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

在干燥对流条件下和平坦地形下，近地面大气边界层（ABL）中经常形成强垂直轴涡旋。它们被称为尘暴，被认为对大陆气溶胶的产生有重大贡献。关于这些大气涡旋的起源和特征知之甚少。由于固定传感器提供的数据不足，而且遥感方法的分辨率不够，因此在自然环境中测量这些物质极为困难。为了模拟这样的结构，模型需要解决这两个问题，大尺度的大气边界层以及小尺度的耗散过程。这是不可能的，直到现在，尽管已经使用了具有超过10亿个网格点和2米空间分辨率的域。该项目旨在进行系统的数值研究，同时首次进行实验室实验，以确定涡旋发生的机制和最低条件，以及它们如何有助于热量和灰尘的垂直输送。本文采用德国汉诺威莱布尼茨大学气象与气候研究所开发的模拟软件PALM（Parallelized LES Model），对大涡模拟和直接数值模拟（DNS）进行了研究。与此同时，将在对流实验“伊尔梅瑙桶”中进行实验室实验，这是一个高8米、宽7米的圆柱形试验室，在伊尔梅瑙工业大学的热与流体动力研究所进行。在这个经典的瑞利-贝纳德实验中，封闭的空气从下面加热，从上面冷却（类似于在大气中），过去已经观察到了具有垂直轴的强烈漩涡。这些涡流将在本项目中进行测量和表征。这是第一次，实验数据允许直接比较和评估在瑞利数Ra= 10 ^[12]以下的DNS中获得的结果。此外，还将对瑞利数高达Ra=10^18的大气边界层进行大涡模拟研究，以保证DNS和实验室实验的结果可以转移到湍流大气区。为了克服以前的分辨率限制，在近壁区域的网格间距将进一步减少到0.1米，通过应用LES嵌套技术。申请人还将解决在巨大的LES数据集中识别移动涡旋的问题，特别是在其初始阶段，但也在其进一步演变期间。

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.