Elliptical instability in rotating spherical fluid shells: application to Earth’s fluid core

If a circular flow of a contained rotating fluid is strained into an elliptical one in such a way that the elliptical streamlines preserve their figures in a laboratory reference frame, the flow becomes unstable. This type of instability is known to be excited as a result of coupling between a pair of inertial modes of the contained fluid by the applied strain. The present work is concerned with the elliptical instability of a thick, spherical, rotating fluid shell. We have conducted experiments and have observed the excitation of the elliptical instability as well as some of the inertial modes of a rotating fluid contained in the above mentioned geometry. The velocities of the fluid particles are captured by means of Digital Particle Imaging Velocimetry (DPIV). Plots of the velocity vectors of the inertial modes obtained experimentally are used to identify the modes predicted using theoretical methods. Such investigations are of geophysical interest, since tidal forcing might be sufficient to excite an elliptical instability in Earth’s fluid core as well as in other planetary interiors.