show that the crossing of gas changes during the migration process as the migration rate slows down. Therefore, classical type II migration where the planet migrates with the viscous drift rate and no gas crosses the gap is no general process but may only occur for special parameters and at a certain time during the orbital evolution of the planet.

Novel Methods for Measuring Depth of Anesthesia by Quantifying Dominant Information Flow in Multichannel EEGs

Directory of Open Access Journals (Sweden)

Kab-Mun Cha

2017-01-01

Full Text Available In this paper, we propose novel methods for measuring depth of anesthesia (DOA by quantifying dominant information flow in multichannel EEGs. Conventional methods mainly use few EEG channels independently and most of multichannel EEG based studies are limited to specific regions of the brain. Therefore the function of the cerebral cortex over wide brain regions is hardly reflected in DOA measurement. Here, DOA is measured by the quantification of dominant information flow obtained from principle bipartition. Three bipartitioning methods are used to detect the dominant information flow in entire EEG channels and the dominant information flow is quantified by calculating information entropy. High correlation between the proposed measures and the plasma concentration of propofol is confirmed from the experimental results of clinical data in 39 subjects. To illustrate the performance of the proposed methods more easily we present the results for multichannel EEG on a two-dimensional (2D brain map.

Magnetically gated accretion in an accreting 'non-magnetic' white dwarf.

Science.gov (United States)

Scaringi, S; Maccarone, T J; D'Angelo, C; Knigge, C; Groot, P J

2017-12-13

White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 10 6 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as 'non-magnetic', because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the 'non-magnetic' accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 10 4 gauss and 1 × 10 5 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

Constraining jet physics in weakly accreting black holes

Science.gov (United States)

Markoff, Sera

2007-04-01

Outflowing jets are observed in a variety of astronomical objects such as accreting compact objects from X-ray binaries (XRBs) to active galactic nuclei (AGN), as well as at stellar birth and death. Yet we still do not know exactly what they are comprised of, why and how they form, or their exact relationship with the accretion flow. In this talk I will focus on jets in black hole systems, which provide the ideal test population for studying the relationship between inflow and outflow over an extreme range in mass and accretion rate. I will present several recent results from coordinated multi-wavelength studies of low-luminosity sources. These results not only support similar trends in weakly accreting black hole behavior across the mass scale, but also suggest that the same underlying physical model can explain their broadband spectra. I will discuss how comparisons between small- and large-scale systems are revealing new information about the regions nearest the black hole, providing clues about the creation of these weakest of jets. Furthermore, comparisons between our Galactic center nucleus Sgr A* and other sources at slightly higher accretion rates can illucidate the processes which drive central activity, and pave the way for new tests with upcoming instruments.

Transitional millisecond pulsars in the low-level accretion state

Science.gov (United States)

Jaodard, Amruta D.; Hessels, Jason W. T.; Archibald, Anne; Bogdanov, Slavko; Deller, Adam; Hernandez Santisteban, Juan; Patruno, Alessandro; D'Angelo, Caroline; Bassa, Cees; Amruta Jaodand

2018-01-01

In the canonical pulsar recycling scenario, a slowly spinning neutron star can be rejuvenated to rapid spin rates by the transfer of angular momentum and mass from a binary companion star. Over the last decade, the discovery of three transitional millisecond pulsars (tMSPs) has allowed us to study recycling in detail. These systems transition between accretion-powered (X-ray) and rotation-powered (radio) pulsar states within just a few days, raising questions such as: what triggers the state transition, when does the recycling process truly end, and what will the radio pulsar’s final spin rate be? Systematic multi-wavelength campaigns over the last decade have provided critical insights: multi-year-long, low-level accretion states showing coherent X-ray pulsations; extremely stable, bi-modal X-ray light curves; outflows probed by radio continuum emission; a surprising gamma-ray brightening during accretion, etc. In my thesis I am trying to bring these clues together to understand the low-level accretion process that recycles a pulsar. For example, recently we timed PSR J1023+0038 in the accretion state and found it to be spinning down ~26% faster compared to the non-accreting radio pulsar state. We are currently conducting simultaneous multi-wavelength campaigns (XMM, HST, Kepler and VLA) to understand the global variability of the accretion flow, as well as high-energy Fermi-LAT observations to probe the gamma-ray emission mechanism. I will highlight these recent developments, while also presenting a broad overview of tMSPs as exciting new laboratories to test low-level accretion onto magnetized neutron stars.

A magnetic model for low/hard state of black hole binaries

Science.gov (United States)

Ye, Yong-Chun; Wang, Ding-Xiong; Huang, Chang-Yin; Cao, Xiao-Feng

2016-03-01

A magnetic model for the low/hard state (LHS) of two black hole X-ray binaries (BHXBs), H1743-322 and GX 339-4, is proposed based on transport of the magnetic field from a companion into an accretion disk around a black hole (BH). This model consists of a truncated thin disk with an inner advection-dominated accretion flow (ADAF). The spectral profiles of the sources are fitted in agreement with the data observed at four different dates corresponding to the rising phase of the LHS. In addition, the association of the LHS with a quasi-steady jet is modeled based on transport of magnetic field, where the Blandford-Znajek (BZ) and Blandford-Payne (BP) processes are invoked to drive the jets from BH and inner ADAF. It turns out that the steep radio/X-ray correlations observed in H1743-322 and GX 339-4 can be interpreted based on our model.

Sediment transport and deposition on a river-dominated tidal flat: An idealized model study

Science.gov (United States)

Sherwood, Christopher R.; Chen, Shih-Nan; Geyer, W. Rockwell; Ralston, David K.

2010-01-01

A 3-D hydrodynamic model is used to investigate how different size classes of river-derived sediment are transported, exported and trapped on an idealized, river-dominated tidal flat. The model is composed of a river channel flanked by sloping tidal flats, a configuration motivated by the intertidal region of the Skagit River mouth in Washington State, United States. It is forced by mixed tides and a pulse of freshwater and sediment with various settling velocities. In this system, the river not only influences stratification but also contributes a significant cross-shore transport. As a result, the bottom stress is strongly ebb-dominated in the channel because of the seaward advance of strong river flow as the tidal flats drain during ebbs. Sediment deposition patterns and mass budgets are sensitive to settling velocity. The lateral sediment spreading scales with an advective distance (settling time multiplied by lateral flow speed), thereby confining the fast settling sediment classes in the channel. Residual sediment transport is landward on the flats, because of settling lag, but is strongly seaward in the channel. The seaward transport mainly occurs during big ebbs and is controlled by a length scale ratio Ld/XWL, where Ld is a cross-shore advective distance (settling time multiplied by river outlet velocity), and XWL is the immersed cross-shore length of the intertidal zone. Sediment trapping requires Ld/XWL stratification and reducing tidal range both favor sediment trapping, whereas varying channel geometries and asymmetry of tides has relatively small impacts. Implications of the modeling results on the south Skagit intertidal region are discussed.

Mass and Momentum Transport in Microcavities for Diffusion-Dominant Cell Culture Applications

Science.gov (United States)

Yew, Alvin G.; Pinero, Daniel; Hsieh, Adam H.; Atencia, Javier

2012-01-01

For the informed design of microfluidic devices, it is important to understand transport phenomena at the microscale. This letter outlines an analytically-driven approach to the design of rectangular microcavities extending perpendicular to a perfusion microchannel for microfluidic cell culture devices. We present equations to estimate the spatial transition from advection- to diffusion-dominant transport inside cavities as a function of the geometry and flow conditions. We also estimate the time required for molecules, such as nutrients or drugs to travel from the microchannel to a given depth into the cavity. These analytical predictions can facilitate the rational design of microfluidic devices to optimize and maintain long-term, physiologically-based culture conditions with low fluid shear stress.

Advective Removal of Intraparticle Uranium from Contaminated Vadose Zone Sediments, Hanford, USA

International Nuclear Information System (INIS)

Ilton, Eugene S.; Qafoku, Nikolla; Liu, Chongxuan; Moore, D. A.; Zachara, John M.

2008-01-01

A column study on U contaminated vadose zone sediments from the Hanford Site, WA, was performed in order to aid the development of a model for predicting U(VI) release rates under a dynamic flow regime and for variable geochemical conditions. The sediments of interest are adjacent to and below tank BX-102, part of the BX tank farm that contained high level liquid radioactive waste. Two sediments, with different U(VI) loadings and intraparticle large fracture vs. smaller fracture ratios, were reacted with three different solutions. The primary reservoir for U(VI) appears to be a micron-sized nanocrystalline Na-U-Si phase, possibly Na-boltwoodite, that nucleated and grew on plagioclase grains that line fractures within sand-sized granitic clasts. The solutions were all calcite saturated and in equilibrium with atmospheric CO2, where one solution was simply DI-water, the second was a synthetic ground water (SGW) with elevated Na, and the third was the same SGW but with both elevated Na and Si. The latter two solutions were employed, in part, to test the effect of saturation state on U(VI) release. For both sediments and all three electrolytes, there was an initial rapid release of U(VI) to the advecting solution followed by a plateau of low U(VI) concentration. U(VI) effluent concentration increased during subsequent stop flow (SF) events. The electrolytes with elevated Na and Si appreciably depressed U(VI) concentrations relative to DI water. The effluent data for both sediments and all three electrolytes was simulated reasonably well by a three domain model (the advecting fluid, fractures, and matrix) that coupled U(VI) dissolution rates, intraparticle U(VI) diffusion, and interparticle advective transport of U(VI); where key transport and dissolution processes had been parameterized in previous batch studies. For the calcite-saturated DI-water, U(VI) concentrations in the effluent remained far below saturation with respect to Na-boltwoodite and release of U(VI) to

Advection-diffusion model for normal grain growth and the stagnation of normal grain growth in thin films

International Nuclear Information System (INIS)

Lou, C.

2002-01-01

An advection-diffusion model has been set up to describe normal grain growth. In this model grains are divided into different groups according to their topological classes (number of sides of a grain). Topological transformations are modelled by advective and diffusive flows governed by advective and diffusive coefficients respectively, which are assumed to be proportional to topological classes. The ordinary differential equations governing self-similar time-independent grain size distribution can be derived analytically from continuity equations. It is proved that the time-independent distributions obtained by solving the ordinary differential equations have the same form as the time-dependent distributions obtained by solving the continuity equations. The advection-diffusion model is extended to describe the stagnation of normal grain growth in thin films. Grain boundary grooving prevents grain boundaries from moving, and the correlation between neighbouring grains accelerates the stagnation of normal grain growth. After introducing grain boundary grooving and the correlation between neighbouring grains into the model, the grain size distribution is close to a lognormal distribution, which is usually found in experiments. A vertex computer simulation of normal grain growth has also been carried out to make a cross comparison with the advection-diffusion model. The result from the simulation did not verify the assumption that the advective and diffusive coefficients are proportional to topological classes. Instead, we have observed that topological transformations usually occur on certain topological classes. This suggests that the advection-diffusion model can be improved by making a more realistic assumption on topological transformations. (author)

Precipitation Sedimentation and Advection in GFS

Science.gov (United States)

Sun, R.; Tallapragada, V.

2016-12-01

Zhao and Carr microphysics scheme as implemented in the NCEP Global Forecasting System (GFS) predicts only the total cloud condensate (cloud water or ice). The precipitation generated in the column fall to the ground instantly. This mean precipitation sedimentation and advection are not considered. As resolution increases the lack of the two physical processes creates problems. The slowly falling precipitation (snow) falls to the wrong surface grid box, which may have led to the observed spotty-precipitation pattern. To solve the problem two prognositic variables, snow and rain, are added. Addition of the two precipitation variable allows their advection. The corresponding sedimentation process are also added. In this study we examine the effect of precipitation advection and sedimentation on the precipitation pattern, associated precipitation skills and clouds.

Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni.

Science.gov (United States)

Muñoz-Darias, T; Casares, J; Mata Sánchez, D; Fender, R P; Armas Padilla, M; Linares, M; Ponti, G; Charles, P A; Mooley, K P; Rodriguez, J

2016-06-02

Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10(-8) solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

Accreting CO material onto ONe white dwarfs towards accretion-induced collapse

Science.gov (United States)

Wu, Cheng-Yuan; Wang, Bo

2018-03-01

The final outcomes of accreting ONe white dwarfs (ONe WDs) have been studied for several decades, but there are still some issues that are not resolved. Recently, some studies suggested that the deflagration of oxygen would occur for accreting ONe WDs with Chandrasekhar masses. In this paper, we aim to investigate whether ONe WDs can experience accretion-induced collapse (AIC) or explosions when their masses approach the Chandrasekhar limit. Employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we simulate the long-term evolution of ONe WDs with accreting CO material. The ONe WDs undergo weak multicycle carbon flashes during the mass-accretion process, leading to mass increase of the WDs. We found that different initial WD masses and mass-accretion rates influence the evolution of central density and temperature. However, the central temperature cannot reach the explosive oxygen ignition temperature due to neutrino cooling. This work implies that the final outcome of accreting ONe WDs is electron-capture induced collapse rather than thermonuclear explosion.

Accreting planets as dust dams in 'transition' disks

International Nuclear Information System (INIS)

Owen, James E.

2014-01-01

We investigate under what circumstances an embedded planet in a protoplanetary disk may sculpt the dust distribution such that it observationally presents as a 'transition' disk. We concern ourselves with 'transition' disks that have large holes (≳ 10 AU) and high accretion rates (∼10 –9 -10 –8 M ☉ yr –1 ), particularly, those disks which photoevaporative models struggle to explain. Adopting the observed accretion rates in 'transition' disks, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small (s ≲ 1 μm) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disk with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a 'transition' disk (NIR dip in the spectral energy distribution (SED), millimeter cavity, and high accretion rate). At suitably high planet masses (≳ 3-4 M J ), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disk that is optically thin to infrared radiation. The planet-disk system will present as a 'transition' disk with a dip in the SED only when the planet mass and planetary accretion rate are high enough. At other times, it will present as a disk with a primordial SED, but with a cavity in the millimeter, as observed in a handful of protoplanetary disks.

ZOMG - II. Does the halo assembly history influence central galaxies and gas accretion?

Science.gov (United States)

Romano-Díaz, Emilio; Garaldi, Enrico; Borzyszkowski, Mikolaj; Porciani, Cristiano

2017-08-01

The growth rate and the internal dynamics of galaxy-sized dark-matter haloes depend on their location within the cosmic web. Haloes that sit at the nodes grow in mass till the present time and are dominated by radial orbits. Conversely, haloes embedded in prominent filaments do not change much in size and are dominated by tangential orbits. Using zoom hydrodynamical simulations including star formation and feedback, we study how gas accretes on to these different classes of objects, which, for simplicity, we dub 'accreting' and 'stalled' haloes. We find that all haloes get a fresh supply of newly accreted gas in their inner regions, although this slowly decreases with time, in particular for the stalled haloes. The inflow of new gas is always higher than (but comparable with) that of recycled material. Overall, the cold-gas fraction increases (decreases) with time for the accreting (stalled) haloes. In all cases, a stellar disc and a bulge form at the centre of the simulated haloes. The total stellar mass is in excellent agreement with expectations based on the abundance-matching technique. Many properties of the central galaxies do not seem to correlate with the large-scale environment in which the haloes reside. However, there are two notable exceptions that characterize stalled haloes with respect to their accreting counterparts: (I) The galaxy disc contains much older stellar populations. (II) Its vertical scaleheight is larger by a factor of 2 or more. This thickening is likely due to the heating of the long-lived discs by mergers and close flybys.

Methods to assess radioisotope migration in cementitious media using radial diffusion and advection

International Nuclear Information System (INIS)

Hinchliff, J.; Felipe-Sotero, M.; Evans, N.D.M.; Read, D.; Drury, D.

2012-01-01

One of the primary aims of this project is to understand how a range of isotopes associated with radioactive wastes, move through the cementitious media potentially present in a geological disposal facility (GDF). This paper describes the development of experimental methods that use radial flow from intact cylinders of cementitious material to evaluate the potential for diffusion and advection of relevant isotopes through Nirex reference vault backfill (NRVB). The small scale and cost effectiveness of the approach means that multiple experiments can be undertaken encompassing the full range of physical (and chemical) variations. The radial flow experimental method uses small pre-cast cylinders of the matrix under investigation. For diffusion an appropriate concentration of the isotope of interest ( 90 Sr in the present experiments) is introduced into a cavity in the centre of the cylinder, which is then sealed, and placed in a solution previously equilibrated with the matrix. The increase in concentration of the isotope in the external solution is then determined at defined time intervals. For advection 90 Sr is similarly introduced into the central core of the cylinder and then equilibrated water is forced under nitrogen pressure, from the central core to the outside of the cylinder where it is collected in a tray prior to analysis. Both experimental set ups and results have been modelled using conventional numerical solutions and the simulation package GoldSim. Concerning diffusion experiments the modelled data reproduces the observed data effectively with a right diffusivity value of 9*10 -11 m 2 /s. Concerning advection results are more mitigated and need further investigation

Ice Accretion on Wind Turbine Blades

DEFF Research Database (Denmark)

Hudecz, Adriána; Koss, Holger; Hansen, Martin Otto Laver

2013-01-01

In this paper, both experimental and numerical simulations of the effects of ice accretion on a NACA 64-618 airfoil section with 7° angle of attack are presented. The wind tunnel tests were conducted in a closed-circuit climatic wind tunnel at Force Technology in Denmark. The changes of aerodynamic...... forces were monitored as ice was building up on the airfoil for glaze, rime and mixed ice. In the first part of the numerical analysis, the resulted ice profiles of the wind tunnel tests were compared to profiles estimated by using the 2D ice accretion code TURBICE. In the second part, Ansys Fluent...... of the rime iced ice profile follows the streamlines quite well, disturbing the flow the least. The TURBICE analysis agrees fairly with the profiles produced during the wind tunnel testing....

An advection-based model to increase the temporal resolution of PIV time series.

Science.gov (United States)

Scarano, Fulvio; Moore, Peter

A numerical implementation of the advection equation is proposed to increase the temporal resolution of PIV time series. The method is based on the principle that velocity fluctuations are transported passively, similar to Taylor's hypothesis of frozen turbulence . In the present work, the advection model is extended to unsteady three-dimensional flows. The main objective of the method is that of lowering the requirement on the PIV repetition rate from the Eulerian frequency toward the Lagrangian one. The local trajectory of the fluid parcel is obtained by forward projection of the instantaneous velocity at the preceding time instant and backward projection from the subsequent time step. The trajectories are approximated by the instantaneous streamlines, which yields accurate results when the amplitude of velocity fluctuations is small with respect to the convective motion. The verification is performed with two experiments conducted at temporal resolutions significantly higher than that dictated by Nyquist criterion. The flow past the trailing edge of a NACA0012 airfoil closely approximates frozen turbulence , where the largest ratio between the Lagrangian and Eulerian temporal scales is expected. An order of magnitude reduction of the needed acquisition frequency is demonstrated by the velocity spectra of super-sampled series. The application to three-dimensional data is made with time-resolved tomographic PIV measurements of a transitional jet. Here, the 3D advection equation is implemented to estimate the fluid trajectories. The reduction in the minimum sampling rate by the use of super-sampling in this case is less, due to the fact that vortices occurring in the jet shear layer are not well approximated by sole advection at large time separation. Both cases reveal that the current requirements for time-resolved PIV experiments can be revised when information is poured from space to time . An additional favorable effect is observed by the analysis in the

Statistics of an advected passive scalar

International Nuclear Information System (INIS)

Kimura, Y.; Kraichnan, R.H.

1993-01-01

An elementary argument shows that non-Gaussian fluctuations in the temperature at a point in space are induced by random advection of a passive temperature field that has a nonlinear mean gradient, whether or not there is molecular diffusion. This is corroborated by exact analysis for the nondiffusive case and by direct numerical simulation for diffusive cases. Eulerian mapping closure gives results close to the simulation data. Non-Gaussian fluctuations of temperature at a point also are induced by a more subtle mechanism that requires both advection and molecular diffusion and is effective even when the statistics are strictly homogeneous. It operates through selectively strong dissipation of regions where intense temperature gradients have been induced by advective straining. This phenomenon is demonstrated by simulations and explored by means of an idealized analytical model

Accretion on to Magnetic White Dwarfs

Directory of Open Access Journals (Sweden)

Wickramasinghe Dayal

2014-01-01

The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

Ultraluminous X-ray sources as neutrino pulsars

Science.gov (United States)

Mushtukov, Alexander A.; Tsygankov, Sergey S.; Suleimanov, Valery F.; Poutanen, Juri

2018-05-01

The classical limit on the accretion luminosity of a neutron star is given by the Eddington luminosity. The advanced models of accretion on to magnetized neutron stars account for the appearance of magnetically confined accretion columns and allow the accretion luminosity to be higher than the Eddington value by a factor of tens. However, the recent discovery of pulsations from ultraluminous X-ray source (ULX) in NGC 5907 demonstrates that the accretion luminosity can exceed the Eddington value up to by a factor of 500. We propose a model explaining observational properties of ULX-1 in NGC 5907 without any ad hoc assumptions. We show that the accretion column at extreme luminosity becomes advective. Enormous energy release within a small geometrical volume and advection result in very high temperatures at the bottom of accretion column, which demand to account for the energy losses due to neutrino emission which can be even more effective than the radiation energy losses. We show that the total luminosity at the mass accretion rates above 1021 g s-1 is dominated by the neutrino emission similarly to the case of core-collapse supernovae. We argue that the accretion rate measurements based on detected photon luminosity in case of bright ULXs powered by neutron stars can be largely underestimated due to intense neutrino emission. The recently discovered pulsating ULX-1 in galaxy NGC 5907 with photon luminosity of {˜ } 10^{41} {erg s^{-1}} is expected to be even brighter in neutrinos and is thus the first known Neutrino Pulsar.

