Characteristics of displacement fluid fronts - from rapid interfacial jumps to steady capillary flows

多孔介质中的流体驱替是由压力松弛和力平衡引发的一系列快速界面跳跃和流体重新分布。在这些快速动力学之后，通常会有一段时间的界面钉扎，然后才会出现新的流体侵入。这些不连续的过程不能用基于Richards方程的标准宏观水流公式来表示，其中水流受白金汉-达西定律驱动。因此，我们建议开发和测试其他模拟策略(改进的孔隙网络中的侵入渗流；泡沫排水方程)来再现孔隙尺度上的相分布和驱替过程。在驱替前锋后面的区域，流体可以沿着稳定的界面发生，表现出白金汉-达西流动和理查兹方程所描述的流动状态。对于不同的过程(渗透、渗吸、蒸发和排水)和多孔介质，我们计划研究Richards方程失效的条件，估计Richards方程的过渡长度和时间以及适用的阈值。这些极限将在实验系列中使用高速相机和中子成像以及理论考虑来确定。在与SP1子项目的合作中，我们的建模策略将通过与格子-Boltzmann模拟的比较来测试。将为子项目SP6和SP7提供实验结果，以确定修正的放大方案，并为SP1和SP2验证多孔介质与大气之间的耦合方法。

Fluid displacement in porous media takes place as a sequence of rapid interfacial jumps with fluid redistributions triggered by pressure relaxation and force balance. These rapid dynamics are often followed by a period of interfacial pinning before a new cascade of fluid invasion occurs. These discontinuous processes cannot be represented by standard macroscopic formulation of water flow based on Richards equation with flow driven by Buckingham-Darcy law. Therefore, we propose to develop and test alternative modeling strategies (modified invasion percolation in a pore network; foam drainage equation) to reproduce phase distribution and displacement processes at pore scale. In the region behind a displacement front, fluid flow may take place along stable interfaces exhibiting flow regimes described by the Buckingham-Darcy flow and the Richards equation. For different processes (infiltration, imbibition, evaporation and drainage) and porous media, we plan to investigate conditions where Richards equation fails, providing estimates for transition length and times and thresholds for applicability of Richards equation. These limits will be determined in experimental series using high speed camera and neutron imaging and theoretical considerations. In collaboration with subproject SP1, our modeling strategy will be tested by comparison with Lattice-Boltzmann simulations. Experimental results will be provided for subprojects SP6 and SP7 to define modified upscaling schemes, and SP1 and SP2 to validate the coupling approaches between porous medium and atmosphere.