Barrel of Ilmenau: a large-scale convection experiment to study dust devil-like flow structures

We present an experimental facility for the validation of numerical simulations on atmospheric dust devils in a controlled laboratory experiment. Dust devils are atmospheric air vortices with a vertical axis, and are formed by intense solar radiation and the resulting vertical temperature gradient. The structure of a typical dust devil is dominated by a radial inflow near the surface and a vertical upward flow within the vortex. These vortices have been studied in recent years using field observations, in situ measurements, and large-eddy simulation (LES). Field tests suffer from the limited area and their unpredictable behavior, while the LES approach cannot resolve the dust devils well enough. Dust devil-like structures may also occur in direct numerical simulation (DNS) with a Rayleigh number of at least Ra = 10^7 in Rayleigh-Bénard convection, with the advantage that the structures can be resolved more precisely. In order to validate the DNS approach and provide measurement data, the airflow is measured inside of a large-scale Rayleigh-Bénard cell of similar geometry (i.e. inside the Barrel of Ilmenau) to the DNS set-up for Rayleigh numbers from Ra = 10^6 to Ra = 10^12. For the measurement of the flow in a large volume, an optical measurement method is used to obtain the trajectories of single particles. Since there are no commercial systems that are suitable for such a large measurement volume, we developed our own system

Wärmetransport in turbulenten Konvektionsströmungen

Abschlussarbeit zur experimentellen Untersuchung des Wärmetransportes in turbulenten Konvektionsströmungen, studiert an einem großskaligem Rayleigh-Bénard-Experiment mit dem besonderem Schwerpunkt auf der Charakterisierung des Geschwindigkeits- und Temperaturfeldes in der Nähe der Kühlplatte, der Analyse der kohärenten Oszillationen in den Zeitreihen beider Größen und der Bestimmung von globalen Ähnlichkeitsparametern in ihrer Abhängigkeit von den thermischen und den geometrischen Randbedingungen.Auch im Buchhandel erschienen: Wärmetransport in turbulenten Konvektionsströmungen / Ronald du Puits. Aachen: Shaker, 2008 DOI: 10.2370/436_282 Preis: 15,00

Viscous boundary layers in turbulent Rayleigh-Bénard convection

Highly resolved local velocity profiles inside the boundary layers in turbulent Rayleigh-Bénard convection in air are presented and discussed. The present work makes progress to our work in the past (see du Puits & Resagk, 2007) that our actual set-up permits the measurement of the wall-normal velocity component w up to a distance of 200 mm away from the wall. All component profiles were performed in a cylindrical box with an aspect ratio Γ = 1, a Prandtl number Pr = 0.7 and Rayleigh numbers Ra = 3 × 10 9 , Ra = 3 × 10 10 . We compare the experimental results with numerics at Ra = 3 × 10 10 directly. We found that the profiles of mean velocity from both experiments and numerics collapse very well with each other and both of the mean horizontal velocity profiles differ from the laminar Blasius prediction at the boundary layer. The wall-normal mean velocity at the central window tends to zero in both experiment and numerics

Thermal boundary layers in turbulent Rayleigh-Bénard convection at aspect ratios between 1 and 9

We report highly resolved temperature measurements in turbulent Rayleigh–Bénard convection in air at a fixed Prandtl number Pr = 0.7. Extending our previous work (du Puits et al 2007 J. Fluid Mech. 572 231–54), we carried out measurements at various aspect ratios while keeping the Rayleigh number constant. We demonstrate that the temperature field inside the convective boundary layers of both horizontal plates is virtually independent on the global flow pattern accompanying the variation in the aspect ratio. Thanks to technical upgrades of the experimental facility as well as a significant improvement of the accuracy and reliability of our temperature measurement — and unlike in our previous work — we find that the measured profiles of the time-averaged temperature field neither follow a clear power-law trend nor fit a linear or a logarithmic scaling over a significant fraction of the boundary-layer thickness. Analyzing the temperature data simultaneously acquired at both horizontal plates, various transitions in the cross-correlation and the auto-correlation function of the temperature signals are observed while varying the aspect ratio Γ. These transitions might be associated with a change in the global flow pattern from a single-roll mode at Γ = 1 toward a double- or a multi-roll mode pattern at higher aspect ratios

Temperature profiles measurements in turbulent Rayleigh-Bénard convection by optical fibre system at the Barrel of IImenau

Modelling of large-scale natural (thermally-generated) turbulent flows (such as the turbulent convection in Earth’s atmosphere, oceans, or Sun) is approached in laboratory experiments in the simplified model system called the Rayleigh-Bénard convection (RBC). We present preliminary measurements of vertical temperature profiles in the cell with the height of 4:7 m, 7:15m in diameter, obtained at the Barrel of Ilmenau (BOI), the worldwide largest experimental setup to study highly turbulent RBC, newly equipped with the Luna ODiSI-B optical fibre system. In our configuration, the system permits to measure the temperature with a high spatial resolution of 5mm along a very thin glass optical fibre with the length of 5m and seems to be perfectly suited for measurement of time series of instantaneous vertical temperature profiles. The system was supplemented with the two Pt100 vertically movable probes specially designed by us for reference temperature profiles measurements