تقرير
Taking control of compressible modes: bulk viscosity and the turbulent dynamo
| العنوان: | Taking control of compressible modes: bulk viscosity and the turbulent dynamo |
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| المؤلفون: | Beattie, James R., Federrath, Christoph, Kriel, Neco, Hew, Justin Kin Jun, Bhattacharjee, Amitava |
| سنة النشر: | 2023 |
| المجموعة: | Astrophysics Physics (Other) |
| مصطلحات موضوعية: | Astrophysics - Astrophysics of Galaxies, Astrophysics - High Energy Astrophysical Phenomena, Physics - Plasma Physics |
| الوصف: | Many polyatomic astrophysical plasmas are compressible and out of chemical and thermal equilibrium, and yet, due to Stokes' hypothesis, a means to carefully control the decay of compressible modes in these systems has largely been neglected. This is especially important for small-scale, turbulent dynamo processes, which are known to be sensitive to the effects of compression. To control the viscous properties of the compressible modes, we perform supersonic, visco-resistive dynamo simulations with additional bulk viscosity $\nu_{\rm bulk}$, deriving a new $\nu_{\rm bulk}$ Reynolds number $\rm{Re}_{\rm bulk}$, and viscous Prandtl number $\rm{P}\nu \equiv \rm{Re}_{\rm bulk} / \rm{Re}_{\rm shear}$, where $\rm{Re}_{\rm shear}$ is the shear viscosity Reynolds number. For $10^{-3} \leq \rm{P}\nu \leq \infty$, we explore a broad range of statistics critical to the dynamo problem, including the integral and spectral energy ratios, growth rates, and the magnetic $E_{\rm mag}(k)$ and kinetic $E_{\rm kin}(k)$ energy spectrum. We derive a general framework for decomposing $E_{\rm mag}$ 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 compressible modes play a dual role, growing and decaying $E_{\rm mag}$, and that field-line stretching is the main driver of growth, even in supersonic dynamos. In the absence of $\nu_{\rm bulk}$, compressible modes pile up on small-scales, creating an apparent spectral bottleneck, which disappears for $\rm{P}\nu \approx 1$. As $\rm{P}\nu$ decreases, compressible modes are dissipated at increasingly larger scales, in turn suppressing incompressible modes through a coupling between viscosity operators. We emphasise the importance of further understanding the role of $\nu_{\rm bulk}$ in compressible astrophysical plasmas. Comment: 28 pages, 18 figures, submitted to MNRAS. Comments welcome. Re-uploaded on 11 Dec with compressed large figures to reduce file size |
| نوع الوثيقة: | Working Paper |
| URL الوصول: | http://arxiv.org/abs/2312.03984 |
| رقم الأكسشن: | edsarx.2312.03984 |
| قاعدة البيانات: | arXiv |
| الوصف غير متاح. |