In this film we show results from an experimental measurement of the flow within a large Rayleigh-Bénard cell.
The air sample with a diameter and a height of 1.1 m (aspect ratio = 1) was cooled by a water flow at the top and electrically heated at the bottom. It was filled with tiny (380 µm) Helium-filled soap bubbles, which follow the flow ideally due to their neutral density and live for on average several minutes. The bubbles are illuminated from above by an array of high-power LEDs.
The illuminated bubbles are imaged by six cameras from different viewing directions, which record the projections of the bubble system at a rate of 30 Hz.
From the different projections, the particle trajectories are reconstructed using the Shake-The-Box method [1], which allows Lagrangian Particle Tracking at high particle image densities. More than 550.000 bubbles could be tracked instantaneously, revealing the flow within the cell. In order to visualize and investigate flow structures, the Lagrangian data is interpolated onto an Eulerian grid, using the FlowFit algorithm [2]. This data assimilation method applies regularizations by the Navier Stokes equations to further enhance spatial resolution beyond the sampling of the tracer bubbles.
The film illustrates exemplary results from various viewpoints, illustrating the beauty of the induced turbulent flow and the wealth and of the data gathered.
[1] Schanz D, Gesemann S, Schröder A. Shake-The-Box: Lagrangian particle tracking at high particle image densities. Exp Fluids, 57(5), 2016
[2] Gesemann S, Huhn F, Schanz D, Schröder A. From noisy particle tracks to velocity, acceleration and pressure fields using B-splines and penalties. 18th Int. Symp. on Appl. of Laser and Imaging Tech. to Fluid Mech. Lisbon, Portugal, July 4 – 7 2016
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