## 4 Discussion  
At present, the inclined-tube settling tank has been widely used in the Yellow River irrigation region with a sediment removal rate of up to 75%, which is significantly higher than that of the settling tank with a conventional structure [33; 34]. In this paper, the ADV was used to measure the flow field in the inclined-tube settling tank. It was found that the flow pattern in the tank was stable, and the flow direction was consistently directed to the downstream in the SSA and ISA. The large circulation zone was not found in the tank, so the short-circuiting phenomenon didn't occur, which was conductive to enlarge the effective sedimentation volume in the tank [35; 36; 37; 38; 39]. The high removal efficiency of fine sediment was closely related to the inclined tubes in the tank with functions of shortening the settling distance and expanding the settling area, but also other structures in the tank, with a great effect on adjusting the flow pattern and improving the settling efficiency. The block effect of the flow adjustment board on the flow could effectively reduce the turbulence intensity and the kinetic energy [40; 41], so when the high-velocity water from the inlet flowed through the flow adjustment board, the velocity became uniform, it maybe help to accelerate the sediment settling and improve the settling efficiency. After the water flowed out of the inclined tubes, it maintained a stable pattern (see Figure 5a), contributing to the sediment settling in this area. This might be the result of the laminar flow in the inclined tubes [13; 42; 43], making the flow pattern stable in the upper ISA. The flow turbulence intensity of the inclined-tube settling tank under the conditions in this paper contributed to the sediment settling. The fine sediment absorbed the turbulent kinetic energy of the flow during the movement, promoting the random turbulence of the sediment particles, increasing the collision probability of sediment particles [44; 45], many research works have found that appropriate turbulence intensity was conductive to the formation of flocs and accelerating the sediment settling [46; 47; 48]. The Reynolds shear stress had a dual effect on the sediment flocculation [49]. On the one hand, the increase of the flow shear strength would increase the collision probability among sediment particles and promote the occurrence and development of the flocculation; on the other  
Figure 6: Distribution of vertical flow characteristics: (**a**) velocity (the contour plot represents the vertical velocity \(w\), and the vector represents velocity \(u\)-\(v\)); (**b**) dimensionless turbulence intensity \(I_{Z}\); and (**c**) dimensionless Reynolds shear stress \(R_{YZ}\). (The unit of velocity is m/s, and the dimensionless turbulence intensity and dimensionless Reynolds shear stress is dimensionless). hand, when the flow shear strength exceeded the internal cohesive force among the flocs, the increase of the shear strength would destroy the large flocs with loose structure, which was not beneficial to the formation of the flocs. By means of the vibration test of a single grille, [PERSON] et al. found that when the Reynolds shear stress was in the range of 3-19 s\({}^{-1}\), the Reynolds shear stress of water would promote sediment flocculation [50]. In the inclined-tube settling tank in this paper, the Reynolds shear stress in most areas was within the range, which was beneficial to the sediment flocculation and to settling at the tank bottom. However, some structures in the inclined-tube settling tank had a negative effect on the flow pattern and were not conducive to the sediment settling. The inlet had a great influence on the flow characteristics in the settling area [51]. In this paper, the flow velocity from the water inlet was high, which not only washed away the sediment that had been deposited at the tank bottom, but also affected the structural stability of the tank, which was not conducive to the long-term safe operation of the tank.