scholarly journals Numerical Simulations of MHD Turbulence in Accretion Disks

2003 ◽  
pp. 329-348 ◽  
Author(s):  
Steven A. Balbus ◽  
John F. Hawley
Keyword(s):  
Numerical Simulations ◽  
Accretion Disks ◽  
Mhd Turbulence

Saturation mechanism and generated viscosity in gravito-turbulent accretion disks

2020 ◽  
Vol 640 ◽  
pp. A53
Author(s):  
L. Löhnert ◽  
S. Krätschmer ◽  
A. G. Peeters
Keyword(s):  
Growth Rate ◽  
Numerical Simulations ◽  
Accretion Disks ◽  
Gravitational Instability ◽  
Turbulent Viscosity ◽  
Radial Distance ◽  
Maximum Growth ◽  
Wave Number ◽  
Radiative Cooling ◽  
Mixing Length

Here, we address the turbulent dynamics of the gravitational instability in accretion disks, retaining both radiative cooling and irradiation. Due to radiative cooling, the disk is unstable for all values of the Toomre parameter, and an accurate estimate of the maximum growth rate is derived analytically. A detailed study of the turbulent spectra shows a rapid decay with an azimuthal wave number stronger than ky−3, whereas the spectrum is more broad in the radial direction and shows a scaling in the range kx−3 to kx−2. The radial component of the radial velocity profile consists of a superposition of shocks of different heights, and is similar to that found in Burgers’ turbulence. Assuming saturation occurs through nonlinear wave steepening leading to shock formation, we developed a mixing-length model in which the typical length scale is related to the average radial distance between shocks. Furthermore, since the numerical simulations show that linear drive is necessary in order to sustain turbulence, we used the growth rate of the most unstable mode to estimate the typical timescale. The mixing-length model that was obtained agrees well with numerical simulations. The model gives an analytic expression for the turbulent viscosity as a function of the Toomre parameter and cooling time. It predicts that relevant values of α = 10−3 can be obtained in disks that have a Toomre parameter as high as Q ≈ 10.

MHD turbulence and accretion disks

1990 ◽  
Vol 59 (1) ◽  
pp. 145-152 ◽  
Author(s):  
G.T. Geertsema ◽  
A. Achterberg
Keyword(s):  
Accretion Disks ◽  
Mhd Turbulence

scholarly journals Spectral properties of magnetohydrodynamic convection with mean horizontal temperature gradient

2006 ◽  
Vol 2 (S239) ◽  
pp. 513-513
Author(s):  
D. Skandera ◽  
W.-Ch. Müller
Keyword(s):  
Temperature Field ◽  
Rayleigh Number ◽  
Temperature Gradient ◽  
Numerical Simulations ◽  
Spectral Properties ◽  
Three Dimensional ◽  
Numerical Code ◽  
Dimensional System ◽  
Horizontal Temperature Gradient ◽  
Mhd Turbulence

AbstractSpectral properties of convective magnetohydrodynamic (MHD) turbulence in two and three dimensions are studied by means of direct numerical simulations (Skandera D. & Müller W.-C. 2006). The investigated system is set up with a mean horizontal temperature gradient in order to avoid a development of elevator instabilities in a fully periodic box. All simulations are performed without mean magnetic field. The applied resolution is 5123 and 20482. The MHD equation are solved by a numerical code (Müller & Biskamp 2000) that uses a standard pseudospectral scheme. For removing of aliasing errors a spherical truncation method is employed. Obtained results are compared with predictions of various existing phenomenological theories for magnetohydrodynamic and convective turbulence (Müller & Biskamp 2000). While the three-dimensional system is found to operate in a Kolmogorov-like regime where buoyant forces have a negligible impact on the turbulence dynamics (relatively low Rayleigh number achieved in the simulation; Ra ∼106), the two-dimensional system exhibits interesting irregular quasi-oscillations between a buoyancy dominated Bolgiano-Obukhov-like regime of turbulence and a standard Iroshnikov-Kraichnan-like regime of turbulence (Müller & Biskamp 2000). The most important parameter determining the turbulent regime of 2D magnetoconvection, apart from a high Rayleigh number, seems to be the mutual alignment of velocity and magnetic fields. The non-linear dynamics and the interplay between individual fields are examined with different transfer functions that confirm basic assumptions about directions of energy transfer in spectral space. Kinetic, magnetic and temperature energy are transported by a turbulent cascade from large to smaller scales. The local/nonlocal character of the transport is tested for several individual terms in the governing equations. Moreover, other statistical quantities, e.g. probability density functions, are computed as well. A passive character of the temperature field in the investigated three-dimensional magnetoconvection is supported by computations of intermittency using extended self-similarity. The intermittency of the Elsasser field z+ is in agreement with results from numerical simulations of isotropic MHD turbulence (Müller & Biskamp 2000). The intermittency of the temperature field is found to approximately agree with results of passive scalar measurements in hydrodynamic turbulence (Ruiz-Chavarria, Baudet & Ciliberto 1996).