CONSTRAINING THE STRUCTURE OF SAGITTARIUS A*'s ACCRETION FLOW WITH MILLIMETER VERY LONG BASELINE INTERFEROMETRY CLOSURE PHASES

Energy Technology Data Exchange (ETDEWEB)

Broderick, Avery E [Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto, ON M5S 3H8 (Canada); Fish, Vincent L; Doeleman, Sheperd S [Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886 (United States); Loeb, Abraham [Institute for Theory and Computation, Harvard University, Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

2011-09-01

Millimeter wave very long baseline interferometry (mm-VLBI) provides access to the emission region surrounding Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, on sub-horizon scales. Recently, a closure phase of 0{sup 0} {+-} 40{sup 0} was reported on a triangle of Earth-sized baselines (SMT-CARMA-JCMT) representing a new constraint upon the structure and orientation of the emission region, independent from those provided by the previously measured 1.3 mm-VLBI visibility amplitudes alone. Here, we compare this to the closure phases associated with a class of physically motivated, radiatively inefficient accretion flow models and present predictions for future mm-VLBI experiments with the developing Event Horizon Telescope (EHT). We find that the accretion flow models are capable of producing a wide variety of closure phases on the SMT-CARMA-JCMT triangle and thus not all models are consistent with the recent observations. However, those models that reproduce the 1.3 mm-VLBI visibility amplitudes overwhelmingly have SMT-CARMA-JCMT closure phases between {+-}30{sup 0}, and are therefore broadly consistent with all current mm-VLBI observations. Improving station sensitivity by factors of a few, achievable by increases in bandwidth and phasing together multiple antennas at individual sites, should result in physically relevant additional constraints upon the model parameters and eliminate the current 180{sup 0} ambiguity on the source orientation. When additional stations are included, closure phases of order 45{sup 0}-90{sup 0} are typical. In all cases, the EHT will be able to measure these with sufficient precision to produce dramatic improvements in the constraints upon the spin of Sgr A*.

Applying a physical continuum model to describe the broadband X-ray spectra of accreting pulsars at high luminosity

Science.gov (United States)

Pottschmidt, Katja; Hemphill, Paul B.; Wolff, Michael T.; Cheatham, Diana M.; Iwakiri, Wataru; Gottlieb, Amy M.; Falkner, Sebastian; Ballhausen, Ralf; Fuerst, Felix; Kuehnel, Matthias; Ferrigno, Carlo; Becker, Peter A.; Wood, Kent S.; Wilms, Joern

2018-01-01

A new window for better understanding the accretion onto strongly magnetized neutron stars in X-ray binaries is opening. In these systems the accreted material follows the magnetic field lines as it approaches the neutron star, forming accretion columns above the magnetic poles. The plasma falls toward the neutron star surface at near-relativistic speeds, losing energy by emitting X-rays. The X-ray spectral continua are commonly described using phenomenological models, i.e., power laws with different types of curved cut-offs at higher energies. Here we consider high luminosity pulsars. In these systems the mass transfer rate is high enough that the accreting plasma is thought to be decelerated in a radiation-dominated radiative shock in the accretion columns. While the theory of the emission from such shocks had already been developed by 2007, a model for direct comparison with X-ray continuum spectra in xspec or isis has only recently become available. Characteristic parameters of this model are the accretion column radius and the plasma temperature, among others. Here we analyze the broadband X-ray spectra of the accreting pulsars Centaurus X-3 and 4U 1626-67 obtained with NuSTAR. We present results from traditional empirical modeling as well as successfully apply the radiation-dominated radiative shock model. We also take the opportunity to compare to similar recent analyses of both sources using these and other observations.

Experimental Investigation of the Performance of Tilt Current Meters in Wave-Dominated Flows

DEFF Research Database (Denmark)

Hansen, Asger Bendix; Carstensen, Stefan

2017-01-01

In recent years, tilt current meters (TCMs) have received renewed attention as they provide an inexpensive method for measuring currents in the coastal zone. However, previous studies focused mainly on current dominated flows or the current component of the flow. This study investigates the perfo...

The Growth of Central Black Hole and the Ionization Instability of Quasar Disk

Science.gov (United States)

Lu, Ye; Cheng, K. S.; Zhang, S. N.

2003-01-01

A possible accretion model associated with the ionization instability of quasar disks is proposed to address the growth of the central black hole harbored in the host galaxy. The evolution of quasars in cosmic time is assumed to change from a highly active state to a quiescent state triggered by the S-shaped ionization instability of the quasar accretion disk. For a given external mass transfer rate supplied by the quasar host galaxy, ionization instability can modify accretion rate in the disk and separates the accretion flows of the disk into three different phases, like a S-shape. We suggest that the bright quasars observed today are those quasars with disks in the upper branch of S-shaped instability, and the faint or 'dormant' quasars are simply the system in the lower branch. The middle branch is the transition state which is unstable. We assume the quasar disk evolves according to the advection-dominated inflow-outflow solutions (ADIOS) configuration in the stable lower branch of S-shaped instability, and Eddington accretion rate is used to constrain the accretion rate in each phase. The mass ratio between black hole and its host galactic bulge is a nature consequence of ADIOS. Our model also demonstrates that a seed black hole (BH) similar to those found in spiral galaxies today is needed to produce a BH with a final mass 2 x 10(exp 8) solar mases.

Contributions to the study of astrophysical plasmas. From accretion-ejection flows to particle acceleration in shocks

International Nuclear Information System (INIS)

Casse, Fabien

2013-01-01

After having outlined that the study of turbulence is a point of convergence between mathematics and physics, and that magnetic turbulence is omnipresent in astrophysical plasmas and also present in the interstellar medium, in stars and in their environment, in accretion disks, at the vicinity of shocks, and so on, the author proposes an overview of his research works which started with a research thesis on magnetised accretion disks and transport of relativistic particles in a magnetic turbulence. So, in this report for an accreditation to supervise research (HDR), he first focuses on physics of systems in accretion, and particularly on magnetised accretion-ejection structures. He evokes his work on a stationary modelling of these structures, on magnetohydrodynamics digital simulation of these systems, and on some instabilities in accretion disks and their interest in astrophysics. In a second part, the author reports his works on numerical assessment of coefficients of spatial diffusion of cosmic rays in a magnetic turbulence, and the description of multi-scale environments such as supernovae debris or different regions of extra-galactic jets.

Generalized network modeling of capillary-dominated two-phase flow.

Science.gov (United States)

Raeini, Ali Q; Bijeljic, Branko; Blunt, Martin J

2018-02-01

We present a generalized network model for simulating capillary-dominated two-phase flow through porous media at the pore scale. Three-dimensional images of the pore space are discretized using a generalized network-described in a companion paper [A. Q. Raeini, B. Bijeljic, and M. J. Blunt, Phys. Rev. E 96, 013312 (2017)2470-004510.1103/PhysRevE.96.013312]-which comprises pores that are divided into smaller elements called half-throats and subsequently into corners. Half-throats define the connectivity of the network at the coarsest level, connecting each pore to half-throats of its neighboring pores from their narrower ends, while corners define the connectivity of pore crevices. The corners are discretized at different levels for accurate calculation of entry pressures, fluid volumes, and flow conductivities that are obtained using direct simulation of flow on the underlying image. This paper discusses the two-phase flow model that is used to compute the averaged flow properties of the generalized network, including relative permeability and capillary pressure. We validate the model using direct finite-volume two-phase flow simulations on synthetic geometries, and then present a comparison of the model predictions with a conventional pore-network model and experimental measurements of relative permeability in the literature.

Generalized network modeling of capillary-dominated two-phase flow

Science.gov (United States)

Raeini, Ali Q.; Bijeljic, Branko; Blunt, Martin J.

2018-02-01

We present a generalized network model for simulating capillary-dominated two-phase flow through porous media at the pore scale. Three-dimensional images of the pore space are discretized using a generalized network—described in a companion paper [A. Q. Raeini, B. Bijeljic, and M. J. Blunt, Phys. Rev. E 96, 013312 (2017), 10.1103/PhysRevE.96.013312]—which comprises pores that are divided into smaller elements called half-throats and subsequently into corners. Half-throats define the connectivity of the network at the coarsest level, connecting each pore to half-throats of its neighboring pores from their narrower ends, while corners define the connectivity of pore crevices. The corners are discretized at different levels for accurate calculation of entry pressures, fluid volumes, and flow conductivities that are obtained using direct simulation of flow on the underlying image. This paper discusses the two-phase flow model that is used to compute the averaged flow properties of the generalized network, including relative permeability and capillary pressure. We validate the model using direct finite-volume two-phase flow simulations on synthetic geometries, and then present a comparison of the model predictions with a conventional pore-network model and experimental measurements of relative permeability in the literature.

Buoyancy limits on magnetic viscosity stress-law scalings in quasi stellar object accretion disk models

International Nuclear Information System (INIS)

Sakimoto, P.J.

1985-01-01

Quasi-Stellar Objects (QSOs) are apparently the excessively bright nuclei of distant galaxies. They are thought to be powered by accretion disks surrounding supermassive black holes: however, proof of this presumption is hampered by major uncertainties in the viscous stress necessary for accretion to occur. Models generally assume an and hoc stress law which scales the stress with the total pressure. Near the black hole, radiation pressure dominates gas pressure; scaling the stress with the radiation pressure results in disk models that are thermally unstable and optically thin. This dissertation shows that a radiation pressure scaling for the stress is not possible if the viscosity is due to turbulent magnetic Maxwell stresses. The argument is one of internal self-consistency. First, four model accretion disks that bound the reasonably expected ranges of viscous stress scalings and vertical structures are constructed. Magnetic flux tubes of various initial field strengths are then placed within these models, nd their buoyancy is modeled numerically. In disks using the radiation pressure stress law scaling, low opacities allow rapid heat flow into the flux tubes: the tubes are extremely buoyant, and magnetic fields strong enough to provide the required stress cannot be retained. If an alternative gas pressure scaling for the stress is assumed, then the disks are optically thick; flux tubes have corresponding lower buoyancy, and magnetic fields strong enough to provide the stress can be retained for dynamically significant time periods

Influence of ice accretion on the noise generated by an airfoil section

International Nuclear Information System (INIS)

Szasz, Robert-Zoltan; Ronnfors, Matilda; Revstedt, Johan

2016-01-01

Highlights: • The noise generated by ice accreted airfoils is investigated using a hybrid approach. • The roughness of the ice surface is found to have an important effect on the radiated noise. • Ice was found to damp lower frequencies and amplify higher ones. - Abstract: We investigate the noise generated by an airfoil section. Three cases are considered, one with a clean airfoil and two cases with airfoils with ice accretion. The amount of ice is the same in the two cases with ice accretion, but the surface of the accreted ice layer is smoother in one of them. The noise is computed using a hybrid approach. First the flow and the acoustic sources are computed. Second, the noise propagation is predicted by solving an inhomogeneous wave equation. The results indicate that in this case the accreted ice layer leads to a decrease of the radiated noise levels, especially in the lower frequency range.

Low-Dissipation Advection Schemes Designed for Large Eddy Simulations of Hypersonic Propulsion Systems

Science.gov (United States)

White, Jeffrey A.; Baurle, Robert A.; Fisher, Travis C.; Quinlan, Jesse R.; Black, William S.

2012-01-01

The 2nd-order upwind inviscid flux scheme implemented in the multi-block, structured grid, cell centered, finite volume, high-speed reacting flow code VULCAN has been modified to reduce numerical dissipation. This modification was motivated by the desire to improve the codes ability to perform large eddy simulations. The reduction in dissipation was accomplished through a hybridization of non-dissipative and dissipative discontinuity-capturing advection schemes that reduces numerical dissipation while maintaining the ability to capture shocks. A methodology for constructing hybrid-advection schemes that blends nondissipative fluxes consisting of linear combinations of divergence and product rule forms discretized using 4th-order symmetric operators, with dissipative, 3rd or 4th-order reconstruction based upwind flux schemes was developed and implemented. A series of benchmark problems with increasing spatial and fluid dynamical complexity were utilized to examine the ability of the candidate schemes to resolve and propagate structures typical of turbulent flow, their discontinuity capturing capability and their robustness. A realistic geometry typical of a high-speed propulsion system flowpath was computed using the most promising of the examined schemes and was compared with available experimental data to demonstrate simulation fidelity.

Mass flow and velocity profiles in Neurospora hyphae: partial plug flow dominates intra-hyphal transport.

Science.gov (United States)

Abadeh, Aryan; Lew, Roger R

2013-11-01

Movement of nuclei, mitochondria and vacuoles through hyphal trunks of Neurospora crassa were vector-mapped using fluorescent markers and green fluorescent protein tags. The vectorial movements of all three were strongly correlated, indicating the central role of mass (bulk) flow in cytoplasm movements in N. crassa. Profiles of velocity versus distance from the hyphal wall did not match the parabolic shape predicted by the ideal Hagen-Poiseuille model of flow at low Reynolds number. Instead, the profiles were flat, consistent with a model of partial plug flow due to the high concentration of organelles in the flowing cytosol. The intra-hyphal pressure gradients were manipulated by localized external osmotic treatments to demonstrate the dependence of velocity (and direction) on pressure gradients within the hyphae. The data support the concept that mass transport, driven by pressure gradients, dominates intra-hyphal transport. The transport occurs by partial plug flow due to the organelles in the cytosol.

Formation of primordial supermassive stars by rapid mass accretion

Energy Technology Data Exchange (ETDEWEB)

Hosokawa, Takashi; Yoshida, Naoki [Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033 (Japan); Yorke, Harold W. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Inayoshi, Kohei; Omukai, Kazuyuki, E-mail: takashi.hosokawa@phys.s.u-tokyo.ac.jp, E-mail: hosokwtk@gmail.com [Department of Physics, Kyoto University, Kyoto 606-8502 (Japan)

2013-12-01

Supermassive stars (SMSs) forming via very rapid mass accretion ( M-dot {sub ∗}≳0.1 M{sub ⊙} yr{sup −1}) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10{sup 4–5} M {sub ☉}. Our stellar evolution calculations show that a star becomes supermassive while passing through the 'supergiant protostar' stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ≅ 100 AU for M {sub *} ≳ 10{sup 4} M {sub ☉}, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 10{sup 4} K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M {sub *} ≳ 10{sup 5} M {sub ☉} can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10{sup 5} M {sub ☉}. An extremely massive black hole should form after the collapse of the inner core.

Measurements on, and modelling of diffusive and advective radon transport in soil

DEFF Research Database (Denmark)

Graff, E.R. van der; Witteman, G.A.A.; Spoel, W.H. van der

1994-01-01

Results are presented of measurements on radon transport in soil under controlled conditions with a laboratory facility consisting of a stainless steel vessel (height and diameter 2 m) filled with a uniform column of sand. At several depths under the sand surface, probes are radially inserted...... into the vessel to measure the radon concentration in the soil gas. To study advective radon transport a perforated circular box is placed in the sand close to the bottom of the vessel. By pressurising this box, an air flow through the sand column is induced. Radon concentration profiles were measured without...... an air flow as a function of time, and for several values of the air flow, equilibrium radon concentration profiles were measured....

Charging and Transport Dynamics of a Flow-Through Electrode Capacitive Deionization System.

Science.gov (United States)

Qu, Yatian; Campbell, Patrick G; Hemmatifar, Ali; Knipe, Jennifer M; Loeb, Colin K; Reidy, John J; Hubert, Mckenzie A; Stadermann, Michael; Santiago, Juan G

2018-01-11

We present a study of the interplay among electric charging rate, capacitance, salt removal, and mass transport in "flow-through electrode" capacitive deionization (CDI) systems. We develop two models describing coupled transport and electro-adsorption/desorption which capture salt removal dynamics. The first model is a simplified, unsteady zero-dimensional volume-averaged model which identifies dimensionless parameters and figures of merits associated with cell performance. The second model is a higher fidelity area-averaged model which captures both spatial and temporal responses of charging. We further conducted an experimental study of these dynamics and considered two salt transport regimes: (1) advection-limited regime and (2) dispersion-limited regime. We use these data to validate models. The study shows that, in the advection-limited regime, differential charge efficiency determines the salt adsorption at the early stage of the deionization process. Subsequently, charging transitions to a quasi-steady state where salt removal rate is proportional to applied current scaled by the inlet flow rate. In the dispersion-dominated regime, differential charge efficiency, cell volume, and diffusion rates govern adsorption dynamics and flow rate has little effect. In both regimes, the interplay among mass transport rate, differential charge efficiency, cell capacitance, and (electric) charging current governs salt removal in flow-through electrode CDI.

On the definition of dominant force regimes for flow boiling heat transfer by using single mini-tubes

Science.gov (United States)

Baba, Soumei; Sawada, Kenichiro; Kubota, Chisato; Kawanami, Osamu; Asano, Hitoshi; Inoue, Koichi; Ohta, Haruhiko

Recent increase in the size of space platforms requires the management of larger amount of waste heat under high heat flux conditions and the transportation of it along a long distance to the radiator. Flow boiling applied to the thermal management system in space attracts much attention as promising means to realize high-performance heat transfer and transport because of large latent heat of vaporization. In microgravity two-phase flow phenomena are quite different from those under 1-g condition because buoyancy effects are significantly reduced and surface tension becomes dominant. By the similar reason, flow boiling characteristics in mini channels are not the same as those in channels of normal sizes. In the present stage, however, the boundary between the regimes of body force dominated and of surface tension dominated is not clear. The design of space thermal devices, operated under the conditions where no effect of gravity is expected, will improve the reliability of their ground tests, provided that the boundaries of dominant force regimes are clarified quantitatively in advance. In flow boiling in mini channels or in parallel channels, back flow could be occurred because of rapid growth of bubbles in a confined space, resulting flow rate fluctuation. Flow boiling heat transfer characteristics in mini channels can be changed considerably by the existence of inlet flow rate fluctuation. It is important to pay attention to experimental accuracy and to use a single circular mini-tube to compare heat transfer characteristics with those of normal size tubes. In the present paper, effects of tube orientations, i.e. vertical upward flow, vertical downward flow and horizontal flow, on flow boiling heat transfer characteristics is investigated for FC72 flowing in single mini-tubes with inner diameters of 0.13 and 0.51 mm to establish a reliable dominant force regime map. If the regime map is described by using dimensionless groups of Bond, Weber and Froude numbers

Visualizing Vector Fields Using Line Integral Convolution and Dye Advection

Science.gov (United States)

Shen, Han-Wei; Johnson, Christopher R.; Ma, Kwan-Liu

1996-01-01

We present local and global techniques to visualize three-dimensional vector field data. Using the Line Integral Convolution (LIC) method to image the global vector field, our new algorithm allows the user to introduce colored 'dye' into the vector field to highlight local flow features. A fast algorithm is proposed that quickly recomputes the dyed LIC images. In addition, we introduce volume rendering methods that can map the LIC texture on any contour surface and/or translucent region defined by additional scalar quantities, and can follow the advection of colored dye throughout the volume.

The role of advection in a two-species competition model

CERN Document Server

Averill, Isabel; Lou, Yuan

2017-01-01

The effects of weak and strong advection on the dynamics of reaction-diffusion models have long been studied. In contrast, the role of intermediate advection remains poorly understood. For example, concentration phenomena can occur when advection is strong, providing a mechanism for the coexistence of multiple populations, in contrast with the situation of weak advection where coexistence may not be possible. The transition of the dynamics from weak to strong advection is generally difficult to determine. In this work the authors consider a mathematical model of two competing populations in a spatially varying but temporally constant environment, where both species have the same population dynamics but different dispersal strategies: one species adopts random dispersal, while the dispersal strategy for the other species is a combination of random dispersal and advection upward along the resource gradient. For any given diffusion rates the authors consider the bifurcation diagram of positive steady states by u...

Cyclic and heteroclinic flows near general static spherically symmetric black holes

Energy Technology Data Exchange (ETDEWEB)

Ahmed, Ayyesha K.; Jamil, Mubasher [National University of Sciences and Technology(NUST), Department of Mathematics, School of Natural Sciences (SNS), Islamabad (Pakistan); Azreg-Ainou, Mustapha [Baskent University, Engineering Faculty, Ankara (Turkey); Faizal, Mir [University of Lethbridge, Department of Physics and Astronomy, Alberta (Canada); University of Waterloo, Department of Physics and Astronomy, Waterloo, ON (Canada)

2016-05-15

We investigate the Michel-type accretion onto a static spherically symmetric black hole. Using a Hamiltonian dynamical approach, we show that the standard method employed for tackling the accretion problem has masked some properties of the fluid flow. We determine new analytical solutions that are neither transonic nor supersonic as the fluid approaches the horizon(s); rather, they remain subsonic for all values of the radial coordinate. Moreover, the three-velocity vanishes and the pressure diverges on the horizon(s), resulting in a flow-out of the fluid under the effect of its own pressure. This is in favor of the earlier prediction that pressure-dominant regions form near the horizon. This result does not depend on the form of the metric and it applies to a neighborhood of any horizon where the time coordinate is timelike. For anti-de Sitter-like f(R) black holes we discuss the stability of the critical flow and determine separatrix heteroclinic orbits. For de Sitter-like f(R) black holes, we construct polytropic cyclic, non-homoclinic, physical flows connecting the two horizons. These flows become non-relativistic for Hamiltonian values higher than the critical value, allowing for a good estimate of the proper period of the flow. (orig.)

Swinging between rotation and accretion power in a binary millisecond pulsar

Directory of Open Access Journals (Sweden)

Papitto A.

2014-01-01

While accreting mass, the X-ray emission of IGR J18245–2452 varies dramatically on time-scales ranging from a second to a few hours. We interpret a state characterised by a lower flux and pulsed fraction, and by sudden increases of the hardness of the X-ray emission, in terms of the onset of a magnetospheric centrifugal inhibition of the accretion flow. Prospects of finding new members of the newly established class of transitional pulsars are also briefly discussed.

Solution of the comoving-frame equation of transfer in spherically symmetric flows. III. Effect of aberration and advection terms

International Nuclear Information System (INIS)

Mihalas, D.; Kunasz, P.B.; Hummer, D.G.

