EAGER: Exploratory Research on DNS Modeling of Turbulent Heat Transfer in Porous Media

多孔介质湍流传热的DNS建模多孔介质对流领域中最具争议的问题之一是宏观湍流问题，这是已知的高速流动增强传输的过程。目前尚不清楚宏观湍流是否会发生在流体饱和多孔介质中，如果是的话，它对对流传热有什么影响。在多孔介质中进行速度测量以确定湍流是否存在是极其困难的，因为它们通常是光学不透明的。同时，现在可以进行高清直接数值模拟（DNS）来研究这一现象。与大多数关于多孔介质湍流的现有工作不同，这些工作依赖于某些未经验证的湍流模型，DNS允许人们通过直接解决所有传输尺度来理解所有复杂性的现象。因此，本项目通过DNS研究了多孔介质中以大流速（高雷诺数）发生的过程，试图准确地描述它们并分析它们是否可以被标记为真正的湍流。这里的假设是孔隙的大小决定了湍流漩涡的最大大小。如果湍流涡流的大小不能超过孔隙的大小，那么多孔介质中的湍流现象可能与透明流体中的湍流现象不同。事实上，这种尺寸限制一定会对能量级联产生影响，因为在透明（多孔介质）流体中，湍流动能主要包含在大涡流中。该研究需要对各种2D和3D多孔介质几何形状进行广泛的DNS研究。以不同孔径的多孔介质为研究对象，研究固体壁面对多孔介质填充通道的影响。出现在多孔介质中的湍流结构将与出现在透明流体中的湍流结构进行比较；本文将分析它们对对流换热的影响。特别是，将研究从较大到较小涡流的动能级联，并分析含能涡流的频谱和大小，以揭示多孔介质流动中湍流能量的产生、传递和耗散机制。通过将温度建模为被动标量并研究瞬时温度分布，该项目旨在确定湍流对流中流体动力和热过程之间的联系。此外，将揭示构成固体多孔基质的大量固体障碍物对湍流动能的影响及其对温度场的影响。将考虑双分散多孔介质中第二雷诺数的可能性，这将通过与更大孔径相对应的湍流漩涡的出现而表现出来。将DNS结果与已发表的实验数据进行比较，以验证数值结果。

DNS Modeling of Turbulent Heat Transfer in Porous MediaOne of the most controversial topics in the field of convection in porous media is the issue of macroscopic turbulence, which is the known process of enhancing transport due to high speed flow. It remains unclear whether macroscopic turbulence can occur in a fluid saturated porous medium and, if so, what impact it has on convection heat transfer. It is extremely difficult to carry out velocity measurements within porous media to ascertain if turbulence exists on not in it, as they are typically optically opaque. At the same time, it is now possible to conduct high-definition direct numerical simulation (DNS) to study this phenomenon. In contrast to most existing work on turbulence in porous media, which relies on certain unverified turbulence models, DNS allows one to understand the phenomenon in all its complexities by directly resolving all the scales of transport. Hence, this project examines the processes that take place in porous media at large flow rates (high Reynolds numbers) via DNS, attempting to accurately describe them and analyze whether they can be labeled as true turbulence. The hypothesis here is that the size of the pores determines the maximum size of the turbulent eddies. If the size of turbulent eddies cannot exceed the size of the pores, then turbulent phenomena in porous media may differ from turbulence in clear fluids. Indeed, this size limitation must have an impact on the energy cascade, for in clear (of porous media) fluids the turbulent kinetic energy is predominantly contained within large eddies.The study entails carrying out extensive DNS studies for various 2D and 3D porous media geometries. Porous media with different pore sizes will be investigated to study the effects of solid walls bounding a porous medium filled channel. Turbulent structures that appear in a porous medium will be compared with those in a clear fluid; their effects on convective heat transfer will be analyzed. In particular, the kinetic energy cascade from larger to smaller eddies will be investigated, and the spectrum and size of energy-containing eddies analyzed to uncover the mechanisms of turbulence energy production, transfer, and dissipation in flow through porous media. By modeling temperature as a passive scalar and investigating instantaneous temperature distributions, this project aims at identifying connections between hydrodynamic and thermal processes in turbulent convection. Also, the effects of a large number of solid obstacles, constituting the solid porous matrix, on the amount of kinetic energy in turbulent flow will be unveiled and also how it affects the temperature field. The possibility of a second Reynolds number in a bi-disperse porous medium will be considered, which would manifest through the appearance of turbulent eddies corresponding to a larger pore size. Comparison of the DNS results with published experimental data will be performed to validate the numerical results.