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

本研究是“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，并基于该项目第一阶段提出的科学公开问题，另一组研究离心稳定状态下的流动，特别是准开普勒流动（η²≤µ≤1），其中该流动状态在该项目的第一阶段没有得到解决。在这里，现有的实验装置（TvTCC）将用于第一部分，以进一步分析现有的流动参数。而一个新的系统，建议建立一个更大的几何形状，允许测量的流动结构具有显着更高的空间和时间分辨率和直接转矩测量。在反向旋转状态下的非常宽的间隙TC流（η = 0.1）的可视化显示了各种流动模式的存在，其中发现这些模式中的一些对于该几何形状是独特的（例如，轴向柱涡、螺旋柱涡等）。负责这组模式的机制仍然是一个悬而未决的问题，将在本工作中解决。此外，在η = 0.1时，角动量输运对自转速率的依赖性在低自转速率下表现出最大输运，此时大尺度环流（LSC）满足了整个气隙，增强了角动量通量的输运。随着反向旋转速率的增加，由于外部旋转稳定效应，LSC从外部圆柱壁分离，并导致动量传输减小，直到达到最小值。出乎意料的是，对于更高的反向旋转速率，角动量传输开始再次增加，尽管外圆柱体旋转的明显稳定效果。流的时空分析表明，存在的向内传播的模式开发的外缸壁附近导致这种角动量传输在高的反向旋转速率的增加。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性造成的。拟议的工作将集中在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和剪切诱导的上层结构，出现在其他有壁流动（管道，通道等）之间的类比。对于离心稳定区的TC流，许多研究试图找到一个亚临界流体动力学不稳定性，把湍流考虑在内。然而，在非常宽的间隙流（η = 0.1），结构已被发现，被假定为有助于整体角动量输运。需要进一步识别和量化这些模式，除了在离心稳定制度的流动的全球角动量输运的量化。

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.