Taking control of compressible modes: bulk viscosity and the turbulent dynamo

Abstract

Many polyatomic astrophysical plasmas are compressible and out of chemical and thermal equilibrium, introducing a bulk viscosity into the plasma via the internal degrees of freedom of the molecular composition, directly impacting the decay of compressible modes, ${\mathbf{v}}_{\parallel }(\mathbf{k})$. This is especially important for small-scale, turbulent dynamo processes in the interstellar medium, which are known to be sensitive to the effects of compression. To control the viscous properties of ${\mathbf{v}}_{\parallel }(\mathbf{k})$, we perform trans-sonic, visco-resistive dynamo simulations with additional bulk viscosity ${\nu }_{\mathrm{bulk}}$, deriving a new ${\nu }_{\mathrm{bulk}}$ Reynolds number ${\mathrm{Re}}_{\mathrm{bulk}}$, and viscous Prandtl number $\mathrm{P}\nu \equiv {\mathrm{Re}}_{\mathrm{bulk}}/{\mathrm{Re}}_{\mathrm{shear}}$, where ${\mathrm{Re}}_{\mathrm{shear}}$ is the shear viscosity Reynolds number. We derive a framework for decomposing $E_{\mathrm{mag}}$ growth rates into incompressible and compressible terms via orthogonal tensor decompositions of $\nabla \otimes \mathbf{v}$, where $\mathbf{v}$ is the fluid velocity. We find that ${\mathbf{v}}_{\parallel }(\mathbf{k})$ play a dual role, growing and decaying $E_{\mathrm{mag}}$, and that field-line stretching is the main driver of growth, even in compressible dynamos. In the absence of ${\nu }_{\mathrm{bulk}}$ ($\mathrm{P}\nu \to \infty$), ${\mathbf{v}}_{\parallel }(\mathbf{k})$ pile up on small-scales, creating a spectral bottleneck, which disappears for $\mathrm{P}\nu \approx 1$. (abridged). We emphasize the importance of further understanding the role of ${\nu }_{\mathrm{bulk}}$ in compressible astrophysical plasmas, which we estimate could be as strong as the shear viscosity in the cold ISM, and highlight that compressible direct numerical simulations without bulk viscosity have unresolved compressible mode dissipation scales.

31 pages, 21 figures, accepted in MNRAS

References in corpus (58)

• Array Programming with NumPy
• scikit-image: Image processing in Python
• The Statistics of Supersonic Isothermal Turbulence
• Simulations of nonhelical hydromagnetic turbulence
• CMasher: Scientific colormaps for making accessible, informative and 'cmashing' plots
• Numerical simulations of compressively driven interstellar turbulence: I. Isothermal gas
• The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016
• A robust numerical scheme for highly compressible magnetohydrodynamics: Nonlinear stability, implementation and tests
• Magnetic field amplification in turbulent astrophysical plasmas
• The sonic scale revealed by the world's largest supersonic turbulence simulation
• The density structure and star formation rate of non-isothermal polytropic turbulence
• Is the Scaling of Supersonic Turbulence Universal?
• The Turbulent Dynamo in Highly Compressible Supersonic Plasmas
• Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B
• Mach number dependence of the onset of dynamo action
• Cosmic ray transport in starburst galaxies
• Turbulent dynamo in a conducting fluid and partially ionized gas
• Models of irradiated molecular shocks
• Anisotropy in Quasi-Static Magnetohydrodynamic Turbulence
• Plasma turbulence in the interstellar medium
• Emergence of microphysical bulk viscosity in binary neutron star post-merger dynamics
• Cosmic ray transport in large-amplitude turbulence with small-scale field reversals
• Turbulence from localized random expansion waves
• Galactic Dynamos
• Simulating radio synchrotron emission in star-forming galaxies: small-scale magnetic dynamo and the origin of the far infrared-radio correlation
• Magnetic Fields in the Formation of the First Stars. I. Theory vs. Simulation
• FORGE'd in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions
• Seed magnetic fields in turbulent small-scale dynamos
• The IMF and multiplicity of stars from gravity, turbulence, magnetic fields, radiation and outflow feedback
• Exact scaling laws and the local structure of isotropic magnetohydrodynamic turbulence
• Magnetic field fluctuations in anisotropic, supersonic turbulence
• Compressible turbulence in the interstellar medium: New insights from a high-resolution supersonic turbulence simulation
• Filaments and striations: anisotropies in observed, supersonic, highly-magnetised turbulent clouds
• Fundamental scales in the kinematic phase of the turbulent dynamo
• Efficient highly-subsonic turbulent dynamo and growth of primordial magnetic fields
• Turbulent diffusion of streaming cosmic rays in compressible, partially ionised plasma
• Crustal magnetic fields do not lead to large magnetic-field amplifications in binary neutron-star mergers
• As a matter of dynamical range -- scale dependent energy dynamics in MHD turbulence
• Molecular cloud formation by compression of magnetized turbulent gas subjected to radiative cooling
• SHOCKFIND - An algorithm to identify magnetohydrodynamic shock waves in turbulent clouds
• The small-scale dynamo in a multiphase supernova-driven medium
• The relation between the turbulent Mach number and observed fractal dimensions of turbulent clouds
• Braginskii viscosity on an unstructured, moving mesh accelerated with super-time-stepping
• The density distribution and physical origins of intermittency in supersonic, highly magnetised turbulence with diverse modes of driving
• The spectrum of magnetized turbulence in the interstellar medium
• Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field
• Numerical viscosity and resistivity in MHD turbulence simulations
• The impact of magnetic fields on cosmological galaxy mergers -- II. Modified angular momentum transport and feedback
• Fundamental MHD scales -- II: the kinematic phase of the supersonic small-scale dynamo
• Dissipative magnetic structures and scales in small-scale dynamos
• Growth or Decay -- I: universality of the turbulent dynamo saturation
• Lagrangian statistics of a shock-driven turbulent dynamo in decaying turbulence
• Role of magnetic pressure forces in fluctuation dynamo saturation
• Dust-Evacuated Zones Near Massive Stars: Consequences of Dust Dynamics on Star-forming Regions
• Supersonic turbulence simulations with GPU-based high-order Discontinuous Galerkin hydrodynamics
• A brief introduction to bulk viscosity of fluids
• Resonant instabilities mediated by drag and electrostatic interactions in laboratory and astrophysical dusty plasmas
• So long Kolmogorov: the forward and backward turbulence cascades in a supernovae-driven, multiphase interstellar medium