Experimental investigations on turbulent Taylor-Couette flows in very wide gaps (part 2)

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The proposed work is the second phase of the project ‘Experimental investigation of turbulent Taylor-Couette (TC) in very wide gaps’. The objectives of this work are divided into two groups: one concentrating on the flow in the centrifugally unstable regime and based on the scientific open questions posed in the project's first phase, and the other researching the flow in the centrifugally stable regime, specifically the quasi-Keplerian flow (η²≤µ≤1), where this flow regime was not addressed in the project’s first phase. Here, the existing experimental apparatus (TvTCC) will be used in the first part to further analyze existing flow parameters. While a new system is proposed to be built with a larger geometry that allows measurements of the flow structures with significantly higher spatial and temporal resolution and direct torque measurements. The visualization of the very wide gap TC flow (η = 0.1) in the counter-rotating regime shows the existence of a variety of flow patterns, where some of these patterns are found to be unique to this geometry (e.g., Axial Columnar vortex, Helical Columnar vortex, etc.). The mechanics responsible for this set of patterns are still an open question and will be addressed in this work. Moreover, the angular momentum transport dependence on the rotation rate in η = 0.1 shows a maximum of transport for low counter-rotating rates, where the large-scale circulation (LSC) fulfills the whole gap and enhances the transport of the angular momentum flux. As counter-rotating rates increase, the LSC detaches from the outer cylinder wall due to the outer rotation stabilizing effect and leads to a decrease in momentum transport until it achieves a minimum. Unexpectedly, and for higher counter-rotating rates, the angular momentum transport starts to increase again, despite the apparently stabilizing effect of the outer cylinder rotation. The flow's space-time analysis indicates that the presence of inward-propagating patterns developed near the outer cylinder wall leads to this increase in angular momentum transport at high counter-rotating rates. Since these novel patterns arise in a centrifugally stable region, it is unclear what instability is responsible for them. The proposed work will focus on studying the dynamics of the outer boundary layers where these patterns appear and further studying the analogy between these patterns and the shear-induced superstructures that appear in other wall-bounded flows (pipe, channel, etc.). For TC flow in the centrifugal stable regime, numerous studies tried to find a subcritical hydrodynamic instability to bring turbulence into account. However, in the very wide-gap flow (η = 0.1), structures have been found that are assumed to contribute to the overall angular momentum transport. Further identification and quantification of these patterns are needed, in addition to the quantification of the global angular momentum transport for the flow in the centrifugally stable regime.