Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B13: Geo- and Astrophysical Rotating Flows |
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Chair: Jonathan Aurnou, UCLA Room: 304 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B13.00001: Magnetic Braking of Jovian Jet Flows Ashna Aggarwal, Susanne Horn, Jonathan Aurnou The azimuthally-directed jet flows of the gas giants, Jupiter and Saturn, are amongst their most dominant surface features. Recent Juno gravity measurements have inferred that the zonal jets of Jupiter extend from the weather layer, where they are directly observed, down at least 3,000 km deep into the H-He molecular atmosphere. In addition, Jupiter's electrical conductivity increases as the molecular envelope transitions to a liquid metal. As electrical conductivity increases, the strength of magnetic forces grows, acting as a resistive brake on the jet flows. We have developed a pseudo-spectral code that solves the Cartesian Navier-Stokes equations in 2-D with buoyancy and a quasi-static magnetic field to quantify the process of magnetic braking, thought to truncate the Jovian jets. We will present the results of a suite of direct numerical simulations (DNS) of shearing convection, similar to Goluskin et al., (J. Fluid Mech. 759, 360, 2014), where we vary the strength of an imposed transverse magnetic field. Depending on the value of the magnetic field, the jets are damped, strongly intermittent, or fully suppressed. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B13.00002: The Elbert Subrange of Magnetostrophic Rotating Magnetoconvection Susanne Horn, Jonathan Aurnou Classical linear stability analysis shows that convection subjected to rotation and a magnetic field is most easily excited when Coriolis and Lorentz forces are approximately in balance and in the form of a large-scale stationary bulk mode. Since estimates for Earth also suggest that the outer liquid metal core is in this so-called magnetostrophic state, there is a long-held belief that these modes optimise planetary magnetic field generation. But a single-mode theory is not likely to be geophysically realistic. Instead, liquid metal flows are characterised by pronounced multimodality with a mix of stationary, oscillatory, and boundary-attached modes. In fact, Donna Elbert (cf. Chandrasekhar, 1961) discovered that there is subrange of magnetostrophic rotating magnetoconvection where two types of stationary modes co-exist: a small-scale geostrophic mode and a large-scale magnetostrophic mode. The parameter space for this subrange coincides with the one for planetary cores, suggesting a crucial link to the magnetic field generation in geo- and astrophysical objects. Here, we revisit linear stability results and further use nonlinear direct numerical simulations to verify which onset characteristics, such as length scales and frequencies, carry over to higher supercriticalities. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B13.00003: Experimental study of the non-linear saturation of the elliptical instability: inertial wave turbulence versus geostrophic turbulence Thomas Le Reun, Benjamin Favier, Michael Le Bars We present an experiment of the turbulent saturation of the flow driven by parametric resonance of inertial waves in a rotating fluid. An ellipsoid filled with water is brought to solid-body rotation and then undergoes harmonic modulation of its rotation rate. This triggers the exponential growth of a pair of inertial waves via the elliptical instability. As the instability reaches non-linear saturation, it creates a turbulence where energy is injected into the resonant waves only. Depending on the amplitude of the rotation rate modulation, two different saturation states are observed. At large forcing amplitudes, the saturation flow mainly consists of a steady, geostrophic anticyclone. Its amplitude vanishes as the forcing amplitude is decreased while remaining above the threshold of the elliptical instability. Below this secondary transition, the saturation flow is a superposition of inertial waves which are in weakly non-linear resonant interaction, a state that could asymptotically lead to inertial wave turbulence. The present study is an experimental confirmation of the model of Le Reun, PRL (2017) who introduced the possibility of these two turbulent regimes. The transition between these two regimes and their relevance to geophysical applications are finally discussed. