Non-linear evolution of instabilities between dust and sound waves

ABSTRACT We study the non-linear evolution of the acoustic ‘resonant drag instability’ (RDI) using numerical simulations. The acoustic RDI is excited in a dust–gas mixture when dust grains stream through gas, interacting with sound waves to cause a linear instability. We study this process in a periodic box by accelerating neutral dust with an external driving force. The instability grows as predicted by linear theory, eventually breaking into turbulence and saturating. As in linear theory, the non-linear behaviour is characterized by three regimes – high, intermediate, and low wavenumbers – the boundary between which is determined by the dust–gas coupling strength and the dust-to-gas mass ratio. The high and intermediate wavenumber regimes behave similarly to one another, with large dust-to-gas ratio fluctuations while the gas remains largely incompressible. The saturated state is highly anisotropic: dust is concentrated in filaments, jets, or plumes along the direction of acceleration, with turbulent vortex-like structures rapidly forming and dissipating in the perpendicular directions. The low-wavenumber regime exhibits large fluctuations in gas and dust density, but the dust and gas remain more strongly coupled in coherent ‘fronts’ perpendicular to the acceleration. These behaviours are qualitatively different from those of dust ‘passively’ driven by external hydrodynamic turbulence, with no back-reaction force from dust on to gas. The virulent nature of these instabilities has interesting implications for dust-driven winds in a variety of astrophysical systems, including around cool stars, in dusty torii around active-galactic-nuclei, and in and around giant molecular clouds.

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Non-linear evolution of the resonant drag instability in magnetized gas

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz666 ◽

2019 ◽

Vol 485 (3) ◽

pp. 3991-3998 ◽

Cited By ~ 6

Author(s):

Darryl Seligman ◽

Philip F Hopkins ◽

Jonathan Squire

Keyword(s):

Linear Theory ◽

Coherent Structures ◽

Magnetic Energy ◽

Density Fluctuations ◽

Linear Evolution ◽

Drag Forces ◽

Non Linear ◽

Magnetohydrodynamic Simulations ◽

First Time ◽

Different Parts

Abstract We investigate, for the first time, the non-linear evolution of the magnetized ‘resonant drag instabilities’ (RDIs). We explore magnetohydrodynamic simulations of gas mixed with (uniform) dust grains subject to Lorentz and drag forces, using the gizmo code. The magnetized RDIs exhibit fundamentally different behaviour than purely acoustic RDIs. The dust organizes into coherent structures and the system exhibits strong dust–gas separation. In the linear and early non-linear regime, the growth rates agree with linear theory and the dust self-organizes into 2D planes or ‘sheets.’ Eventually the gas develops fully non-linear, saturated Alfvénic, and compressible fast-mode turbulence, which fills the underdense regions with a small amount of dust, and drives a dynamo that saturates at equipartition of kinetic and magnetic energy. The dust density fluctuations exhibit significant non-Gaussianity, and the power spectrum is strongly weighted towards the largest (box scale) modes. The saturation level can be understood via quasi-linear theory, as the forcing and energy input via the instabilities become comparable to saturated tension forces and dissipation in turbulence. The magnetized simulation presented here is just one case; it is likely that the magnetic RDIs can take many forms in different parts of parameter space.

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Angular momentum transport, layering, and zonal jet formation by the GSF instability: non-linear simulations at a general latitude

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1327 ◽

2020 ◽

Vol 495 (1) ◽

pp. 1468-1490

Author(s):

A J Barker ◽

C A Jones ◽

S M Tobias

Keyword(s):

Angular Momentum ◽

Linear Instability ◽

Momentum Transport ◽

Boussinesq Model ◽

Angular Momentum Transport ◽

Hot Jupiters ◽

Linear Evolution ◽

Non Linear ◽

Red Giant ◽

Subgiant Stars

ABSTRACT We continue our investigation into the non-linear evolution of the Goldreich–Schubert–Fricke (GSF) instability in differentially rotating radiation zones. This instability may be a key player in transporting angular momentum in stars and giant planets, but its non-linear evolution remains mostly unexplored. In a previous paper we considered the equatorial instability, whereas here we simulate the instability at a general latitude for the first time. We adopt a local Cartesian Boussinesq model in a modified shearing box for most of our simulations, but we also perform some simulations with stress-free, impenetrable, radial boundaries. We first revisit the linear instability and derive some new results, before studying its non-linear evolution. The instability is found to behave very differently compared with its behaviour at the equator. In particular, here we observe the development of strong zonal jets (‘layering’ in the angular momentum), which can considerably enhance angular momentum transport, particularly in axisymmetric simulations. The jets are, in general, tilted with respect to the local gravity by an angle that corresponds initially with that of the linear modes, but which evolves with time and depends on the strength of the flow. The instability transports angular momentum much more efficiently (by several orders of magnitude) than it does at the equator, and we estimate that the GSF instability could contribute to the missing angular momentum transport required in both red giant and subgiant stars. It could also play a role in the long-term evolution of the solar tachocline and the atmospheric dynamics of hot Jupiters.

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Hydrodynamization Time Near a Critical Point From a Holographic Bjorken Flow

Proceedings ◽

10.3390/proceedings2019010048 ◽

2019 ◽

Vol 10 (1) ◽

pp. 48

Author(s):

Renato Critelli

Keyword(s):

Critical Point ◽

Recent Progress ◽

Gravitational Theory ◽

Hydrodynamic Behavior ◽

Initial State ◽

Strongly Coupled ◽

Linear Evolution ◽

Non Linear ◽

Strongly Coupled Plasma ◽

Far From Equilibrium

This proceedings reviews recent progress in the study of far-from-equilibrium hydrodynamization process of strongly interacting matter in the vicinity of a critical point. From a full non-linear evolution of a gravitational theory dual to a conformal strongly coupled plasma, and starting from a non-equilibrium initial state, it is verified that the time it takes for the plasma to acquire hydrodynamic behavior greatly increases near the critical point.

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