1976-01-01

We investigate the importance of the advection and aberration terms, which are of order V/c, in the comoving-frame transfer equation in spherical geometry. Characteristic trajectories are found which reduce the spatial derivatives to a perfect differential, and a generalization of the numerical procedure developed in the earlier papers of this series that permits the integration of the transfer equation on these characteristics is presented. For cases in which V/cvery-much-less-than1, a perturbation solution is developed which reduces the problem to that solved in the first paper in this series. For velocities of the form V (r) approx.r/subn/(n=0,1,2), it is shown that the magnitude of the effects arising from the advection and aberration terms is about 5V/c relative to the solution with these terms omitted. In stellar winds V/capproximately-less-than0.01; hence we conclude that aberration and advection terms may safely be ignored, and that consideration of the Doppler-shift term alone is adequate in the computation of spectra from such expanding atmospheres

Symbiotic star CI Cygni: S-process episode or accretion event

Energy Technology Data Exchange (ETDEWEB)

Kenyon, S J; Webbink, R F; Gallagher, J S; Truran, J W

1982-02-01

Evidence that the symbiotic star C I Cygni is an eclipsing binary is reviewed. It is shown that the s-process episode described by Audouze et al. (1981) during its 1975 outburst arises from superposition of normal gM4 absorption features on the continuum of the hot component during eclipse ingress, and not to sudden enhancements of rare earth elements. The peculiar velocity displacements of absorption lines with different excitation potentials during this episode are identified as signatures of an optically thick accretion disk, which dominates the visible spectrum during outburst. The data presented by Audouze et al., and the shape of the light curve thus provide evidence that the outburst is accretion-powered.

Tidal variations of flow convergence, shear, and stratification at the Rio de la Plata estuary turbidity front

Science.gov (United States)

FramiñAn, Mariana B.; Valle-Levinson, Arnoldo; Sepúlveda, HéCtor H.; Brown, Otis B.

2008-08-01

Intratidal variability of density and velocity fields is investigated at the turbidity front of the Río de la Plata Estuary, South America. Current velocity and temperature-salinity profiles collected in August 1999 along a repeated transect crossing the front are analyzed. Horizontal and vertical gradients, stability of the front, convergence zones, and transverse flow associated to the frontal boundary are described. Strong horizontal convergence of the across-front velocity and build up of along-front velocity shear were observed at the front. In the proximity of the front, enhanced transverse (or along-front) flow created jet-like structures at the surface and near the bottom flowing in opposite directions. These structures persisted throughout the tidal cycle and were advected upstream (downstream) by the flood (ebb) current through a distance of ˜10 km. During peak flood, the upper layer flow reversed from its predominant downstream direction and upstreamflow occupied the entire water column; outside the peak flood, two-layer estuarine circulation dominated. Changes in density field were observed in response to tidal straining, tidal advection, and wind-induced mixing, but stratification remained throughout the tidal cycle. This work demonstrates the large spatial variability of the velocity field at the turbidity front; it provides evidence of enhanced transverse circulation along the frontal boundary; and reveals the importance of advective and frictional intratidal processes in the dynamics of the central part of the estuary.

On the Calculation of the Fe K-alpha Line Emissivity of Black Hole Accretion Disks

Energy Technology Data Exchange (ETDEWEB)

Krawczynski, H.; Beheshtipour, B., E-mail: krawcz@wustl.edu [Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, 1 Brookings Drive, CB 1105, St. Louis, MO 63130 (United States)

2017-11-01

Observations of the fluorescent Fe K α emission line from the inner accretion flows of stellar mass black holes in X-ray binaries and supermassive black holes in active galactic nuclei have become an important tool to study the magnitude and inclination of the black hole spin, and the structure of the accretion flow close to the event horizon of the black hole. Modeling spectral, timing, and soon also X-ray polarimetric observations of the Fe K α emission requires the calculation of the specific intensity in the rest frame of the emitting plasma. We revisit the derivation of the equation used for calculating the illumination of the accretion disk by the corona. We present an alternative derivation leading to a simpler equation, and discuss the relation to previously published results.

Inelastic accretion of inertial particles by a towed sphere

Science.gov (United States)

Vallée, Robin; Henry, Christophe; Hachem, Elie; Bec, Jérémie

2018-02-01

The problem of accretion of small particles by a sphere embedded in a mean flow is studied in the case where the particles undergo inelastic collisions with the solid object. The collision efficiency, which gives the flux of particles experiencing at least one bounce on the sphere, is found to depend upon the sphere Reynolds number only through the value of the critical Stokes number below which no collision occurs. In the absence of molecular diffusion, it is demonstrated that multiple bounces do not provide enough energy dissipation for the particles to stick to the surface within a finite time. This excludes the possibility of any kind of inelastic collapse, so that determining an accretion efficiency requires modeling more precisely particle-surface microphysical interactions. A straightforward choice is to assume that the particles stick when their kinetic energy at impact is below a threshold. In this view, numerical simulations are performed to describe the statistics of impact velocities at various values of the Reynolds number. Successive bounces are shown to enhance accretion. These results are put together to provide a general qualitative picture on how the accretion efficiency depends upon the nondimensional parameters of the problem.

Constraints on the inner accretion flow of 4U/MXB 1636-53 (V 801 Arae) from a comparison of X-ray burst and persistent emission

NARCIS (Netherlands)

Damen, E.; Wijers, R.A.M.J.; van Paradijs, J.; Penninx, W.; Oosterbroek, T.; Lewin, W.H.G.; Jansen, F.

1990-01-01

A detailed analysis is presented of the importance of Comptonization in burst and persistent spectra of the low-mass X-ray binary 4U/MXB 1636-53, and from this analysis it is inferred that the inner accretion flow is geometrically thin. It is found that burst spectra of 1636-53 are very nearly

Advective and diffusive contributions to reactive gas transport during pyrite oxidation in the unsaturated zone

DEFF Research Database (Denmark)

Binning, Philip John; Postma, Diederik Jan; Russel, T.F.

2007-01-01

Pyrite oxidation in unsaturated mine waste rock dumps and soils is limited by the supply of oxygen from the atmosphere. In models, oxygen transport through the subsurface is often assumed to be driven by diffusion. However, oxygen comprises 23.2% by mass of dry air, and when oxygen is consumed at...... parameters; for example, the time to approach steady state depends exponentially on the distance between the soil surface and the subsurface reactive zone. Copyright 2007 by the American Geophysical Union....... at depth in the unsaturated zone, a pressure gradient is created between the reactive zone and the ground surface, causing a substantial advective air flow into the subsurface. To determine the balance between advective and diffusive transport, a one-dimensional multicomponent unsaturated zone gas...

RKC time-stepping for advection-diffusion-reaction problems

International Nuclear Information System (INIS)

Verwer, J.G.; Sommeijer, B.P.; Hundsdorfer, W.

2004-01-01

The original explicit Runge-Kutta-Chebyshev (RKC) method is a stabilized second-order integration method for pure diffusion problems. Recently, it has been extended in an implicit-explicit manner to also incorporate highly stiff reaction terms. This implicit-explicit RKC method thus treats diffusion terms explicitly and the highly stiff reaction terms implicitly. The current paper deals with the incorporation of advection terms for the explicit method, thus aiming at the implicit-explicit RKC integration of advection-diffusion-reaction equations in a manner that advection and diffusion terms are treated simultaneously and explicitly and the highly stiff reaction terms implicitly

Modeling of flow-dominated MHD instabilities at WiPPAL using NIMROD

Science.gov (United States)

Flanagan, K.; McCollam, K. J.; Milhone, J.; Mirnov, V. V.; Nornberg, M. D.; Peterson, E. E.; Siller, R.; Forest, C. B.

2017-10-01

Using the NIMROD (non-ideal MHD with rotation - open discussion) code developed at UW-Madison, we model two different flow scenarios to study the onset of MHD instabilities in flow-dominated plasmas in the Big Red Ball (BRB) and the Plasma Couette Experiment (PCX). Both flows rely on volumetric current drive, where a large current is drawn through the plasma across a weak magnetic field, injecting J × B torque across the whole volume. The first scenario uses a vertical applied magnetic field and a mostly radial injected current to create Couette-like flows which may excite the magnetorotational instability (MRI). In the other scenario, a quadrupolar field is applied to create counter-rotating von Karman-like flow that demonstrates a dynamo-like instability. For both scenarios, the differences between Hall and MHD Ohm's laws are explored. The implementation of BRB geometry in NIMROD, details of the observed flows, and instability results are shown. This work was funded by DoE and NSF.

NUMERICAL SIMULATION OF ICE ACCRETION ON AIRFOIL

Directory of Open Access Journals (Sweden)

Nicusor ALEXANDRESCU

2009-09-01

Full Text Available This work consists in the simulation of the ice accretion in the leading edge of aerodynamic profiles and our proposed model encompasses: geometry generation, calculation of the potential flow around the body, boundary layer thickness computation, water droplet trajectory computation, heat and mass balances and the consequent modification of the geometry by the ice growth. The flow calculation is realized with panel methods, using only segments defined over the body contour. The viscous effects are considered using the Karman-Pohlhausen method for the laminar boundary layer. The local heat transfer coefficient is obtained by applying the Smith-Spalding method for the thermal boundary layer. The ice accretion limits and the collection efficiency are determined by computing water droplet trajectories impinging the surface. The heat transfer process is analyzed with an energy and a mass balance in each segment defining the body. Finally, the geometry is modified by the addition of the computed ice thickness to the respective panel. The process by repeating all the steps. The model validation is done using a selection of problems with experimental solution, CIRA (the CESAR project. Hereinafter, results are obtained for different aerodynamic profiles, angles of attack and meteorological parameters

Accreting Black Holes

OpenAIRE

Begelman, Mitchell C.

2014-01-01

I outline the theory of accretion onto black holes, and its application to observed phenomena such as X-ray binaries, active galactic nuclei, tidal disruption events, and gamma-ray bursts. The dynamics as well as radiative signatures of black hole accretion depend on interactions between the relatively simple black-hole spacetime and complex radiation, plasma and magnetohydrodynamical processes in the surrounding gas. I will show how transient accretion processes could provide clues to these ...

Accretion from an inhomogeneous medium

International Nuclear Information System (INIS)

Livio, M.; Soker, N.; Koo, M. de; Savonije, G.J.

1986-01-01

The problem of accretion by a compact object from an inhomogeneous medium is studied in the general γnot=1 case. The mass accretion rate is found to decrease with increasing γ. The rate of accretion of angular momentum is found to be significantly lower than the rate at which angular momentum is deposited into the Bondi-Hoyle, symmetrical, accretion cylinder. The consequences of the results are studied for the cases of neutron stars accreting from the winds of early-type companions and white dwarfs and main-sequence stars accreting from winds of cool giants. (author)

Predicting salt advection in groundwater from saline aquaculture ponds

Science.gov (United States)

Verrall, D. P.; Read, W. W.; Narayan, K. A.

2009-01-01

SummaryThis paper predicts saltwater advection in groundwater from leaky aquaculture ponds. A closed form solution for the potential function, stream function and velocity field is derived via the series solutions method. Numerically integrating along different streamlines gives the location (or advection front) of saltwater throughout the domain for any predefined upper time limit. Extending this process produces a function which predicts advection front location against time. The models considered in this paper are easily modified given knowledge of the required physical parameters.

Virus Dynamics Are Influenced by Season, Tides and Advective Transport in Intertidal, Permeable Sediments.

Science.gov (United States)

Vandieken, Verona; Sabelhaus, Lara; Engelhardt, Tim

2017-01-01

Sandy surface sediments of tidal flats exhibit high microbial activity due to the fast and deep-reaching transport of oxygen and nutrients by porewater advection. On the other hand during low tide, limited transport results in nutrient and oxygen depletion concomitant to the accumulation of microbial metabolites. This study represents the first attempt to use flow-through reactors to investigate virus production, virus transport and the impact of tides and season in permeable sediments. The reactors were filled with intertidal sands of two sites (North beach site and backbarrier sand flat of Spiekeroog island in the German Wadden Sea) to best simulate advective porewater transport through the sediments. Virus and cell release along with oxygen consumption were measured in the effluents of reactors during continuous flow of water through the sediments as well as in tidal simulation experiments where alternating cycles with and without water flow (each for 6 h) were operated. The results showed net rates of virus production (0.3-13.2 × 10 6 viruses cm -3 h -1 ) and prokaryotic cell production (0.3-10.0 × 10 5 cells cm -3 h -1 ) as well as oxygen consumption rates (56-737 μmol l -1 h -1 ) to be linearly correlated reflecting differences in activity, season and location of the sediments. Calculations show that total virus turnover was fast with 2 to 4 days, whereas virus-mediated cell turnover was calculated to range between 5-13 or 33-91 days depending on the assumed burst sizes (number of viruses released upon cell lysis) of 14 or 100 viruses, respectively. During the experiments, the homogenized sediments in the reactors became vertically structured with decreasing microbial activities and increasing impact of viruses on prokaryotic mortality with depth. Tidal simulation clearly showed a strong accumulation of viruses and cells in the top sections of the reactors when the flow was halted indicating a consistently high virus production during low tide. In

On the Maximum Mass of Accreting Primordial Supermassive Stars

Energy Technology Data Exchange (ETDEWEB)

Woods, T. E.; Heger, Alexander [Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800 (Australia); Whalen, Daniel J. [Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Portsmouth PO1 3FX (United Kingdom); Haemmerlé, Lionel; Klessen, Ralf S. [Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische. Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg (Germany)

2017-06-10

Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z ∼ 6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution, and collapse of accreting, non-rotating supermassive stars under accretion rates of 0.01–10 M {sub ⊙} yr{sup −1} using the stellar evolution code Kepler . Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000–330,000 M {sub ⊙} for accretion rates of 0.1–10 M {sub ⊙} yr{sup −1}, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.

Hopf bifurcation in a delayed reaction-diffusion-advection population model

Science.gov (United States)

Chen, Shanshan; Lou, Yuan; Wei, Junjie

2018-04-01

In this paper, we investigate a reaction-diffusion-advection model with time delay effect. The stability/instability of the spatially nonhomogeneous positive steady state and the associated Hopf bifurcation are investigated when the given parameter of the model is near the principle eigenvalue of an elliptic operator. Our results imply that time delay can make the spatially nonhomogeneous positive steady state unstable for a reaction-diffusion-advection model, and the model can exhibit oscillatory pattern through Hopf bifurcation. The effect of advection on Hopf bifurcation values is also considered, and our results suggest that Hopf bifurcation is more likely to occur when the advection rate increases.

Spatio-temporal variability in accretion and erosion of coastal foredunes in the Netherlands: regional climate and local topography.

Science.gov (United States)

Keijsers, Joep G S; Poortinga, Ate; Riksen, Michel J P M; Maroulis, Jerry

2014-01-01

Depending on the amount of aeolian sediment input and dune erosion, dune size and morphology change over time. Since coastal foredunes play an important role in the Dutch coastal defence, it is important to have good insight in the main factors that control these changes. In this paper the temporal variations in foredune erosion and accretion were studied in relation to proxies for aeolian transport potential and storminess using yearly elevation measurements from 1965 to 2012 for six sections of the Dutch coast. Longshore differences in the relative impacts of erosion and accretion were examined in relation to local beach width. The results show that temporal variability in foredune accretion and erosion is highest in narrow beach sections. Here, dune erosion alternates with accretion, with variability displaying strong correlations with yearly values of storminess (maximum sea levels). In wider beach sections, dune erosion is less frequent, with lower temporal variability and stronger correlations with time series of transport potential. In erosion dominated years, eroded volumes decrease from narrow to wider beaches. When accretion dominates, dune-volume changes are relatively constant alongshore. Dune erosion is therefore suggested to control spatial variability in dune-volume changes. On a scale of decades, the volume of foredunes tends to increase more on wider beaches. However, where widths exceed 200 to 300 m, this trend is no longer observed.

RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

International Nuclear Information System (INIS)

Cunningham, Andrew J.; Klein, Richard I.; McKee, Christopher F.; Krumholz, Mark R.; Teyssier, Romain

2012-01-01

We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of ∼2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of ∼0.2 β 1/2 compared to the Bondi value, where β is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

Energy Technology Data Exchange (ETDEWEB)

Cunningham, Andrew J.; Klein, Richard I. [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States); McKee, Christopher F. [Department of Astronomy, University of California Berkeley, Berkeley, CA 94720 (United States); Krumholz, Mark R. [Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA 94560 (United States); Teyssier, Romain, E-mail: ajcunn@gmail.com [Service d' Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette (France)

2012-01-10

We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of {approx}2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of {approx}0.2 {beta}{sup 1/2} compared to the Bondi value, where {beta} is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

The Emerging Paradigm of Pebble Accretion

NARCIS (Netherlands)

Ormel, C.W.; Pessah, M.; Gressel, O.

2017-01-01

Pebble accretion is the mechanism in which small particles ("pebbles") accrete onto big bodies big (planetesimals or planetary embryos) in gas-rich environments. In pebble accretion accretion , accretion occurs by settling and depends only on the mass of the gravitating body gravitating , not its

The symbiotic star CI Cygni: S-process episode or accretion event

International Nuclear Information System (INIS)

Kenyon, S.J.; Webbink, R.F.; Gallagher, J.S.; Truran, J.W.

1982-01-01

Evidence that the symbiotic star C I Cygni is an eclipsing binary is reviewed. It is shown that the 's-process episode' described by Audouze et al. (1981) during its 1975 outburst arises from superposition of normal gM4 absorption features on the continuum of the hot component during eclipse ingress, and not to sudden enhancements of rare earth elements. The peculiar velocity displacements of absorption lines with different excitation potentials during this episode are identified as signatures of an optically thick accretion disk, which dominates the visible spectrum during outburst. The data presented by Audouze et al., and the shape of the light curve thus provide evidence that the outburst is accretion-powered. (orig.)

The nature and role of advection in advection-diffusion equations used for modelling bed load transport

Science.gov (United States)

Ancey, Christophe; Bohorquez, Patricio; Heyman, Joris

2016-04-01

The advection-diffusion equation arises quite often in the context of sediment transport, e.g., for describing time and space variations in the particle activity (the solid volume of particles in motion per unit streambed area). Stochastic models can also be used to derive this equation, with the significant advantage that they provide information on the statistical properties of particle activity. Stochastic models are quite useful when sediment transport exhibits large fluctuations (typically at low transport rates), making the measurement of mean values difficult. We develop an approach based on birth-death Markov processes, which involves monitoring the evolution of the number of particles moving within an array of cells of finite length. While the topic has been explored in detail for diffusion-reaction systems, the treatment of advection has received little attention. We show that particle advection produces nonlocal effects, which are more or less significant depending on the cell size and particle velocity. Albeit nonlocal, these effects look like (local) diffusion and add to the intrinsic particle diffusion (dispersal due to velocity fluctuations), with the important consequence that local measurements depend on both the intrinsic properties of particle displacement and the dimensions of the measurement system.

NUMERICAL SIMULATIONS OF NATURALLY TILTED, RETROGRADELY PRECESSING, NODAL SUPERHUMPING ACCRETION DISKS

International Nuclear Information System (INIS)

Montgomery, M. M.

2012-01-01

Accretion disks around black hole, neutron star, and white dwarf systems are thought to sometimes tilt, retrogradely precess, and produce hump-shaped modulations in light curves that have a period shorter than the orbital period. Although artificially rotating numerically simulated accretion disks out of the orbital plane and around the line of nodes generate these short-period superhumps and retrograde precession of the disk, no numerical code to date has been shown to produce a disk tilt naturally. In this work, we report the first naturally tilted disk in non-magnetic cataclysmic variables using three-dimensional smoothed particle hydrodynamics. Our simulations show that after many hundreds of orbital periods, the disk has tilted on its own and this disk tilt is without the aid of radiation sources or magnetic fields. As the system orbits, the accretion stream strikes the bright spot (which is on the rim of the tilted disk) and flows over and under the disk on different flow paths. These different flow paths suggest the lift force as a source to disk tilt. Our results confirm the disk shape, disk structure, and negative superhump period and support the source to disk tilt, source to retrograde precession, and location associated with X-ray and He II emission from the disk as suggested in previous works. Our results identify the fundamental negative superhump frequency as the indicator of disk tilt around the line of nodes.

Diffusion Dominant Solute Transport Modelling in Fractured Media Under Deep Geological Environment - 12211

Energy Technology Data Exchange (ETDEWEB)

Kwong, S. [National Nuclear Laboratory (United Kingdom); Jivkov, A.P. [Research Centre for Radwaste and Decommissioning and Modelling and Simulation Centre, University of Manchester (United Kingdom)

2012-07-01

Deep geologic disposal of high activity and long-lived radioactive waste is gaining increasing support in many countries, where suitable low permeability geological formation in combination with engineered barriers are used to provide long term waste contaminant and minimise the impacts to the environment and risk to the biosphere. This modelling study examines the solute transport in fractured media under low flow velocities that are relevant to a deep geological environment. In particular, reactive solute transport through fractured media is studied using a 2-D model, that considers advection and diffusion, to explore the coupled effects of kinetic and equilibrium chemical processes. The effects of water velocity in the fracture, matrix porosity and diffusion on solute transport are investigated and discussed. Some illustrative modelled results are presented to demonstrate the use of the model to examine the effects of media degradation on solute transport, under the influences of hydrogeological (diffusion dominant) and microbially mediated chemical processes. The challenges facing the prediction of long term degradation such as cracks evolution, interaction and coalescence are highlighted. The potential of a novel microstructure informed modelling approach to account for these effects is discussed, particularly with respect to investigating multiple phenomena impact on material performance. The GRM code is used to examine the effects of media degradation for a geological waste disposal package, under the combined hydrogeological (diffusion dominant) and chemical effects in low groundwater flow conditions that are typical of deep geological disposal systems. An illustrative reactive transport modelling application demonstrates the use of the code to examine the interplay of kinetic controlled biogeochemical reactive processes with advective and diffusive transport, under the influence of media degradation. The initial model results are encouraging which show the

Modeling the Effects of Ice Accretion on the Low Pressure Compressor and the Overall Turbofan Engine System Performance

Science.gov (United States)

Veres, Joseph P.; Jorgenson, Philip C. E.; Wright, William B.

2011-01-01

The focus of this study is on utilizing a mean line compressor flow analysis code coupled to an engine system thermodynamic code, to estimate the effects of ice accretion on the low pressure compressor, and quantifying its effects on the engine system throughout a notional flight trajectory. In this paper a temperature range in which engine icing would occur was assumed. This provided a mechanism to locate potential component icing sites and allow the computational tools to add blockages due to ice accretion in a parametric fashion. Ultimately the location and level of blockage due to icing would be provided by an ice accretion code. To proceed, an engine system modeling code and a mean line compressor flow analysis code were utilized to calculate the flow conditions in the fan-core and low pressure compressor and to identify potential locations within the compressor where ice may accrete. In this study, an "additional blockage" due to the accretion of ice on the metal surfaces, has been added to the baseline aerodynamic blockage due to boundary layer, as well as the blade metal blockage. Once the potential locations of ice accretion are identified, the levels of additional blockage due to accretion were parametrically varied to estimate the effects on the low pressure compressor blade row performance operating within the engine system environment. This study includes detailed analysis of compressor and engine performance during cruise and descent operating conditions at several altitudes within the notional flight trajectory. The purpose of this effort is to develop the computer codes to provide a predictive capability to forecast the onset of engine icing events, such that they could ultimately help in the avoidance of these events.