Heating mechanisms in accretion disks around young stellar objects

2018 ◽  
Vol 14 (S345) ◽  
pp. 255-256
Author(s):  
Natália F.S. Andrade ◽  
Rafael Rechiche de Campos ◽  
Vera Jatenco-Pereira
Keyword(s):  
Accretion Disks ◽  
Alfven Waves ◽  
Young Stellar Objects ◽  
Alfvén Waves ◽  
Rotational Instability ◽  
Mhd Turbulence ◽  
Stellar Objects ◽  
Heating Source ◽  
Order Of Magnitude ◽  
Magnetic Filed

AbstractAccretion disks are observed around young stellar objects such as T Tauri stars. In order to complete the star formation, particles in the disk need to loose angular momentum in order to be accreted into the central object. The magneto-rotational instability (MRI) is probably the mechanism responsible for a magneto-hydrodynamic (MHD) turbulence that leads to disk accretion, which implies the disk particles to be coupled with the magnetic filed lines. As the temperature in the disk is low, we considered, besides the viscous heating mechanism often included in the models by means of the α - prescription, the damping of Alfvén waves as an additional heating source. In particular, we show that the mechanism derived that couples the turbulent and non-linear damping mechanisms of Alfvén waves proved to be very efficient, generating temperatures almost one order of magnitude higher than those mechanisms considered independently.

Numerical simulations of magnetic accretion disks

1994 ◽  
Vol 433 ◽  
pp. 746 ◽  
Author(s):  
James M. Stone ◽  
Michael L. Norman
Keyword(s):  
Numerical Simulations ◽  
Accretion Disks

scholarly journals Dynamo Action in Accretion Disks

1993 ◽  
Vol 157 ◽  
pp. 209-210
Author(s):  
Ulf Torkelsson
Keyword(s):  
Numerical Simulations ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Standard Theory ◽  
Dynamo Action ◽  
Preliminary Results

Employing the standard theory for thin accretion disks I estimate the relevant parameters for a dynamo in an accretion disk. These estimates could then be compared to the results of numerical simulations. Some preliminary results of such simulations (Torkelsson & Brandenburg 1992) are presented too.

scholarly journals Angular Momentum Transport by MHD Turbulence in Accretion Disks

2004 ◽  
Vol 155 ◽  
pp. 409-410 ◽  
Author(s):  
Takayoshi Sano ◽  
Shu-ichiro Inutsuka ◽  
Neal J. Turner ◽  
James M. Stone
Keyword(s):  
Angular Momentum ◽  
Accretion Disks ◽  
Momentum Transport ◽  
Mhd Turbulence ◽  
Angular Momentum Transport

scholarly journals Numerical Simulations of MHD Accretion Disks

2009 ◽  
Vol 5 (H15) ◽  
pp. 237-238
Author(s):  
J. F. Hawley
Keyword(s):  
Numerical Simulations ◽  
Accretion Disks

AbstractNumerical simulations play an increasingly important role in investigating accretion disks and associated phenomena such as jets. This paper provides a few examples of recent results that have been obtained with simulations, both local or global.

MHD Turbulence in an Accretion Disk

1995 ◽  
Vol 12 (2) ◽  
pp. 159-164 ◽  
Author(s):  
John F. Hawley ◽  
Steven A. Balbus
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Shear Flow ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Reynolds Stresses ◽  
Mhd Turbulence ◽  
Mhd Instability ◽  
Standing Problem ◽  
Disk Shear

AbstractA long-standing problem in the theory of astrophysical accretion disks has been to determine the nature of the stress that transports orbital angular momentum outward. The discovery of a local MHD instability is strong evidence that transport occurs through turbulent Maxwell and Reynolds stresses. Using numerical simulations, we have demonstrated that a weak seed magnetic field in an accretion disk shear flow is unstable and leads to sustained MHD turbulence at dynamically important levels.

scholarly journals Properties of Hall-MHD Turbulence at Sub-Ion Scales: Spectral Transfer Analysis

Atmosphere ◽  
2021 ◽  
Vol 12 (12) ◽  
pp. 1632
Author(s):  
Emanuele Papini ◽  
Petr Hellinger ◽  
Andrea Verdini ◽  
Simone Landi ◽  
Luca Franci ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Transfer Rate ◽  
Space Plasma ◽  
Theoretical Models ◽  
Magnetic Fluctuations ◽  
Energy Transfer Rate ◽  
Mhd Turbulence ◽  
Developed Turbulence ◽  
First Time ◽  
Hall Mhd

We present results of a multiscale study of Hall-magnetohydrodynamic (MHD) turbulence, carried out on a dataset of compressible nonlinear 2D Hall-MHD numerical simulations of decaying Alfvénic turbulence. For the first time, we identify two distinct regimes of fully developed turbulence. In the first one, the power spectrum of the turbulent magnetic fluctuations at sub-ion scales exhibits a power law with a slope of ∼−2.9, typically observed both in solar wind and in magnetosheath turbulence. The second regime, instead, shows a slope of −7/3, in agreement with classical theoretical models of Hall-MHD turbulence. A spectral-transfer analysis reveals that the latter regime occurs when the energy transfer rate at sub-ion scales is dominated by the Hall term, whereas in the former regime, the governing process is the dissipation (and the system exhibits large intermittency). Results of this work are relevant to the space plasma community, as they may potentially reconcile predictions from theoretical models with results from numerical simulations and spacecraft observations.

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