Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session T11: Astrophysical Fluid Dynamics |
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Chair: Evan Anders, Northwestern; Daniel Lecoanet, Northwestern Room: 138 |
Monday, November 21, 2022 4:10PM - 4:23PM |
T11.00001: Convectively-driven internal gravity waves in a massive star Evan H Anders, Daniel Lecoanet, Keaton J Burns, Jeff S Oishi, Geoffrey Vasil, Benjamin P Brown Recent observations of massive stars contain a "red noise" signal which may be an observable manifestation of convectively-driven gravity waves from the stellar core. In this work, we examine the nature of waves driven by core convection and the observable signatures of those waves as they transfer from the radiative-convective interface to the stellar surface. To do so, we study 3D fully compressible spherical simulations of a model of a star, containing a convective core and a stable radiative envelope. We measure both the "pure" wave flux spectrum generated by the convection and the long-term saturated wave signatures at the stellar surface, and compare these results to linear theory. We find good agreement between the linear theory and our fully nonlinear simulations. We speculate about whether the amplitude of waves driven by core convection is large enough to be observable as red noise. |
Monday, November 21, 2022 4:23PM - 4:36PM |
T11.00002: Automatic spectral methods for non-radial structure and stability problems Keaton J Burns, Daniel Lecoanet, Geoffrey Vasil, Benjamin P Brown, Jeff S Oishi Non-radial boundary-value and eigenvalue problems in spherical geometries frequently arise in stellar and planetary science. Common applications include solving for background structures, computing oscillation modes, and studying tidal and convective instabilities. These problems are multidimensional and highly coupled, requiring complex numerical methods that are difficult to develop and modify. Here we will describe recent additions to the Dedalus code that support general non-radial boundary value and eigenvalue problems in spherical shells and full balls. We will briefly describe the underlying numerical methods which support general tensorial PDEs with non-constant coefficients in these geometries. We will then describe several example applications to rotating structure and stability problems in stars and giant planets. |
Monday, November 21, 2022 4:36PM - 4:49PM |
T11.00003: Propagation of a Thermohaline Mixing Front in 3D DNS and 1D Models Imogen G Cresswell, Adrian E Fraser, Evan H Anders, Benjamin P Brown Surface abundances of red-giant branch stars cannot be explained by standard mixing models and require "anomalous mixing" that is believed to be thermohaline mixing. In these stars, a destabilizing compositional source is provided by nuclear burning, and this source causes a thermohaline-unstable zone to grow in size. Here, we consider a system where a thermohaline-unstable layer with a continuous compositional source gradually propagates into a thermohaline-stable layer. We present a suite of 3D simulations across different molecular diffusivities (parameterised by Pr and diffusivity ratio, τ) and initial gradients (parameterised by the density ratio, R0). We also present 1D simulations where temperature and composition are evolved according to a diffusion equation with a turbulent diffusivity given by local models from Brown et al (2013). We compare the propagation of the thermohaline front predicted by 1D models to those found in 3D DNS and see how well these match across parameter space. |
Monday, November 21, 2022 4:49PM - 5:02PM |
T11.00004: Layer formation in a stably-stratified fluid cooled from above. Towards an analog for Jupiter and other gas giants Rafael Fuentes, Andrew Cumming, Evan H Anders The presence of composition gradients in the interiors of gas giants can affect and even suppress convective motions. Under appropriate circumstances, a gradient of heavy elements can trigger the formation of a series of convective layers separated by sharp diffusive interfaces. This state of the fluid, known as layered convection, has been proposed as a mechanism to explain several observational problems in planetary sciences. However, those solutions assume that layered convection can persist over evolutionary time scales. Further, it is not guaranteed that secondary convective layers can form and survive under the vigorous mixing and turbulence of an outer convection zone. Here we use numerical simulations to investigate this problem. |
Monday, November 21, 2022 5:02PM - 5:15PM |
T11.00005: A `dark-energy-free` turbulence similarity solution for an infinite homogeneous expanding universe using Einstein's Averaged Field Equations William K George, Gunnar Johansson A similarity solution is proposed [1] in which both time and space independent variables are scaled with a single time-dependent length scale, δ(t). It grows linearly with gravitational time t, but exponentially with atomic clock (or proper) time, τ = \int_0^t dt' / δ(t'). The resulting Ricci tensor is identically zero, so there is no critical density criterion. The energy density is shown to decrease by a factor of 10120 from the quantum field BIG BANG estimate (the so-called `Worst prediction in the history of physics') to the currently measured values. There is no need for dark matter or dark energy. The Hubble parameter is H / Ho =to / t = 1+z where to is the age of the universe and z is the redshift. Ho=63.4 provides an excellent fit to all the data, and corresponds to an age of the universe of 15.4 billion years. Excellent agreement is also shown with the supernovae data, previously believed to imply the universe is accelerating. This theory suggests strongly that it is not. |
Monday, November 21, 2022 5:15PM - 5:28PM |
T11.00006: The Stability of Prendergast Magnetic Fields Emma Kaufman, Daniel Lecoanet, Evan H Anders Convection is thought to generate large scale, core magnetic fields, which persist in stars after they evolve off the main sequence. It's thought that the remnants of these dynamo fields may take the form of the Prendergast magnetic field (Prendergast 1956), a combination of poloidal and toroidal field components which are expected to stabilize each other. Previous analytic and numerical stability calculations have suggested this magnetic field is stable. We present long timescale numerical calculations which show a linear instability of this magnetic field, and determine the effect of changing diffusivity, Brunt–Väisälä frequency, azimuthal wavenumber, and boundary conditions on the instability. We show that the instability persists over a large parameter space and is sufficiently rapid to destabilize the magnetic field on timescales shorter than the stellar evolution time. |
