Study of Particle Movement Pattern in Vortex Settling Basin Based on Particle Tracking Technique

The quality of water required for agriculture, industry, drinking, etc., has made it necessary for the solids particle in the water flow to enter its permissible level in irrigation and drainage or urban water networks. The Vortex Settling Basin (VSB) makes use of the vortex flow inside the chamber for the separation of sediment particles from the flow.Elaborate studies have been made on investigation flow pattern at VSB that includes: (Paul, 1988 and 1991; Athar et al., 2003; Gheisi, et al, 2006; Ziaei et al., 2007; Chapokpour et al., 2011; Mulligan et al, 2016; Rehman et al., 2017; Huang et al., 2017; S. R. Nikou et al., 2021).Elaborate studies in hydraulic sciences apply particle tracking and image processing method for investigation ( Sun et al., 2015; Shin et al., 2016; Mulligan et al., 2016 and 2018; Rosberry et al., 2019; Witz et al., 2018; Duinmeijer et al., 2019).The investigation of vortex flow is very sensitive to measuring instruments, for example, ADV, which is the most common instrument for measuring the velocity, increases disturbance of flow. Therefore, it is recommended to use the non-interference particle tracking method to measure velocity components.

The experiments were performed in the hydraulic laboratory of the water science and engineering department at the Ferdowsi University of Mashhad, Iran on an acrylic laboratory setup. Spherical particles with a relative density of 1.41, at distances of 37 cm and 1.5 m from the chamber (sediment injection site in the study of Athar et al., (2003)) and in these two longitudinal distances, left at 9 points and each experiment is repeated 5 times (Figure 2).In this research, two iPhone 7 Plus cameras have been used for taking photos. The camera of this phone has one of the most advanced cameras in terms of expertise and technology. In this study, due to the high volume of data at different points, image processing is presented in the highest probability of trapping (point 4), the position of this point is as follows: 5 cm from the floor, 2 cm from its right wall (sloping to the chamber), 18 cm from the sloping wall to the outlet channel and 6.5 cm to the water level. The input flow to the channel is 8 and 13.7 l/s.

The highest probability of trapping for a particle at two longitudinal distances is at point 4 with a probability of 60%. The process of particle displacement and the time series of three velocity diagrams in the vortex settling basin of the present study are sinusoidal. In sections 150°-210° and 330°-30°, the particle is inclined toward the wall and in other sections, it is inclined toward the orifice, affected by the location of input and output channels (S. R. Nikou et al., 2021).There is a meaningful correlation between the two components vx and vy, and in almost all places where the x velocity component is extreme, in the same position y component is zero, and vice versa. This result is quite justifiable given the motion of vortex flow. The extreme values of the velocity component in the x direction become closer as they approach the orifice, indicating an increase in velocity near the orifice and the chamber floor and a smaller curved path around the vortex core. Notably, the absolute value of the maximum velocity in the x direction is 1.61 m/s and in the z direction is 0.13 m/s, which indicates that the particle tends to enter the orifice more in a rotating passage than falling position, having said that, centrifugal force is dominated over the action of dewatering. The mean relative error of water surface profiles by image processing method compared to laboratory data is estimated to be 0.002 and 1.36%, respectively, which confirms well with the experimental measurements.

The results showed that the distribution of the velocity components of the particle in all three dimensions has a sinusoidal trend. The higher value of the maximum velocity in the x-direction than the z-direction indicates the dominance of the centrifugal force over the dewatering operation in the vortex flow.According to the obtained results, particle tracking and image processing can be used as an accurate method with a higher operational speed to investigate the flow patterns and determine the water surface profile in vortex settling basins.