We report on numerical investigations of pattern formation in the classical Rayleigh-Benard convection with cylindrical geometry in the regime of low Prandtl numbers and moderate aspect ratio. Beyond the onset of convection, we found straight and bent rolls as stable patterns. By increasing the Rayleigh number, we studied the generation of defects, their dynamics in the form of climbing and gliding, the existence of stable targets and spirals as well as the occurrence of core instabilities, a variety of pattern types that were also observed in experiments.