Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E30: Geophysical Fluid Dynamics: Rotating Flows |
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Sponsoring Units: DFD Chair: Eckart Meiberg, University of California, Santa Barbara Room: 311 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E30.00001: Zonal Flow Velocimetry in Spherical Couette Flow using Acoustic Modes Matthew M. Adams, Anthony R. Mautino, Douglas R. Stone, Santiago A. Triana, Vedran Lekic, Daniel P. Lathrop We present studies of spherical Couette flows using the technique of acoustic mode Doppler velocimetry. This technique uses rotational splittings of acoustic modes to infer the azimuthal velocity profile of a rotating flow, and is of special interest in experiments where direct flow visualization is impractical. The primary experimental system consists of a 60 cm diameter outer spherical shell concentric with a 20 cm diameter sphere, with air or nitrogen gas serving as the working fluid. The geometry of the system approximates that of the Earth's core, making these studies geophysically relevant. A turbulent shear flow is established in the system by rotating the inner sphere and outer shell at different rates. Acoustic modes of the fluid volume are excited using a speaker and measured via microphones, allowingdetermination of rotational splittings. Preliminary results comparing observed splittings with those predicted by theory are presented. While the majority of these studies were performed in the 60 cm diameter device using nitrogen gas, some work has also been done looking at acoustic modes in the 3 m diameter liquid sodium spherical Couette experiment. Prospects for measuring zonal velocity profiles in a wide variety of experiments are discussed. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E30.00002: Shear secondary instability in a precessing cylinder flow Waleed Mouhali, Thierry Lehner For a certain value of the forcing parameter, cyclones regime has been observed in our experiment involving water in a precessing cylinder. They result from an instability. We propose here to study the nature of this so-called instability. We consider first the mode coupling of two inertial waves with azimuthal wavenumber m$=$0 and m$=$1 (mode forced by the precession) in the inviscid regime (at high Re number limit) creates a differential rotation regime which has been observed in the same experiment at small enough Poincar\'{e} number $\varepsilon$ (ratio of the precession to the rotation frequencies). Secondly, the radial profile of the corresponding axial mean flow vorticity shows an inflexion point leading to a localized inflectional secondary instability. We show that when $\varepsilon$ is increased from low values the forced mode m$=$0 becomes the most instable in this induced differential rotation, which can be responsible for the observed eruptions of jets from the lateral walls of the cylinder leading to the cyclones formation within the volume from the development of an inviscid secondary shear instability. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E30.00003: Turbulent Flows Driven by the Mechanical Forcing of an Ellipsoidal Container Benjamin Favier, Michael Le Bars, Alexander Grannan, Adolfo Ribeiro, Jonathan Aurnou We present a combination of laboratory experiments and numerical simulations modelling geophysically relevant mechanical forcings. Libration and tides correspond to the periodic perturbation of a body's rotation rate and shape, and are both due to gravitational interactions with orbiting companions. Such mechanical forcings can convey a fraction of the rotational energy available and generate intense turbulence in the fluid interior of satellites and planets. We investigate the fluid motions inside a librating or tidally deformed triaxial ellipsoidal container filled with an incompressible fluid. In both cases, the turbulent flow is driven by the elliptic instability which is a triadic resonance between two inertial modes and the base flow. We characterize the transition to turbulence as triadic resonances develop while also investigating both intermittent and sustained regimes. It is shown that the flow is largely independent of the properties of the mechanical forcing, hinting at a possible universal behaviour of the saturated elliptical instability. The existence of such intense flows may play an important role in understanding the thermal and magnetic evolution of celestial bodies. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E30.00004: Influence of the multipole order of the source on the decay of an inertial wave beam in a rotating fluid Nathanael Machicoane, Pierre-Philippe Cortet, Bruno Voisin, Frederic Moisy Inertial wave beams emitted from localized sources are relevant to a broad range of geo and astrophysical flows. These beams are excited at critical lines, where the local slope of solid boundaries equals the propagation angle of the wave, in rotating fluid domains affected by a global harmonic forcing (e.g. precession, libration, tidal motion). We show here theoretically and experimentally that the decay of the amplitude of such wave beams depends on the multipole order of the source. We analyze the far-field viscous decay of a two-dimensional inertial wave beam emitted by a harmonic line source in a rotating fluid. By identifying the relevant conserved quantities along the wave beam, we show how the beam structure and decay exponent are governed by the multipole order of the source. Two wavemakers are considered experimentally, a pulsating and an oscillating cylinder, aiming to produce a monopole and a dipole source, respectively. The relevant conserved quantity which discriminates between these two sources is the instantaneous flow rate along the wave beam, which is non-zero for the monopole and zero for the dipole. For each source, the beam structure and decay exponent, measured using particle image velocimetry, are found in good agreement with the predictions. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E30.00005: Do inertial wave interactions control the rate of energy dissipation of rotating turbulence? Pierre-Philippe Cortet, Antoine Campagne, Nathanael Machicoane, Basile Gallet, Frederic Moisy The scaling law of the energy dissipation rate, $\epsilon\propto U^3/L$ (with $U$ and $L$ the characteristic velocity and lengthscale), is one of the most robust features of fully developed turbulence. How this scaling is affected by a background rotation is still a controversial issue with importance for geo and astrophysical flows. At asymptotically small Rossby numbers $Ro=U/\Omega L$, i.e. in the weakly nonlinear limit, wave-turbulence arguments suggest that $\epsilon$ should be reduced by a factor $Ro$. Such scaling has however never been evidenced directly, neither experimentally nor numerically. We report here direct measurements of the injected power, and therefore of $\epsilon$, in an experiment where a propeller is rotating at a constant rate in a large volume of fluid rotating at $\Omega$. In co-rotation, we find a transition between the wave-turbulence scaling at small $Ro$ and the classical Kolmogorov law at large $Ro$. The transition between these two regimes is characterized from experiments varying the propeller and tank dimensions. In counter-rotation, the scenario is much richer with the observation of an additional peak of dissipation, similar to the one found in Taylor-Couette experiments. [Preview Abstract] |
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