Simulations of small solid accretion on to planetesimals in the presence of gas

Science.gov (United States)

Hughes, A. G.; Boley, A. C.

2017-12-01

The growth and migration of planetesimals in a young protoplanetary disc are fundamental to planet formation. In all models of early growth, there are several processes that can inhibit grains from reaching larger sizes. Nevertheless, observations suggest that growth of planetesimals must be rapid. If a small number of 100 km sized planetesimals do manage to form in the disc, then gas drag effects could enable them to efficiently accrete small solids from beyond their gravitationally focused cross-section. This gas-drag-enhanced accretion can allow planetesimals to grow at rapid rates, in principle. We present self-consistent hydrodynamics simulations with direct particle integration and gas-drag coupling to estimate the rate of planetesimal growth due to pebble accretion. Wind tunnel simulations are used to explore a range of particle sizes and disc conditions. We also explore analytic estimates of planetesimal growth and numerically integrate planetesimal drift due to the accretion of small solids. Our results show that, for almost every case that we consider, there is a clearly preferred particle size for accretion that depends on the properties of the accreting planetesimal and the local disc conditions. For solids much smaller than the preferred particle size, accretion rates are significantly reduced as the particles are entrained in the gas and flow around the planetesimal. Solids much larger than the preferred size accrete at rates consistent with gravitational focusing. Our analytic estimates for pebble accretion highlight the time-scales that are needed for the growth of large objects under different disc conditions and initial planetesimal sizes.

Isothermal Bondi Accretion in Jaffe and Hernquist Galaxies with a Central Black Hole: Fully Analytical Solutions

Energy Technology Data Exchange (ETDEWEB)

Ciotti, Luca; Pellegrini, Silvia, E-mail: luca.ciotti@unibo.it [Department of Physics and Astronomy, University of Bologna, via Piero Gobetti 93/2, I-40129 Bologna (Italy)

2017-10-10

One of the most active fields of research of modern-day astrophysics is that of massive black hole formation and coevolution with the host galaxy. In these investigations, ranging from cosmological simulations, to semi-analytical modeling, to observational studies, the Bondi solution for accretion on a central point-mass is widely adopted. In this work we generalize the classical Bondi accretion theory to take into account the effects of the gravitational potential of the host galaxy, and of radiation pressure in the optically thin limit. Then, we present the fully analytical solution, in terms of the Lambert–Euler W -function, for isothermal accretion in Jaffe and Hernquist galaxies with a central black hole. The flow structure is found to be sensitive to the shape of the mass profile of the host galaxy. These results and the formulae that are provided, most importantly, the one for the critical accretion parameter, allow for a direct evaluation of all flow properties, and are then useful for the abovementioned studies. As an application, we examine the departure from the true mass accretion rate of estimates obtained using the gas properties at various distances from the black hole, under the hypothesis of classical Bondi accretion. An overestimate is obtained from regions close to the black hole, and an underestimate outside a few Bondi radii; the exact position of the transition between the two kinds of departure depends on the galaxy model.

Magnetized SASI: its mechanism and possible connection to some QPOs in XRBs

Science.gov (United States)

Dhang, Prasun; Sharma, Prateek; Mukhopadhyay, Banibrata

2018-05-01

The presence of a surface at the inner boundary, such as in a neutron star or a white dwarf, allows the existence of a standing shock in steady spherical accretion. The standing shock can become unstable in 2D or 3D; this is called the standing accretion shock instability (SASI). Two mechanisms - advective-acoustic and purely acoustic - have been proposed to explain SASI. Using axisymmetric hydrodynamic and magnetohydrodynamic simulations, we find that the advective-acoustic mechanism better matches the observed oscillation time-scales in our simulations. The global shock oscillations present in the accretion flow can explain many observed high frequency (≳100 Hz) quasi-periodic oscillations (QPOs) in X-ray binaries. The presence of a moderately strong magnetic field adds more features to the shock oscillation pattern, giving rise to low frequency modulation in the computed light curve. This low frequency modulation can be responsible for ˜100 Hz QPOs (known as hHz QPOs). We propose that the appearance of hHz QPO determines the separation of twin peak QPOs of higher frequencies.

Blazar Variability from Turbulence in Jets Launched by Magnetically Arrested Accretion Flows

International Nuclear Information System (INIS)

Riordan, Michael O’; Pe’er, Asaf; McKinney, Jonathan C.

2017-01-01

Blazars show variability on timescales ranging from minutes to years, the former being comparable to and in some cases even shorter than the light-crossing time of the central black hole. The observed γ -ray light curves can be described by a power-law power density spectrum (PDS), with a similar index for both BL Lacs and flat-spectrum radio quasars. We show that this variability can be produced by turbulence in relativistic jets launched by magnetically arrested accretion flows (MADs). We perform radiative transport calculations on the turbulent, highly magnetized jet launching region of a MAD with a rapidly rotating supermassive black hole. The resulting synchrotron and synchrotron self-Compton emission, originating from close to the black hole horizon, is highly variable. This variability is characterized by PDS, which is remarkably similar to the observed power-law spectrum at frequencies less than a few per day. Furthermore, turbulence in the jet launching region naturally produces fluctuations in the plasma on scales much smaller than the horizon radius. We speculate that similar turbulent processes, operating in the jet at large radii (and therefore a high bulk Lorentz factor), are responsible for blazar variability over many decades in frequency, including on minute timescales.

Blazar Variability from Turbulence in Jets Launched by Magnetically Arrested Accretion Flows

Energy Technology Data Exchange (ETDEWEB)

Riordan, Michael O’; Pe’er, Asaf [Physics Department, University College Cork, Cork (Ireland); McKinney, Jonathan C., E-mail: michael_oriordan@umail.ucc.ie [Department of Physics and Joint Space-Science Institute, University of Maryland, College Park, MD 20742 (United States)

2017-07-10

Blazars show variability on timescales ranging from minutes to years, the former being comparable to and in some cases even shorter than the light-crossing time of the central black hole. The observed γ -ray light curves can be described by a power-law power density spectrum (PDS), with a similar index for both BL Lacs and flat-spectrum radio quasars. We show that this variability can be produced by turbulence in relativistic jets launched by magnetically arrested accretion flows (MADs). We perform radiative transport calculations on the turbulent, highly magnetized jet launching region of a MAD with a rapidly rotating supermassive black hole. The resulting synchrotron and synchrotron self-Compton emission, originating from close to the black hole horizon, is highly variable. This variability is characterized by PDS, which is remarkably similar to the observed power-law spectrum at frequencies less than a few per day. Furthermore, turbulence in the jet launching region naturally produces fluctuations in the plasma on scales much smaller than the horizon radius. We speculate that similar turbulent processes, operating in the jet at large radii (and therefore a high bulk Lorentz factor), are responsible for blazar variability over many decades in frequency, including on minute timescales.

Limiting Accretion onto Massive Stars by Fragmentation-Induced Starvation

Energy Technology Data Exchange (ETDEWEB)

Peters, Thomas; /ZAH, Heidelberg; Klessen, Ralf S.; /ZAH, Heidelberg /KIPAC, Menlo Park; Mac Low, Mordecai-Mark; /Amer. Museum Natural Hist.; Banerjee, Robi; /ZAH, Heidelberg

2010-08-25

Massive stars influence their surroundings through radiation, winds, and supernova explosions far out of proportion to their small numbers. However, the physical processes that initiate and govern the birth of massive stars remain poorly understood. Two widely discussed models are monolithic collapse of molecular cloud cores and competitive accretion. To learn more about massive star formation, we perform simulations of the collapse of rotating, massive, cloud cores including radiative heating by both non-ionizing and ionizing radiation using the FLASH adaptive mesh refinement code. These simulations show fragmentation from gravitational instability in the enormously dense accretion flows required to build up massive stars. Secondary stars form rapidly in these flows and accrete mass that would have otherwise been consumed by the massive star in the center, in a process that we term fragmentation-induced starvation. This explains why massive stars are usually found as members of high-order stellar systems that themselves belong to large clusters containing stars of all masses. The radiative heating does not prevent fragmentation, but does lead to a higher Jeans mass, resulting in fewer and more massive stars than would form without the heating. This mechanism reproduces the observed relation between the total stellar mass in the cluster and the mass of the largest star. It predicts strong clumping and filamentary structure in the center of collapsing cores, as has recently been observed. We speculate that a similar mechanism will act during primordial star formation.

Wind accretion: Theory and observations

Science.gov (United States)

Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

2015-07-01

A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass

Accuracy of spectral and finite difference schemes in 2D advection problems

DEFF Research Database (Denmark)

Naulin, V.; Nielsen, A.H.

2003-01-01

In this paper we investigate the accuracy of two numerical procedures commonly used to solve 2D advection problems: spectral and finite difference (FD) schemes. These schemes are widely used, simulating, e.g., neutral and plasma flows. FD schemes have long been considered fast, relatively easy...... that the accuracy of FD schemes can be significantly improved if one is careful in choosing an appropriate FD scheme that reflects conservation properties of the nonlinear terms and in setting up the grid in accordance with the problem....

INTERFERENCE AS AN ORIGIN OF THE PEAKED NOISE IN ACCRETING X-RAY BINARIES

Energy Technology Data Exchange (ETDEWEB)

Veledina, Alexandra, E-mail: alexandra.veledina@gmail.com [Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-10691 Stockholm (Sweden)

2016-12-01

We propose a physical model for the peaked noise in the X-ray power density spectra of accreting X-ray binaries. We interpret its appearance as an interference of two Comptonization continua: one coming from the upscattering of seed photons from the cold thin disk and the other fed by the synchrotron emission of the hot flow. Variations of both X-ray components are caused by fluctuations in mass accretion rate, but there is a delay between them corresponding to the propagation timescale from the disk Comptonization radius to the region of synchrotron Comptonization. If the disk and synchrotron Comptonization are correlated, the humps in the power spectra are harmonically related and the dips between them appear at frequencies related as odd numbers 1:3:5. If they are anti-correlated, the humps are related as 1:3:5, but the dips are harmonically related. Similar structures are expected to be observed in accreting neutron star binaries and supermassive black holes. The delay can be easily recovered from the frequency of peaked noise and further used to constrain the combination of the viscosity parameter and disk height-to-radius ratio α ( H / R ){sup 2} of the accretion flow. We model multi-peak power spectra of black hole X-ray binaries GX 339–4 and XTE J1748–288 to constrain these parameters.

Radio emission from Sgr A*: pulsar transits through the accretion disc

Science.gov (United States)

Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

2017-06-01

Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity Lsd ˜ 3 × 1035 erg s-1 and a wind Lorentz factor of γw ˜ 104 a quasi-steady synchrotron emission is established with luminosities in the 1-10 GHz range comparable to the observed one.

OBSERVATION OF MAGNETIC RECONNECTION DRIVEN BY GRANULAR SCALE ADVECTION

International Nuclear Information System (INIS)

Zeng Zhicheng; Cao Wenda; Ji Haisheng

2013-01-01

We report the first evidence of magnetic reconnection driven by advection in a rapidly developing large granule using high spatial resolution observations of a small surge event (base size ∼ 4'' × 4'') with the 1.6 m aperture New Solar Telescope at the Big Bear Solar Observatory. The observations were carried out in narrowband (0.5 Å) He I 10830 Å and broadband (10 Å) TiO 7057 Å. Since He I 10830 Å triplet has a very high excitation level and is optically thin, its filtergrams enable us to investigate the surge from the photosphere through the chromosphere into the lower corona. Simultaneous space data from the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory were used in the analysis. It is shown that the surge is spatio-temporally associated with magnetic flux emergence in the rapidly developing large granule. During the development of the granule, its advecting flow (∼2 km s –1 ) squeezed the magnetic flux into an intergranular lane area, where a magnetic flux concentration was formed and the neighboring flux with opposite magnetic polarity was canceled. During the cancellation, the surge was produced as absorption in He I 10830 Å filtergrams while simultaneous EUV brightening occurred at its base. The observations clearly indicate evidence of a finest-scale reconnection process driven by the granule's motion.

EVOLUTION OF WARPED ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI. I. ROLES OF FEEDING AT THE OUTER BOUNDARIES

International Nuclear Information System (INIS)

Li, Yan-Rong; Wang, Jian-Min; Cheng, Cheng; Qiu, Jie

2013-01-01

We investigate the alignment processes of spinning black holes and their surrounding warped accretion disks in a frame of two different types of feeding at the outer boundaries. We consider (1) fixed flows in which gas is continually fed with a preferred angular momentum, and (2) free flows in which there is no gas supply and the disks diffuse freely at their outer edges. As expected, we find that for the cases of fixed flows the black hole disk systems always align on timescales of several 10 6 yr, irrespective of the initial inclinations. If the initial inclination angles are larger than π/2, the black hole accretion transits from retrograde to prograde fashion, and the accreted mass onto the black holes during these two phases is comparable. On the other hand, for the cases of free flows, both alignments and anti-alignments can occur, depending on the initial inclinations and the ratios of the angular momentum of the disks to that of the black holes. In such cases, the disks will be consumed within timescales of 10 6 yr by black holes accreting at the Eddington limit. We propose that there is a close connection between the black hole spin and the lifetime for which the feeding persists, which determines the observable episodic lifetimes of active galactic nuclei. We conclude that careful inclusion of the disk feeding at the outer boundaries is crucial for modeling the evolution of the black hole spin.

Ice accretion modeling for wind turbine rotor blades

Energy Technology Data Exchange (ETDEWEB)

Chocron, D.; Brahimi, T.; Paraschivoiu, I.; Bombardier, J.A. [Ecole Polytechnique de Montreal (Canada)

1997-12-31

The increasing application of wind energy in northern climates implies operation of wind turbines under severe atmospheric icing conditions. Such conditions are well known in the Scandinavian countries, Canada and most of Eastern European countries. An extensive study to develop a procedure for the prediction of ice accretion on wind turbines rotor blades appears to be essential for the safe and economic operation of wind turbines in these cold regions. The objective of the present paper is to develop a computer code capable of simulating the shape and amount of ice which may accumulate on horizontal axis wind turbine blades when operating in icing conditions. The resulting code is capable to predict and simulate the formation of ice in rime and glaze conditions, calculate the flow field and particle trajectories and to perform thermodynamic analysis. It also gives the possibility of studying the effect of different parameters that influence ice formation such as temperature, liquid water content, droplet diameter and accretion time. The analysis has been conducted on different typical airfoils as well as on NASA/DOE Mod-0 wind turbine. Results showed that ice accretion on wind turbines may reduce the power output by more than 20%.

Radial Transport and Meridional Circulation in Accretion Disks

Energy Technology Data Exchange (ETDEWEB)

Philippov, Alexander A. [Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540 (United States); Rafikov, Roman R., E-mail: sashaph@princeton.edu [Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 (United States)

2017-03-10

Radial transport of particles, elements and fluid driven by internal stresses in three-dimensional (3D) astrophysical accretion disks is an important phenomenon, potentially relevant for the outward dust transport in protoplanetary disks, origin of the refractory particles in comets, isotopic equilibration in the Earth–Moon system, etc. To gain better insight into these processes, we explore the dependence of meridional circulation in 3D disks with shear viscosity on their thermal stratification, and demonstrate a strong effect of the latter on the radial flow. Previous locally isothermal studies have normally found a pattern of the radial outflow near the midplane, switching to inflow higher up. Here we show, both analytically and numerically, that a flow that is inward at all altitudes is possible in disks with entropy and temperature steeply increasing with height. Such thermodynamic conditions may be typical in the optically thin, viscously heated accretion disks. Disks in which these conditions do not hold should feature radial outflow near the midplane, as long as their internal stress is provided by the shear viscosity. Our results can also be used for designing hydrodynamical disk simulations with a prescribed pattern of the meridional circulation.

Accretion by rotating magnetic neutron stars. III. Accretion torques and period changes in pulsating X-ray sources

International Nuclear Information System (INIS)

Ghosh, P.; Lamb, F.K.

1979-01-01

We use the solutions of the two-dimensional hydromagnetic equations obtained previously to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. We find that the magnetic coupling between the star and the plasma outside the inner edge of the disk is appreciable. As a result of this coupling the spin-up torque on fast rotators is substantially less than that on slow rotators; for sufficiently high stellar angular velocities or sufficiently low accretion rates this coupling dominates that de to the plasma and the magnetic field at the inner edge of the disk, braking the star's rotation even while accretion, and hence X-ray emission, continues.We apply these results to pulsating X-ray sources, and show that the observed secular spin-up rates of all the sources in which this rate has been measured can be accounted for quantitatively if one assumes that these sources are accreting from Keplerian disks and have magnetic moments approx.10 29 --10 32 gauss cm 3 . The reduction of the torque on fast rotators provides a natural explanation of the spin-up rate of Her X-1, which is much below that expected for slow rotators. We show further that a simple relation between the secular spin-up rate : P and the quantity PL/sup 3/7/ adequately represents almost all the observational data, P and L being the pulse period and the luminosity of the source, respectively. This ''universal'' relation enables one to estimate any one of the parameters P, P, and L for a given source if the other two are known. We show that the short-term period fluctuations observed in Her X-1, Cen X-3, Vela X-1, and X Per can be accounted for quite naturally as consequences of torque variations caused by fluctuations in the mass transfer rate. We also indicate how the spin-down torque at low luminosities found here may account for the paradoxical existence of a large number of long-period sources with short spin-up time scales

Accretion-Ejection Instability in magnetized accretion disk around compact objects

International Nuclear Information System (INIS)

Varniere, Peggy

2002-01-01

The major problem in accretion physics come from the origin of angular momentum transfer in the disk. My PhD deal with a mechanism (the Accretion-Ejection Instability, AEI) able to explain and link together accretion in the inner region of the disk and ejection. This instability occurs in magnetized accretion disk near equipartition with gas pressure. We first study the impact of some relativistic effects on the instability, particularly on the m = 1 mode. And compared the results with the Quasi-Periodic Oscillation (QPO) observed in micro-quasars. In the second part we study analytically and numerically the Alfven wave emission mechanism which re-emit the angular momentum and energy taken from the inner region of the disk into the corona. The last part deals with MHD numerical simulation. First of all a 2D non-linear disk simulation which contribute to QPO modelization. The last chapter is about a beginning collaboration on 3D simulation in order to study the Alfven wave emission in the corona. (author) [fr

SHOCK-DRIVEN ACCRETION IN CIRCUMPLANETARY DISKS: OBSERVABLES AND SATELLITE FORMATION

Energy Technology Data Exchange (ETDEWEB)

Zhu, Zhaohuan [Department of Physics and Astronomy, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, NV 89154 (United States); Ju, Wenhua; Stone, James M., E-mail: zhzhu@physics.unlv.edu [Department of Astrophysical Sciences, 4 Ivy Lane, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States)

2016-12-01

Circumplanetary disks (CPDs) control the growth of planets, supply material for satellites to form, and provide observational signatures of young forming planets. We have carried out two-dimensional hydrodynamical simulations with radiative cooling to study CPDs and suggested a new mechanism to drive the disk accretion. Two spiral shocks are present in CPDs, excited by the central star. We find that spiral shocks can at least contribute to, if not dominate, the angular momentum transport and energy dissipation in CPDs. Meanwhile, dissipation and heating by spiral shocks have a positive feedback on shock-driven accretion itself. As the disk is heated up by spiral shocks, the shocks become more open, leading to more efficient angular momentum transport. This shock-driven accretion is, on the other hand, unsteady due to production and destruction of vortices in disks. After being averaged over time, a quasi-steady accretion is reached from the planet’s Hill radius all the way to the planet surface, and the disk α  coefficient characterizing angular momentum transport is ∼0.001–0.02. The disk surface density ranges from 10 to 1000 g cm{sup −2} in our simulations, which is at least three orders of magnitude smaller than the “minimum-mass subnebula” model used to study satellite formation; instead it is more consistent with the “gas-starved” satellite formation model. Finally, we calculate the millimeter flux emitted by CPDs at ALMA and EVLA wavelength bands and predict the flux for several recently discovered CPD candidates, which suggests that ALMA is capable of discovering these accreting CPDs.

SHOCK-DRIVEN ACCRETION IN CIRCUMPLANETARY DISKS: OBSERVABLES AND SATELLITE FORMATION

International Nuclear Information System (INIS)

Zhu, Zhaohuan; Ju, Wenhua; Stone, James M.

2016-01-01

Circumplanetary disks (CPDs) control the growth of planets, supply material for satellites to form, and provide observational signatures of young forming planets. We have carried out two-dimensional hydrodynamical simulations with radiative cooling to study CPDs and suggested a new mechanism to drive the disk accretion. Two spiral shocks are present in CPDs, excited by the central star. We find that spiral shocks can at least contribute to, if not dominate, the angular momentum transport and energy dissipation in CPDs. Meanwhile, dissipation and heating by spiral shocks have a positive feedback on shock-driven accretion itself. As the disk is heated up by spiral shocks, the shocks become more open, leading to more efficient angular momentum transport. This shock-driven accretion is, on the other hand, unsteady due to production and destruction of vortices in disks. After being averaged over time, a quasi-steady accretion is reached from the planet’s Hill radius all the way to the planet surface, and the disk α  coefficient characterizing angular momentum transport is ∼0.001–0.02. The disk surface density ranges from 10 to 1000 g cm −2 in our simulations, which is at least three orders of magnitude smaller than the “minimum-mass subnebula” model used to study satellite formation; instead it is more consistent with the “gas-starved” satellite formation model. Finally, we calculate the millimeter flux emitted by CPDs at ALMA and EVLA wavelength bands and predict the flux for several recently discovered CPD candidates, which suggests that ALMA is capable of discovering these accreting CPDs.

Concentration polarization, surface currents, and bulk advection in a microchannel

DEFF Research Database (Denmark)

Nielsen, Christoffer Peder; Bruus, Henrik

2014-01-01

. A remarkable outcome of the investigations is the discovery of strong couplings between bulk advection and the surface current; without a surface current, bulk advection is strongly suppressed. The numerical simulations are supplemented by analytical models valid in the long channel limit as well...... as in the limit of negligible surface charge. By including the effects of diffusion and advection in the diffuse part of the electric double layers, we extend a recently published analytical model of overlimiting current due to surface conduction....

