Dynamic Nonequilibrium During Unsaturated Water Flow
Dynamic Nonequilibrium During Unsaturated Water Flow
复制标题
不饱和水流期间的动态非平衡
DOI：
--
复制
发表时间：
1999
期刊：
影响因子：
--
通讯作者：
B. Schultze
中科院分区：
文献类型：
--
作者：
B. Schultze
传统上，持水特性是在平衡或稳态流动条件下测量的，并假设所确定的特性可用于现场条件下的瞬变水流模型。然而，用常规方法和用瞬变方法获得的滞留数据的比较表明，存在“动态效应”，表现为静态和动态水力功能之间的差异。本文回顾了以往关于动态效应的证据，说明了在多步出/入流实验中动态效应的存在，并讨论了一个专门为量化动态效应而设计的实验。最后，与经典的Richards模型和两相流模型的模拟结果进行了比较，结果表明，气流阻力的大小在很大程度上是由气流阻力引起的。一般假定多孔介质的水力性质是静态的，即只取决于多孔介质中孔隙的分布和几何排列以及湿/干历史(滞后)，前提是固体基质是刚性的，流体性质是恒定的，外部条件不随时间变化。这意味着，在单调排水或吸胀过程中，水分含量θ和基质势ψ之间的关系不受水流动力学的影响，而θ(ψ)是由一个独特的特征来表示的。传统上，持水特性是在平衡条件下在实验室中测量的，通常认为所确定的特性可以用于模拟现场条件下的瞬变水流。然而，动态方法和常规方法获得的排水滞留数据的比较表明，在动态情况下，在给定的吸力处会有更多的水被保持。根据Klute[1986]的说法，“这种观察的原因和影响仍然是一个调查的主题。”如果瞬变水流过程中的含水率和水势之间发生动态不平衡，那么目前使用的描述饱和/非饱和水流的标准模拟技术在瞬变水流条件下就不能可靠地应用。本文的目的是研究瞬变流试验中的动态效应，这些试验通常用于水力参数的反模拟识别。与过去的大多数调查相反，我们使用未受干扰的柱子和
Traditionally, the water retention characteristic is measured under equilibrium or steady-state flow conditions, and it is assumed that the so-determined properties can be used to model transient water flow under field conditions. However, comparison of retention data obtained with the conventional method and using transient methods indicate the presence of “dynamic effects” which show up as differences between static and dynamic hydraulic functions. This paper reviews previous evidence of dynamic effects, illustrates the presence of dynamic effects during a multistep outflow/inflow experiment, and discusses an experiment specifically designed to quantify dynamic effects. Finally, a comparison of simulations with the classical Richards model and a two-phase flow model show that dynamic effects are to a considerable part caused by non-negligible resistances of air flow. INTRODUCTION Hydraulic properties of porous media are generally assumed to be of a static nature, i.e., to depend only on the distribution and geometric arrangement of pores in the porous medium and on the wetting/drying history (hysteresis), provided the solid matrix is rigid, fluid properties are constant, and external conditions do not change with time. This implies that the relationship between water content, θ, and matric potential, ψ, during monotonous draining or imbibition processes is not affected by the dynamics of water flow, and that θ (ψ) is expressed by a unique characteristic. Traditionally, the water retention characteristic is measured in the laboratory under equilibrium conditions, and it is generally assumed that the so-determined properties can be used to model transient water flow under field conditions. However, comparison of drainage retention data obtained with dynamic methods and using the conventional approach indicate that more water will be held at a given suction in the dynamic case. According to Klute [1986], "the reasons for and implication of this observation continue to be a subject of investigation." If dynamic nonequilibrium occurs between the water content and the water potential during transient water flow, the presently used standard simulation technique for describing saturated/unsaturated flow cannot be reliably applied under transient flow conditions. The aim of this paper is to investigate dynamic effects during transient flow experiments, which are generally used to identify hydraulic parameters by inverse simulation. Contrary to most investigations in the past, we use undisturbed columns and