Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T07: Astrophysical Fluid Dynamics: General |
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Chair: Pascale Garaud, University of California Santa Cruz Room: 103A |
Monday, November 20, 2023 4:25PM - 4:38PM |
T07.00001: The surface signal of gravity waves generated by massive star core convection Evan H Anders, Daniel Lecoanet, Matteo Cantiello, Keaton J Burns, Benjamin A Hyatt, Emma Kaufman, Richard Townsend, Benjamin P Brown, Geoffrey Vasil, Jeffrey Oishi, Adam S Jermyn The cores of stars at least 20% more massive than the Sun are convective, and turbulent motions in the core launch gravity waves which may imprint upon the stellar surface in observable ways. We present the first 3D simulations of massive star convection extending from the star's center to near its surface, with realistic stellar luminosities. We measure both the luminosity of waves generated by the convective core and the features of those waves at the outer boundary of the simulations. Using the gravity wave eigenmodes of the simulation wave cavity, we construct a transfer function which accurately predicts the wave signal at the outer boundary given the wave luminosity. We then use this method to predict the observable consequences of gravity waves at the surfaces of massive stars. |
Monday, November 20, 2023 4:38PM - 4:51PM |
T07.00002: Evidence for the Magneto-rotational Instability in the Solar Magnetic Cycle Kyle Augustson, Geoffrey Vasil, Daniel Lecoanet The Sun’s magnetic cycle displays a pattern of propagating sunspots, starting around 30^{○ }latitude and annihilating near the equator 11 years later. Relative longitudinal flows, called torsional oscillations, track sunspot migration and undoubtedly share a common cause. Notably, helioseismology reveals that low-latitude torsional oscillations only occur within the outer 5–10% in radius, coinciding with an inwardly increasing angular velocity called the Near-Surface Shear Layer (NSSL). Negative differential rotation gradient of sufficient strength with a polar magnetic field signifies the Magneto-Rotational Instability (MRI)—crucial in astrophysical accretion disks. Together, these two facts address the general question: where and how is the solar dynamo operating? Here, we provide evidence that the MRI operates within the NSSL and is essential to understanding the solar dynamo. |
Monday, November 20, 2023 4:51PM - 5:04PM |
T07.00003: The effect of magnetic field on solar inertial modes Emma Kaufman, Daniel Lecoanet, Evan H Anders The Sun's magnetic field is responsible for solar weather, such as coronal mass ejections and solar flares, which pose a great threat to our power grid and aircraft. Accurate predictions of solar weather could reduce the severity of the threat, but a complete understanding of the solar magnetic field is needed to accurately model these events. |
Monday, November 20, 2023 5:04PM - 5:17PM |
T07.00004: Comparing models with DNS of stratified turbulence at low Prandtl number Pascale Garaud, Colm-Cille P Caulfield, Greg P Chini, Laura Cope, Kasturi Shah Quantifying transport by strongly stratified turbulence in stellar and planetary interiors is paramount to the development of accurate stellar evolution models. Recent theoretical work by Chini et al. (2022) and Shah et al. (in prep, see also abstract in this meeting) using multiscale asymptotic modeling has helped create a map of parameter space delimiting various regimes where specific force balances are expected. Each regime is predicted to give rise to distinct scaling laws for the characteristic vertical eddy scale, vertical velocity, and buoyancy fluctuation, as functions of the input parameters (the Reynolds number, the Prandtl number, and the Froude number). In this work, we compare their model with various existing and new datasets obtained using DNS. The DNS are forced with a constant-in-time horizontal body force in the streamwise direction that varies sinusoidally in the spanwise direction, but is invariant in the direction of gravity. A constant stratification is imposed, and all fluctuations around the background state are assumed to be otherwise triply-periodic in the domain. All simulations are run until a statistically stationary state is achieved. The Prandtl number is low (Pr ≤ 0.1), consistent with our interest in stellar and planetary fluid flows. We find that the DNS generally agree with the model predictions of Shah et al., with the caveat that it is not possible to achieve high enough Reynolds numbers to probe all possible regimes. |
Monday, November 20, 2023 5:17PM - 5:30PM |
T07.00005: Stably stratified turbulence at low Prandtl number in stellar interiors Valentin Skoutnev We extend the scaling relations of strongly (stably) stratified turbulence from the geophysical regime of unity Prandtl number to the astrophysical regime of extremely small Prandtl number applicable to stars and gas giants. Such turbulence can be driven by e.g. dynamical shear instabilities in radiative zones of stars. A transition to a new turbulent regime is found to occur when the Prandtl number drops below the inverse of the buoyancy Reynolds number, i.e. PrRb<1, which signals a shift of the dominant balance in the buoyancy equation. Application of critical balance arguments then derives new predictions for the anisotropic energy spectrum and dominant balance of the Boussinesq equations in the PrRb≪1 regime. We find that all the standard scaling relations from the unity Pr limit of strongly stratified turbulence simply carry over if the Froude number, Fr, is replaced by a modified Froude number, Fr_{M}≡Fr/(PrRb)^{1/4}. The geophysical and astrophysical regimes are thus smoothly connected across the PrRb=1 transition. Applications to vertical transport in stellar radiative zones via turbulent diffusion is discussed. |
