Genesis and Features of Dust Devil Like Vortices in Convective Boundary Layers - A Comparative Study Using DNS/LES and Laboratory Experiments

点击翻译按钮获取中文摘要

Intense vortices with vertical axes frequently form in the near surface atmospheric boundary layer (ABL) under dry convective conditions and in flat terrain. They are known as dust devils and are believed to significantly contribute to the production of continental aerosol. Only little is known about the origin and features of these atmospheric vortices. They are extremely difficult to measure in their natural environment since stationary sensors provide insufficient data and remote sensing methods do not have sufficient resolution. In order to simulate such structures, models are required resolving both, the large scales of the atmospheric boundary layer as well as the small scales of the dissipative processes. This was impossible until now although domains with more than one billion grid points and 2 m spatial resolution have been used. The project aims to perform systematic numerical studies, accompanied - for the first time - by laboratory experiments, to determine the mechanism and minimum conditions under which the vortices occur, and how they contribute to the vertical transport of heat and dust. Based on the simulation software PALM (PArallelized LES Model) developed at the Institut fuer Meteorologie und Klimatologie of Leibniz Universitaet Hannover both, LES and direct numerical simulations (DNS) will be carried out. Concurrently, laboratory experiments will be undertaken in the convection experiment "Barrel of Ilmenau" - an eight meters high and seven meters wide, cylindrical test cell operated at the Institut für Thermo- und Fluiddynamik at Technische Universitaet Ilmenau. In this classical Rayleigh-Bénard experiment in which confined air is heated from below and cooled from above (similar as in the atmosphere), intense vortices with vertical axes have been already observed in the past. These vortices will be measured and characterized in this project. For the first time, this experimental data permits a direct comparison with and an evaluation of the results obtained in the DNS for Rayleigh numbers up to Ra=10^12. Additionally, LES studies for the ABL with Rayleigh numbers up to Ra=10^18 will be carried out to guarantee that results from DNS and the laboratory experiment can be transferred to the turbulent atmospheric regime. In order to overcome previous resolution limitations, the grid spacing in the near wall region will be further reduced down to 0.1m by applying an LES nesting technique. The applicants will also tackle the problem of the identification of the moving vortices within the huge LES data sets, especially in their initial phase, but also during their further evolution.