Observed Dust Devil-Like Flow Structures In Large-Scale Turbulent Rayleigh-Bénard Convection

Turbulent Rayleigh-Bénard (RB) convection, a fluid layer heated from below and cooled from above, might be an appropriate tool to reproduce flow phenomena as occurring in a convective atmospheric boundary layer. One of these phenomena is the appearance of so-called dust devils, vortex like flow structures with typical dimensions up to a few tens of meters in diameter and up to a few hundreds of meters in height. Unlike their big brothers, the tornados, start to evolve close to the ground, but the physics of their evolution are still not fully understood. Since field measurements of dust devils are extremely hard to realize (due to their very rare and spontaneous appearance at arbitrarily varying locations), continuous flow measurements in large-scale laboratory experiments may help to overcome this problem. Such measurements in flows of several m³ in volume require optical measurement techniques like particle tracking velocimetry which gains access to the 3-dimensional Lagrangian velocity field. Special attention has to be drawn to the particle seeding of the flow and the imaging of particles. Particles must have neutral density compared to the fluid, and must be large enough to be detected by cameras over a range of several meters. They also should exhibit a prolonged lifetime.

In this study, we present experiments conducted in the “Barrel of Ilmenau”, the world-wide largest Rayleigh-Bénard laboratory experiment with 7.15 m in diameter and up to 6.3 m in height using air as working fluid. Neutrally-buoyant helium-filled soap bubbles were injected into the flow and tracked using four cameras. Illumination was carried out with five high-power LEDs in pulsed mode. Reconstruction of the Lagrangian velocity field was performed with Shake-The-Box algorithm. Thus, the major focus of the presented work was to get access to the entire volumetric Lagrangian velocity field in the convection cell on a very long time. Our aim is to discover very rare flow events in turbulent convection, like e.g. dust devil-like vortices or other turbulent superstructures. Measurement data was analyzed to conclude about the applicability of particle tracking velocimetry as suitable measurement technique for large volumetric flows. Finally, an example of a discovered dust-devil like vortex is presented in detail.