Core Dynamics of Multi-Armed Spirals in Rayleigh-Benard Convection
  We report experimental observations of the core dynamics of multi-armed, rotating spirals in Rayleigh-Benard convection for a fluid with Prandtl number near one. In addition to the large-scale rotation of the spirals, we found a cyclic core motion within a central area of radius r = appr. (n lambda/2), where n is the number of spiral arms ending in the core and lambda is the wavelength of the pattern. The dynamics of the core was much faster than the large-scale spiral rotation. We observed multi-armed spirals for which the two periods were commensurate and others for which they were incommensurate. 
Brendan B. Plapp, Eberhard Bodenschatz, (Laboratory of Atomic and   Solid State Physics, Cornell University, USA)