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

拟议的工作是“非常宽间隙湍流泰勒-库埃特（TC）实验研究”项目的第二阶段。这项工作的目标分为两组：一组集中在离心不稳定状态下的流动，并基于项目第一阶段提出的科学开放性问题，另一组研究离心稳定状态下的流动，特别是准开普勒流（η²≤µ≤1），其中这种流动状态在项目第一阶段没有解决。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。然而，一种新的系统被提议建立一个更大的几何结构，允许以更高的空间和时间分辨率和直接扭矩测量来测量流动结构。在反向旋转区域的非常宽间隙TC流（η = 0.1）的可视化显示了多种流动模式的存在，其中一些模式被发现是这种几何结构所特有的（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开放的问题，将在本工作中解决。在η = 0.1时，角动量输运对转速的依赖表明，在低反转速时，角动量输运最大，此时大尺度环流（LSC）填补了整个间隙，增强了角动量通量的输运。随着反旋转速率的增加，由于外旋转稳定效应，LSC与外圆柱壁分离，导致动量输运减少，直到达到最小值。出乎意料的是，对于更高的反向旋转速率，角动量输运开始再次增加，尽管外圆柱体旋转有明显的稳定作用。流动的时空分析表明，在高反旋转速率下，外柱壁附近形成的向内传播模式的存在导致了角动量输运的增加。由于这些新的模式出现在一个离心稳定的区域，目前还不清楚是什么不稳定造成了它们。建议的工作将集中于研究这些模式出现的外边界层的动力学，并进一步研究这些模式与出现在其他有壁流动（管道，通道等）中剪切诱导的上层结构之间的类比。对于处于离心稳定状态的TC流，许多研究试图找到一个亚临界流体动力不稳定性来考虑湍流。然而，在极宽间隙流（η = 0.1）中，已经发现了一些结构，这些结构被认为有助于总体角动量输运。除了对离心稳定状态下流动的全局角动量输运进行量化外，还需要对这些模式进行进一步的识别和量化。

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.