Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D3: Astrophysical Fluid DynamicsGeophysical
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Chair: Benjamin Brown, University of Colorado Room: 403 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D3.00001: `Un-Darkening’ the Cosmos: New laws of physics for an expanding universe William George Dark matter is believed to exist because Newton’s Laws are inconsistent with the visible matter in galaxies. Dark energy is necessary to explain the universe expansion. George, W.K. (2016) “Could time be logarithmic?”, J Cosmology, 26, 6 14118-14132 (also available from www.turbulence-online.com) ) suggested that the equations themselves might be in error because they implicitly assume that time is measured in linear increments. This presentation couples the possible non-linearity of time with an expanding universe. Maxwell’s equations for an expanding universe with constant speed of light are shown to be invariant only if time itself is non-linear. Both linear and exponential expansion rates are considered. A linearly expanding universe corresponds to logarithmic time, while exponential expansion corresponds to exponentially varying time. Revised Newton’s laws using either leads to different definitions of mass and kinetic energy, both of which appear time-dependent if expressed in linear time. And provide the possibility of explaining the astronomical observations without either dark matter or dark energy. We would have never noticed the differences on earth, since the leading term in both expansions is linear in $\delta/t_o$ where $t_o$ is the current age. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D3.00002: Convective overshoot in the interiors of solar-type stars Benjamin Brown, Evan Anders, Keaton Burns, Daniel Lecoanet, Jeffrey Oishi, Geoffrey Vasil Convection is an important phenomena in stars. Within stellar convection zones, the transport of heat, magnetic fields and other quantities is highly non-local. At the boundaries of convection zones, the stratification becomes stable and internal gravity waves dominate the dynamics. The extent to which convection overshoots into stably stratified regions is important for understanding the transport and storage of magnetic fields in solar-like dynamos. Here, using the Dedalus pseudospectral framework, we consider the properties of convective overshoot in simulations of fully-compressible convection at low Mach number in domains that span significant density stratifications. We find that little overshoot occurs under conditions typical for solar-type stars, limiting the role of convective overshoot in governing global-scale solar dynamos. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D3.00003: Convective dynamics and chemical disequilibrium in the atmospheres of substellar objects Baylee Bordwell, Benjamin P. Brown, Jeffrey S. Oishi The thousands of substellar objects now known provide a unique opportunity to test our understanding of atmospheric dynamics across a range of environments. The chemical timescales of certain species transition from being much shorter than the dynamical timescales to being much longer than them at a point in the atmosphere known as the quench point. This transition leads to a state of dynamical disequilibrium, the effects of which can be used to probe the atmospheric dynamics of these objects. Unfortunately, due to computational constraints, models that inform the interpretation of these observations are run at dynamical parameters which are far from realistic values. In this study, we explore the behavior of a disequilibrium chemical process with increasingly realistic planetary conditions, to quantify the effects of the approximations used in current models. We simulate convection in 2-D, plane-parallel, polytropically-stratified atmospheres, into which we add reactive passive tracers that explore disequilibrium behavior. We find that as we increase the Rayleigh number, and thus achieve more realistic planetary conditions, the behavior of these tracers does not conform to the classical predictions of disequilibrium chemistry. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D3.00004: ABSTRACT WITHDRAWN |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D3.00005: Double-diffusive staircases in gas giant planets Pascale Garaud, Ryan Moll, Chris Mankovich, Jonathan Fortney We present Direct Numerical Simulations of the transport of heat and heavy elements across a double-diffusive interface or a double-diffusive staircase, in conditions that are close to those one may expect to find near the boundary between the heavy-element rich core and the hydrogen-helium envelope of giant planets such as Jupiter. We find that the non-dimensional ratio of the buoyancy flux associated with heavy element transport to the buoyancy flux associated with heat transport lies roughly between 0.5 and 1, which is much larger than previous estimates derived by analogy with geophysical double-diffusive convection. We also find that the structure of double-diffusive interfaces at low Prandtl number differs significantly from those observed at high Prandtl number. Using these results, we find that the entire core of Jupiter would be eroded within less than 1Myr assuming that the core-envelope boundary is composed of a single interface, but could be entirely preserved in the presence of a staircase. This is particularly pertinent in the context of present and anticipated results from the Juno mission. