Nonmodal growth and optimal perturbations in magnetohydrodynamic shear flows
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arXiv
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| Autori principali: | , , |
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| Natura: | Preprint |
| Pubblicazione: |
2025
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| _version_ | 1866915769319686144 |
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| author | Fraser, Adrian E. Kaminski, Alexis K. Oishi, Jeffrey S. |
| author_facet | Fraser, Adrian E. Kaminski, Alexis K. Oishi, Jeffrey S. |
| contents | In astrophysical shear flows, the Kelvin-Helmholtz (KH) instability is generally suppressed by magnetic tension provided a sufficiently strong streamwise magnetic field. This is often used to infer upper (or lower) bounds on field strengths in systems where shear-driven fluctuations are (or are not) observed, on the basis that perturbations cannot grow in the absence of linear instability. On the contrary, by calculating the maximum growth that small-amplitude perturbations can achieve in finite time for such a system, we show that perturbations can grow in energy by orders of magnitude even when the flow is sub-Alfvénic, raising the possibility that shear-driven turbulence may be found even in the presence of strong magnetic fields, and challenging inferences from the observed presence or absence of shear-driven fluctuations. We further show that magnetic fields introduce additional nonmodal growth mechanisms relative to the hydrodynamic case, and that 2D simulations miss key aspects of these growth mechanisms. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_03141 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Nonmodal growth and optimal perturbations in magnetohydrodynamic shear flows Fraser, Adrian E. Kaminski, Alexis K. Oishi, Jeffrey S. Fluid Dynamics Solar and Stellar Astrophysics Plasma Physics Space Physics In astrophysical shear flows, the Kelvin-Helmholtz (KH) instability is generally suppressed by magnetic tension provided a sufficiently strong streamwise magnetic field. This is often used to infer upper (or lower) bounds on field strengths in systems where shear-driven fluctuations are (or are not) observed, on the basis that perturbations cannot grow in the absence of linear instability. On the contrary, by calculating the maximum growth that small-amplitude perturbations can achieve in finite time for such a system, we show that perturbations can grow in energy by orders of magnitude even when the flow is sub-Alfvénic, raising the possibility that shear-driven turbulence may be found even in the presence of strong magnetic fields, and challenging inferences from the observed presence or absence of shear-driven fluctuations. We further show that magnetic fields introduce additional nonmodal growth mechanisms relative to the hydrodynamic case, and that 2D simulations miss key aspects of these growth mechanisms. |
| title | Nonmodal growth and optimal perturbations in magnetohydrodynamic shear flows |
| topic | Fluid Dynamics Solar and Stellar Astrophysics Plasma Physics Space Physics |
| url | https://arxiv.org/abs/2510.03141 |