Turbulence in Accretion Discs. The Global Baroclinic Instability

Science.gov (United States)

Klahr, Hubert; Bodenheimer, Peter

The transport of angular momentum away from the central object is a sufficient condition for a protoplanetary disk to accrete matter onto the star and spin it down. Magnetic fields cannot be of importance for this process in a large part of the cold and dusty disk where the planets supposedly form. Our new hypothesis on the angular momentum transport based on radiation hydro simulations is as follows: We present the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We show by analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that this baroclinic flow is unstable and produces turbulence. These findings are currently tested for numerical effects by performing barotropic simulations which show that imposed turbulence rapidly decays. The turbulence in baroclinic disks draws energy from the background shear, transports angular momentum outward and creates a radially inward bound accretion of matter, thus forming a self consistent process. Gravitational energy is transformed into turbulent kinetic energy, which is then dissipated, as in the classical accretion paradigm. We measure accretion rates in 2D and 3D simulations of dot M= - 10-9 to -10-7 Msolar yr-1 and viscosity parameters of α = 10-4 - 10-2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (r-φ) plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

Theoretical, numerical and experimental study of accretion shocks dynamics in magnetic cataclysmic variables

International Nuclear Information System (INIS)

Busschaert, Clotilde

2013-01-01

Magnetic cataclysmic variables are interacting binary Systems containing a highly magnetized white dwarf which accretes material from a companion. Material is led along magnetic field lines and falls onto the magnetic pole(s) supersonically forming an accretion column. As the material hits the surface, a reverse shock is formed and the shocked region is structured by the cooling effect of radiation processes. This work is a multidisciplinary study of the dynamics of the accretion column. Firstly, a numerical study of the accretion column structure at the astrophysical scale is presented. The observational consequences are discussed. This approach is completed by experiments using radiative flows generated by powerful lasers. The relevance of such experiments is based on the establishment of scaling laws. News scaling laws in the frame of radiative ideal or resistive MHD are exposed. The results of the sizing and the interpretation of the POLAR experimental campaign of 2012 on LULI2000 installation are presented. (author) [fr

Snow accretion on overhead wires

Energy Technology Data Exchange (ETDEWEB)

Sakamoto, Y. [Meteorological Research Inst. for Technology Co. Ltd., Tokyo (Japan); Tachizaki, S.; Sudo, N. [Tohoku Electric Power Co. Ltd., Miyagi (Japan)

2005-07-01

Wet snow accretion can cause extensive damage to transmission systems. This paper reviewed some of the difficulties faced by researchers in the study of wet snow accretion on overhead lines in Japan. The study of snow accretion phenomena is complicated by the range of phase changes in water. Snowflakes produced in an upper atmospheric layer with a temperature below freezing do not melt when they go through a lower atmospheric layer with a temperature above freezing, but are in a mixed state of solid and liquid due to the latent heat of melting. The complicated properties of water make studies of snow accretion difficult, as well as the fact that snow changes its physical properties rapidly, due to the effects of ambient temperature, rainfall, and solar radiation. The adhesive forces that cause snow accretion include freezing; bonding through freezing; sintering; condensation and freezing of vapor in the air; mechanical intertwining of snowflakes; capillary action due to liquids; coherent forces between ice particles and water formed through the metamorphosis of snowflakes. In addition to these complexities, differences in laboratory room environments and natural snow environments can also pose difficulties for researchers. Equations describing the relationship between the density of accreted snow and the meteorological parameters involved were presented, as well as empirical equations which suggested that snow accretion efficiency has a dependency on air temperature. An empirical model for estimating snow loads in Japan was outlined, as well as various experiments observing show shedding. Correlations for wet snow accretion included precipitation intensity; duration of precipitation; air temperature; wind speed and wind direction in relation to the overhead line. Issues concerning topography and wet snow accretion were reviewed. It was concluded that studies of snow accretion will benefit by the collection of data in each matrix of the relevant parameters. 12 refs

The Dirichlet problem of a conformable advection-diffusion equation

Directory of Open Access Journals (Sweden)

Avci Derya

2017-01-01

Full Text Available The fractional advection-diffusion equations are obtained from a fractional power law for the matter flux. Diffusion processes in special types of porous media which has fractal geometry can be modelled accurately by using these equations. However, the existing nonlocal fractional derivatives seem complicated and also lose some basic properties satisfied by usual derivatives. For these reasons, local fractional calculus has recently been emerged to simplify the complexities of fractional models defined by nonlocal fractional operators. In this work, the conformable, a local, well-behaved and limit-based definition, is used to obtain a local generalized form of advection-diffusion equation. In addition, this study is devoted to give a local generalized description to the combination of diffusive flux governed by Fick’s law and the advection flux associated with the velocity field. As a result, the constitutive conformable advection-diffusion equation can be easily achieved. A Dirichlet problem for conformable advection-diffusion equation is derived by applying fractional Laplace transform with respect to time t and finite sin-Fourier transform with respect to spatial coordinate x. Two illustrative examples are presented to show the behaviours of this new local generalized model. The dependence of the solution on the fractional order of conformable derivative and the changing values of problem parameters are validated using graphics held by MATLcodes.

Advective transport of CO2 in permeable media induced by atmospheric pressure fluctuations: 1. An analytical model

Science.gov (United States)

W. J. Massman

2006-01-01

Advective flows within soils and snowpacks caused by pressure fluctuations at the upper surface of either medium can significantly influence the exchange rate of many trace gases from the underlying substrate to the atmosphere. Given the importance of many of these trace gases in understanding biogeochemical cycling and global change, it is crucial to quantify (as much...

Cellwise conservative unsplit advection for the volume of fluid method

DEFF Research Database (Denmark)

Comminal, Raphaël; Spangenberg, Jon; Hattel, Jesper Henri

2015-01-01

We present a cellwise conservative unsplit (CCU) advection scheme for the volume of fluid method (VOF) in 2D. Contrary to other schemes based on explicit calculations of the flux balances, the CCU advection adopts a cellwise approach where the pre-images of the control volumes are traced......-overlapping donating regions and pre-images with conforming edges to their neighbors, resulting in the conservativeness and the boundedness (liquid volume fraction inside the interval [0, 1]) of the CCU advection scheme. Finally, the update of the liquid volume fractions is computed from the intersections of the pre......-image polygons with the reconstructed interfaces. The CCU scheme is tested on several benchmark tests for the VOF advection, together with the standard piecewise linear interface calculation (PLIC). The geometrical errors of the CCU compare favorably with other unsplit VOF-PLIC schemes. Finally, potential...

Nested Architecture of Pyroclastic Bedforms Generated by a Single Flow Event: Outcrop Examples from the Izu Volcanic Islands, Japan

Science.gov (United States)

Nemoto, Y.; Yoshida, S.

2009-12-01

We claim that compound bedforms, where small bedforms (e.g., dunes and antidunes) occur within and around the larger bedforms, are common in pyroclastic-flow deposits, using Quaternary-Holocene outcrop examples from the modern Izu volcanic island chain some 100-150 km SSW of Tokyo. The nested occurrence of bedforms have been well documented for siliciclastic deposits, as exemplified by compound dunes where small dunes (c. cm- dm thick) occur between the avalanche surfaces within larger dunes, indicating that these dunes of different sizes were produced simultaneously. However, compound dunes have rarely been reported from pyroclastic deposits. In contrast, we have discovered that compound dunes are common in pyroclastic flow deposits in the late Pleistocene & Holocene outcrops in Niijima and Oshima of the Izu volcanic island chain. Moreover, these outcrops contain abundant compound antidunes, which have been reported from neither siliciclastic or pyroclastic deposits. This is probably because flume studies, where most of published antidune studies are based, focus on small (c. cm-dm high) antidunes. In Niijima Island, we examined pyroclastic-flow deposits shed from Mt. Miyatsuka (14 ka) and Mt. Mukai (886 A.D.). Both groups of deposits contain abundant antidune stratifications, which commonly form nested structures in a two- or three-fold hierarchy, with subordinate crossbeddings originated from dune migrations. Each class of antidunes is characterized by multiple scour surfaces and vertical aggradations around mounds of lag deposits above erosion surfaces, and typically has both upstream and downstream accretion components with different proportions. The late Pleistocene pyroclastic outcrops of the nearby Oshima Island exhibit similar patterns. The geometry of the accretion surfaces vary significantly in the outcrops of both Niijima and Oshima. Whereas the antidunes dominated by upstream accretion are characterized by (1) gently inclined accretion surface and (2

Advection and dispersion of bed load tracers

Science.gov (United States)

Lajeunesse, Eric; Devauchelle, Olivier; James, François

2018-05-01

We use the erosion-deposition model introduced by Charru et al. (2004) to numerically simulate the evolution of a plume of bed load tracers entrained by a steady flow. In this model, the propagation of the plume results from the stochastic exchange of particles between the bed and the bed load layer. We find a transition between two asymptotic regimes. The tracers, initially at rest, are gradually set into motion by the flow. During this entrainment regime, the plume is strongly skewed in the direction of propagation and continuously accelerates while spreading nonlinearly. With time, the skewness of the plume eventually reaches a maximum value before decreasing. This marks the transition to an advection-diffusion regime in which the plume becomes increasingly symmetrical, spreads linearly, and advances at constant velocity. We analytically derive the expressions of the position, the variance, and the skewness of the plume and investigate their asymptotic regimes. Our model assumes steady state. In the field, however, bed load transport is intermittent. We show that the asymptotic regimes become insensitive to this intermittency when expressed in terms of the distance traveled by the plume. If this finding applies to the field, it might provide an estimate for the average bed load transport rate.

Holocene reef accretion: southwest Molokai, Hawaii, U.S.A.

Science.gov (United States)

Engels, Mary S.; Fletcher, Charles H.; Field, Michael E.; Storlazzi, Curt D.; Grossman, Eric E.; Rooney, John J.B.; Conger, Christopher L.; Glenn, Craig

2004-01-01

extension across Hawaii in general, is controlled by wave-induced near-bed shear stress related to refracted North Pacific swell. Holocene accretion patterns here also reflect the long-term influence of wave-induced near-bed shear stress from north swell during late Holocene time. This finding is consistent with other studies (e.g., Grigg 1998; Cabioch et al. 1999) that reflect the dominance of swell energy and sea level in controlling modern and late Holocene accretion elsewhere in Hawaii and across the Pacific and Indian oceans. Notably, however, this result is refined and clarified for Hawaii in the hypothesis of Rooney et al. (2003) stating that enhancement of the El Niño Southern Oscillation beginning approximately 5000 years ago led to increased north swell energy and signaled the end to net accretion along exposed coastlines in Hawaii. The exposure of Hale O Lono to north swell and the age of sea floor there (ca. 4,800 cal yr BP), coupled with the lack of north swell incidence at Hikauhi and the continuous accretion that has occurred there over the last millennium, strongly supports the ENSO reef hypothesis as outlined by Rooney et al. (2003). Other factors controlling Holocene reef accretion at the study site are relative sea-level position and rate of rise, and wave sheltering by Laau Point. Habitat suitable for reef accretion on the southwest shore of Molokai has shrunk throughout the Holocene.

Heat flow in vapor dominated areas of the Yellowstone Plateau volcanic field: implications for the thermal budget of the Yellowstone Caldera

Science.gov (United States)

Hurwitz, Shaul; Harris, Robert; Werner, Cynthia Anne; Murphy, Fred

2012-01-01

Characterizing the vigor of magmatic activity in Yellowstone requires knowledge of the mechanisms and rates of heat transport between magma and the ground surface. We present results from a heat flow study in two vapor dominated, acid-sulfate thermal areas in the Yellowstone Caldera, the 0.11 km2 Obsidian Pool Thermal Area (OPTA) and the 0.25 km2 Solfatara Plateau Thermal Area (SPTA). Conductive heat flux through a low permeability layer capping large vapor reservoirs is calculated from soil temperature measurements at >600 locations and from laboratory measurements of soil properties. The conductive heat output is 3.6 ± 0.4 MW and 7.5 ± 0.4 MW from the OPTA and the SPTA, respectively. The advective heat output from soils is 1.3 ± 0.3 MW and 1.2 ± 0.3 MW from the OPTA and the SPTA, respectively and the heat output from thermal pools in the OPTA is 6.8 ± 1.4 MW. These estimates result in a total heat output of 11.8 ± 1.4 MW and 8.8 ± 0.4 MW from OPTA and SPTA, respectively. Focused zones of high heat flux in both thermal areas are roughly aligned with regional faults suggesting that faults in both areas serve as conduits for the rising acid vapor. Extrapolation of the average heat flux from the OPTA (103 ± 2 W·m−2) and SPTA (35 ± 3 W·m−2) to the ~35 km2 of vapor dominated areas in Yellowstone yields 3.6 and 1.2 GW, respectively, which is less than the total heat output transported by steam from the Yellowstone Caldera as estimated by the chloride inventory method (4.0 to 8.0 GW).

Dark Energy Domination In The Virgocentric Flow

Science.gov (United States)

Byrd, Gene; Chernin, A. D.; Karachentsev, I. D.; Teerikorpi, P.; Valtonen, M.; Dolgachev, V. P.; Domozhilova, L. M.

2011-04-01

Dark energy (DE) was first observationally detected at large Gpc distances. If it is a vacuum energy formulated as Einstein's cosmological constant, Λ, DE should also have dynamical effects at much smaller scales. Previously, we found its effects on much smaller Mpc scales in our Local Group (LG) as well as in other nearby groups. We used new HST observations of member 3D distances from the group centers and Doppler shifts. We find each group's gravity dominates a bound central system of galaxies but DE antigravity results in a radial recession increasing with distance from the group center of the outer members. Here we focus on the much larger (but still cosmologically local) Virgo Cluster and systems around it using new observations of velocities and distances. We propose an analytic model whose key parameter is the zero-gravity radius (ZGR) from the cluster center where gravity and DE antigravity balance. DE brings regularity to the Virgocentric flow. Beyond Virgo's 10 Mpc ZGR, the flow curves to approach a linear global Hubble law at larger distances. The Virgo cluster and its outer flow are similar to the Local Group and its local outflow with a scaling factor of about 10; the ZGR for Virgo is 10 times larger than that of the LG. The similarity of the two systems on the scales of 1 to 30 Mpc suggests that a quasi-stationary bound central component and an expanding outflow applies to a wide range of groups and clusters due to small scale action of DE as well as gravity. Chernin, et al 2009 Astronomy and Astrophysics 507, 1271 http://arxiv.org/abs/1006.0066 http://arxiv.org/abs/1006.0555

Magnetohydrodynamics of accretion disks

International Nuclear Information System (INIS)

Torkelsson, U.

1994-04-01

The thesis consists of an introduction and summary, and five research papers. The introduction and summary provides the background in accretion disk physics and magnetohydrodynamics. The research papers describe numerical studies of magnetohydrodynamical processes in accretion disks. Paper 1 is a one-dimensional study of the effect of magnetic buoyancy on a flux tube in an accretion disk. The stabilizing influence of an accretion disk corona on the flux tube is demonstrated. Paper 2-4 present numerical simulations of mean-field dynamos in accretion disks. Paper 11 verifies the correctness of the numerical code by comparing linear models to previous work by other groups. The results are also extended to somewhat modified disk models. A transition from an oscillatory mode of negative parity for thick disks to a steady mode of even parity for thin disks is found. Preliminary results for nonlinear dynamos at very high dynamo numbers are also presented. Paper 3 describes the bifurcation behaviour of the nonlinear dynamos. For positive dynamo numbers it is found that the initial steady solution is replaced by an oscillatory solution of odd parity. For negative dynamo numbers the solution becomes chaotic at sufficiently high dynamo numbers. Paper 4 continues the studies of nonlinear dynamos, and it is demonstrated that a chaotic solution appears even for positive dynamo numbers, but that it returns to a steady solution of mixed parity at very high dynamo numbers. Paper 5 describes a first attempt at simulating the small-scale turbulence of an accretion disk in three dimensions. There is only find cases of decaying turbulence, but this is rather due to limitations of the simulations than that turbulence is really absent in accretion disks

A generalized advection dispersion equation

Indian Academy of Sciences (India)

This paper examines a possible effect of uncertainties, variability or heterogeneity of any dynamic system when being included in its evolution rule; the notion is illustrated with the advection dispersion equation, which describes the groundwater pollution model. An uncertain derivative is defined; some properties of.

OBSERVATION OF MAGNETIC RECONNECTION DRIVEN BY GRANULAR SCALE ADVECTION

Energy Technology Data Exchange (ETDEWEB)

Zeng Zhicheng; Cao Wenda [Center for Solar-Terrestrial Research, New Jersey Institute of Technology, 323 Martin Luther King Blvd., Newark, NJ 07102 (United States); Ji Haisheng [Big Bear Solar Observatory, 40386 North Shore Lane, Big Bear City, CA 92314 (United States)

2013-06-01

We report the first evidence of magnetic reconnection driven by advection in a rapidly developing large granule using high spatial resolution observations of a small surge event (base size {approx} 4'' Multiplication-Sign 4'') with the 1.6 m aperture New Solar Telescope at the Big Bear Solar Observatory. The observations were carried out in narrowband (0.5 A) He I 10830 A and broadband (10 A) TiO 7057 A. Since He I 10830 A triplet has a very high excitation level and is optically thin, its filtergrams enable us to investigate the surge from the photosphere through the chromosphere into the lower corona. Simultaneous space data from the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory were used in the analysis. It is shown that the surge is spatio-temporally associated with magnetic flux emergence in the rapidly developing large granule. During the development of the granule, its advecting flow ({approx}2 km s{sup -1}) squeezed the magnetic flux into an intergranular lane area, where a magnetic flux concentration was formed and the neighboring flux with opposite magnetic polarity was canceled. During the cancellation, the surge was produced as absorption in He I 10830 A filtergrams while simultaneous EUV brightening occurred at its base. The observations clearly indicate evidence of a finest-scale reconnection process driven by the granule's motion.

3D Modeling of Accretion Disks and Circumbinary Envelopes in Close Binaries

Science.gov (United States)

Bisikalo, D.

2010-12-01

A number of observations prove the complex flow structure in close binary stars. The gas dynamic structure of the flow is governed by the stream of matter from the inner Lagrange point, the accretion disk, the circum-disk halo, and the circumbinary envelope. Observations reflect the current state of a binary system and for their interpretation one should consider the gas dynamics of flow patterns. Three-dimensional numerical gasdynamical modeling is used to study the gaseous flow structure and dynamics in close binaries. It is shown that the periodic variations of the positions of the disk and the bow shock formed when the inner parts of the circumbinary envelope flow around the disk result in variations in both the rate of angular-momentum transfer to the disk and the flow structure near the Lagrange point L3. All these factors lead to periodic ejections of matter from the accretion disk and circum-disk halo into the outer layers of the circumbinary envelope. The results of simulations are used to estimate the physical parameters of the circumbinary envelope, including 3D matter distribution in it, and the matter-flow configuration and dynamics. The envelope becomes optically thick for systems with high mass-exchange rates, M⊙=10-8 Msun/year, and has a significant influence on the binary's observed features. The uneven phase distributions of the matter and density variations due to periodic injections of matter into the envelope are important for interpretations of observations of CBSs.

Radio properties of central dominant galaxies in cluster cooling flows

International Nuclear Information System (INIS)

O'dea, C.P.; Baum, S.A.

1986-01-01

New VLA observations of central dominant (cd) galaxies currently thought to be in cluster cooling flows are combined with observations from the literature to examine the global properties of a heterogeneous sample of 31 cd galaxies. The radio sources tend to be of low or intermediate radio power and have small sizes (median extent about 25 kpc). The resolved sources tend to have distorted morphologies (e.g., wide-angle tails and S shapes). It is not yet clear whether the radio emission from these cd galaxies is significantly different from those not thought to be in cluster cooling flows. The result of Jones and Forman (1984), that there is a possible correlation between radio power and excess X-ray luminosity in the cluster center (above a King model fit to the X-ray surface brightness), is confirmed. 43 references

The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

Science.gov (United States)

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

2016-04-01

We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ˜ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ˜ 108-1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108-109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar 108 M⊙ can contribute a considerable fraction (˜20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

Rapid growth of seed black holes in the early universe by supra-exponential accretion.

Science.gov (United States)

Alexander, Tal; Natarajan, Priyamvada

2014-09-12

Mass accretion by black holes (BHs) is typically capped at the Eddington rate, when radiation's push balances gravity's pull. However, even exponential growth at the Eddington-limited e-folding time t(E) ~ few × 0.01 billion years is too slow to grow stellar-mass BH seeds into the supermassive luminous quasars that are observed when the universe is 1 billion years old. We propose a dynamical mechanism that can trigger supra-exponential accretion in the early universe, when a BH seed is bound in a star cluster fed by the ubiquitous dense cold gas flows. The high gas opacity traps the accretion radiation, while the low-mass BH's random motions suppress the formation of a slowly draining accretion disk. Supra-exponential growth can thus explain the puzzling emergence of supermassive BHs that power luminous quasars so soon after the Big Bang. Copyright © 2014, American Association for the Advancement of Science.

Accretion onto CO White Dwarfs using MESA

Science.gov (United States)

Feng, Wanda; Starrfield, Sumner

2018-06-01

The nature of type Ia Supernovae (SNe Ia) progenitor systems and their underlying mechanism are not well understood. There are two competing progenitor scenarios: the single-degenerate scenario wherein a white dwarf (WD) star accretes material from a companion star, reaching the Chandrasekhar mass limit; and, the double-degenerate scenario wherein two WDs merge. In this study, we investigate the single-degenerate scenario by accretion onto carbon-oxygen (CO) WDs using the Modules for Experiments in Stellar Astrophysics (MESA). We vary the WD mass, composition of the accreting material, and accretion rate in our models. Mixing between the accreted material and the WD core is informed by multidimensional studies that suggest occurance after thermonuclear runaway (TNR) ensues. We compare the accretion of solar composition material onto CO WDs with the accretion of mixed solar and core material after TNR. As many of our models eject less material than accreted, our study supports that accretion onto CO WDs is a feasible channel for SNe I progenitors.

