论文标题
双曲线纤细的PDE模型和保守的重力偏向饱和地下水流量的保守数值方法
A hyperbolic-elliptic PDE model and conservative numerical method for gravity-dominated variably-saturated groundwater flow
论文作者
论文摘要
当一个相比另一个相比更重,更粘稠时,理查兹方程通常用于表示不饱和多孔介质中的两相流体流动。但是,由于毛细管压力项中的退化,它无法描述某些毛细血管功能的完全饱和流动。从数学上讲,重力为主的变量饱和流很有趣,因为它们的控制部分微分方程从不饱和区域的双曲线转换为饱和区域的椭圆形。此外,润湿阵线的存在引入了强烈的空间梯度通常会导致数值不稳定。在这项工作中,我们开发了一个强大的多维数学模型,并在可忽略不计的毛细作用力的极限下实现了一种众所周知的有效和保守的数值方法。饱和区域中的椭圆问题通过仅使用不饱和细胞中固定的头部边界条件求解与饱和细胞相对应的还原系统有效地集成到我们的框架中。总而言之,这个耦合的双曲线纤维化PDE框架提供了高效的,基于物理的理查德方程的扩展,以模拟完全饱和的区域。最后,我们提供了一套易于实施但具有挑战性的基准测试问题,涉及一个和二维的饱和流。这些简单的问题伴随着它们相应的分析解决方案,可以被证明是代码验证,模型验证(V&V)的关键以及可变饱和流的模拟器的性能比较。我们的数值解决方案与提出问题的分析结果表现出了很好的比较。在分层的异质土壤中二维浸润的最后一个测试问题显示了多个断开饱和区域的形成和演变。
Richards equation is often used to represent two-phase fluid flow in an unsaturated porous medium when one phase is much heavier and more viscous than the other. However, it cannot describe the fully saturated flow for some capillary functions without specialized treatment due to degeneracy in the capillary pressure term. Mathematically, gravity-dominated variably saturated flows are interesting because their governing partial differential equation switches from hyperbolic in the unsaturated region to elliptic in the saturated region. Moreover, the presence of wetting fronts introduces strong spatial gradients often leading to numerical instability. In this work, we develop a robust, multidimensional mathematical model and implement a well-known efficient and conservative numerical method for such variably saturated flow in the limit of negligible capillary forces. The elliptic problem in saturated regions is integrated efficiently into our framework by solving a reduced system corresponding only to the saturated cells using fixed head boundary conditions in the unsaturated cells. In summary, this coupled hyperbolic-elliptic PDE framework provides an efficient, physics-based extension of the hyperbolic Richards equation to simulate fully saturated regions. Finally, we provide a suite of easy-to-implement yet challenging benchmark test problems involving saturated flows in one and two dimensions. These simple problems, accompanied by their corresponding analytical solutions, can prove to be pivotal for the code verification, model validation (V&V) and performance comparison of simulators for variably saturated flow. Our numerical solutions show an excellent comparison with the analytical results for the proposed problems. The last test problem on two-dimensional infiltration in a stratified, heterogeneous soil shows the formation and evolution of multiple disconnected saturated regions.