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

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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?