Vortex breakdown occurs in highly swirling flows in various engineering applications, such
as hydraulic turbines working in either high-load or part-load conditions, leading to pressure
pulsations and performance degradation. In the present study, the formation of a helical
vortex breakdown having rotational and plunging components (axial oscillation) in a straight
diffuser is investigated with transient simulation from its columnar state to a helical vortex
breakdown state. It is similar to the helical precessing vortex rope occurring in hydraulic …