Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session T11: Astrophysical Fluid Dynamics |
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Chair: Phil Marcus, University of California, Berkeley Room: North 125 AB |
Tuesday, November 23, 2021 12:40PM - 12:53PM |
T11.00001: Convective penetration exists and we found it Evan H Anders, Adam S Jermyn, Daniel Lecoanet, Benjamin P Brown Most stars host convection zones in which heat is transported directly by fluid motion. Parameterizations like mixing length theory adequately describe convective flows in the bulk of these regions, but the behavior of convective boundaries is not well understood. Here we present 3D numerical simulations which exhibit penetration zones: regions where the entire luminosity could be carried by radiation, but where the temperature gradient is approximately adiabatic and convection is present. To parameterize this effect, we define the "penetration parameter" P which compares how far the radiative gradient deviates from the adiabatic gradient on either side of the Schwarzschild convective boundary. Following Roxburgh (1989) and Zahn (1991), we construct an energy-based theoretical model in which the extent of penetration is controlled by P. We test this theory using 3D numerical simulations which employ a simplified Boussinesq model of stellar convection. We find significant convective penetration in all simulations. Our simple theory describes the simulations well. In stellar contexts, we expect P ≈ 1 and in this regime our results suggest that convection zones may extend beyond the Schwarzschild boundary by up to ∼20-30% of a mixing length. |
Tuesday, November 23, 2021 12:53PM - 1:06PM |
T11.00002: The distinguished limit for energetically robust large scale dynamos driven by rapidly rotating convection Michael Calkins, Ming Yan Natural dynamos, such as the Earth's outer core, generate global scale magnetic field despite the inferred presence of small scale convectively-driven turbulence. Helicity, which is a measure of the correlation between the velocity and vorticity fields, is thought to be an important ingredient for the generation of these global scale magnetic fields. Previous simulations show that as the buoyancy forcing is increased the relative helicity decreases and the small scales of the magnetic field become energetically dominant. However, asymptotic theory predicts that energetically robust large scale dynamos, defined as large scale dynamos in which the largest scale of the magnetic field is energetically dominant relative to the small scale magnetic field, are preferred when a particular, or distinguished, limit is taken that is characterized by rapid rotation and an induction equation balance in which stretching and diffusion are comparable on the small scales. Here we use a suite of direct numerical simulations of rotating convection-driven dynamos in the asymptotic regime that confirms this theory, demonstrating that energetically robust large scale dynamos can be generated for strongly forced turbulent convection despite small relative helicity. |
Tuesday, November 23, 2021 1:06PM - 1:19PM |
T11.00003: Force Balances and Flow Regimes in MHD Convective Simulations Imogen G Cresswell, Evan H Anders, Benjamin P Brown, Jeff S Oishi, Geoffrey Vasil Strong background magnetic fields inhibit convection, as seen in sunspots, and are responsible for the difference in convective patterns between plage regions and the quiet Sun. The strength of this background field determines how dynamically important the Lorentz force is. It is crucial to the developing theory of magnetoconvection to understand the interplay between the Lorentz and buoyancy forces as the field strength changes, and how this balance affects the dynamics of the flow. In this work, we use Dedalus to study MHD Rayleigh-Bernard convection. We perform a suite of 2D simulations in which the strength of the background magnetic field and the strength of convective driving (quantified by the Chandrasekhar number and the Rayleigh number, respectively) vary by many orders of magnitude. We measure & report the first and second-order force balances in order to understand how magnetically constrained the system is. The degree of magnetic constraint felt by the nonlinear convective solution depends on whether there is a leading-order balance between the Lorentz and buoyancy forces. We quantify regimes in which these simulations exist based on the primary & secondary force balance, and determine scaling laws for the velocity and the induced magnetic field within these regimes. |
Tuesday, November 23, 2021 1:19PM - 1:32PM |
T11.00004: MHD effects on fingering convection in stars: the problem with parasites Adrian E Fraser, Pascale Garaud Fingering convection is an important source of mixing in stars, which are low Prandtl number fluids. Quantifying transport by fingering convection is therefore crucial to a better understanding of stellar evolution. In the absence of magnetic fields, Brown et al. have shown that fingering-induced fluxes measured in DNS are well-predicted by a so-called "parasitic" model, in which the primary fingering instability is assumed to saturate due to the growth of secondary shear instabilities between the fingers. Recent work [Harrington & Garaud, ApJ 2019] extended this analysis to include a vertical magnetic field, and found that a similar parasitic model could account for the increase in turbulent fluxes with magnetic field strength observed in simulations. However, this analysis was limited to a small region of parameter space. Here we show that lowering the magnetic Prandtl number and exploring a broader range of stratifications reveals discrepancies between the parasitic saturation model and the DNS results. We propose some explanations, and discuss implications of our findings for other systems where parasitic saturation models are used, such as the MRI in accretion disks and GSF instability in stars. |
Tuesday, November 23, 2021 1:32PM - 1:45PM |
