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A Hopf bifurcation with translational invariance has been widely considered as an appropriate model for the appearance of spiral vortices in counter-rotating Taylor-Couette flow. Our experimental work demonstrates that flow conditions close to the axial boundaries are responsible for the type of bifurcation scenario, i.e., either asymmetric pure traveling waves or more complex behavior, such as defect states or symmetric mixed states appearing from a Hopf bifurcation. The measurements were performed in the first Taylor-Couette experiment with independently rotating endwalls confining the system in axial direction. The rotation rate of the (synchronous) endwalls is found to be an essential control parameter for the spatial amplitude distribution of the traveling waves and also reflects symmetry of the corresponding flow pattern appearing from the Hopf bifurcation.

The origin and the transition to turbulence in wall-bounded shear flows is one of the outstanding problems of classical physics. In order to shed light on this transition, recent investigations have analyzed the decay of localized turbulent structures. At low Reynolds numbers, an exponential distribution of survival times has been observed in pipe and plane Couette flows [1, 2, 3, 4]. In phase space this is related to the decay from a chaotic saddle. However, pipe flow is an open flow, posing many experimental and numerical challenges for the study of bifurcation events. In a closed system we have found a flow state that shows transient turbulent behavior also at low Reynolds numbers. It appears as turbulent bursting in the Taylor-Couette system without an external forcing for counter-rotating cylinders above the centrifugal instability. Therefore it competes with coherent states of the system. Here we present spatiotemporal properties and lifetime behavior of this flow.

We present a type of spiral vortex state that appears from a supercritical Hopf bifurcation below the linear instability of circular Couette flow in a Taylor-Couette system with rigid end plates. These spirals have been found experimentally as well as numerically as "pure" states but also coexist with "classical" spirals (or axially standing waves for smaller systems) which typically appear from linear instability in counterrotating Taylor-Couette flow. These spiral states have an axial distribution of the strongly localized amplitude in the vicinity of the rigid end plates that confine the system in the axial direction. Furthermore, they show significantly different oscillation frequencies compared to the critical spiral frequencies. Despite the localization of the amplitude near the ends, the states appear as global states with spirals that propagate either toward the middle from each end of the system or vice versa. In contrast to classical spirals, these states exhibit a spatial or a spatiotemporal reflection symmetry.