Precession

The basic problem is delineated in the figure above. The earth consists approximately of three layers: the inner core (IC), the outer core, and the mantle. Outer core and mantle meet at the core mantle boundary (CMB). The outer core, i.e. the region inbetween rIC and rCMB, is liquid. The motion of this liquid (mostly iron) is believed to be at the origin of the earth's magnetic field. The earth revolves once a day about the axis joining geographic north and south poles, and this axis itself precesses about the normal to the ecliptic which forms an angle of 23.5o with the diurnal rotation axis. The problem is to determine the fluid flow inside the outer core driven by the precessional motion of the solid boundaries.

While geophysics is the main motivation for studying precession driven flows, engineering also provides applications, for instance the dynamics of spin stabilized spacecrafts containing liquid fuel during maneuvers intended to reorient the spin axis.

Numerical simulations are underway in order to determine the fluid flow inside a precessing container. Both spherical and ellipsoidal containers are being considered. The spherical boundary forces flow only through viscous torques whereas an ellipsoidal boundary drives the fluid with pressure torques as well. The main goal of the simulations is to classify the instabilities the flow may undergo, and to determine their influence on the dynamo mechanism. Some results are given in the references.