Monday, November 20, 2023 5:30PM - 5:43PM |
T07.00006: Meridional circulation revisited Deepayan Banik, Kristen Menou The investigation of time-dependent meridional circulation and differential rotation in radiative zones remains a central and challenging topic in stellar evolution theory. To address this, we apply the 'downward control principle' from atmospheric science under a geostrophic f-plane approximation. We confirm the known physics result that steady-state meridional circulation decays with a length scale of N/2Ω × √Pr, assuming molecular viscosity as the dominant drag mechanism. Prior to steady-state, the circulation and zonal wind (differential rotation) spread jointly via radiative diffusion, adhering to thermal wind balance. The corresponding hyper-diffusion process is reasonably well approximated by regular diffusion on scales comparable to the pressure scale-height. We derive an inhomogeneous diffusion equation for the zonal flow, offering closed-form time-dependent solutions within a finite depth domain, facilitating rapid prototyping of meridional circulation patterns. In the weak drag limit, the time to reach rotational steady-state may exceed the Eddington-Sweet time, being governed instead by the longer drag time. We also conclude that the current Sun is only one-tenth as young as its rotational steady-state and will exhaust all its fuel before reaching that stage. Our streamlined meridional circulation solutions provide leading-order internal rotation profiles, enabling the study of fluid/MHD instabilities (or waves) in angular momentum redistribution within stellar radiative zones. Despite geometric limitations and simplifying assumptions, our thin-layer geostrophic approach is expected to yield qualitatively useful insights for understanding deep meridional circulation in stars. |
Monday, November 20, 2023 5:43PM - 5:56PM |
T07.00007: From terrestrial to astrophysical interfacial waves Anthony F Bonfils, Dhrubaditya Mitra, Woosok Moon, John S Wettlaufer A white dwarf may accrete material (principally hydrogen and helium) from a companion star, increasing the density and temperature and possibly giving rise to runaway nuclear burning. The resulting energy is accompanied by ejection of material called novae ejecta, which can contain a significant amount of heavier elements, such as carbon, nitrogen, and oxygen, presumably entrained and brought upward from the deeper layers of the white dwarf. This process, which is the presumed to be the primary source of carbon and oxygen in the universe, is poorly understood. Rosner and others [1] proposed that the Miles instability of surface waves forced by a shear flow is responsible for the mixing of elements at the surface of the white dwarf. The Miles instability originated in physical oceanography as a mechanism for the growth of wind waves [2,3]. In this talk, I will explain how to generalize that instability to astrophysical settings. Next, I will take advantage of the large scales in astrophysical flows to perform a short wave analysis and obtain a general asymptotic formula for the growth rate of the Miles instability. Along the way, I will discuss the potential relevance of another oceanographic instability — the so-called rippling instability [4] — for this astrophysical puzzle. |
Monday, November 20, 2023 5:56PM - 6:09PM |
T07.00008: Double-diffusive convection with fixed flux boundary conditions Arstanbek Tulekeyev, Adrian E Fraser, Pascale Garaud Oscillatory double-diffusive convection occurs in planetary and stellar interiors that have a stabilizing composition gradient and a destabilizing temperature gradient. The nonlinear dynamics of this instability have been previously studied using triply periodic DNS which showed that the stratification spontaneously forms density layers that gradually merge over time. However, this model setup is somewhat unrealistic because it allows both heat and compositional fluxes through the domain to adjust to the evolving dynamics. By contrast, in planets and stars it is controlled by nuclear reaction rates and/or surface cooling. In this work, therefore, we study double-diffusive convection with fixed-flux boundary conditions for low Prandtl number. We use Dedalus, an open-source library for solving differential equations with spectral methods, to implement the model. Our simulations typically evolve through multiple phases: oscillatory double-diffusive convection, emergence of convective boundary layers and their growth, layer interface erosion, and fully convective phase. We propose simple models to describe the evolution of the system in each phase. |
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