Influence of coaxial cylinders on vortex breakdown in a closed flow
  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. 
Cecilia Cabeza, Gustavo Sarasua, Arturo C. Marti, Italo Bove, Sylvana   Varela, Gabriel Usera, Anton Vernet