VARIATION IN THE SETTLING BEHAVIOR OF PARTICULATE SUSPENSION IN SATURATED GRAVEL LAYERS

Abstract

The gravitational settling of solid particles in saturated porous media, which is common in various engineering processes and natural phenomena, was investigated experimentally. This study examines the settling pattern of fine particles suspended in saturated porous media composed of gravel layers. The experiments were conducted by varying the physical properties of particles and fluid, particle concentration, and the pore scale of porous media. The three-dimensional settling behaviors of suspended particles in an invisible porous media were observed by using refractive index matching together with sheet laser scanning techniques. The experimental results showed that various settling patterns were shown depending on the particle size and concentration. Under conditions of large particle size with a small concentration, suspended particles settle individually through gaps between gravels in the direction of gravity with the velocity similar to that of an isolated particle. However, small particles with large concentrations settle collectively through the pore region as if they are one continuous fluid. In this case, the particles migrate in both vertical and lateral directions with the velocity much larger than the Stokes settling velocity. The settling velocity was quantified by image analysis of reconstructed three-dimensional images in porous media. The results indicated that the settling velocity of particles greatly depends on the settling pattern. Finally, we found that the dimensionless parameter, which describes the collectivity of suspended particles in porous media, quantitatively predicts both the transition of the settling pattern and the settling velocity.