## 1 Introduction  
The settling tank, which separates particles from the sandy water of the tank by the force of gravity, is the most conventional and common environment-friendly project and plays an important role in the sustainable utilization of sandy water. At present, the design of the settling tank with a conventional structure for removing coarse sediment has become mature. [PERSON] et al. [1] proposed a circular-ring desilting and sediment ejection basin, whose sediment removal rate may be up to 100% for sediment with particle size of over 1 mm; [PERSON] [2] designed a gravity desilting filter tank suitable for Xinjiang region, China, with a range of permissible granule diameter of 0.1-0.15 mm when the sediment is out of the tank with the overflow; [PERSON] et al. [3] enhanced sedimentation by slowing down velocity of the flow from the inlet using parallel retrofit baffles; [PERSON] et al. [4] presented a circular settling tank with peripheral inflow and central effluent. However, the settling velocity of the fine sediment in the water is low, and the settling process is easily affected by the flow turbulence. Therefore, to efficiently remove the fine sediment, the conventional tank tends to cover a large area and can be costly. First proposed by [PERSON] [5], inclined tubes could produce an ideal flow pattern, reduce the settling distance and expand the settling area [6]. The results of [PERSON] et al. [7], [PERSON] et al. [8] and [PERSON] et al. [9] showed that inclined tubes could significantly accelerate the solid-liquid separation efficiency. Compared with the conventional structure, the inclined-tube settling tank has great advantages in fine sandy water treatment. The Yellow River is the main source of irrigation water in the Yellow River Irrigation Region. The particle size of most sediment in the Yellow River is 50-100 \(\upmu\)m, so Yellow River water is typical fine sandy water, which can easily cause the blockage of drip irrigation emitters [10,11]. In order to avoid the above problem and promote the sustainable utilization of Yellow River water, a kind of inclined-tube settling tank has been used in the Yellow River Irrigation Region and works very well in practice. However, there are a few research works about the inclined-tube settling tank. [PERSON] et al. [12] tested the removal efficiency of different kinds of suspended solids in the tank. [PERSON] et al. [13] and [PERSON] [14] proposed the calculation formula of sediment removal efficiency in the tank by theoretical analysis. However, there is still a lack of sufficient research on the inclined-tube settling tank, and there are few research works on the flow characteristics in the tank, which limits the improvement of the structure and the settling efficiency of the tank. The settling efficiency of the settling tank mainly depends on the movement characteristics of the suspended solids and flow in the tank [15]. When the sediment concentration is low and the particle size is extremely fine, the suspended solids have no effect on the flow movement, and the sediment movement track is closely related to the flow pattern [16]. The flow in the tank is turbulent, directly affecting the sediment concentration and deposition [17,18], so it is very important to understand the flow characteristics to improve the efficiency of the sediment settling. Researchers have always attached great importance to the observation of the flow characteristics in the tank. Water-sediment movement in the tank can be directly observed by the numerical simulation, and the mathematical model for the simulation has greatly developed in the past 50 years [19, 20, 21, 22, 23]. However, due to the lack of experimental data, these mathematical models have not yet been completely validated, so the numerical model has not been widely applied to the design of the actual tank. Rather than numerical simulation, many authors were therefore still in favor of describing the flow field with experimental results by measurement instruments.