[IGPP Everyone] [EPSS Everyone] Planetary Science seminar: TODAY

[Kevin McKeegan]

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*PLANETARY SCIENCE SEMINAR*

*Thursday, May 10*

*noon in Slichter 3853*

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*Emily Hawkins and Ashna Aggarwal*

*Dept. of Earth, Planetary, and Space Sciences*

*UCLA*



Emily Hawkins:  *"E**xperimental Investigations of Rapidly Rotating
Convective Turbulence in Planetary Interiors"*

The magnetic fields of planets are generated and sustained by fluid motions
in their electrically conducting liquid metal interiors.  The key
characteristics of such flows, thought to be governed to leading order by
rapid rotation and turbulent convection, are not well understood at present.
  Our new laboratory device, ‘NoMag’, is designed to span a wider and more
extreme range of parameters than previously studied, thus allowing us to
explore essential features of rotating convection in a setting similar to
that of core flows, i.e. one that is both rapidly rotating and highly
turbulent.  Specifically, ‘NoMag’ is constructed to simulate a local, polar
parcel of planetary core convecting fluid under the influence of axial
rotation and buoyancy forcing. As such, a cylindrical geometry is
constructed with a fixed diameter of *D ≈ *60 cm and heights ranging between
 *H ≈*5 cm to *H ≈ *185 cm. Using this device, we explore the properties of
rotating convection in water, with Ekman numbers (viscous
diffusion/Coriolis force) ranging between *E **≅*3*×*10*−*8 (i.e. rapidly
rotating) to *E **≅* 10*−*3 (i.e. weakly rotating) and Rayleigh numbers
(thermal buoyancy/ thermal and viscous diffusion) between *Ra **≅* 105 (i.e.
weakly convecting) to *Ra **≅* 1013 (i.e. turbulently convecting). We
utilize laser doppler velocimetry (LDV) to obtain point measurements of
bulk convective velocities, resulting in measured Reynolds numbers
(inertia/viscous diffusion) ranging between *Re **≅* 102 to *Re  **≅* 5 *× *
104 , with the onset of turbulence occurring near *Re **∼** O*(103 ). For
the first time, we couple velocity and heat transfer measurements by the
simultaneous collection of temperature time series at the fluid boundaries
and at multiple locations within the fluid bulk. In this talk, I will
present recent experimental results using our *H **≅* 20 cm tall tank that
test inevitably coupled heat transfer and convective velocity scaling
predictions relevant to rapidly rotating systems.



*Ashna Aggarwal.  “**Magnetic Braking of Jupiter's Jet Flows”*

The azimuthally-directed zonal winds of the gas giants, Jupiter and Saturn,
are amongst their most dominant surface features.  Recent Juno gravity
measurements have inferred that the zonal  winds  of Jupiter extend  from
the  weather  layer  where they are observed  down at  least  3,000 km deep
into  the H-He  molecular  atmosphere.   In addition, Jupiter’s electrical
conductivity increases as a function of spherical radius, r, as the  molecular
envelope transitions to a liquid  metal.  As electrical conductivity
increases, the strength of magnetic forces grows, which act as a resistive
brake on the azimuthal jet flows.  The process of magnetic braking, thought
to play a key role in the spherical truncation of the jets, will be
quantified  with this study.  As such, I have developed a pseudo-spectral
code that solves the Cartesian Navier-Stokes  equations  in 2-D with
buoyancy  and a quasi-static magnetic  field. I conduct di- rect numerical
simulations  (DNS) of shearing  convection  and vary the strength of the
imposed magnetic field, whose intensity is controlled  by the  value  of the
  Chandrasekhar number,  Q,  (estimated ratio  of Lorentz  and  viscous
forces) in order to investigate the  effects of a magnetic  field on the
damping  of the shear flow. In this talk,  I will present preliminary
results  of the first magneto-hydrodynamic case, carried out  at  Rayleigh
number,  Ra = 106  (the  ratio  of buoyancy  to diffusion),  Prandtl number,
  Pr = 1 (the ratio of viscous to thermal diffusion), and Q = 103 , where
the jet flows are strongly magnetically damped.

--
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Kevin D. McKeegan
Professor of Cosmochemistry & Geochemistry
Dept. of Earth, Planetary, and Space Sciences
UCLA
Los Angeles, CA 90095-1567
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mckeegan at epss.ucla.edu
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