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B13.00004: How the Great Red Spot of Jupiter Stays Alive while Losing Energy through Viscous and Radiative Dissipation Aidi Zhang, Philip Marcus During the last decade, the cloud cover over the Great Red Spot (GRS) of Jupiter has shrunk significantly. This observation, along with recent observations that the GRS has been repeatedly shedding large (100,000 km$^2$) chunks of itself, has caused many planetary scientists to speculate that the GRS will vanish in the next 10 years. Here we argue against that hypothesis and demonstrate that GRS, which is a large anticyclone, maintains itself with a weak (and not directly observable) secondary circulation that is consistent with all of the observations. Numerical simulations of the anelastic ideal gas equations are used to show that this secondary circulation both re-energizes the GRS and creates new anti-cyclonic vorticity via baroclinic dynamics that can only exist in a vertically stably-stratified, rotating atmosphere. The secondary circulation brings energy from the atmosphere outside the GRS into its interior. The energy flux into the GRS balances the loss of energy of the GRS from viscosity and radiative damping. The rate of viscous loss of kinetic energy from the GRS is small compared to the rate of loss of potential and thermal energy due to radiative damping. Without the secondary circulation of the GRS, radiative damping would cause the GRS to decay in 4-5 years. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B13.00005: A quasi-geostrophic thermally driven model for rapid dynamics in Earth’s core Meredith Plumley, Andrew Jackson, Stefano Maffei Simulating the physics of Earth’s core remains an elusive challenge that precludes numerical solution of the full governing equations. Model reduction must be embraced to study the dynamics occurring in the core. One alternative for rotating flows is to adopt a columnar approximation for the velocity field, sometimes referred to as the quasi-geostrophic approximation. This technique is used to derive a completely new model for the rapid dynamics in thermally driven convection on the equatorial plane by using axial averages. We show that the onset of thermal convection found using local theory applied to this model agrees quantitatively with the values found in full numerical investigations and with asymptotic theory. Time evolution results from this model can provide insight into the dynamics on short timescales in the core. We investigate the scaling laws of supercritical convection in parameter regimes unachievable in 3-D models. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B13.00006: Machine learning predictions for magnetic field time evolution in a Three-Meter liquid sodium spherical Couette experiment Artur Perevalov, Ruben Rojas, Brian Hunt, Daniel Lathrop The source of the Earth's magnetic field is the turbulent flow of liquid metal in the outer core and its interaction with the present magnetic field. Our experiment's goal is to create an Earth-like dynamo to explore the mechanisms of generating magnetic fields and to understand the involved dynamics. Full numerical prediction is a challenging problem as it is strongly nonlinear and not computationally feasible to resolve. We present the implementation of various prediction techniques, including a reservoir computer deep learning algorithm, to probe the feasibility of using magnetic field data from the experiment to predict future measurements. The experiment is a three-meter diameter outer sphere and a one-meter diameter inner core model with the gap filled with liquid sodium. The spheres can rotate independently up to 4 and 14 Hz respectively, giving a Reynolds number up to $ 1.5*10^8 $. Two external electromagnets apply magnetic fields, while an array of Hall sensors measure the resulting magnetic fields. We use this data to train a reservoir computer to predict Hall sensor measurements and mimic waves in the experiment. Surprisingly, accurate predictions can be made for several magnetic dipole time scales. This shows that such a MHD system’s behavior can be predicted. [Preview Abstract] |
Saturday, November 23, 2019 5:58PM - 6:11PM |
B13.00007: A New Hope for Dynamo Action in the Three-Meter Model of Earth’s Core: Increasing Helicity. Ruben Rojas, Artur Perevalov, Daniel Lathrop The dynamo generation of magnetic fields in turbulent flows of conducting fluids and plasmas is an important feature of stars and planetary cores. Our Three-Meter diameter spherical Couette experiment uses liquid sodium to mimic some of the dynamics of these flows, giving insight into these natural phenomena. While we have seen substantial magnetic field gain, dynamo states have yet not been achieved. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest roughening the inner sphere, which can be achieved by adding baffles on the inner sphere. Those studies showed a reduction in the threshold for dynamo action by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. Thus, we seek to achieve a dynamo state in our three-meter experiment by adding baffles on the inner sphere. In this work, we use a 40-cm spherical Couette water apparatus to characterize the effect of different baffle designs on flow dynamics. We present velocity profiles and torque measurements which give us insight into baffle design of the three-meter experiment. We propose a new baffle design that improves the likelihood of observing dynamo action by breaking the symmetry in parameter space of the experiment and generating flows of different topology. [Preview Abstract] |
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