The swirls in the boundary layer lead to turbulences whose origin and formation can be caused by incidental disturbances on the surface of the body or by impetus within the flow itself. This can lead to random separation, local inverse flows up to the formation of regular swirls, or cyclical vibrations of the swirl. The four phase pictures of a transversally streamed rectangular cylinder at low Reynold's numbers illustrate the relationships between the boundary layer and the dead water area formed in the train with the caster swirl. This caster swirl cyclically changes its running direction (as in the figure). E.g. for Re>1,000, real swirls are formed for cycle cylinders.