Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E17: Astrophysical Fluid Dynamics |
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Chair: Joseph Barranco, San Francisco State University Room: 2002 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E17.00001: Fluid Instabilities inside Astrophysical Explosions Ke-Jung Chen, Stan Woosley, Alexander Heger, Ann Almgren, Weiqun Zheng We present our results from the simulations of fluid instabilities inside supernovae with a new radiation-hydrodynamic code, CASTRO. Massive stars are ten times more massive than Sun. Observational and theoretical studies suggest that these massive stars tend to end their lives with energetic explosions, so-called supernovae. Many fluid instabilities occur during the supernova explosions. The fluid instabilities can be driven by hydrodynamics, nuclear burning, or radiation. In this talk, we discuss about the possible physics of fluid instabilities found in our simulations and how the resulting mixing affects the observational signatures of supernovae. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E17.00002: End-effects versus stratification in quasi-Keplerian Taylor--Couette flow Colin Leclercq, Rich Kerswell There has been much controversy in the past decade over the impact of end-wall boundary conditions on transition to turbulence in centrifugally stable Taylor--Couette experiments, e.g. Paoletti \& Lathrop, PRL (2011); Ji {\it et al.}, Nature (2006); Balbus, Nature (2011). In this configuration, the meridional flow driven by the axial boundaries is no longer confined to their vicinity, potentially leading to turbulence through a classical supercritical route at high rotation rates (Avila {\it et al.}, POF (2008); Avila, PRL (2012)). But the question of subcritical transition in the limit of infinite cylinders remains of fundamental importance to the theory of weakly ionised accretion disks, as it may help to understand the inferred existence of turbulence there. We investigate theoretically the effect of stratification on azimuthally symmetric quasi-Keplerian base flows in the finite-length Taylor--Couette system. The challenge is to find a practical way to suppress the meridional flow, while not triggering the stratorotational instability. Different strategies will be discussed, including layered density profiles obtained with a stratifying agent of variable concentration and linear density profiles caused by a temperature difference between the top and bottom boundaries. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E17.00003: The Fluid Dynamics of Saturn's ``String of Pearls'' Christopher Gebhart, Philip Marcus A long-lived feature in Saturn's northern hemisphere is a row of 20 - 30 discrete dark clouds at latitude 33~$^{\circ}$ that extends approximately 1/3 of the way around the planet. It was the named the ``String of Pearls'' (SoP). It was suggested by others that these clouds are associated with a row of cyclonic vortices. However, generally a row of vortices with the same sign is unstable, and the vortices merge. Using a version of Correlation Image Velocimetry (CIV) that we developed to extract velocities from satellite images of clouds (Advection Corrected CIV), we have created a velocity field map of the SoP. From that map, we believe that the SoP lies on a strong, wavy westward jet stream that is sandwiched between a row of anticyclones on its northern side and a row of cyclones on its southern side, i.e., between a Karman Vortex Sheet (KVS). We previously showed that a KVS that sandwiches a jet stream is stable using 2D, quasigeostrophic simulations. Here, we present preliminary results on the stability and dynamics of a KVS using numerical simulations of the fully 3D, anelastic equations. We compare our simulations with the observations of the SoP. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E17.00004: Zombie Vortices: The {\it Dead} Zones of Protoplanetary Disks are Not Dead Chung-Hsiang Jiang, Philip Marcus, Suyang Pei, Joe Barranco, Pedram Hassanzadeh, Daniel Lecoanet Numerical simulations, using both the anelastic and fully compressible equations of motion, show that the ``dead zones'' of protoplanetary disks (PPDs) around forming stars are unstable and filled with vortex-dominated turbulence with Mach and Rossby numbers of order 0.2 -- 0.3. The {\it dead zones} are regions in which the temperature is too cool for the gas to ionize and be destabilized by instabilities associated with the magnetic field. The ``dead zones'' were thought, by most authors, to be stable to all purely-hydrodynamic instabilities because the flow has an angular momentum that increases with increasing radius in a PPD and is therefore stable by Rayleigh's theorem. However, that theorem in not applicable to stratified flows, such as those in a PPD. We summarize our simulations with emphasis on the finite-amplitude trigger of the instability and show that when the trigger is Kolmogorov noise, the Mach number of the noise that is needed to create instability is proportional to $Re^{-1/2}$, where $Re$ is the Reynolds number of the initial noise. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E17.00005: Zombie Vortices: Angular Momentum Transport and Planetesimal Formation Joseph Barranco, Philip Marcus, Suyang Pei, Chung-Hsiang Jiang, Pedram Hassanzadeh, Daniel Lecoanet Zombie vortices may fill the dead zones of protoplanetary disks, where they may play important roles in star and planet formation. We will investigate this new, purely hydrodynamic instability and explore the conditions necessary to resurrect the dead zone and fill it with large amplitude vortices that may transport angular momentum and allow mass to accrete onto the protostar. One unresolved issue is whether angular momentum transport is mediated via asymmetries in the vortices, vortex-vortex interactions, or acoustic waves launched by the vortices. Vortices may also play a crucial role in the formation of planetesimals, the building blocks of planets. It is still an open question how grains grow to kilometer-size. We will investigate the interactions of dust with vortices generated via our new hydrodynamic instability, and bridge the gap between micron-sized grains and kilometer-sized planetesimals. [Preview Abstract] |
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