Evolution of Unmagnetized and Magnetized Shear Layers

Palotti, M. L.; Heitsch, Fabian; Zweibel, Ellen G.; Huang, Yi-Min

doi:10.1086/529066

Cited by 30 publications

(39 citation statements)

References 27 publications

(41 reference statements)

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“…11(c, f ), the mean flow evolution is substantially different, with mixing of momentum over a wider region (about twice as wide as for the hydrodynamic case), leaving smaller shears near the centre of the channel. This is consistent with previous studies [e.g., 16].…”

Section: A Mean Flow Changessupporting

confidence: 94%

“…, 7]. However, in the presence of a background magnetic field, various studies have shown how vortices can be disrupted, by which we mean either a reduction in strength or spatial coherence, or completely destroyed [8][9][10][11][12][13][14][15][16]. Here we show explicitly how this disruption depends on both the field strength and on the magnetic Reynolds number Rm.…”

Section: Introductionmentioning

confidence: 76%

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Vortex disruption by magnetohydrodynamic feedback

2017

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In an electrically conducting fluid, vortices stretch out a weak, large-scale magnetic field to form strong current sheets on their edges. Associated with these current sheets are magnetic stresses, which are subsequently released through reconnection, leading to vortex disruption, and possibly even destruction. This disruption phenomenon is investigated here in the context of two-dimensional, homogeneous, incompressible magnetohydrodynamics. We derive a simple order of magnitude estimate for the magnetic stresses -and thus the degree of disruption -that depends on the strength of the background magnetic field (measured by the parameter M , a ratio between the Alfvén speed and a typical flow speed) and on the magnetic diffusivity (measured by the magnetic Reynolds number Rm). The resulting estimate suggests that significant disruption occurs when M 2 Rm = O(1). To test our prediction, we analyse direct numerical simulations of vortices generated by the breakup of unstable shear flows with an initially weak background magnetic field. Using the Okubo-Weiss vortex coherence criterion, we introduce a vortex disruption measure, and show that it is consistent with our predicted scaling, for vortices generated by instabilities of both a shear layer and a jet.

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Section: A Mean Flow Changessupporting

confidence: 94%

Section: Introductionmentioning

confidence: 76%

Vortex disruption by magnetohydrodynamic feedback

2017

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“…This is definitely the most extreme case: magnetic field components parallel to the shear flows can stabilize against the KHI, and for compressible flows, the system will be stable for all those wavenumbers whose effective Mach number is larger than some critical value (Gerwin 1968). For a recent detailed discussion of the KHI, see, e.g., Palotti et al (2008).…”

Section: Kelvin-helmholtz Instabilitymentioning

confidence: 99%

Fragmentation of Shocked Flows: Gravity, Turbulence, and Cooling

Heitsch

Hartmann

Burkert

2008

Astrophysical Journal

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The observed rapid onset of star formation in molecular clouds requires rapid formation of dense fragments that can collapse individually before being overtaken by global gravitationally driven flows. Many previous investigations have suggested that supersonic turbulence produces the necessary fragmentation, without addressing however the source of this turbulence. Motivated by our previous (numerical) work on the flow-driven formation of molecular clouds, we investigate the expected timescales of the dynamical and thermal instabilities leading to the rapid fragmentation of gas swept up by large-scale flows and compare them with global gravitational collapse timescales. We identify parameter regimes in gas density, temperature, and spatial scale within which a given instability will dominate. We find that dynamical instabilities disrupt large-scale coherent flows through generation of turbulence, while strong thermal fragmentation amplifies the resulting low-amplitude density perturbations, thus leading to small-scale, highdensity fragments as seeds for local gravity to act upon. Global gravity dominates only on the largest scales; large-scale, gravitationally driven flows promote the formation of groups and clusters of stars formed by turbulence, thermal fragmentation, and rapid cooling.

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“…This is an interesting problem in its own right. Hughes & Tobias (2001) considered the linear evolution of magnetized shear instabilities, whilst the non-linear problem has also been studied in unstratified compressible fluids (Frank et al 1996;Ryu, Jones & Frank 2000;Palotti et al 2008) as well as in isothermal stratified layers (Brüggen & Hillebrandt 2001). The recent review article by Gilman & Cally (2007) describes global magnetohydrodynamic shear instabilities in the tachocline.…”

Section: Introductionmentioning

confidence: 99%

Interactions between magnetohydrodynamic shear instabilities and convective flows in the solar interior

Silvers

Bushby

Proctor

2009

Monthly Notices of the Royal Astronomical Society

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Motivated by the interface model for the solar dynamo, this paper explores the complex magnetohydrodynamic interactions between convective flows and shear-driven instabilities. Initially, we consider the dynamics of a forced shear flow across a convectively stable polytropic layer, in the presence of a vertical magnetic field. When the imposed magnetic field is weak, the dynamics are dominated by a shear flow (Kelvin-Helmholtz type) instability. For stronger fields, a magnetic buoyancy instability is preferred. If this stably stratified shear layer lies below a convectively unstable region, these two regions can interact. Once again, when the imposed field is very weak, the dynamical effects of the magnetic field are negligible and the interactions between the shear layer and the convective layer are relatively minor. However, if the magnetic field is strong enough to favour magnetic buoyancy instabilities in the shear layer, extended magnetic flux concentrations form and rise into the convective layer. These magnetic structures have a highly disruptive effect upon the convective motions in the upper layer.Key words: convection -instabilities -MHD -Sun: interior -Sun: magnetic fields. I N T RO D U C T I O NThe 11 year solar magnetic cycle is driven by a hydromagnetic dynamo. However, the exact nature of this dynamo mechanism is still not fully understood, and there are several scenarios that seek to explain the observed behaviour. The well-known 'interface' dynamo model (Parker 1993) is based on the idea that the dynamo operates in a region that straddles the base of the solar convection zone and the stably stratified region that lies beneath (for some recent reviews see Ossendrijver 2003;Proctor 2006; Dormy & Soward 2007;Silvers 2008). Although this is a conceptually appealing model for the solar dynamo, the only numerical investigations of the interface dynamo have been based upon mean-field dynamo theory (see e.g. Charbonneau & MacGregor 1997;Chan et al. 2004;Zhang, Liao & Schubert 2004;Bushby 2006). In mean-field theory, several aspects of the dynamo model (particularly the effects of turbulent convection) are parametrized. However, the resulting coefficients are poorly determined by both theory and observations. Due to the involved computational costs, it has not yet been possible to demonstrate the operation of the interface dynamo by carrying out threedimensional simulations of compressible magnetohydrodynamics. Given these computational constraints, it makes sense to investigate different components of the interface dynamo in isolation.

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Evolution of Unmagnetized and Magnetized Shear Layers

Cited by 30 publications

References 27 publications

Vortex disruption by magnetohydrodynamic feedback

Vortex disruption by magnetohydrodynamic feedback

Fragmentation of Shocked Flows: Gravity, Turbulence, and Cooling

Interactions between magnetohydrodynamic shear instabilities and convective flows in the solar interior

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