## 2 Applications of RIM
### Hydraulic/Mechanical Engineering: PorousMedia-Fluid(Surface or Pipe Flow) Interaction  
Porous media flow is prevalent in nature, engineered materials, animals, and plants. There are various types of porous media, including rocks (such as all kinds of mineral deposits), soils, vegetation, biological materials, and artificial porous media materials. It is essential to get a proper understanding of interaction characteristics when fluids flow over or through porous media because systems like this are utilized in a variety of engineering applications. For example, porous media systems can be found in topics as diverse as gravel beds of natural streams in hydraulic engineering [32; 33], turbomachinery design in aerodynamics [34], fuel cells in the energy field [35], and pebble bed reactor cores [36]. In the realm of hydraulic engineering, the interactions between water and soil hold significant importance. Natural soil is a three-phase system consisting of solids, liquids, and gases. Surface water flows, when seeping through the pores of the soil, play a crucial role in regulating water movement and the transformation of aqueous solutes. In the past, engineering focused mainly on the macroscopic motion and characteristics of seepage flow, measuring average medium properties such as porosity, permeability, average velocity, and fluid pressure. For example, most previous studies in the literature [37; 38; 39; 40; 41] have primarily described the seepage effect on main flow or sediment transport over a porous bed based on macroscopically averaged seepage flow velocities or turbulence characteristics. This is partly because the microscopic features of pore flow were difficult to measure. Due to differences in pore structure, flow characteristics vary significantly with location. Any type of invasive measuring technique may significantly change the flow characteristics in the local porous media domain. Therefore, non-invasive optical techniques prove to be a suitable method for measurements. Furthermore, with the aid of the RIM technique for transparency of simulated soils, nowadays researchers have been able to capture the flow field hidden in the porous media. Consequently, it can be anticipated that more precise equations may be proposed in the future to account for the effect of localized flow on sediment transport. This will be followed by a review of several RIM materials and typical applications of RIM-based flow visualization in porous media flow. Glass beads are frequently utilized as a solid material in the examination of fluid flow in porous media due to their transparency and ease of access. [PERSON] and [PERSON] (2018) used glass beads to investigate turbulent flows over and through rough porous beds. The study utilized the RIM technique to align the refractive index of the solid beads with that of the interstitial fluid. PIV was used to visualize and measure flow within the porous matrix. Careful handling and preparation of the glass beads is crucial to maintaining the integrity of the porous structure and ensuring accurate flow measurements. Due to the intellectual property issues involved, it is not possible for us to present the images of the RIM effect from the cited papers directly here, but we have purchased solid and liquid materials for validation. RIM tests for glass beads were performed following the methodology described in this article, as shown in Figure 1. The results demonstrate the significant potential for developing RIM-based optical methods for flow visualization in porous media. Hydrogel beads show promise as a material for investigating porous media flow due to their optical transparency and environmental friendliness. In their research, [PERSON] et al. [PERSON] et al. (2018) used hydrogel beads made of superabsorbent polymers that expand upon hydration to imitate the refractive index of water. This characteristic enabled the observation of internal flow fields in the porous medium, providing insights into the behavior of interstitial flows in granular media.