Investigating Sedimentation Behavior of Montmorillonite Flocs between Flat Plates in a 2D System Using Image Analysis

Abstract

The sedimentation of flocs in aquatic environments is a fundamental phenomenon that has not yet been fully elucidated. This study quantitatively examines sedimentation behavior, particularly focusing on sedimentation turbulence, in a two-dimensional system between flat plates, utilizing image analysis. Experiments were conducted in a rectangular container with montmorillonite suspensions coagulated in a sodium chloride solution. The settling motion of flocs was visualized using a green laser from above and captured horizontally with a digital camera. The study employed Particle Image Velocimetry (PIV) to analyze the velocity field in floc sedimentation, using the flocs as tracers to calculate the mean velocity at the sediment–supernatant interface. The results showed that the mean PIV value is affected by rising particles caused by sedimentation turbulence, indicating that PIV analysis of flow fields using flocs as tracers is reliable. The maximum settling velocity was found to increase with the initial interface height and the thickness of the container. The study further notes that flow velocity fluctuations increase during rapid sedimentation, marked by repeated collisions, separation, and the flocculation of variably sized flocs, offering a clear explanation of sedimentation turbulence. Additionally, Fourier analysis of vertical spectra in the container reflects the formation and collapse of flocs.

Hydrodynamic adjustment of mean flow and turbulence around a sinking boulder during local scouring

“…Current studies are mostly based on clear water, xed-bed or eld experiments (Papanicolaou et al, 2012;Baki et al, 2015), and there are few studies on the ow structure of boulder reaches under dynamic beds. This paper analyzes the changes in turbulence and bed shear stress in the instantaneous ow eld (Lee & Hsu, 1994;Nezu and Sanjou, 2011;Roknujjaman, et al, 2024) and explain the change in water ow structure in the process of boulder scouring through experiments to more clearly reveal the interaction mechanism between boulders and water ow under a moving bed.…”