Key points are not available for this paper at this time.
The spiral form of vortex breakdown observed in the numerical simulations of Ruith et al. ( J. Fluid Mech. , vol. 486, 2003, p. 331) is interpreted as a nonlinear global mode originating at the convective/absolute instability transition point in the lee of the vortex breakdown bubble. The local absolute frequency at the transition station is shown to yield a satisfactory prediction of the precession frequency measured in the three-dimensional direct numerical simulations.
Gallaire et al. (Wed,) studied this question.