ASTROPHYSICS

The daily rotation of a planet leads to the well known Coriolis force, which changes the direction of the wind in regions of high or low pressure and also has a great influence on the flow of liquid iron in the earth's core. The effect of precession is similar, although more subtle: Due to the inertia of the flow, it aims to keep its old direction of rotation while the rotational direction of the planet itself changes continually, leading to complex flows. They have been proposed as a possible source for planetary magnetic field which are of great relevance for the existence of life.

Such a precessing flow is one of the cases were it is evident from theoretical work that the approximation of a perfectly spherical planet misses out on some essential physics. A spheroidal or ellipsoidal shape remains a challenge for the numerical simulation of geophysical flows, since certain simplifications applied in a sphere are not usable and therefore more computational power is necessary to reach the relevant parameters. The scientists in Münster therefore modified an existing code called Nek5000 to handle a spheroidal geometry, a problem that is much less established and researched than flows in spheres. The code has already been proven (e.g. on the Jülich Blue Gene/P Extreme Scaling Workshop 2010) to offer excellent parallel scaling capabilities to take full advantage of a modern high performance system for up to hundreds of thousands processing cores.

The computational capabilities of the JUQUEEN cluster at the Jülich Supercomputing Center allowed the researchers to study the basic dynamics of precessional flow when approaching more realistic geometries and parameters, where they found complex small-scale dynamics. One of the most interesting and far reaching findings from the simulations was the confirmation of a hysteresis effect that had already been predicted by theoretical considerations in the sixties. This means that the resulting flow in a planet depends not only on its current state, but also on its history, in this case on whether the time period of precession is increasing or decreasing – this is tantamount to an orbital companion moving closer to its neighboring planet or farther away from it.

Scientific Contact:

Jan Vormann 
Institut für Geophysik
Westfälische Wilhelms-Universität Münster 
Corrensstraße 24, D-48149 Münster (Germany)
e-mail: jan.vormann [at] uni-muenster.de
https://www.uni-muenster.de/Physik.GP/