An experimental study of the dynamics of librating bodies

Recently, various studies have modelled the fluid dynamics of librating astrophysical bodies. However, S.V. Subbotin and V.G. Kozlov, hydrodynamicists at the Perm State Humanitarian Pedagogical University Laboratory of Vibrational Hydromechanics, in Russia, respond that most of this prior work excludes an inner core unfixed to the rotation axis, which, according to Subottin, is crucial, because the cores of planets are free to oscillate.

In Physics of Fluids, Subbotin and Kozlov report on their experimental study of the dynamics of a free solid core and a liquid in a spherical cavity rotating about a horizontal axis.

With their experimental setup, which consisted of a spherical cavity rotating about a horizontal axis in a gravitational field, the researchers could establish rotation about a horizontal axis for the spherical cavity containing the liquid and the free solid core and study the core dynamics under various libration parameters.

“In our experiment, the centrifugal force keeps the light free core on the rotation axis, while the librations and the static gravity field in the laboratory reference frame cause it to oscillate,” said Subbotin.

Using high-speed video and particle image velocimetry, the researchers collected results pertaining to the dynamical nature of librating bodies. Their findings include uncovering the effects of harmonic forcing and the additivity of mean zonal flow via librations.

Subbotin and Kozlov say that the difference between their experimental model and the dynamics of actual planets is great, but their work deepens our understanding of the mechanical motion of the solid core of a planet and its gravitational interaction with other astrophysical bodies.

Source: “Librations induced zonal flow and differential rotation of free inner core in rotating spherical cavity,” by V. G. Kozlov and S. V. Subbotin, Physics of Fluids (2017). The article can be accessed at https://doi.org/10.1063/1.5000864 .