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D3.00006: Jovian vortices by simulated annealing P. J. Morrison, G. R. Flierl, R. V. Swaminathan We explore the conditions required for isolated vortices to exist in sheared zonal flows and the stability of the underlying zonal winds. This is done using the standard 2-layer quasigeostrophic model with the lower layer depth becoming infinite; however, this model differs from the usual layer model because the lower layer is not assumed to be motionless but has a steady configuration of alternating zonal flows [1]. Steady state vortices are obtained by a simulated annealing computational method introduced in [2], generalized and applied in [3] in fluid flow, and used in the context of magnetohydrodynamics in [4,5]. Various cases of vortices with a constant potential vorticity anomaly atop zonal winds and the stability of the underlying winds are considered using a mix of computational and analytical techniques. [1] A.P. Stamp and T.E. Dowling, J. Geophys. Res. {\bf98}, 847 (1993). \noindent [2] G.K. Vallis, G. Carnevale, W.R. Young, J. Fluid Mech. 207 (1989) 133. [3] G. Flierl and P. Morrison, Physica D {\bf240}, 212 (2011). [4] M. Furakawa and P. J. Morrison, Plasma Phys.\ Control. Fusion {\bf59}, 054001 (2017). [5] C. Bressan, M. Kraus, P. J. Morrison, O. Maj, and E. Sonnendr\"ucker, poster contribution DPG Bremen Conference (2017) [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D3.00007: Nonlinear hydrodynamic instability and turbulence in eccentric astrophysical discs with vertical structure Aaron Wienkers, Gordon Ogilvie The classical theory of astrophysical discs (including Saturn's rings, protoplanetary systems, and high-energy accretion discs around black holes) assumes circular orbital motion around a central mass. However, certain systems are known to contain eccentric forcing, necessitating a generalization to the shearing box model to include the oscillatory local geometry associated with this eccentricity. The hydrodynamic equations in this model are non-standard because of the use of time-dependent, non-orthogonal coordinates, and are known to lead to hydrodynamic instability involving the growth of internal waves. Here we present the results of the first ever local nonlinear simulations in an eccentric shearing box representing an elliptic disc with vertical structure. The nonlinear saturation of this parametric instability inherent to eccentric discs generates further self-regulating azimuthal zonal flows, and results in stable limit cycle behavior. We explore this energy pathway from the global eccentric mode into turbulence and finally the zonal flows, and discuss the viability of this instability to balance the eccentricity growth in systems exhibiting mean-motion orbital resonances such as the eccentric Lindblad resonance. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D3.00008: Non-linear interactions of Rossby waves in astrophysical fluid dynamics Arakel Petrosyan, Dmitry Klimachkov We have obtained extended set of rotating magnetohydrodynamic shallow water equations in external magnetic field. The MHD shallow water equations are revised by supplementing them with the equations that are consequences of the magnetic field divergence-free conditions and reveal the existence of vertical component of the magnetic field in such approximation providing its relation with the horizontal magnetic field. It is shown that the presence of a vertical magnetic field significantly changes the dynamics of the wave processes in astrophysical plasma compared to the neutral fluid. We have investigated the interaction of magneto-Rossby waves in the MHD shallow water flows in external vertical magnetic field and in horizontal (toroidal and poloidal) magnetic field on a $\beta $-plane. Using the asymptotic multiscale method we obtained the non-linear interaction equations for the waves amplitudes. The analysis of the amplitudes equations shows that on $\beta $-plane there are two types of instabilities: one magneto-Rossby wave decays into two magneto-Rossby waves and magneto-Rossby wave amplifies in field of two magneto-Rossby waves. These instabilities occur in the external vertical magnetic field and in the horizontal magnetic field. For all types of instabilities the growth rates are found.. [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D3.00009: The SPH consistency problem and some astrophysical applications Jaime Klapp, Leonardo Sigalotti, Otto Rendon, Ruslan Gabbasov, Ayax Torres We discuss the SPH kernel and particle consistency problem and demonstrate that SPH has a limiting second-order convergence rate. We also present a solution to the SPH consistency problem. We present examples of how SPH implementations that are not mathematically consistent may lead to erroneous results. The new formalism has been implemented into the Gadget 2 code, including an improved scheme for the artificial viscosity. We present results for the ``Standard Isothermal Test Case'' of gravitational collapse and fragmentation of protostellar molecular cores that produce a very different evolution than with the standard SPH theory. A further application of accretion onto a black hole is presented. [Preview Abstract] |
Sunday, November 19, 2017 4:12PM - 4:25PM |
D3.00010: ABSTRACT WITHDRAWN |
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