Filament formation in wind–cloud interactions– II. Clouds with turbulent density, velocity, and magnetic fields

Abstract

We present a set of numerical experiments designed to systematically investigate how turbulence and magnetic fields influence the morphology, energetics, and dynamics of filaments produced in wind-cloud interactions. We cover three-dimensional, magnetohydrodynamic systems of supersonic winds impacting clouds with turbulent density, velocity, and magnetic fields. We find that log-normal density distributions aid shock propagation through clouds, increasing their velocity dispersion and producing filaments with expanded cross sections and highly-magnetised knots and sub-filaments. In self-consistently turbulent scenarios the ratio of filament to initial cloud magnetic energy densities is ∼ 1. The effect of Gaussian velocity fields is bound to the turbulence Mach number: Supersonic velocities trigger a rapid cloud expansion; subsonic velocities only have a minor impact. The role of turbulent magnetic fields depends on their tension and is similar to the effect of radiative losses: the stronger the magnetic field or the softer the gas equation of state, the greater the magnetic shielding at wind-filament interfaces and the suppression of Kelvin-Helmholtz instabilities. Overall, we show that including turbulence and magnetic fields is crucial to understanding cold gas entrainment in multi-phase winds. While cloud porosity and supersonic turbulence enhance the acceleration of clouds, magnetic shielding protects them from ablation and causes Rayleigh-Taylor-driven sub-filamentation. Wind-swept clouds in turbulent models reach distances ∼ 15 − 20 times their core radius and acquire bulk speeds ∼ 0.3 − 0.4 of the wind speed in one cloud-crushing time, which are three times larger than in non-turbulent models. In all simulations the ratio of turbulent magnetic to kinetic energy densities asymptotes at ∼ 0.1 − 0.4, and convergence of all relevant dynamical properties requires at least 64 cells per cloud radius.

On the dynamics and survival of fractal clouds in galactic winds

“…The numerical resolution is R 64 (i.e., 64 cells cover the cloud radius), which corresponds to a uniform grid resolution of (N X 1 × N X 2 × N X 3 ) = (640 × 1920×640). This resolution is adequate to describe the overall evolution of 3D turbulent cloud models as shown in Banda-Barragán et al (2018) for a similar configuration. Note also that uniform-grid simulations, albeit more expensive than moving-mesh simulations, have the advantage of capturing the high-density gas in the cloud, the wind-cloud interface (where instabilities grow), and the low-density mixed gas at identical resolution in all models.…”

“…Vortical motions remove gas from the cloud and the wind deposits it downstream, thus forming a long-standing, turbulent filamentary tail at the rear side of the cloud. In fractal cloud models wind and cloud gas mix more effectively than in uniform models, so the resulting filament in these models has a more complex structure populated by a collection of knots and sub-filaments (see also Cooper et al 2009;Banda-Barragán et al 2018). Figure 2.…”

“…In such case, the effective drag force acting upon the cloud is enhanced, aiding cloud acceleration. On the other hand, cloud models with supersonic velocity fields and/or strong, turbulent magnetic fields also favour scenarios in which clouds undergo a period of fast acceleration (Banda-Barragán et al 2018). In these models the initial turbulent kinetic and magnetic energy thermalises, thus allowing the cloud to expand, accelerate, and reach high velocities over short time-scales, without being significantly disrupted by dynamical instabilities, e.g., Kelvin-Helmholtz (hereafter KH) and Rayleigh-Taylor (hereafter RT) instabilities.…”

“…Cooper et al (2009) assumed a Kolmogorov-like log-normal distribution for the cloud, motivated by studies on incompressible turbulence. Schneider & Robertson (2017) included turbulent, solenoidal cloud models from Robertson & Goldreich (2012), and in Banda- Barragán et al (2018) we only probed clouds characterised by a single PDF taken from a simulation of isothermal turbulence with mixed forcing (by Federrath & Klessen 2012). Nevertheless, changes in the standard deviation of the PDFs and in the power-law index of the spectra of densities are expected for different regimes of turbulence (see Federrath, Klessen & Schmidt 2008Federrath & Klessen 2012).…”

The Launching of Cold Clouds by Galaxy Outflows. III. The Influence of Magnetic Fields

“…Aligned magnetic fields have the capability to form a high magnetic pressure flux rope, while transverse fields are stretched along the front of the cloud, resulting in a magnetic pressure that is comparable to the ram pressure from the wind. Self-contained and turbulent magnetic fields have been found to suppress the disruption of the clouds and result in smaller fragments comoving with the wind (Li et al 2013;McCourt et al 2015;Banda-Barragán et al 2018). It is clear that magnetic fields play an important role in the evolution of the entrained clouds, though most studies have been limited to the early stages of the interaction.…”