Experimental Analysis of Turbulent Superstructures in Thermal Convection by Time-Resolved Lagrangian Particle Tracking up to Very High Rayleigh Numbers

在此基础上，我们的目标是应用几种创新的测量技术来捕获经典湍流状态和最终状态下的瑞利-巴姆纳德对流（RBC）中的湍流上层结构（TSS）。结合“摇盒”（STB）拉格朗日粒子跟踪方法，以长寿命的微观肥皂泡作为示踪粒子，用于密集种子流动，数据同化工具FlowFit和温度敏感涂料（TSP），将实现高空间和时间分辨率的大规模测量。生成的数据将有助于提高对复杂现象的理解，如湍流上层结构或大尺度环流（LSC）、热羽流和湍流背景波动之间的相互作用，通过直接观察它们的动态相互作用。两个不同的样品需要处理。较低的Ra（< 2·10^8）是用水作为工作流体的对流电池获得的。它有一个二次的水平截面和一个可变的宽高比在4-10的范围内。此外，将使用纵向宽高比在5和10之间可变的立方体对流电池。它应在马克斯-普朗克动力学和自组织研究所的U-Boot内运行，采用加压六氟化硫作为工作流体。这个程序允许访问最大为5·10^13的瑞利数。具体而言，本提案应解决以下目标和研究问题。在实验室条件下生成和观察RBC的湍流上层结构建立STB / FlowFit和TSP作为实验工具箱，研究RBC3中的拉格朗日和欧拉流动结构和统计。扩展STB / FlowFit方法，通过使用温度敏感颗粒来确定与速度场平行的3D温度场。在中等宽高比（4..10）到非常高的Ra（~10^13）下研究湍流RBC。生成的TSS如何与瑞利数进行标度？从LSC过渡到TSS，即随着宽高比的增加，大尺度流结构的形态和动力学是如何变化的？驱动小尺度和大尺度相干流/上层结构（如热羽流、LSC和TSS或相邻LSC）之间动态相互作用的机制是什么？4. 侧向约束的实际几何形状（矩形、二次型、圆形）对湍流上层建筑的尺寸和侧向布置有多大影响？大尺度流结构（LSC / TSS）是如何与对流开始上方观测到的模式联系起来的？

With this proposal, we aim at application of several innovative measurement techniques to capturing turbulent superstructures (TSS) in Rayleigh-Bénard convection (RBC) in the classical turbulent regime and the ultimate state. Combination of the “Shake-The-Box” (STB) Lagrangian Particle Tracking method for densely seeded flows with long-lived microscopic soap bubbles as tracer particles, the data assimilation tool FlowFit and Temperature-Sensitive Paints (TSP), large scale measurements with high spatial and temporal resolution shall be enabled. The generated data will help improving the understanding of complex phenomena such as interactions between turbulent superstructures or large scale circulations (LSC), thermal plumes and turbulent background fluctuations by directly observing their dynamic interplay. Two different samples shall be addressed. The lower Ra (< 2·10^8) are accessed with a convection cell using water as working fluid. It has a quadratic horizontal section and a variable aspect ratio in the range of 4-10. Further, a cuboidal convection cell with a longitudinal aspect ratio variable between 5 and 10 will be used. It shall be operated within the ‘U-Boot’ of the Max-Planck-Institute for Dynamics and Self-Organization, employing pressurized Sulphur-Hexafluoride as working fluid. This procedure allows to access Rayleigh numbers up to 5·10^13. In specific, the following Goals and research questions shall be addressed with this proposal.Goals:1. Generate and observe turbulent superstructures in RBC under laboratory conditions 2. Establish STB / FlowFit and TSP as an experimental toolbox to study Lagrangian and Eulerian flow structures and statistics in RBC3. Extend the STB / FlowFit methodology to determine 3D temperature fields in parallel to the velocity fields by using temperature-sensitive particles4. Study turbulent RBC at moderately large aspect ratios (4..10) up to very high Ra (~10^13)Research questions:1. How do the generated TSS scale with the Rayleigh-number?2. How does the morphology and dynamics of the large scale flow structures change upon transition from LSC to TSS, i.e. with increasing aspect ratio?3. What are the mechanisms driving the dynamic interplay between small and large scale coherent flow / superstructures, such as thermal plumes, LSC and TSS or adjacent LSC? 4. How far does the actual geometry of the lateral confinement (rectangular, quadratic, circular) impact on the dimension and lateral arrangement of turbulent superstructures?5. How are the large scale flow structures (LSC / TSS) linked to the patterns observed just above the onset of convection?