Core dynamics of multi-armed spirals in Rayleigh-Bénard 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 nλ/2 where n is the number of spiral arms ending in the core and λ is the wavelength of the pattern. The dynamics of the core was much faster than the large-scale spiral rotation. We observed multiarmed spirals for which the two periods were commensurate and others for which they were incommensurate.