T11.00005: The combined effects of horizontal and vertical shear instabilities at low Prandtl number Pascale Garaud, Saniya Khan Recent work has demonstrated the importance of horizontal shear in driving vertical transport in stratified fluids, especially at low Prandtl number (which is relevant in the interiors of stars and planets). Various regions of parameter space have been identified, each with its own set of scaling laws governing the dominant balance of term in the momentum and buoyancy equations, respectively. An important outstanding question of interest, however, is that of the combined effects of large-scale horizontal and vertical shear, which are often present at the same time in stars. In this paper, we demonstrate that vertical shear only influences the outcome provided it is stronger than the emergent vertical shear developing between unstable meanders of the horizontal flow. As such, it can be shown that the latter is not likely to play a role in most stellar interior conditions, and that theories that only incorporate transport by large-scale horizontal shear flows are sufficient when modeling stellar mixing. |
Tuesday, November 23, 2021 1:45PM - 1:58PM |
T11.00006: Angular momentum transport in electrically-conducting fluids Christophe Gissinger The radial transport of angular momentum is a central quantity in astrophysics, as it is an essential ingredient in the dynamics of many objects, among which the best known are accretion discs or radiative stars. In both cases, the mechanism that generates the turbulence and the amount of angular momentum transported outward remain to be clearly identified. This problem has also attracted a lot of attention on a more fundamental point if view, where it is questionable whether there is a so-called ultimate regime in which the angular momentum flux becomes independant of molecular viscosity. Taylor-Couette setups have long been considered as a powerful tool to study this problem. In this talk, I will briefly review these efforts, and present two recent studies investigating this problem of angular momentum transport from a different perspective: I will first describe a new laboratory experiment in which the Couette flow is driven by an electromagnetic force rather than the rotation of the boundaries, leading to an interesting analogue of astrophysical disks. In a second part, I will describe numerical simulations of stratified spherical Couette flow aiming to model a radiative star. For some parameters, a magnetic field is self- generated by the star and produces a significant angular momentum flux, leading to a drastic spin-down of the inner part of the star |
Tuesday, November 23, 2021 1:58PM - 2:11PM |
T11.00007: Magneto-Gravity Waves in Stellar Interiors Emma Kaufman, Daniel Lecoanet Internal gravity waves change with variations of the background in which they propagate. We present 3 dimensional simulations of internal gravity waves with different background magnetic field configurations, including no background magnetic field, and a uniform vertical magnetic field. We compare the simulations to analytic solutions. Our results show that the frequency of the wave will effect the kinetic and magnetic energy distribution within the simulation. The average energy of the waves is large at certain resonant frequencies. The waves also exhibit critical cylindrical radii, which move towards the outer layers of the star as frequency increases, in accordance with analytic solutions. |
Tuesday, November 23, 2021 2:11PM - 2:24PM |
T11.00008: High Mach number convection in stellar and planetary atmospheres Whitney T Powers, Benjamin P Brown
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Tuesday, November 23, 2021 2:24PM - 2:37PM |
T11.00009: The effects of a vertical magnetic field on Oscillatory Double-Diffusive Convection at low Prandtl number Amishi Sanghi, Adrian E Fraser, Pascale Garaud
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Tuesday, November 23, 2021 2:37PM - 2:50PM |
T11.00010: Radiation-Driven Dust Hydrodynamics in late-phase AGB stars Hanif Zargarnezhad, Jacob A McFarland, Anegla Speck, Finnis Stribling The interplay of stellar luminosity variations and dust hydrodynamics in Asymptotic Giant Branch (AGB) stars, and the consequences for dust survival and mass-loss remain mysterious. In this work, we broadly investigate the role of dust and radiation in the formation of hydrodynamic features, known as cometary knots, observable in planetary nebulae (PNe), post AGB remnants. Luminosity variations arise due to turbulent thermal convection and manifest as granules, observable in the photosphere of stars, such as our own. Scaling laws suggest a size and energy for granules in AGB stars, far larger than those observed on our own sun. These intensity perturbations are one possible source for the formation of hydrodynamic features in PNe. Previous research studies have considered similar physics in dust-driven winds at shorter length and time scales, using either 1D simulations, or 2D simulations with a single mixed particle-gas fluid. Here we present a Eulerian-Lagrangian method for studying this problem at larger length and time scales. Simulations are performed using the FLASH code, in part developed by the Flash Center at the University of Chicago. The Particle-in-Cell method was used with the two-dimensional Euler equations and solved using the directionally split piecewise-parabolic method (PPM). This method was then modified for the astrophysics regime by implementing radiation and non-continuum drag models for the particle and gas phases. The effects of a perturbed radiation field due to strong turbulent eddies of granulation have been investigated to determine if these could be responsible for the formation of cometary knots observed in planetary nebulae. |
Tuesday, November 23, 2021 2:50PM - 3:03PM |
T11.00011: Longevity of Stratified Anticyclones with Thermal Dissipation and Cyclones with Viscous Dissipation and Their Relevance to Jupiter Aidi Zhang, Philip S Marcus We consider families of cyclones and anticyclones that are axisymmetric equilibria in an unbounded, uniformly rotating domain governed by the Boussinesq equations with vertical density stratifications, parameterized by the Brunt-Vaisala (BV) frequency N(z) along the central axis of the vortex, and by the BV N∞(z) asymptotically far from the vortex. We show that the longevities of the anticyclones can be far longer than the characteristic Newton cooling of the fluid and that the longevities cyclones can be far longer than the characteristic viscous decay time. We report how the vortex lifetimes scale with the parameters of the systems and how they relate to the Jovian observations of mid-latitude anticyclones and polar cyclones. |
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