Dynamics and Selection of Giant Spirals in Rayleigh-Bénard Convection

For Rayleigh-B\'enard convection of a fluid with Prandtl number σ0ex0ex=0ex0ex1.4, we report experimental and theoretical results on a pattern selection mechanism for cell-filling, giant, rotating spirals. We show that the pattern selection in a certain limit can be explained quantitatively by a phase-diffusion mechanism. This mechanism for pattern selection is very different from that for spirals in excitable media.