Visualization and modeling of the evaporation process using representative 2D Micromodels and universal scaling laws

蒸发是控制土壤水分收支的关键过程，其特点是大气边界条件与“固相-水-气”三相系统的流动和输运性质复杂地相互作用。中心目标是利用代表性的二维微模型和网络模型对真实土壤结构的蒸发过程进行实验和理论研究。三维孔隙结构通过微x射线计算机断层扫描（micro-CT）测量，并通过新的拓扑二维-三维转换映射到二维微模型。将实验结果与蒸发网络模型的理论结果进行了比较，该模型首次考虑了厚膜表面的蒸发和实验孔径分布。二维微模型具有内部和外部粗糙度，如天然土壤和松散岩石。首次将厚膜流动作为固体表面微观结构的函数进行了研究。因此，REV-微模型（REV：代表初等体积）分别采用硅（内部粗糙度为0.1微米）、玻璃（0.5微米）和玻璃陶瓷（3微米）制成。作为工作假设，假定(i)不同的大气条件（扩散流、层流和湍流；增加的潜在蒸发速率）引起不同的干燥动力学；（ii）临界水势取决于厚膜流和厚膜表面。研究了农业用地（富尔伯格田）的两种土壤类型：(i)沙土（a层）和（ii）中砂（c层）。采用微ct（最大分辨率5微米）和数学形态学方法（Minkowski函数）对孔隙结构进行了分析。在相关毛细数范围内，对水平和垂直水流进行了微观模型试验；(10 ^ 7…10 ^ 4;增加蒸发速率)。可视化实验用两台高分辨率单反相机（1800万像素）和一台荧光显微镜进行。厚膜动力学用“粗糙表面润湿”的标准理论来描述。所有实验(1)进行聚类分析，(2)根据通用标度定律计算重力相关长度，(3)确定干燥/位移锋的分形维数。基于一个具有代表性的网络模型，导出了连续介质理论的两个重要参数（REV-scale）——有效渗透率和有效扩散系数。预期的结果既具有基本的兴趣又具有很大的实际意义，因为它们提高了对过程的理解，从而提高了rev尺度上蒸发的预测建模。

Evaporation is a key process controlling the soil water budget and is characterized by a complex interplay of atmospheric boundary conditions and the flow- and transport properties of the 3-phase system "solid phase - water - gas". The central objective is the experimental and theoretical investigation of the evaporation process for real soil structures using representative 2D-micromodels and network models. The 3D-pore structure is measured by micro-X-ray computer tomography (micro-CT) and mapped to the 2D-micromodel by a new topological 2D-3D transformation. The experimental results are compared with the theoretical results of an evaporation-network model, which for the first time takes into account the evaporation over the thick-film surface and the experimental pore size distribution. The 2D-micromodels have both an inner and an outer roughness as natural soils and loose rock. For the first time the thick-film flow is investigated as a function of the microstructure of the solid surface. Therefore, REV-micromodels (REV: representative elementary volume) are produced in silicon (inner roughness = 0.1 micro-m), glass (0.5) and glass ceramic (3). As a working hypothesis it is assumed that (i) different atmospheric conditions (diffusive flow, laminar flow and turbulent flow; increasing potential evaporation rate) cause different drying dynamics and that (ii) the critical water potential depends on the thick-film flow and on the thick-film-surface. Two soil types from agricultural land (Fuhrberger Feld) are investigated: (i) sandy soil (A-horizon) and (ii) medium sand (C-horizon). The pore structure is analyzed using micro-CT (maximal resolution: 5 micro-m) and methods of mathematical morphology (Minkowski functions). The micro-model experiments are carried out for both the horizontal and the vertical water flow in the range of relevant capillary numbers; (10^-7 ... 10^-4; increasing evaporation rate). The visualization experiments are carried out with two high-resolution SLR cameras (18 megapixels) and a fluorescence microscope. The Thick-film dynamics is described by the standard theory of "wetting on rough surfaces". For all experiments (i) a cluster analysis is conducted, (ii) the gravitational correlation length is calculated by universal scaling laws, and (iii) the fractal dimension of the drying/displacement front is determined. Based on a representative network model the two important parameters of the continuum theory (REV-scale) - the effective permeability and the effective diffusion coefficient - are derived. The expected results are both of fundamental interest and of great practical relevance, as they improve process understanding and consequently the predictive modeling of evaporation at REV-scale.