Relativistic, accreting disks

International Nuclear Information System (INIS)

Abramowicz, M.A; Jaroszynski, M.; Sikora, M.

1978-01-01

An analytic theory of the hydrodynamical structure of accreting disks (without self-gravitation but with pressure) orbiting around and axially symmetric, stationary, compact body (e.g. black hole) is presented. The inner edge of the marginally stable accreting disk (i.e. disk with constant angular momentum density) has a sharp cusp located on the equatorial plane between rsub(ms) and rsub(mb). The existence of the cusp is also typical for any angular momentum distribution. The physical importance of the cusp follows from the close analogy with the case of a close binary system (L 1 Lagrange point on the Roche lobe). The existence of the cusp is thus a crucial phenomenon in such problems as boundary condition for the viscous stresses, accretion rate etc. (orig.) [de

Mode-locking in advection-reaction-diffusion systems: An invariant manifold perspective

Science.gov (United States)

Locke, Rory A.; Mahoney, John R.; Mitchell, Kevin A.

2018-01-01

Fronts propagating in two-dimensional advection-reaction-diffusion systems exhibit a rich topological structure. When the underlying fluid flow is periodic in space and time, the reaction front can lock to the driving frequency. We explain this mode-locking phenomenon using the so-called burning invariant manifolds (BIMs). In fact, the mode-locked profile is delineated by a BIM attached to a relative periodic orbit (RPO) of the front element dynamics. Changes in the type (and loss) of mode-locking can be understood in terms of local and global bifurcations of the RPOs and their BIMs. We illustrate these concepts numerically using a chain of alternating vortices in a channel geometry.

The effect of cooling management on blood flow to the dominant follicle and estrous cycle length at heat stress.

Science.gov (United States)

Honig, Hen; Ofer, Lior; Kaim, Moshe; Jacobi, Shamay; Shinder, Dima; Gershon, Eran

2016-07-15

The use of ultrasound imaging for the examination of reproductive organs has contributed substantially to the fertility management of dairy cows around the world. This method has many advantages such as noninvasiveness and immediate availability of information. Adding Doppler index to the ultrasound imaging examination, improved the estimation of blood volume and flow rate to the ovaries in general and to the dominant follicle in particular. The aim of this study was to examine changes in the blood flow to the dominant follicle and compare them to the follicular development throughout the cycle. We further set out to examine the effects of different types of cooling management during the summer on the changes in blood flow to the dominant follicle. For this purpose, 24 Israeli-Holstein dairy cows, under heat stress, were randomly assigned one of two groups: one was exposed to five cooling sessions per day (5CS) and the other to eight cooling sessions per day (8CS). Blood flow to the dominant follicle was measured daily using Doppler index throughout the estrous cycle. No differences in the preovulatory dominant follicle diameter were detected between the two cooling management regimens during the cycle. However, the length of the first follicular wave was significantly longer, whereas the second follicular wave was nonsignificantly shorter in the 5CS group as compared to the 8CS group. In addition, no difference in blood flow was found during the first 18 days of the cycle between the two groups. However, from Day 20 until ovulation a higher rate of blood flow was measured in the ovaries of cows cooled 8 times per day as compared to the 5CS group. No differences in progesterone levels were noted. Finally, the estrous cycle length was shorter in the 8CS group as compared to the 5CS group. Our data suggest that blood flow to the dominant follicle and estrous cycle length is affected by heat stress. Using the appropriate cooling management during heat stress can

A mixed markers and volume-of-fluid method for the reconstruction and advection of interfaces in two-phase and free-boundary flows

International Nuclear Information System (INIS)

Aulisa, Eugenio; Manservisi, Sandro; Scardovelli, Ruben

2003-01-01

In this work we present a new mixed markers and volume-of-fluid (VOF) algorithm for the reconstruction and advection of interfaces in the two-dimensional space. The interface is described by using both the volume fraction function C, as in VOF methods, and surface markers, which locate the interface within the computational cells. The C field and the markers are advected by following the streamlines. New markers are determined by computing the intersections of the advected interface with the grid lines, then other markers are added inside each cut cell to conserve the volume fraction C. A smooth motion of the interface is obtained, typical of the marker approach, with a good volume conservation, as in standard VOF methods. In this article we consider a few typical two-dimensional tests and compare the results of the mixed algorithm with those obtained with VOF methods. Translations, rotations and vortex tests are performed showing that many problems of the VOF technique can be solved and a good accuracy in the geometrical motion and mass conservation can be achieved

Grain-size segregation and levee formation in geophysical mass flows

Science.gov (United States)

Johnson, C.G.; Kokelaar, B.P.; Iverson, Richard M.; Logan, M.; LaHusen, R.G.; Gray, J.M.N.T.

2012-01-01

Data from large-scale debris-flow experiments are combined with modeling of particle-size segregation to explain the formation of lateral levees enriched in coarse grains. The experimental flows consisted of 10 m3 of water-saturated sand and gravel, which traveled ∼80 m down a steeply inclined flume before forming an elongated leveed deposit 10 m long on a nearly horizontal runout surface. We measured the surface velocity field and observed the sequence of deposition by seeding tracers onto the flow surface and tracking them in video footage. Levees formed by progressive downslope accretion approximately 3.5 m behind the flow front, which advanced steadily at ∼2 m s−1during most of the runout. Segregation was measured by placing ∼600 coarse tracer pebbles on the bed, which, when entrained into the flow, segregated upwards at ∼6–7.5 cm s−1. When excavated from the deposit these were distributed in a horseshoe-shaped pattern that became increasingly elevated closer to the deposit termination. Although there was clear evidence for inverse grading during the flow, transect sampling revealed that the resulting leveed deposit was strongly graded laterally, with only weak vertical grading. We construct an empirical, three-dimensional velocity field resembling the experimental observations, and use this with a particle-size segregation model to predict the segregation and transport of material through the flow. We infer that coarse material segregates to the flow surface and is transported to the flow front by shear. Within the flow head, coarse material is overridden, then recirculates in spiral trajectories due to size-segregation, before being advected to the flow edges and deposited to form coarse-particle-enriched levees.

The Graded Alluvial River: Variable Flow and the Dominant Discharge

Science.gov (United States)

Blom, A.; Arkesteijn, L.; Viparelli, E.

2016-12-01

We derive analytical formulations for the graded or equilibrium longitudinal profile of a mixed-sediment alluvial river under variable flow. The formulations are applicable to reaches upstream from the backwater zone. The model is based on the conservation equations for the mass of two distinct sediment modes, sand and gravel, at the bed surface to account for the effects of grain size selective transport and abrasion of gravel particles. The effects of a variable flow rate are included by (a) treating the flow as a continuously changing yet steady water discharge (i.e. here termed an alternating steady discharge) and (b) assuming the time scale of changes in channel slope and bed surface texture to be much larger than the one of changes in flow rate. The equations are simplified realizing that at equilibrium the river profile finds itself in a dynamic steady state with oscillations around constant mean values of channel slope and bed surface texture. A generalized sediment transport relation representing the stochastic nature of sediment transport allows for explicit or analytical solutions to the streamwise decrease of both the channel slope and the bed surface mean grain size under variable flow for reaches unaffected by backwater effects. This modelling approach also provides a definition of a channel-forming or dominant water discharge, i.e., that steady water discharge that is equivalent in its effect on the equilibrium channel slope to the full hydrograph.

Accretion onto a Kiselev black hole

Energy Technology Data Exchange (ETDEWEB)

Jiao, Lei [Hebei University, College of Physical Science and Technology, Baoding (China); Yang, Rongjia [Hebei University, College of Physical Science and Technology, Baoding (China); Hebei University, Hebei Key Lab of Optic-Electronic Information and Materials, Baoding (China)

2017-05-15

We consider accretion onto a Kiselev black hole. We obtain the fundamental equations for accretion without the back-reaction. We determine the general analytic expressions for the critical points and the mass accretion rate and find the physical conditions the critical points should fulfill. The case of a polytropic gas are discussed in detail. It turns out that the quintessence parameter plays an important role in the accretion process. (orig.)

Study on accretion of ice on oscillating aerofoils; Shindoyokujo no chakuhyo ni kansuru kenkyu

Energy Technology Data Exchange (ETDEWEB)

Nishikawa, S; Sugihara, H [Kandenko Co. Ltd., Tokyo (Japan); Kimura, S; Sugiyama, Y [Kanagawa Institute of Technology, Kanagawa (Japan); Tsuboi, K [Ibaraki University, Ibaraki (Japan). Faculty of Engineering

1997-11-25

The effects of pitching and heaving motions of an aerofoil on accretion of ice thereon are investigated for the design of wind turbine blades. The conditions under which an aerofoil shows a heaving motion are 10m/s as wind velocity, -12degC as ambient temperature, 0.5Hz as frequency and 5mm as amplitude. Initial growth of ice is stochastic to a great extent; ice grows rapidly around the seed which is formed by chance somewhere on an laerofoil. The effects of ice appear in the flow field in the vicinity of the aerofoil front edge, as it continues to grow, providing different conditions for water droplets to collide with each other. Average ice shapes can be classified by environment, although at low repeatability. Range of ice accretion varies depending on aerofoil motion, whether it is pitching, heaving or at rest. The most important factor that determines behavior of ice accretion is conditions of the flow field near the aerofoil front edge. In a pitching or heaving motion, its speed is an important factor for growth of ice. 2 refs., 5 figs.

Papaloizou-Pringle instability suppression by the magnetorotational instability in relativistic accretion discs

Science.gov (United States)

Bugli, M.; Guilet, J.; Müller, E.; Del Zanna, L.; Bucciantini, N.; Montero, P. J.

2018-03-01

Geometrically thick tori with constant specific angular momentum have been widely used in the last decades to construct numerical models of accretion flows on to black holes. Such discs are prone to a global non-axisymmetric hydrodynamic instability, known as Papaloizou-Pringle instability (PPI), which can redistribute angular momentum and also lead to an emission of gravitational waves. It is, however, not clear yet how the development of the PPI is affected by the presence of a magnetic field and by the concurrent development of the magnetorotational instability (MRI). We present a numerical analysis using three-dimensional GRMHD simulations of the interplay between the PPI and the MRI considering, for the first time, an analytical magnetized equilibrium solution as initial condition. In the purely hydrodynamic case, the PPI selects as expected the large-scale m = 1 azimuthal mode as the fastest growing and non-linearly dominant mode. However, when the torus is threaded by a weak toroidal magnetic field, the development of the MRI leads to the suppression of large-scale modes and redistributes power across smaller scales. If the system starts with a significantly excited m = 1 mode, the PPI can be dominant in a transient phase, before being ultimately quenched by the MRI. Such dynamics may well be important in compact star mergers and tidal disruption events.

Gravity signatures of terrane accretion

Science.gov (United States)

Franco, Heather; Abbott, Dallas

1999-01-01

In modern collisional environments, accreted terranes are bracketed by forearc gravity lows, a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane. The size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate. Along the Central American trench, the accretion of Gorgona Island caused a seaward trench jump of 48 to 66 km. The relict trench axes show up as gravity lows behind the trench with minimum values of -78 mgal (N of Gorgona) and -49 mgal (S of Gorgona) respectively. These forearc gravity lows have little or no topographic expression. The active trench immediately seaward of these forearc gravity lows has minimum gravity values of -59 mgal (N of Gorgona) and -58 mgal (S of Gorgona), respectively. In the north, the active trench has a less pronounced gravity low than the sediment covered forearc. In the Mariana arc, two Cretaceous seamounts have been accreted to the Eocene arc. The northern seamount is most likely a large block, the southern seamount may be a thrust slice. These more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench. However, the minimum values of the Mariana forearc gravity lows are modest only by comparison to the Mariana Trench (-216 mgal); their absolute values are more negative than at Gorgona Island (-145 to -146 mgal). We speculate that the forearc gravity lows and seaward trench jumps near Gorgona Island were produced by the accretion of a hotspot island from a strong plate. The Mariana gravity lows and seaward trench jumps (or thrust slices) were the result of breaking a relatively weak plate close to the seamount edifice. These gravity lows resulting from accretion events should be preserved in older accreted terranes.

Relativistic, accreting disks

Energy Technology Data Exchange (ETDEWEB)

Abramowicz, M A; Jaroszynski, M; Sikora, M [Polska Akademia Nauk, Warsaw

1978-02-01

An analytic theory of the hydrodynamical structure of accreting disks (without self-gravitation but with pressure) orbiting around an axially symmetric, stationary, compact body (e.g. black hole) is presented. The inner edge of the marginally stable accreting disk (i.e. disk with constant angular momentum density) has a sharp cusp located on the equatorial plane between r/sub ms/ and r/sub mb/. The existence of the cusp is also typical for any angular momentum distribution. The physical importance of the cusp follows from the close analogy with the case of a close binary system (L/sub 1/ Lagrange point on the Roche lobe). The existence of the cusp is thus a crucial phenomenon in such problems as boundary condition for the viscous stresses, accretion rate, etc.

A COMMON SOURCE OF ACCRETION DISK TILT

International Nuclear Information System (INIS)

Montgomery, M. M.; Martin, E. L.

2010-01-01

Many different system types retrogradely precess, and retrograde precession could be from a tidal torque by the secondary on a misaligned accretion disk. However, a source that causes and maintains disk tilt is unknown. In this work, we show that accretion disks can tilt due to a force called lift. Lift results from differing gas stream supersonic speeds over and under an accretion disk. Because lift acts at the disk's center of pressure, a torque is applied around a rotation axis passing through the disk's center of mass. The disk responds to lift by pitching around the disk's line of nodes. If the gas stream flow ebbs, then lift also ebbs and the disk attempts to return to its original orientation. To first approximation, lift does not depend on magnetic fields or radiation sources but does depend on the mass and the surface area of the disk. Also, for disk tilt to be initiated, a minimum mass transfer rate must be exceeded. For example, a 10 -11 M sun disk around a 0.8 M sun compact central object requires a mass transfer rate greater than ∼ 8 x 10 -11 M sun yr -1 , a value well below the known mass transfer rates in cataclysmic variable dwarf novae systems that retrogradely precess and exhibit negative superhumps in their light curves and a value well below mass transfer rates in protostellar-forming systems.

A NEWLY FORMING COLD FLOW PROTOGALACTIC DISK, A SIGNATURE OF COLD ACCRETION FROM THE COSMIC WEB

International Nuclear Information System (INIS)

Martin, D. Christopher; Matuszewski, Mateusz; Morrissey, Patrick; Neill, James D.; Moore, Anna; Steidel, Charles C.; Trainor, Ryan

2016-01-01

How galaxies form from, and are fueled by, gas from the intergalactic medium (IGM) remains one of the major unsolved problems in galaxy formation. While the classical Cold Dark Matter paradigm posits galaxies forming from cooling virialized gas, recent theory and numerical simulations have highlighted the importance of cold accretion flows—relatively cool ( T ∼ few × 104 K) unshocked gas streaming along filaments into dark matter halos, including hot, massive, high-redshift halos. These flows are thought to deposit gas and angular momentum into the circumgalactic medium resulting in disk- or ring-like structures, eventually coalescing into galaxies forming at filamentary intersections. We earlier reported a bright, Ly α emitting filament near the QSO HS1549+19 at redshift z = 2.843 discovered with the Palomar Cosmic Web Imager. We now report that the bright part of this filament is an enormous ( R > 100 kpc) rotating structure of hydrogen gas with a disk-like velocity profile consistent with a 4 × 10"1"2 M _⊙ halo. The orbital time of the outer part of the what we term a “protodisk” is comparable to the virialization time and the age of the universe at this redshift. We propose that this protodisk can only have recently formed from cold gas flowing directly from the cosmic web.

A NEWLY FORMING COLD FLOW PROTOGALACTIC DISK, A SIGNATURE OF COLD ACCRETION FROM THE COSMIC WEB

Energy Technology Data Exchange (ETDEWEB)

Martin, D. Christopher; Matuszewski, Mateusz; Morrissey, Patrick; Neill, James D. [Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Mail Code 278-17, Pasadena, California 91125 (United States); Moore, Anna [Caltech Optical Observatories, Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Mail Code 11-17, Pasadena, California 91125 (United States); Steidel, Charles C. [Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Mail Code 249-17, Pasadena, California 91125 (United States); Trainor, Ryan, E-mail: cmartin@srl.caltech.edu [Department of Astronomy, University of California, Berkeley, 501 15 Campbell Hall, Berkeley, CA 94720 (United States)

2016-06-10

How galaxies form from, and are fueled by, gas from the intergalactic medium (IGM) remains one of the major unsolved problems in galaxy formation. While the classical Cold Dark Matter paradigm posits galaxies forming from cooling virialized gas, recent theory and numerical simulations have highlighted the importance of cold accretion flows—relatively cool ( T ∼ few × 104 K) unshocked gas streaming along filaments into dark matter halos, including hot, massive, high-redshift halos. These flows are thought to deposit gas and angular momentum into the circumgalactic medium resulting in disk- or ring-like structures, eventually coalescing into galaxies forming at filamentary intersections. We earlier reported a bright, Ly α emitting filament near the QSO HS1549+19 at redshift z = 2.843 discovered with the Palomar Cosmic Web Imager. We now report that the bright part of this filament is an enormous ( R > 100 kpc) rotating structure of hydrogen gas with a disk-like velocity profile consistent with a 4 × 10{sup 12} M {sub ⊙} halo. The orbital time of the outer part of the what we term a “protodisk” is comparable to the virialization time and the age of the universe at this redshift. We propose that this protodisk can only have recently formed from cold gas flowing directly from the cosmic web.

The Physics of Wind-Fed Accretion

International Nuclear Information System (INIS)

Mauche, Christopher W.; Liedahl, Duane A.; Akiyama, Shizuka; Plewa, Tomasz

2008-01-01

We provide a brief review of the physical processes behind the radiative driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and accretion of a fraction of the stellar wind by a compact object, typically a neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we describe a program to develop global models of the radiatively-driven photoionized winds and accretion flows of HMXBs, with particular attention to the prototypical system Vela X-l. The models combine XSTAR photoionization calculations, HULLAC emission models appropriate to X-ray photoionized plasmas, improved models of the radiative driving of photoionized winds, FLASH time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo radiation transport. We present two- and three-dimensional maps of the density, temperature, velocity, ionization parameter, and emissivity distributions of representative X-ray emission lines, as well as synthetic global Monte Carlo X-ray spectra. Such models help to better constrain the properties of the winds of HMXBs, which bear on such fundamental questions as the long-term evolution of these binaries and the chemical enrichment of the interstellar medium.

The relative role of galaxy mergers and cosmic flows in feeding black holes

International Nuclear Information System (INIS)

Bellovary, Jillian; Brooks, Alyson; Volonteri, Marta; Governato, Fabio; Quinn, Thomas; Wadsley, James

2013-01-01

Using a set of zoomed-in cosmological simulations of high-redshift progenitors of massive galaxies, we isolate and trace the history of gas that is accreted by central supermassive black holes. We determine the origins of the accreted gas, in terms of whether it entered the galaxy during a merger event or was smoothly accreted. Furthermore, we designate whether the smoothly accreted gas is accreted via a cold flow or is shocked upon entry into the halo. For moderate-mass (10 6 -10 7 M ☉ ) black holes at z ∼ 4, there is a preference to accrete cold flow gas as opposed to gas of shocked or merger origin. However, this result is a consequence of the fact that the entire galaxy has a higher fraction of gas from cold flows. In general, each black hole tends to accrete the same fractions of smooth- and merger-accreted gas as is contained in its host galaxy, suggesting that once gas enters a halo it becomes well-mixed, and its origins are erased. We find that the angular momentum of the gas upon halo entry is a more important factor; black holes preferentially accrete gas that had low angular momentum when it entered the galaxy, regardless of whether it was accreted smoothly or through mergers.

Solutes and cells - aspects of advection-diffusion-reaction phenomena in biochips

DEFF Research Database (Denmark)

Vedel, Søren

2012-01-01

the dependencies on density. This shows that the varied single-cell behavior including the overall modulations imposed by density arise as a natural consequence of pseudopod-driven motility in a social context. The final subproject concerns the combined effects of advection, diffusion and reaction of several......Cell’), and the overall title of the project is Solutes and cells — aspects of advection-diffusion-reaction phenomena in biochips. The work has consisted of several projects focusing on theory, and to some extend analysis of experimental data, with advection-diffusion-reaction phenomena of solutes as the recurring theme...... quantitatively interpret the proximal concentration of specific solutes, and integrate this to achieve biological functions. In three specific examples, the author and co-workers have investigated different aspects of the influence of advection, diffusion and reaction on solute distributions, as well...

The effect of catastrophic collisional fragmentation and diffuse medium accretion on a computational interstellar dust system

Science.gov (United States)

Liffman, Kurt

1990-01-01

The effects of catastrophic collisional fragmentation and diffuse medium accretion on a the interstellar dust system are computed using a Monte Carlo computer model. The Monte Carlo code has as its basis an analytic solution of the bulk chemical evolution of a two-phase interstellar medium, described by Liffman and Clayton (1989). The model is subjected to numerous different interstellar processes as it transfers from one interstellar phase to another. Collisional fragmentation was found to be the dominant physical process that shapes the size spectrum of interstellar dust. It was found that, in the diffuse cloud phase, 90 percent of the refractory material is locked up in the dust grains, primarily due to accretion in the molecular medium. This result is consistent with the observed depletions of silicon. Depletions were found to be affected only slightly by diffuse cloud accretion.

Mixed ice accretion on aircraft wings

Science.gov (United States)

Janjua, Zaid A.; Turnbull, Barbara; Hibberd, Stephen; Choi, Kwing-So

2018-02-01

Ice accretion is a problematic natural phenomenon that affects a wide range of engineering applications including power cables, radio masts, and wind turbines. Accretion on aircraft wings occurs when supercooled water droplets freeze instantaneously on impact to form rime ice or runback as water along the wing to form glaze ice. Most models to date have ignored the accretion of mixed ice, which is a combination of rime and glaze. A parameter we term the "freezing fraction" is defined as the fraction of a supercooled droplet that freezes on impact with the top surface of the accretion ice to explore the concept of mixed ice accretion. Additionally we consider different "packing densities" of rime ice, mimicking the different bulk rime densities observed in nature. Ice accretion is considered in four stages: rime, primary mixed, secondary mixed, and glaze ice. Predictions match with existing models and experimental data in the limiting rime and glaze cases. The mixed ice formulation however provides additional insight into the composition of the overall ice structure, which ultimately influences adhesion and ice thickness, and shows that for similar atmospheric parameter ranges, this simple mixed ice description leads to very different accretion rates. A simple one-dimensional energy balance was solved to show how this freezing fraction parameter increases with decrease in atmospheric temperature, with lower freezing fraction promoting glaze ice accretion.

