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

这项工作是“大间隙湍流Taylor-Couette(TC)的实验研究”项目的第二阶段。这项工作的目标分为两组：一组专注于离心力不稳定区域的流动，并基于项目第一阶段提出的科学开放问题，另一组研究离心力稳定区域的流动，特别是准开普勒流(η²≤µ≤1)，该流动状态在项目第一阶段中没有讨论。在这里，第一部分将使用现有的实验装置(TvTCC)来进一步分析现有的流动参数。而一个新的系统被提议建立一个更大的几何结构，允许测量具有显著更高的空间和时间分辨率的流动结构和直接的扭矩测量。在对旋流区的甚宽间隙TC流(η=0.1.1)的可视化显示中，存在多种流型，其中一些流型是该几何形状所特有的(例如，轴向柱状涡、螺旋柱状涡等)。负责这组模式的机制仍然是一个悬而未决的问题，将在本工作中讨论。此外，在η=0.1时，角动量输运对自转速度的依赖关系表现出最大输运，此时大尺度环流填补了整个空隙，增强了角动量通量的输运。随着对转转速的增加，由于外旋转稳定化作用，LSC从外筒壁上脱离，动量输运减弱，直至最小。出乎意料的是，对于较高的反向旋转速度，角动量输运开始再次增加，尽管外筒旋转具有明显的稳定作用。流动的时空分析表明，内向传播模式的存在在圆柱体外壁附近发展，导致在高对角转速下角动量输运的增加。由于这些新的模式出现在离心稳定的区域，目前还不清楚是什么不稳定导致了它们。拟议的工作将集中于研究出现这些图案的外边界层的动力学，并进一步研究这些图案与其他壁面边界流(管道、渠道等)中出现的剪切诱导的上部结构之间的相似之处。对于离心稳定区的TC流动，许多研究试图寻找一种考虑湍流的亚临界流体动力不稳定性。然而，在极宽间隙流动中(η=0.1.1)，已发现结构被认为对整体角动量输运有贡献。除了对离心力稳定区域内流动的全球角动量传输进行量化外，还需要进一步确定和量化这些模式。

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.