Turbulent Transport by Diffusive Stratified Shear Flows: From Local to Global Models. I. Numerical Simulations of a Stratified Plane Couette Flow

Garaud, Pascale; Gagnier, D.; Verhoeven, Jan

doi:10.3847/1538-4357/837/2/133

Cited by 40 publications

(89 citation statements)

References 26 publications

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“…While we have checked that standard shear instabilities are never excited in the simulations presented because the gradient Richardson number J =N 2 /Ŝ 2 m = Pr(1 − R −1 0 )/Ŝ 2 m is always significantly larger than one, diffusive shear instabilities could take place, as explained in Section 2 (see, e.g. Garaud & Kulenthirarajah 2016;Garaud et al 2017). These are difficult to identify unambiguously, however, since they are subcritical in nature, and it is not clear how to distinguish them beyond doubt from other dynamics.…”

Section: Modelmentioning

confidence: 87%

The Interaction between Shear and Fingering (Thermohaline) Convection

Garaud

Kumar

Sridhar

2019

ApJ

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Fingering convection is a turbulent mixing process that can occur in stellar radiative regions whenever the mean molecular weight increases with radius. In some cases, it can have a significant observable impact on stellar structure and evolution. The efficiency of mixing by fingering convection as a standalone process has been studied by Brown et al. (2013), but other processes such as rotation, magnetic fields and shear can affect it. In this paper, we present a first study of the effect of shear on fingering (thermohaline) convection in astrophysics. Using Direct Numerical Simulations we find that a moderate amount of shear (that is not intrinsically shear-unstable) always decreases the mixing efficiency of fingering convection, as a result of the tilt it imparts to the fingering structures. We propose a simple analytical extension of the Brown et al. (2013) model in the presence of shear that satisfactorily explains the numerically-derived turbulent compositional mixing coefficient for moderate shearing rates, and can trivially be implemented in stellar evolution codes. We also measure from the numerical simulations a turbulent viscosity, and find that the latter is strongly tied to the turbulent compositional mixing coefficient. Observational implications and caveats of the model are discussed.

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Section: Modelmentioning

confidence: 87%

The Interaction between Shear and Fingering (Thermohaline) Convection

Garaud

Kumar

Sridhar

2019

ApJ

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“…Zahn 1992). In such a case, Garaud et al (2017) and Gagnier & Garaud (2018) proposed to model the local shear-driven turbulence by introducing the turbulent viscosity…”

Section: Other Dissipative Mechanismsmentioning

confidence: 99%

“…Several studies have examined the turbulence in radiative zones (e.g. Zahn 1992; Mathis et al 2004;Garaud et al 2017;Gagnier & Garaud 2018;Mathis et al 2018). Yet, these models focus on shear-driven turbulence.…”

Section: Turbulent Mixing Due To Nonlinear Tidal Flowsmentioning

confidence: 99%

Fossil field decay due to nonlinear tides in massive binaries

Vidal

Cébron

ud‐Doula

et al. 2019

A&A

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Context. Surface magnetic fields have been detected in 5 to 10% of isolated massive stars, hosting outer radiative envelopes. They are often thought to have a fossil origin, resulting from the stellar formation phase. Yet, magnetic massive stars are scarcer in (close) short-period binaries, as reported by the BinaMIcS (Binarity and Magnetic Interaction in various classes of Stars) collaboration. Aims. Different physical conditions in the molecular clouds giving birth to isolated stars and binaries are commonly invoked. In addition, we propose that the observed lower magnetic incidence in close binaries may be due to nonlinear tides. Indeed, close binaries are probably prone to tidal instability, a fluid instability growing upon the equilibrium tidal flow via nonlinear effects. Yet, stratified effects have hitherto been largely overlooked. Methods. We theoretically and numerically investigate tidal instability in rapidly rotating, stably stratified fluids permeated by magnetic fields. We use the short-wavelength stability method to propose a comprehensive (local) theory of tidal instability at the linear onset, discussing damping effects. Then, we propose a mixing-length theory for the mixing generated by tidal instability in the nonlinear regime. We successfully assess our theoretical predictions against proofof-concept, direct numerical simulations. Finally, we compare our predictions with the observations of short-period, double-lined spectroscopic binary systems. Results. Using new analytical results, cross-validated by a direct integration of the stability equations, we show that tidal instability can be generated by nonlinear couplings of inertia-gravity waves with the equilibrium tidal flow in short-period massive binaries, even against the Joule diffusion. In the nonlinear regime, a fossil magnetic field can be dissipated by the turbulent magnetic diffusion induced by the saturated tidal flows. Conclusions. We predict that the turbulent Joule diffusion of fossil fields would occur in a few million years for several short-period massive binaries. Therefore, turbulent tidal flows could explain the observed dearth of some short-period magnetic binaries.

