Intense Flows Driven by Mechanical Forcing in Non-axisymmetric Containers

Abstract:

Here we present laboratory experimental results that simulate two geophysically relevant mechanical

forcings that can drive intense fluid motions in the interior fluid layer of non-axisymmetric containers; libration

and tidal distortions. Longitudinal libration refers to the small periodic oscillations of a satellite's mean rotation

rate as it orbits a primary body and is replicated using an oscillating hard acrylic ellipsoid. Tidal forcing refers to

the rotating gravitational distortion of a body in orbit and is replicated using a deformable silicone sphere. We

use a particle image velocimetry (PIV) technique to measure the 2D velocity field in the nearly equatorial plane

over hundreds of librational and tidal cycles.

 First, while the theoretical base flow for each mechanism is nearly identical, we verify the base flow

induced by the tidal distortion and a time-averaged zonal flow that scales as the square of the tidal forcing and is

expected to be small in planets. Additionally, for a fixed tidal distortion, a polar vortex first identified by Suess

(1970) is re-examined that may drive an intense vortex at planetary settings.

 Second, we investigate the characteristics of turbulence in the bulk fluid layer generated via an elliptical

instability of librational and tidal forcing. An elliptic instability is the triadic resonance of two inertial modes

whose non-dimensional frequencies are between [-2-2] with the mechanically induced base flow. This is called

libration driven elliptical instability (LDEI) and tidal driven elliptical instability (TDEI) respectively. We

characterize the evolution of the turbulent flow that displays either intermittent large cycles of growth and decay

or smaller cycles of saturation while also investigating the cascade of energy inside the inertial mode frequency

regime. The existence of these types of intense flows may play an important in understanding the thermal

evolution and magnetic field generation in bodies subject to mechanical forcing and not considered in standard

models of convectively forced magnetic field generation.