37,435 research outputs found

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

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    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-BĂ©nard 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-BĂ©nard 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

    Viscous boundary layers in turbulent Rayleigh-BĂ©nard convection

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    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
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