Experimental study of the settling of twin spherical particles released side by side: The impact of particle size, fluid viscosity, initial spacing, and particle density

The settling of solid particles in a fluid is an important process that needs to be considered in many fields of research. For example, the interactions among particles and between particles and the surrounding fluid are important topics in studying suspended sediment transport and water clarification. In this paper, the settling processes and interactions of twin spherical particles released side by side were experimentally studied. The Reynolds number varied in the range of 1–300, which is within the transition zone. Particle Tracking Velocimetry (PTV) and Particle Image Velocimetry (PIV) were utilized to capture the settlement trajectory, and provide insight into the flow fields around the particles. The influences of particle size, fluid viscosity, initial spacing, and particle density on the settling process were systematically investigated. The experimental results reveal that the initial spacing between the twin particles ( l 0 ∗) and the Reynolds number ( Re ) are the two most important factors affecting particle settling. The interaction between particles comprises only repulsion when the initial spacing is small, while the density of particles has little effect on the final settling state when the initial spacing is not very small. The flow fields around different particles are similar for the same Re , leading to similar final settlement behaviors, except for the case of l 0 ∗ = 0, when the influence of particle rotation cannot be ignored. Except for the case of l 0 ∗ = 0, although the particle density has little effect on the final settling behavior, it affects the repulsive process during settling. The final repulsive distance between the twin particles is highly dependent on Re and l 0 ∗. Two critical Re values exist (∼10 and ∼100), where the repulsive distance is negatively correlated with Re when Re < 10, but it is positively correlated with Re when Re > 100. However, the repulsive distance is always negatively correlated with l 0 ∗. These findings can improve understanding of more complex phenomena such as the particle group settling process and sediment transport.