Advances in Understanding Pore-Scale Dispersion

多孔介质是由固体和空隙（或孔隙）组成的任何材料。海绵、沙滩沙和炉过滤器是多孔材料（多孔介质）的常见例子。其他的例子还有石油储层、供应地下水的含水层，以及减少汽车尾气排放的催化转换器。随着计算机速度和能力的最新进步，科学家们现在有方法模拟和理解水和溶解物质以及小颗粒通过多孔介质的方式。这可以用来帮助制造具有特定有用品质的多孔材料，并了解天然多孔材料的行为。化学物质在多孔介质中流过复杂的孔隙网络时扩散的方式被称为分散。分散很重要，因为它会影响多孔材料的固体界面上化学反应和传质发生的速度。例如，对弥散的理解对于理解呼吸过程中血液是如何在肺部充氧至关重要。第二个例子是，了解扩散对于了解泄漏到地下的污染物的扩散和最终清理是很重要的。在本研究中，将收集真实多孔材料的三维x射线图像（很像医学CAT扫描），并使用最先进的计算方法模拟多孔介质内的分散过程。这些信息将有助于发展描述多孔材料弥散的新理论，并将在工程应用（如过滤）和基础科学（如了解我们的肺是如何工作的！）尽管多孔介质中的弥散问题已经被研究了100多年，但在基本连续介质力学的层面上，人们对多孔介质中弥散与流场复杂变化之间的相互作用仍未完全理解。直到最近才有可能在复杂的三维多孔介质的几何结构中进行流动的数值模拟，这些介质具有足够的尺寸和分辨率，从而产生将多孔介质中的微观物理过程与可观察到的材料宏观行为作为一个整体联系起来所需的数据。本研究将结合理论、数值计算和实验数据，以前所未有的详细程度研究单分散和多分散多孔介质中的分散过程。由于多孔介质中的分散与许多与实践水文工作者相关的地下过程有关（例如，生物修复、污染物扩散、河流和潜流带之间的质量传递），因此了解分散过程非常重要，这样我们就可以确定它如何影响地下的运输。这项研究也适用于水文学以外的工程系统（如色谱和催化）。目标是通过：1)以前所未有的分辨率实验测量（通过x射线断层扫描）随机多孔介质床；2)采用最先进的数值方法对这些多孔介质中的流动和分散过程进行直接数值模拟（通过稳定惯性流动范围）；3)发展改进的描述理论（通过体积平均法），从多孔材料的代表性结构预测分散过程的宏观行为。

A porous medium is any material consisting of solids and voids (or pores). Sponges, beach sand, and furnace filters are familiar examples of porous materials (porous media). Other examples are petroleum reservoirs, aquifers that supply groundwater, and catalytic converters that reduce car emissions. With the recent advances in computer speed and power, scientists now have methods to simulate and understand the way that water and dissolved matter, as well as small particles, move through porous media. This can be used to help fabricate porous materials with specific useful qualities and to understand the behavior of natural porous materials. The way that chemicals spread in porous media as they flow though the intricate network of the pores is called dispersion. Dispersion is important because it influences how quickly chemical reactions and mass transfer happen at the solid interfaces of a porous material. For example, understanding of dispersion is essential to understanding how blood is oxygenated in the lungs during breathing. As a second example, understanding dispersion is important for understanding the spread and eventual cleanup of contaminants spilled into the subsurface. In this research, 3-dimensional X-ray images (much like a medical CAT scan) of real porous materials will be collected, and state-of-the-art computational methods will simulate the dispersion process within the porous media. This information will be useful in developing new theories for describing dispersion in porous materials, and this will have potential utility in engineering applications (such as filtration) and in fundamental science (such as understanding how our lungs work!) Although dispersion in porous media has been studies for over 100 years, the interaction of diffusion with the complex variations in the flow field within a porous medium are still not understood at the level of basic continuum mechanics. It has only recently become possible to conduct numerical simulations of flow within the geometry of a complex, three-dimensional porous medium of sufficient size and resolution to generate the kind of data needed to connect the microscale physical processes within a porous medium with the observable macroscopic behavior of the material as a whole. This research will combine theory, numerical computation, and experimental data to examine the process of dispersion in monodisperse and polydisperse porous media at an unprecedented level of detail. Because dispersion in porous media is relevant to a host of subsurface processes relevant to practicing hydrologists (e.g., bioremediation, contaminant spreading, mass transfer between rivers and the hyporheic zone), it is important to understand the dispersion process so that we can determine how it affects transport in the subsurface. This research also has applications to engineered systems outside of hydrology (such as chromatography and catalysis). The goal is to bring new understanding about the dispersion process by: 1) experimentally measuring (via X-ray tomography) beds of random porous media at unprecedented resolution; 2) conducting direct numerical simulations of the flow and dispersion process in these porous media (up through the steady inertial flow range) using state-of-the art numerical methods; and 3) developing modified descriptive theory (via the method of volume averaging) to predict the macroscopic behavior of the dispersion process from representative structure of porous materials.