2.5-D modelling of rime ice accretion on a swept airfoil

International Nuclear Information System (INIS)

Szilder, K.; Lozowski, E.P.

2004-01-01

In this paper, we present a three-dimensional morphogenetic model which simulates discrete rime ice accretion structures forming on a swept airfoil. Rime is an ice deposit caused by the impingement and freezing of supercooled cloud droplets at accretion temperatures below 0 o C. Depending on conditions, the droplets may freeze as spheres or deform on the surface, but for rime to form, surface liquid flow is limited or non-existent. In the model, roughness elements that develop initially on the wing surface evolve into rime feathers and other complex, three-dimensional structures. On swept wings, these resemble the so-called 'lobster tails' or 'scallops' that are observed in wind tunnels and in flight. We show the results of sensitivity tests, performed as a function of two model parameters: sweep angle and freezing range parameter. Our exploratory research has shown that the model predicts realistic-looking, three-dimensional ice structures on swept wings. This capability does not exist in current in-flight ice accretion models. (author)

A New Paradigm for Gamma Ray Bursts: Long Term Accretion Rate Modulation by an External Accretion Disk

Science.gov (United States)

Cannizzo, John; Gehrels, Neil

2009-01-01

We present a new way of looking at the very long term evolution of GRBs in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep "breaks" in the long term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an "external disk" whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power law decay to the GRB light curves. In this model, the different canonical power law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power law segment.

Migration of accreting giant planets

Science.gov (United States)

Crida, A.; Bitsch, B.; Raibaldi, A.

2016-12-01

We present the results of 2D hydro simulations of giant planets in proto-planetary discs, which accrete gas at a more or less high rate. First, starting from a solid core of 20 Earth masses, we show that as soon as the runaway accretion of gas turns on, the planet is saved from type I migration : the gap opening mass is reached before the planet is lost into its host star. Furthermore, gas accretion helps opening the gap in low mass discs. Consequently, if the accretion rate is limited to the disc supply, then the planet is already inside a gap and in type II migration. We further show that the type II migration of a Jupiter mass planet actually depends on its accretion rate. Only when the accretion is high do we retrieve the classical picture where no gas crosses the gap and the planet follows the disc spreading. These results impact our understanding of planet migration and planet population synthesis models. The e-poster presenting these results in French can be found here: L'e-poster présentant ces résultats en français est disponible à cette adresse: http://sf2a.eu/semaine-sf2a/2016/posterpdfs/156_179_49.pdf.

VARIABILITY FROM NON-AXISYMMETRIC FLUCTUATIONS INTERACTING WITH STANDING SHOCKS IN TILTED BLACK HOLE ACCRETION DISKS

Energy Technology Data Exchange (ETDEWEB)

Henisey, Ken B. [Natural Science Division, Pepperdine University, Malibu, CA 90263 (United States); Blaes, Omer M. [Department of Physics, University of California, Santa Barbara, CA 93106 (United States); Fragile, P. Chris [Department of Physics and Astronomy, College of Charleston, Charleston, SC 29424 (United States)

2012-12-10

We study the spatial and temporal behavior of fluid in fully three-dimensional, general relativistic, magnetohydrodynamical simulations of both tilted and untilted black hole accretion flows. We uncover characteristically greater variability in tilted simulations at frequencies similar to those predicted by the formalism of trapped modes, but ultimately conclude that its spatial structure is inconsistent with a modal interpretation. We find instead that previously identified, transient, overdense clumps orbiting on roughly Keplerian trajectories appear generically in our global simulations, independent of tilt. Associated with these fluctuations are acoustic spiral waves interior to the orbits of the clumps. We show that the two non-axisymmetric standing shock structures that exist in the inner regions of these tilted flows effectively amplify the variability caused by these spiral waves to markedly higher levels than in untilted flows, which lack standing shocks. Our identification of clumps, spirals, and spiral-shock interactions in these fully general relativistic, magnetohydrodynamical simulations suggests that these features may be important dynamical elements in models that incorporate tilt as a way to explain the observed variability in black hole accretion flows.

How young the accretion-powered pulsars could be?

Science.gov (United States)

Kostina, M. V.; Ikhsanov, N. R.

2017-12-01

A question about the age of accretion-powered X-ray pulsars has recently been reopened by a discovery of the X-ray pulsar SXP 1062 in the SMC. This High Mass X-ray Binary (HMXB) contains a neutron star rotating with the period of 1062 s and is associated with a supernova remnant of the age ∼ 104 yr. An attempt to explain the origin of this young long-period X-ray pulsar within the traditional scenario of three basic states (ejector, propeller and accretor) encounters difficulties. Even if this pulsar were born as a magnetar the spin-down time during the propeller stage would exceed 104 yr. Here we explore a more circuitous way of the pulsar spin evolution in HMXBs, in which the propeller stage in the evolutionary track is avoided. We find this way to be possible if the stellar wind of the massive companion to the neutron star is magnetized. The geometry of plasma flow captured by the neutron star in this case differs from spherically symmetrical and the magnetospheric radius of the neutron star is smaller than that evaluated in the convention accretion scenarios. We show that the age of an accretion-powered pulsar in this case can be as small as ∼ 104 years without the need of invoking initial magnetic field in excess of 1013 G.

Memory effects in chaotic advection of inertial particles

International Nuclear Information System (INIS)

Daitche, Anton; Tél, Tamás

2014-01-01

A systematic investigation of the effect of the history force on particle advection is carried out for both heavy and light particles. General relations are given to identify parameter regions where the history force is expected to be comparable with the Stokes drag. As an illustrative example, a paradigmatic two-dimensional flow, the von Kármán flow is taken. For small (but not extremely small) particles all investigated dynamical properties turn out to heavily depend on the presence of memory when compared to the memoryless case: the history force generates a rather non-trivial dynamics that appears to weaken (but not to suppress) inertial effects, it enhances the overall contribution of viscosity. We explore the parameter space spanned by the particle size and the density ratio, and find a weaker tendency for accumulation in attractors and for caustics formation. The Lyapunov exponent of transients becomes larger with memory. Periodic attractors are found to have a very slow, t −1/2 type convergence towards the asymptotic form. We find that the concept of snapshot attractors is useful to understand this slow convergence: an ensemble of particles converges exponentially fast towards a snapshot attractor, which undergoes a slow shift for long times. (paper)

Can Hall effect trigger Kelvin-Helmholtz instability in sub-Alfvénic flows?

Science.gov (United States)

Pandey, B. P.

2018-05-01

In the Hall magnetohydrodynamics, the onset condition of the Kelvin-Helmholtz instability is solely determined by the Hall effect and is independent of the nature of shear flows. In addition, the physical mechanism behind the super- and sub-Alfvénic flows becoming unstable is quite different: the high-frequency right circularly polarized whistler becomes unstable in the super-Alfvénic flows whereas low-frequency, left circularly polarized ion-cyclotron wave becomes unstable in the presence of sub-Alfvénic shear flows. The growth rate of the Kelvin-Helmholtz instability in the super-Alfvénic case is higher than the corresponding ideal magnetohydrodynamic rate. In the sub-Alfvénic case, the Hall effect opens up a new, hitherto inaccessible (to the magnetohydrodynamics) channel through which the partially or fully ionized fluid can become Kelvin-Helmholtz unstable. The instability growth rate in this case is smaller than the super-Alfvénic case owing to the smaller free shear energy content of the flow. When the Hall term is somewhat smaller than the advection term in the induction equation, the Hall effect is also responsible for the appearance of a new overstable mode whose growth rate is smaller than the purely growing Kelvin-Helmholtz mode. On the other hand, when the Hall diffusion dominates the advection term, the growth rate of the instability depends only on the Alfvén -Mach number and is independent of the Hall diffusion coefficient. Further, the growth rate in this case linearly increases with the Alfvén frequency with smaller slope for sub-Alfvénic flows.

Pebble Accretion in Turbulent Protoplanetary Disks

Science.gov (United States)

Xu, Ziyan; Bai, Xue-Ning; Murray-Clay, Ruth A.

2017-09-01

It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the “Hill regime”), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time {τ }s˜ 0.1{--}1) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to {τ }s˜ 0.01) can be modestly reduced by strong turbulence for low-mass cores.

Lagrangian statistics and flow topology in forced two-dimensional turbulence.

Science.gov (United States)

Kadoch, B; Del-Castillo-Negrete, D; Bos, W J T; Schneider, K

2011-03-01

A study of the relationship between Lagrangian statistics and flow topology in fluid turbulence is presented. The topology is characterized using the Weiss criterion, which provides a conceptually simple tool to partition the flow into topologically different regions: elliptic (vortex dominated), hyperbolic (deformation dominated), and intermediate (turbulent background). The flow corresponds to forced two-dimensional Navier-Stokes turbulence in doubly periodic and circular bounded domains, the latter with no-slip boundary conditions. In the double periodic domain, the probability density function (pdf) of the Weiss field exhibits a negative skewness consistent with the fact that in periodic domains the flow is dominated by coherent vortex structures. On the other hand, in the circular domain, the elliptic and hyperbolic regions seem to be statistically similar. We follow a Lagrangian approach and obtain the statistics by tracking large ensembles of passively advected tracers. The pdfs of residence time in the topologically different regions are computed introducing the Lagrangian Weiss field, i.e., the Weiss field computed along the particles' trajectories. In elliptic and hyperbolic regions, the pdfs of the residence time have self-similar algebraic decaying tails. In contrast, in the intermediate regions the pdf has exponential decaying tails. The conditional pdfs (with respect to the flow topology) of the Lagrangian velocity exhibit Gaussian-like behavior in the periodic and in the bounded domains. In contrast to the freely decaying turbulence case, the conditional pdfs of the Lagrangian acceleration in forced turbulence show a comparable level of intermittency in both the periodic and the bounded domains. The conditional pdfs of the Lagrangian curvature are characterized, in all cases, by self-similar power-law behavior with a decay exponent of order -2.

Interhemispheric Asymmetry of the Sunward Plasma Flows for Strongly Dominant IMF BZ > 0

Science.gov (United States)

Yakymenko, K. N.; Koustov, A. V.; Fiori, R. A. D.

2018-01-01

Super Dual Auroral Radar Network (SuperDARN) convection maps obtained simultaneously in both hemispheres are averaged to infer polar cap ionospheric flow patterns under strongly dominant positive interplanetary magnetic field (IMF) Bz component. The data set consisted of winter observations in the Northern Hemisphere simultaneously with summer observations in the Southern Hemisphere. Long-lasting high-latitude dayside reverse convection cells are shown to have faster sunward flows at near-magnetic noon hours in the summer/Southern Hemisphere. Sunward flows typically deviate from the midnight-noon meridian toward 10-11 h of magnetic local time in the summer/Southern Hemisphere and are more aligned with the midnight-noon meridian in the winter/Northern Hemisphere. Flow deviations in the winter/Northern Hemisphere can be both toward prenoon and postnoon hours, and there is no clear relationship between flow deviation and the IMF By component. No strong preference for the sunward flow occurrence depending on the IMF Bx polarity was found. In addition, the rate of the sunward flow speed increase in response to an increase in driving conditions was found to be comparable for the IMF Bx > 0 and Bx < 0.

Accretion history of mid-Holocene coral reefs from the southeast Florida continental reef tract, USA

Science.gov (United States)

Stathakopoulos, A.; Riegl, B. M.

2015-03-01

Sixteen new coral reef cores were collected to better understand the accretion history and composition of submerged relict reefs offshore of continental southeast (SE) Florida. Coral radiometric ages from three sites on the shallow inner reef indicate accretion initiated by 8,050 Cal BP and terminated by 5,640 Cal BP. The reef accreted up to 3.75 m of vertical framework with accretion rates that averaged 2.53 m kyr-1. The reef was composed of a nearly even mixture of Acropora palmata and massive corals. In many cases, cores show an upward transition from massives to A. palmata and may indicate local dominance by this species prior to reef demise. Quantitative macroscopic analyses of reef clasts for various taphonomic and diagenetic features did not correlate well with depth/environmental-related trends established in other studies. The mixed coral framestone reef lacks a classical Caribbean reef zonation and is best described as an immature reef and/or a series of fused patch reefs; a pattern that is evident in both cores and reef morphology. This is in stark contrast to the older and deeper outer reef of the SE Florida continental reef tract. Accretion of the outer reef lasted from 10,695-8,000 Cal BP and resulted in a larger and better developed structure that achieved a distinct reef zonation. The discrepancies in overall reef morphology and size as well as the causes of reef terminations remain elusive without further study, yet they likely point to different climatic/environmental conditions during their respective accretion histories.

Testing the deep-crustal heating model using quiescent neutron-star very-faint X-ray transients and the possibility of partially accreted crusts in accreting neutron stars

Science.gov (United States)

Wijnands, R.; Degenaar, N.; Page, D.

2013-07-01

It is assumed that accreting neutron stars in low-mass X-ray binaries are heated due to the compression of the existing crust by the freshly accreted matter which gives rise to a variety of nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements (the deep-crustal heating scenario). In this paper we discuss if neutron stars in the so-called very-faint X-ray transients (VFXTs; those transients have outburst peak 2-10 keV X-ray luminosities short-term (less than a few tens of thousands of years) and the one throughout their lifetime. The latter is particularly important because it can be so low that the neutron stars might not have accreted enough matter to become massive enough that enhanced core cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processes have been inferred. However, the amount of matter can also not be too low because then the crust might not have been replaced significantly by accreted matter and thus a hybrid crust of partly accreted and partly original, albeit further compressed matter, might be present. This would inhibit the full range of pycnonuclear reactions to occur and therefore possibly decrease the amount of heat deposited in the crust. More detailed calculations of the heating and cooling properties of such hybrid crusts have to be performed to be conclusive. Furthermore, better understanding is needed about how a hybrid crust affects other properties such as the thermal conductivity. A potentially interesting way to observe the effects of a hybrid crust on the heating and cooling of an accreting neutron star is to observe the crust cooling of such a neutron star after a prolonged (years to decades) accretion episode and compare the results with similar studies performed for neutron stars with a fully accreted crust. We also show that some individual neutron

Advective-diffusive transport of D2O in unsaturated media under evaporation condition

International Nuclear Information System (INIS)

Koarashi, Jun; Atarashi-Andoh, Mariko; Amano, Hikaru; Yamazawa, Hiromi; Iida, Takao

2003-01-01

Advective-diffusive transport of HTO in unsaturated media was investigated empirically using deuterated water (D 2 O) and columns filled with glass beads. The tortuosity factor was evaluated by numerical model calculations corresponding to first experiment for diffusion under no-evaporation condition. Temporal variations in depth profiles of D 2 O concentrations in the columns were observed by second experiment, which considers the transferring and spreading of D 2 O by pore-water flow caused by evaporation. Measurements and model calculations indicated that diffusion was about two times more efficient than dispersion for D 2 O spreading process under this evaporation condition. (author)

SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

International Nuclear Information System (INIS)

Matt, Sean P.; Pinzón, Giovanni; Greene, Thomas P.; Pudritz, Ralph E.

2012-01-01

We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

Source to Accretion Disk Tilt

OpenAIRE

Montgomery, M. M.; Martin, E. L.

2010-01-01

Many different system types retrogradely precess, and retrograde precession could be from a tidal torque by the secondary on a misaligned accretion disk. However, a source to cause and maintain disk tilt is unknown. In this work, we show that accretion disks can tilt due to a force called lift. Lift results from differing gas stream supersonic speeds over and under an accretion disk. Because lift acts at the disk's center of pressure, a torque is applied around a rotation axis passing through...

X-Ray Spectra from MHD Simulations of Accreting Black Holes

Science.gov (United States)

Schnittman, Jeremy D.; Krolik, Julian H.; Noble, Scott C.

2012-01-01

We present the results of a new global radiation transport code coupled to a general relativistic magneto-hydrodynamic simulation of an accreting, nonrotating black hole. For the first time, we are able to explain from first principles in a self-consistent way the X-ray spectra observed from stellar-mass black holes, including a thermal peak, Compton reflection hump, power-law tail, and broad iron line. Varying only the mass accretion rate, we are able to reproduce the low/hard, steep power-law, and thermal-dominant states seen in most galactic black hole sources. The temperature in the corona is T(sub e) 10 keV in a boundary layer near the disk and rises smoothly to T(sub e) greater than or approximately 100 keV in low-density regions far above the disk. Even as the disk's reflection edge varies from the horizon out to approximately equal to 6M as the accretion rate decreases, we find that the shape of the Fe Ka line is remarkably constant. This is because photons emitted from the plunging region are strongly beamed into the horizon and never reach the observer. We have also carried out a basic timing analysis of the spectra and find that the fractional variability increases with photon energy and viewer inclination angle, consistent with the coronal hot spot model for X-ray fluctuations.

COMPETITIVE ACCRETION IN A SHEET GEOMETRY AND THE STELLAR IMF

International Nuclear Information System (INIS)

Hsu, Wen-Hsin; Hartmann, Lee; Heitsch, Fabian; Gomez, Gilberto C.

2010-01-01

We report a set of numerical experiments aimed at addressing the applicability of competitive accretion to explain the high-mass end of the stellar initial mass function in a sheet geometry with shallow gravitational potential, in contrast to most previous simulations which have assumed formation in a cluster gravitational potential. Our flat cloud geometry is motivated by models of molecular cloud formation due to large-scale flows in the interstellar medium. The experiments consisted of smoothed particle hydrodynamics simulations of gas accretion onto sink particles formed rapidly from Jeans-unstable dense clumps placed randomly in the finite sheet. These simplifications allow us to study accretion with a minimum of free parameters and to develop better statistics on the resulting mass spectra. We considered both clumps of equal mass and Gaussian distributions of masses and either uniform or spatially varying gas densities. In all cases, the sink mass function develops a power-law tail at high masses, with dN/dlog M ∝ M -Γ . The accretion rates of individual sinks follow M-dot ∝M 2 at high masses; this results in a continual flattening of the slope of the mass function toward an asymptotic form Γ ∼ 1 (where the Salpeter slope is Γ = 1.35). The asymptotic limit is most rapidly reached when starting from a relatively broad distribution of initial sink masses. In general, the resulting upper mass slope is correlated with the maximum sink mass; higher sink masses are found in simulations with flatter upper mass slopes. Although these simulations are of a highly idealized situation, the results suggest that competitive accretion may be relevant in a wider variety of environments than previously considered, and in particular that the upper mass distribution may generally evolve toward a limiting value of Γ ∼ 1.

Modeling Effectivity of Atmospheric Advection-Diffusion Processes

International Nuclear Information System (INIS)

Brojewski, R.

1999-01-01

Some methods of solving the advection-diffusion problems useful in the field of atmospheric physics are presented and analyzed in the paper. The most effective one ( from the point of view of computer applications) was chosen. This is the method of problem decomposition with respect to the directions followed by secondary decomposition of the problem with respect to the physical phenomena. Introducing some corrections to the classical numerical methods of solving the problems, a hybrid composed of the finite element method for the advection problems and the implicit method with averaging for the diffusion processes was achieved. This hybrid method and application of the corrections produces a very effective means for solving the problems of substance transportation in atmosphere. (author)

Relativistic Outflows from ADAFs

Science.gov (United States)

Becker, Peter; Subramanian, Prasad; Kazanas, Demosthenes

2001-04-01

Advection-dominated accretion flows (ADAFs) have a positive Bernoulli parameter, and are therefore gravitationally bound. The Newtonian ADAF model has been generalized recently to obtain the ADIOS model that includes outflows of energy and angular momentum, thereby allowing accretion to proceed self-consistently. However, the utilization of a Newtonian gravitational potential limits the ability of this model to describe the inner region of the disk, where any relativistic outflows are likely to originate. In this paper we modify the ADIOS scenario to incorporate a seudo - Newtonian potential, which approximates the effects of general relativity. The analysis yields a unique, self - similar solution for the structure of the coupled disk/wind system. Interesting features of the new solution include the relativistic character of the outflow in the vicinity of the radius of marginal stability, which represents the inner edge of the quasi-Keplerian disk in our model. Our self - similar model may therefore help to explain the origin of relativistic jets in active galaxies. At large distances the radial dependence of the accretion rate approachs the unique form dot M ∝ r^1/2, with an associated density variation given by ρ ∝ r-1. This density variation agrees with that implied by the dependence of the X-ray hard time lags on the Fourier frequency for a number of accreting galactic black hole candidates. While intriguing, the results of our self-similar model need to be confirmed in the future by incorporating a detailed physical description of the energization mechanism that drives the outflow, which is likely to be powered by the shear of the underlying accretion disk.

Passive advection-dispersion in networks of pipes: Effect of connectivity and relationship to permeability

Science.gov (United States)

Bernabé, Y.; Wang, Y.; Qi, T.; Li, M.

2016-02-01

The main purpose of this work is to investigate the relationship between passive advection-dispersion and permeability in porous materials presumed to be statistically homogeneous at scales larger than the pore scale but smaller than the reservoir scale. We simulated fluid flow through pipe network realizations with different pipe radius distributions and different levels of connectivity. The flow simulations used periodic boundary conditions, allowing monitoring of the advective motion of solute particles in a large periodic array of identical network realizations. In order to simulate dispersion, we assumed that the solute particles obeyed Taylor dispersion in individual pipes. When a particle entered a pipe, a residence time consistent with local Taylor dispersion was randomly assigned to it. When exiting the pipe, the particle randomly proceeded into one of the pipes connected to the original one according to probabilities proportional to the outgoing volumetric flow in each pipe. For each simulation we tracked the motion of at least 6000 solute particles. The mean fluid velocity was 10-3 ms-1, and the distance traveled was on the order of 10 m. Macroscopic dispersion was quantified using the method of moments. Despite differences arising from using different types of lattices (simple cubic, body-centered cubic, and face-centered cubic), a number of general observations were made. Longitudinal dispersion was at least 1 order of magnitude greater than transverse dispersion, and both strongly increased with decreasing pore connectivity and/or pore size variability. In conditions of variable hydraulic radius and fixed pore connectivity and pore size variability, the simulated dispersivities increased as power laws of the hydraulic radius and, consequently, of permeability, in agreement with previously published experimental results. Based on these observations, we were able to resolve some of the complexity of the relationship between dispersivity and permeability.

