Naoyuki WATANABE

Direct observation of porous media flow focusing on particle structure

Yutaka FUKUMOTO

 Porous materials in the field of civil engineering include soil, rock, concrete, asphalt, etc. The aging of these materials is closely related to the movement of permeate flow within them. In other fields of porous materials, such as fuel cells, biological tissues, filters, membranes, and fibers, understanding the behavior of internal fluid flow can provide important engineering insights.　

 Tortuosity is an index used to observe the behavior of seepage flow. The degree of flexure is correlated with the diffusion coefficient and hydraulic conductivity in the porous media, and is an important index for understanding the internal behavior of the porous media in terms of mass transfer. However, the methods to measure the tortuosity are mainly based on geometrical definitions or numerical simulations, and there are few cases where actual experiments are conducted to measure the numerical values. In addition, most empirical formulas for tortuosity are functions of porosity and surface area, and do not reflect information on the local geometric structure.

 In addition, since the internal erosion phenomenon caused by seepage flow occurs within the ground, it is not possible to directly confirm the extent of damage. In addition, the interaction between geotechnical engineering, such as erosion and flow of soil particles, and fluid engineering, such as flow in porous media, is not well understood because of the lack of fundamental knowledge. In terms of the safety of civil engineering structures such as embankments, the emphasis has been placed on external damage such as overtopping and slides, but improving safety against internal erosion damage due to seepage flow is also an important issue.

In order to improve the safety of the soil skeleton, an experimental method to visualize the seepage flow inside the skeleton, which cannot be directly observed, has been established by using refractive index matching technology (RIMS) through the research until last year. 

In the last fiscal year, experiments were conducted on the behavior of seepage flow in one-layer and two-layer particle configurations. Especially at the boundary in the two-layer structure, the experimentally determined hydraulic tortuosity was found to be independent of changes in the filling ratio. In the present study, we focused on the particle structure by changing the structure of transparent sand, which was simulated as a soil structure, and compared the results with those of previous studies for each structure, mainly by analyzing the hydraulic tortuosity at the boundary of each layer and each particle structure.