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“…Their predictions are now being tested using direct numerical simulations (Prat & Lignières 2013Prat et al 2016;Garaud et al 2017;Gagnier & Garaud 2018). However, very few studies have examined the instabilities arising from horizontal shear (e.g., Zahn 1992;Maeder 2003;.…”

Section: Introductionmentioning

confidence: 99%

Horizontal shear instabilities in rotating stellar radiation zones

Park

Prat

Mathis

2020

A&A

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Context. Stellar interiors are the seat of efficient transport of angular momentum all along with their evolution. In this context, understanding the dependence of the turbulent transport triggered by the instabilities of the vertical and horizontal shears of the differential rotation in stellar radiation zones as a function of their rotation, stratification, and thermal diffusivity is mandatory. Indeed, it constitutes one of the cornerstones of the rotational transport and mixing theory which is implemented in stellar evolution codes to predict the rotational and chemical evolutions of stars. Aims. We investigate horizontal shear instabilities in rotating stellar radiation zones by considering the full Coriolis acceleration with both the dimensionless horizontal Coriolis componentf and the vertical component f . Methods. We performed a linear stability analysis using linearized equations derived from the Navier-Stokes and heat transport equations in the rotating non-traditional f -plane. We considered a horizontal shear flow with a hyperbolic tangent profile as the base flow. The linear stability was analyzed numerically in wide ranges of parameters, and we performed an asymptotic analysis for large vertical wavenumbers using the Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) approximation for non-diffusive and highly diffusive fluids. Results. As in the traditional f -plane approximation, we identify two types of instabilities: the inflectional and inertial instabilities. The inflectional instability is destabilized asf increases and its maximum growth rate increases significantly, while the thermal diffusivity stabilizes the inflectional instability similarly to the traditional case. The inertial instability is also strongly affected; for instance, the inertially unstable regime is also extended in the non-diffusive limit as 0 < f < 1 +f 2 /N 2 , where N is the dimensionless Brunt-Väisälä frequency. More strikingly, in the high-thermal-diffusivity limit, it is always inertially unstable at any colatitude θ except at the poles (i.e., 0 • < θ < 180 • ). We also derived the critical Reynolds numbers for the inertial instability using the asymptotic dispersion relations obtained from the WKBJ analysis. Using the asymptotic and numerical results, we propose a prescription for the effective turbulent viscosities induced by the inertial and inflectional instabilities that can be possibly used in stellar evolution models. The characteristic time of this turbulence is short enough so that it is efficient to redistribute angular momentum and mix chemicals in stellar radiation zones.

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Turbulent Transport by Diffusive Stratified Shear Flows: From Local to Global Models. I. Numerical Simulations of a Stratified Plane Couette Flow

Cited by 40 publications

References 26 publications

The Interaction between Shear and Fingering (Thermohaline) Convection

The Interaction between Shear and Fingering (Thermohaline) Convection

Fossil field decay due to nonlinear tides in massive binaries

Horizontal shear instabilities in rotating stellar radiation zones

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