Advection endash diffusion past a strip. II. Oblique incidence

International Nuclear Information System (INIS)

Knessl, C.; Keller, J.B.

1997-01-01

Advection and diffusion of particles past an impenetrable strip is considered when the strip is oblique to the advection or drift velocity. The particle concentration p(x,y) is determined asymptotically for large values of vL/D, where v is the drift velocity, D is the diffusion coefficient, and 2L is the width of the strip. The results complement those of Part I, which treated a strip normal to the drift velocity. copyright 1997 American Institute of Physics

MASSIVE BLACK HOLES IN STELLAR SYSTEMS: 'QUIESCENT' ACCRETION AND LUMINOSITY

International Nuclear Information System (INIS)

Volonteri, M.; Campbell, D.; Mateo, M.; Dotti, M.

2011-01-01

Only a small fraction of local galaxies harbor an accreting black hole, classified as an active galactic nucleus. However, many stellar systems are plausibly expected to host black holes, from globular clusters to nuclear star clusters, to massive galaxies. The mere presence of stars in the vicinity of a black hole provides a source of fuel via mass loss of evolved stars. In this paper, we assess the expected luminosities of black holes embedded in stellar systems of different sizes and properties, spanning a large range of masses. We model the distribution of stars and derive the amount of gas available to a central black hole through a geometrical model. We estimate the luminosity of the black holes under simple, but physically grounded, assumptions on the accretion flow. Finally, we discuss the detectability of 'quiescent' black holes in the local universe.

Does the mass of a black hole decrease due to the accretion of phantom energy?

International Nuclear Information System (INIS)

Gao Changjun; Chen Xuelei; Faraoni, Valerio; Shen Yougen

2008-01-01

According to Babichev et al., the accretion of a phantom test fluid onto a Schwarzschild black hole will induce the mass of the black hole to decrease, however the backreaction was ignored in their calculation. Using new exact solutions describing black holes in a background Friedmann-Robertson-Walker universe, we find that the physical black hole mass may instead increase due to the accretion of phantom energy. If this is the case, and the future universe is dominated by phantom dark energy, the black hole apparent horizon and the cosmic apparent horizon will eventually coincide and, after that, the black hole singularity will become naked in finite comoving time before the big rip occurs, violating the cosmic censorship conjecture.

Two-relaxation-time lattice Boltzmann method and its application to advective-diffusive-reactive transport

Science.gov (United States)

Yan, Zhifeng; Yang, Xiaofan; Li, Siliang; Hilpert, Markus

2017-11-01

The lattice Boltzmann method (LBM) based on single-relaxation-time (SRT) or multiple-relaxation-time (MRT) collision operators is widely used in simulating flow and transport phenomena. The LBM based on two-relaxation-time (TRT) collision operators possesses strengths from the SRT and MRT LBMs, such as its simple implementation and good numerical stability, although tedious mathematical derivations and presentations of the TRT LBM hinder its application to a broad range of flow and transport phenomena. This paper describes the TRT LBM clearly and provides a pseudocode for easy implementation. Various transport phenomena were simulated using the TRT LBM to illustrate its applications in subsurface environments. These phenomena include advection-diffusion in uniform flow, Taylor dispersion in a pipe, solute transport in a packed column, reactive transport in uniform flow, and bacterial chemotaxis in porous media. The TRT LBM demonstrated good numerical performance in terms of accuracy and stability in predicting these transport phenomena. Therefore, the TRT LBM is a powerful tool to simulate various geophysical and biogeochemical processes in subsurface environments.

Kinetic description of quasi-stationary axisymmetric collisionless accretion disk plasmas with arbitrary magnetic field configurations

International Nuclear Information System (INIS)

Cremaschini, Claudio; Miller, John C.; Tessarotto, Massimo

2011-01-01

A kinetic treatment is developed for collisionless magnetized plasmas occurring in high-temperature, low-density astrophysical accretion disks, such as are thought to be present in some radiatively inefficient accretion flows onto black holes. Quasi-stationary configurations are investigated, within the framework of a Vlasov-Maxwell description. The plasma is taken to be axisymmetric and subject to the action of slowly time-varying gravitational and electromagnetic fields. The magnetic field is assumed to be characterized by a family of locally nested but open magnetic surfaces. The slow collisionless dynamics of these plasmas is investigated, yielding a reduced gyrokinetic Vlasov equation for the kinetic distribution function. For doing this, an asymptotic quasi-stationary solution is first determined, represented by a generalized bi-Maxwellian distribution expressed in terms of the relevant adiabatic invariants. The existence of the solution is shown to depend on having suitable kinetic constraints and conditions leading to particle trapping phenomena. With this solution, one can treat temperature anisotropy, toroidal and poloidal flow velocities, and finite Larmor-radius effects. An asymptotic expansion for the distribution function permits analytic evaluation of all the relevant fluid fields. Basic theoretical features of the solution and their astrophysical implications are discussed. As an application, the possibility of describing the dynamics of slowly time-varying accretion flows and the self-generation of magnetic field by means of a ''kinetic dynamo effect'' are discussed. Both effects are shown to be related to intrinsically kinetic physical mechanisms.

Enhanced transpiration by riparian buffer trees in response to advection in a humid temperate agricultural landscape

Science.gov (United States)

Hernandez-Santana, V.; Asbjornsen, H.; Sauer, T.; Isenhart, T.; Schilling, K.; Schultz, Ronald

2011-01-01

Riparian buffers are designed as management practices to increase infiltration and reduce surface runoff and transport of sediment and nonpoint source pollutants from crop fields to adjacent streams. Achieving these ecosystem service goals depends, in part, on their ability to remove water from the soil via transpiration. In these systems, edges between crop fields and trees of the buffer systems can create advection processes, which could influence water use by trees. We conducted a field study in a riparian buffer system established in 1994 under a humid temperate climate, located in the Corn Belt region of the Midwestern U.S. (Iowa). The goals were to estimate stand level transpiration by the riparian buffer, quantify the controls on water use by the buffer system, and determine to what extent advective energy and tree position within the buffer system influence individual tree transpiration rates. We primarily focused on the water use response (determined with the Heat Ratio Method) of one of the dominant species (Acer saccharinum) and a subdominant (Juglans nigra). A few individuals of three additional species (Quercus bicolor, Betula nigra, Platanus occidentalis) were monitored over a shorter time period to assess the generality of responses. Meteorological stations were installed along a transect across the riparian buffer to determine the microclimate conditions. The differences found among individuals were attributed to differences in species sap velocities and sapwood depths, location relative to the forest edge and prevailing winds and canopy exposure and dominance. Sapflow rates for A. saccharinum trees growing at the SE edge (prevailing winds) were 39% greater than SE interior trees and 30% and 69% greater than NW interior and edge trees, respectively. No transpiration enhancement due to edge effect was detected in the subdominant J. nigra. The results were interpreted as indicative of advection effects from the surrounding crops. Further, significant

Strouhal number of bridge cables with ice accretion at low flow turbulence

Czech Academy of Sciences Publication Activity Database

Górski, P.; Pospíšil, Stanislav; Kuznetsov, Sergeii; Tatara, M.; Marušić, Ante

2016-01-01

Roč. 22, č. 2 (2016), s. 253-272 ISSN 1226-6116 R&D Projects: GA MŠk(CZ) LO1219; GA ČR(CZ) GC13-34405J; GA ČR(CZ) GA14-12892S Keywords : bridge cable * ice accretion * Strouhal number * angle of attack * vortex shedding frequency Subject RIV: JM - Building Engineering Impact factor: 0.868, year: 2016 http://koreascience.or.kr/article/ArticleFullRecord.jsp?cn=KJKHCF_2016_v22n2_253

A possible origin of viscosity in Keplerian accretion disks due to secondary perturbation: Turbulent transport without magnetic fields

International Nuclear Information System (INIS)

Mukhopadhyay, Banibrata; Saha, Kanak

2011-01-01

The origin of hydrodynamic turbulence in rotating shear flow is a long standing puzzle. Resolving it is especially important in astrophysics when the flow's angular momentum profile is Keplerian which forms an accretion disk having negligible molecular viscosity. Hence, any viscosity in such systems must be due to turbulence, arguably governed by magnetorotational instability, especially when temperature T > or approx. 10 5 . However, such disks around quiescent cataclysmic variables, protoplanetary and star-forming disks, and the outer regions of disks in active galactic nuclei are practically neutral in charge because of their low temperature, and thus are not expected to be coupled with magnetic fields enough to generate any transport due to the magnetorotational instability. This flow is similar to plane Couette flow including the Coriolis force, at least locally. What drives their turbulence and then transport, when such flows do not exhibit any unstable mode under linear hydrodynamic perturbation? We demonstrate that the three-dimensional secondary disturbance to the primarily perturbed flow that triggers elliptical instability may generate significant turbulent viscosity in the range 0.0001 ∼ t ∼< 0.1, which can explain transport in accretion flows.

AS ABOVE, SO BELOW: EXPLOITING MASS SCALING IN BLACK HOLE ACCRETION TO BREAK DEGENERACIES IN SPECTRAL INTERPRETATION

International Nuclear Information System (INIS)

Markoff, Sera; Silva, Catia V.; Nowak, Michael A.; Gallo, Elena; Plotkin, Richard M.; Hynes, Robert; Wilms, Jörn; Maitra, Dipankar; Drappeau, Samia

2015-01-01

Over the past decade, evidence has mounted that several aspects of black hole (BH) accretion physics proceed in a mass-invariant way. One of the best examples of this scaling is the empirical “fundamental plane of BH accretion” relation linking mass, radio, and X-ray luminosity over eight orders of magnitude in BH mass. The currently favored theoretical interpretation of this relation is that the physics governing power output in weakly accreting BHs depends more on relative accretion rate than on mass. In order to test this theory, we explore whether a mass-invariant approach can simultaneously explain the broadband spectral energy distributions from two BHs at opposite ends of the mass scale but that are at similar Eddington accretion fractions. We find that the same model, with the same value of several fitted physical parameters expressed in mass-scaling units to enforce self-similarity, can provide a good description of two data sets from V404 Cyg and M81*, a stellar and supermassive BH, respectively. Furthermore, only one of several potential emission scenarios for the X-ray band is successful, suggesting it is the dominant process driving the fundamental plane relation at this accretion rate. This approach thus holds promise for breaking current degeneracies in the interpretation of BH high-energy spectra and for constructing better prescriptions of BH accretion for use in various local and cosmological feedback applications

2.5-D modelling of rime ice accretion on a swept airfoil

Energy Technology Data Exchange (ETDEWEB)

Szilder, K. [National Research Council of Canada, Inst. for Aerospace Research, Ottawa, Ontario (Canada)]. E-mail: Krzysztof.Szilder@nrc-cnrc.gc.ca; Lozowski, E.P. [Univ. of Alberta, Dept. of Earth and Atmospheric Sciences, Edmonton, Alberta (Canada)

2004-07-01

In this paper, we present a three-dimensional morphogenetic model which simulates discrete rime ice accretion structures forming on a swept airfoil. Rime is an ice deposit caused by the impingement and freezing of supercooled cloud droplets at accretion temperatures below 0{sup o}C. Depending on conditions, the droplets may freeze as spheres or deform on the surface, but for rime to form, surface liquid flow is limited or non-existent. In the model, roughness elements that develop initially on the wing surface evolve into rime feathers and other complex, three-dimensional structures. On swept wings, these resemble the so-called 'lobster tails' or 'scallops' that are observed in wind tunnels and in flight. We show the results of sensitivity tests, performed as a function of two model parameters: sweep angle and freezing range parameter. Our exploratory research has shown that the model predicts realistic-looking, three-dimensional ice structures on swept wings. This capability does not exist in current in-flight ice accretion models. (author)

Perils at the heart of the Milky Way: Systematic effects for studying low-luminosity accretion onto Sgr A*

Science.gov (United States)

Corrales, Lia; Mon, Brayden; Haggard, Daryl; Baganoff, Frederick K.; Garmire, Gordon; Degenaar, Nathalie; Reynolds, Mark

2017-08-01

The supermassive black hole at the center of our galaxy, Sgr A*, is surprisingly under-luminous. This problem has motivated a host of theoretical models to explain low-level radiatively inefficient accretion flows onto compact objects. We discuss how the Galactic Center sight line, which is optically thick to the scattering of soft X-rays (tau ~ 5), affects high resolution studies of the accretion flow around Sgr A*. X-ray light from compact objects in the dense GC environment is scattered by foreground dust, producing scattering echoes that are time delayed relative to the X-ray source's light curve. We discuss the scattering halo of SWIFT J174540.7-290015, which underwent the brightest X-ray outburst within 30’' of Sgr A*. Preliminary fits to the scattering halo suggest that a small amount of foreground dust, within 250 pc of the GC, affects the X-ray surface brightness profile within 10’' of any GC point source. The associated time delay is on the order of several hours, which is important for understanding the quiescent accretion flow of Sgr A*. We take advantage of the Chandra Galactic Center XVP dataset to explore the effect of the interstellar medium on the inferred characteristics of Sgr A*.

Vulnerability assessment of the Toluca Valley aquifer combining a parametric approach and advective transport

International Nuclear Information System (INIS)

Gárfias, J.; Llanos, H.; Franco, R.; Martel, R.

2017-01-01

Groundwater vulnerability assessment is an important task in water resources and land management. Depending on the availability of data and the complexity of the hydrogeological conditions, different approaches can be adopted. As an alternative, this study involves the use of a combined approach based on vulnerability methods and advective particle tracking to better understand the susceptibility to contamination in the Toluca valley aquifer. An intrinsic vulnerability map (DRASTIC) was used to identify areas that are more susceptible to ground water contamination. To estimate advective particle tracking, we developed a 3D flow model using VisualModflow and MODPATH to describe the regional flow of groundwater. The vulnerability map demonstrates the problematic application and interpretation of qualitative the vulnerability method of the parametric system group, which indicates a difference of approximately 23% when compared with the modified vulnerability map. Potential contamination sources based on landfill sites were comparatively high; approximately 76% are located in areas that could be susceptible to contamination through vertical infiltration, especially those that are located along the Lerma system of wells. Industrial parks located in the centre of the valley (83%), where continuous extraction of groundwater and land subsidence occurs, have been classified as high vulnerability zones, increasing the risk of contaminants from surface sources reaching the groundwater. In order to understand the susceptibility to contamination in the aquifer, various delineation approaches should be adopted and all the results that validate each other should be considered, thus making a good strategy for implementing different degrees of protection measures. [es

Verification of a dust transport model against theoretical solutions in multidimensional advection diffusion problems

Energy Technology Data Exchange (ETDEWEB)

Xu, Z., E-mail: zhanjie.xu@kit.ed [Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe (Germany); Travis, J.R. [Ingenieurbuero DuBois-Pitzer-Travis, 63071 Offenbach (Germany); Breitung, W.; Jordan, T. [Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe (Germany)

2010-12-15

Potentially explosive dust aerosol mobilization in the vacuum vessel is an important safety issue of the ITER facility, especially in scenarios of loss of vacuum accidents. Therefore dust mobilization modeling is ongoing in Research Center Karlsuhe. At first the aerosol particle model in the GASFLOW computer code is introduced briefly. To verify the particle model, a series of particle diffusion problems are simulated in one-, two- and three-dimensions. In each problem a particle source is initially exposed to an advective gas flow. Then a dust cloud is formed in the down stream. To obtain the theoretical solution about the particle concentration in the dust cloud, the governing diffusion partial differential equations with an additional advection term are solved by using Green's function method. Different spatial and temporal characters about the particle sources are also considered, e.g., instantaneous or continuous sources, line, or volume sources and so forth. The GASFLOW simulation results about the particle concentrations and the corresponding Green's function solutions are compared case by case. Very good agreements are found between the theoretical solutions and the GASGLOW simulations, when the drag force between the micron-sized particles and the conveying gas flow meets the Stokes' law about resistance. This situation is corresponding to a very small Reynolds number based on the particle diameter, with a negligible inertia effect of the particles. This verification work shows that the particle model of the GASFLOW code can reproduce numerically particle transport and diffusion in a good way.

Time-series of turbulent flow in a pipe measured with PIV

DEFF Research Database (Denmark)

Meyer, Knud Erik; Westerweel, Jerry

1999-01-01

Measurements with particle image velocimetry of the fully developed flow of water in a pipe with a Reynolds number of 5370 are presented. The measurements are taken with a frame rate high enough to capture the same flow structure on 2 or 3 frames. This makes it possible to estimate advection...... velocities of the flow structures and distances between the structures. Ejection andsweep structures are identified and it is found that average advection velocity for the structures corresponds the local mean velocity at a distance of 0.15 diameter from the wall. It is also noted that ejections tend...

Compton-heated winds and coronae above accretion disks. II. Instability and oscillations

International Nuclear Information System (INIS)

Shields, G.A.; Mckee, C.F.; Lin, D.N.C.; Begelman, M.C.; California Univ., Berkeley; California Univ., Santa Cruz; Colorado Univ., Boulder)

1986-01-01

The stability and evolution of windy accretion disks is investigated in detail. The basic disk evolution equations are briefly recapitulated, and an idealized analytic treatment of the wind and viscosity is used to show that steady disk flow is indeed unstable for sufficiently large ratio of the mass loss rate in the wind to the central accretion rate. Numerical solutions for a more realistic and appropriate expression for the mass loss rate and the standard ad hoc alpha model prescription for the viscosity are presented. The application of these results to real systems with Compton-heated winds is discussed, and a general formula for the oscillation period is given. The prediction is compared with observed periodic behavior of Galactic X-ray sources and AGNs. 17 references

Design of Experiments Relevant to Accreting Stream-Disk Impact in Interacting Binaries

Science.gov (United States)

Krauland, Christine; Drake, R. P.; Kuranz, C. C.; Grosskopf, M. J.; Young, R.; Plewa, T.

2010-05-01

In many Cataclysmic Binary systems, mass transfer via Roche lobe overflow onto an accretion disk occurs. This produces a hot spot from the heating created by the supersonic impact of the infalling flow with the rotating accretion disk, which can produce a radiative reverse shock in the infalling flow. This collision region has many ambiguities as a radiation hydrodynamic system. Depending upon conditions, it has been argued (Armitgae & Livio, ApJ 493, 898) that the shocked region may be optically thin, thick, or intermediate, which has the potential to significantly alter its structure and emissions. Laboratory experiments have yet to produce colliding flows that create a radiative reverse shock or to produce obliquely incident colliding flows, both of which are aspects of these Binary systems. We have undertaken the design of such an experiment, aimed at the Omega-60 laser facility. The design elements include the production of postshock flows within a dense material layer or ejecta flows by release of material from a shocked layer. Obtaining a radiative reverse shock in the laboratory requires producing a sufficiently fast flow (> 100 km/s) within a material whose opacity is large enough to produce energetically significant emission from experimentally achievable layers. In this poster we will discuss the astrophysical context, the experimental design work we have done, and the challenges of implementing and diagnosing an actual experiment. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, by the National Laser User Facility Program in NNSA-DS and by the Predictive Sciences Academic Alliances Program in NNSA-ASC. The corresponding grant numbers are DE-FG52-09NA29548, DE-FG52-09NA29034, and DE-FC52-08NA28616.

Simulating X-ray bursts during a transient accretion event

Science.gov (United States)

Johnston, Zac; Heger, Alexander; Galloway, Duncan K.

2018-06-01

Modelling of thermonuclear X-ray bursts on accreting neutron stars has to date focused on stable accretion rates. However, bursts are also observed during episodes of transient accretion. During such events, the accretion rate can evolve significantly between bursts, and this regime provides a unique test for burst models. The accretion-powered millisecond pulsar SAX J1808.4-3658 exhibits accretion outbursts every 2-3 yr. During the well-sampled month-long outburst of 2002 October, four helium-rich X-ray bursts were observed. Using this event as a test case, we present the first multizone simulations of X-ray bursts under a time-dependent accretion rate. We investigate the effect of using a time-dependent accretion rate in comparison to constant, averaged rates. Initial results suggest that using a constant, average accretion rate between bursts may underestimate the recurrence time when the accretion rate is decreasing, and overestimate it when the accretion rate is increasing. Our model, with an accreted hydrogen fraction of X = 0.44 and a CNO metallicity of ZCNO = 0.02, reproduces the observed burst arrival times and fluences with root mean square (rms) errors of 2.8 h, and 0.11× 10^{-6} erg cm^{-2}, respectively. Our results support previous modelling that predicted two unobserved bursts and indicate that additional bursts were also missed by observations.

Advectional enhancement of eddy diffusivity under parametric disorder

International Nuclear Information System (INIS)

Goldobin, Denis S

2010-01-01

Frozen parametric disorder can lead to the appearance of sets of localized convective currents in an otherwise stable (quiescent) fluid layer heated from below. These currents significantly influence the transport of an admixture (or any other passive scalar) along the layer. When the molecular diffusivity of the admixture is small in comparison to the thermal one, which is quite typical in nature, disorder can enhance the effective (eddy) diffusivity by several orders of magnitude in comparison to the molecular diffusivity. In this paper, we study the effect of an imposed longitudinal advection on the delocalization of convective currents, both numerically and analytically, and report a subsequent drastic boost of the effective diffusivity for weak advection.

Turbulent characteristics of shear-thinning fluids in recirculating flows

Energy Technology Data Exchange (ETDEWEB)

Pereira, A.S. [Inst. Superior de Engenharia do Porto (Portugal). Dept. de Engenharia Quimica; Pinho, F.T. [Centro de Estudos de Fenomenos de Transporte, Departamento de Engenharia Mecanica e Gestao Industrial, Faculdade de Engenharia da Universidade do Porto, Rua dos Bragas, 4050-123 Porto (Portugal)

2000-03-01

A miniaturised fibre optic laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1-0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers. (orig.)

Cold gas accretion in galaxies

NARCIS (Netherlands)

Sancisi, Renzo; Fraternali, Filippo; Oosterloo, Tom; van der Hulst, Thijs

Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: (1) A large number of galaxies are accompanied by

The diversity of quasars unified by accretion and orientation.