Monday, November 21, 2022 5:28PM - 5:41PM |
T11.00007: Inferring Stellar Magnetic Fields with Magnetic - Internal Gravity Wave Interactions Daniel Lecoanet, Ian Freeman, Dominic Bowman, Timothy Van Reeth The brightness of stars varies with time. This is often due to the waves which propagate within stars. It is easy to measure the standing modes of stars because they oscillate at a specific frequency. Here we consider how a standing internal gravity wave may change in the presence of a magnetic field. For weak magnetic field strengths, the wavelength of the internal gravity changes, which would lead to a modification of the frequency of the standing mode. For strong magnetic field strengths, the internal gravity wave converts into a resonant Alfven waves. These resonant waves form a continuum, and the converted mode would likely strongly damp due to phase mixing. Thus, we predict strong magnetic fields will preclude internal gravity wave standing modes. We apply this reasoning to a specific star which does not have as many standing modes as expected. We can explain the lack of standing modes if the star has a strong magnetic field near its convective core. |
Monday, November 21, 2022 5:41PM - 5:54PM |
T11.00008: Hydrodynamic stability constraints on the three-dimensional structure of planetary vortices Aidi Zhang, Philip S Marcus Observations by the Juno spacecraft have inspired new 3D models of the Jovian vortices, including the Great Red Spot. However, these models are heuristic and not stable equilibria of Euler’s equation. Stability with respect to convection is a concern because anticyclones always locally de-stratify the fluid, and we consistently find that if a vortex is locally unstable at any height with respect to convection, it breaks apart and does not reform. Using the anelastic approximation, we compute families of 3D vortices embedded in a Jovian-like stratified shear flow. We show that a vortex’s horizontal cross-sectional area must decrease to zero at the top and bottom of the vortex, while the vertical vorticity must be nearly uniform as a function of height. Otherwise, the vortex will shear apart or look non-physical. For vortices in which the vertical vorticity, or potential vorticity is nearly uniform over in the horizontal directions, (e.g., one with solid-body rotation or the Jovian Red Oval), we have found only family of vortices that is stable with respect to convective and shear instabilities. We have developed a simple analytic approximation of these vortices that agrees well with the numerical simulations and captures the 3D vertical structure of this stable family of vortices. |
Monday, November 21, 2022 5:54PM - 6:07PM Not Participating |
T11.00009: The influence of density ratio on instabilities present in supernova remnants. Samuel Petter, Quinton Dzurny, Gokul Pathikonda, Benjamin Musci, Devesh Ranjan The presented work focuses on the comparison of high-speed (10 kHz) velocity measurements at different Atwood numbers. Data collected in the Georgia Tech Blast Wave Facility is analyzed for growth of the mixing width based on TKE estimations, and circulation is compared for different fluid structure formations resulting from different density ratios. Detonators are used to generate blast waves which mix a variable density interface by depositing baroclinic vorticity. Here, there are two primary fluid instabilities present that make up the BDI, the Richtmyer-Meshkov (RMI) and Rayleigh-Taylor Instabilities (RTI). The unique cylindrical geometry provides an important platform to collect data for validation in predictive models in flows subjected to Bell-Plesset effects, and variable density effects. The experimental data is aimed at eventually estimating the various velocity statistics, and exploring vorticity models that can be used to predict growth rates. |
Monday, November 21, 2022 6:07PM - 6:20PM |
T11.00010: Internal Heating, why Convective Driving Models Matter for Astrophysics Whitney T Powers, Evan H Anders, Benjamin P Brown Convection drives mixing in stars, brown dwarfs, and gas giant planets. Convection in stars is driven by internal heating caused by changes in radiative conductivity or nuclear burning. In planets, convection is driven by changes in radiative conductivity and heat from gravitational contraction. However, most multi-dimensional models of stellar and planetary convection use hard boundaries, driving convection with fixed temperatures or fluxes at the top and bottom of the domain which can drive non-physical flows. With an internally heated convection model heat is deposited throughout the domain, which mimics the natural processes in stars and planets. In this work we study the fundamental properties of internally heated convection. This model gives us independent control over flow speed and turbulence in our simulations and provides consistent behavior in power spectra. This will enable us to conduct better experiments to study convection in astrophysical systems. |
Monday, November 21, 2022 6:20PM - 6:33PM |
T11.00011: Predicting the dark matter particle mass, size, and other properties from the mass and energy cascade and two-thirds law in dark matter flow Zhijie Xu How can the fluid mechanics help us understand the dark matter mystery - the biggest quest of contemporary astrophysics? After years of null results in the search for thermal WIMPs, a different prospective might be required. We present a new fluid mechanics approach to estimate the dark matter particle mass, size and many other relevant properties based on the nature of flow of dark matter. A comparison with hydrodynamic turbulence is presented to reveal the unique features of dark matter flow, i.e. an inverse mass and energy cascade from small to large scales with a scale-independent rate of energy cascade εu=-4.6x10-7m2/s3. If gravity is the only interaction involved and viscosity is absent, the energy cascade leads to a two-thirds law for pairwise velocity that can be extended down to the smallest scale, where quantum effects become important. Combining the rate of energy cascade εu, Planck constant h, and gravitational constant G on that scale, mass of dark matter particles is found to be around 1012GeV with a size around 10-13m. This strongly suggests a heavy dark matter scenario with a mass much greater than standard WIMPs. The accompanying slides and datasets for this work can be found on Zenodo.org by searching "dark matter flow" or https://doi.org/10.5281/zenodo.6569901. |
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