  The effect of fixed cylindrical rods located at the centerline axis on vortex
breakdown (VB) is studied experimentally and numerically. We find that the VB
is enhanced for very small values of the rod radius $d$, while it is suppressed
for values of $d$ beyond a critical value. In order to characterize this
effect, the critical Reynolds number for the appearance of vortex breakdown as
a function of the radius of the fixed rods and the different aspect ratios was
accurately determined, using digital particle image velocimetry. The numerical
and experimental results are compared showing an excellent agreement. In
addition, a simple model in order to show that this effect also appears in open
pipe flows is presented in the appendix.
