Nonlinear effects on the precessional Instability in magnetized turbulence

Autor: Claude Cambon, Abdelaziz Salhi, Amor Khlifi
Přispěvatelé: Laboratoire de Mecanique des Fluides et d'Acoustique (LMFA), École Centrale de Lyon (ECL), Université de Lyon-Université de Lyon-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Institut National des Sciences Appliquées de Lyon (INSA Lyon), Université de Lyon-Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Centre National de la Recherche Scientifique (CNRS), département de physique de la faculté des sciences de Tunis, Université de Tunis
Jazyk: angličtina
Rok vydání: 2020
Předmět:
Zdroj: Atmosphere
Atmosphere, MDPI 2020, 11, pp.14. ⟨10.3390/atmos11010014⟩
Atmosphere, Vol 11, Iss 1, p 14 (2019)
Volume 11
Issue 1
ISSN: 2073-4433
DOI: 10.3390/atmos11010014⟩
Popis: By means of direct numerical simulations (DNS), we study the impact of an imposed uniform magnetic field on precessing magnetohydrodynamic homogeneous turbulence with a unit magnetic Prandtl number. The base flow which can trigger the precessional instability consists of the superposition of a solid-body rotation around the vertical ( x 3 ) axis (with rate &Omega
) and a plane shear (with rate S = 2 &epsilon
&Omega
) viewed in a frame rotating (with rate &Omega
p = &epsilon
) about an axis normal to the plane of shear and to the solid-body rotation axis and under an imposed magnetic field that aligns with the solid-body rotation axis ( B ‖ &Omega
) . While rotation rate and Poincaré
number are fixed, &Omega
= 20 and &epsilon
= 0 . 17 , the B intensity was varied, B = 0 . 1 , 0 . 5 , and 2 . 5 , so that the Elsasser number is about &Lambda
= 0 . 1 , 2 . 5 and 62 . 5 , respectively. At the final computational dimensionless time, S t = 2 &epsilon
t = 67 , the Rossby number Ro is about 0 . 1 characterizing rapidly rotating flow. It is shown that the total (kinetic + magnetic) energy ( E ) , production rate ( P ) due the basic flow and dissipation rate ( D ) occur in two main phases associated with different flow topologies: (i) an exponential growth and (ii) nonlinear saturation during which these global quantities remain almost time independent with P &sim
D . The impact of a "strong" imposed magnetic field ( B = 2 . 5 ) on large scale structures at the saturation stage is reflected by the formation of structures that look like filaments and there is no dominance of horizontal motion over the vertical (along the solid-rotation axis) one. The comparison between the spectra of kinetic energy E ( &kappa
) ( k &perp
) , E ( &kappa
k ‖ = 1 , 2 ) and E &kappa
k ‖ = 0 ) at the saturation stage reveals that, at large horizontal scales, the major contribution to E ( &kappa
) does not come only from the mode k ‖ = 0 but also from the k ‖ = 1 mode which is the most energetic. Only at very large horizontal scales at which E ( &kappa
) &sim
E 2 D ( &kappa
) , the flow is almost two-dimensional. In the wavenumbers range 10 &le
k &perp
&le
40 , the spectra E ( &kappa
) and E ( &kappa
k ‖ = 0 ) respectively follow the scaling k &perp
2 and k &perp
3 . Unlike the velocity field the magnetic field remains strongly three-dimensional for all scales since E 2 D ( m ) ( k &perp
) ≪ E ( m ) ( k &perp
) . At the saturation stage, the Alfvé
n ratio between kinetic and magnetic energies behaves like k ‖ - 2 for B k ‖ / ( 2 &epsilon
) <
1 .
Databáze: OpenAIRE
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