Experiments on the subcritical transition to spiral turbulence in Taylor-Couette flow by parametric modulation and chaos control

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Disentangling the complexity in turbulent flows is one of the fundamental problems in physical science with a large relevance to many applications. The transition to turbulence occurs abruptly in many shear flows, such as Couette and Poiseuille flows, even though the laminar flow is linearly stable. Though it is now well accepted that disconnected flow states of finite-amplitude can exist in Navier-Stokes equation there is an ongoing debate on the role of these states for this so-called subcritical transition to turbulence in shear flows. The aim of this project is to provide a definite experimental evidence for the existence of finite-amplitude solutions in a linearly stable shear flow. The experiments will be performed in the spiral turbulence regime of counter-rotating Taylor-Couette flow which exists also below the 'classical' centrifugal instability of the system. Destabilisation of the laminar flow in the linearly stable regime by temporal variations of Reynolds numbers, such as sudden changes and period modulations, as well as stabilisation of disordered flow by means of time-delayed feedback chaos control in this closed flow experiment will allow an investigation of the asymptotic solution and bifurcation structure of such disconnected states of finite amplitude. This study will shed light on their role in the subcritical transition to turbulence in shear flows.