Circular aquaculture tanks have excellent flow patterns, no dead space, uniform water exchange, and are easy to produce secondary flows, enabling residual bait and manure and its solid particles to converge to the bottom of the central outfall and discharge smoothly(Zhang et al., 2022b). The effect of sewage collection in a circular aquaculture tank is directly related to the water quality in the tank, which is the key issue in factory recirculating aquaculture. The jet pipe is a common hydraulic drive equipment for factory recirculating water aquaculture, and it is a widely used hydraulic drive device for actual factory aquaculture tank(Oca and Masalo, 2013). Its power comes from the pump drive, and its working principle is the same as that of submersible thruster (Hu et al., 2022; Tang et al., 2023) and waterwheel type oxygenator (Ali, 2013), both of which drive the water movement to realize sewage collection. However, if the jet pipe is not properly laid, it will result in dead space for sewage collection near the wall of the factory farming tank; the water flow velocity in the dead space is low, and the sewage deposited at the bottom of the farming tank is difficult to remove, with little dissolved oxygen. Rational design is the key to achieving optimal flow field flow, effective circulating water exchange, uniform feed distribution, good water quality, and low energy consumption in the Qinghai Lake naked carp factory aquaculture plant. Therefore, the flow rate Q of the jet pipe, the deployment distance ratio d/r (d is the distance between the midpoint of the jet pipe and the nearest tank wall, and r is the radius of the circular aquaculture tank), the deployment angle θ, and the jet pipe structure are four factors that need to be focused on.
The study results in this paper show that for the transverse single pipe with single-hole jet, the best sewage movement and accumulation situation was observed when the flow rate Q along the circular aquaculture tank was 7 L/s, the deployment distance ratio was 1/4, and the deployment angle θ was 0°. This is almost consistent with the results of (Zhu et al., 2021), who used particle image velocimetry (PIV) to measure the flow field in a circular aquaculture tank under different inlet pipe settings through model tests to investigate the effect of the deployment distance and deployment angle of the jet pipe on the hydrodynamic characteristics of a two-pipe inlet circular recirculating water aquaculture tank. The results indicated that the average flow velocity was high and did not change significantly when the distance of the inlet pipe was d = 1/4 r and the angle of the inlet pipe was α = 0°~40°. The configuration of the experimental set-up is consistent with that reported in (Oca and Masalo, 2007, 2013), with the main vortex formed occupying most of the rotating area of the rotating unit when L/W was increased from 0.95 to 1.43, without creating a significant dead volume in the slot. Based on CFD technology, (Hu et al., 2021) investigated the flow field distribution characteristics and the movement characteristics of solid particles in a circular circulating water aquaculture tank by using the STAR-CCM + numerical simulation software, and it was found that the average flow velocity in the tank was larger and the hydrodynamic characteristics were better when the inlet pipe angle θ = 0° and the distance ratio d = 3/8 r. These results are in general agreement with those of this paper. The layout angles in the above study were all around 0°, and the small differences in the layout distance ratios were most likely caused by different test conditions.
(Xue et al., 2021; Zhang et al., 2022b)studied the hydrodynamic characteristics of the inlet diameter ratio on the rectangular round-tangent angle aquaculture tank under the single pipe inlet mode by adopting numerical simulation software. In their study, the inlet diameter ratio of the aquaculture tank (parameter C/B, where C is the horizontal distance from the jet hole location to the aquaculture tank, and B is the side length of the short side of the aquaculture tank) was set from 0.02 to 0.04 to facilitate the best flow field conditions for the aquaculture tank system with a square round-tangent angle. In the study conducted by (Gui et al., 2020). on the flow field characteristics of a single channel square aquaculture tank by optimizing the arc angle, it was found that the relative arc width ratio set at 0.20 to 0.25 led to a better flow field and higher space utilization. (Zhang et al., 2022a). A numerical simulation model was developed with reference to the experimental model to investigate the hydrodynamic performance of single-channel recirculating aquaculture systems with different space utilisation. A comprehensive performance index is proposed, which can effectively evaluate the space utilisation and hydrodynamic characteristics of different culture tanks. The results show that:the turbulence intensity of the flow field gradually decreases as the radius of the tank fillet increases, (Gorle et al., 2020; Summerfelt et al., 2000) used CFD to simulate the effects of the tank geometry and tank inlet and outlet structure on the particle flushing and hydraulic mixing performance of the aquaculture tank. In actual production and physical model tests, θ = 0°, θ = 20°, and 45° are the three most common inlet angles that lead to a good sewage collection effect in the circular recirculating water aquaculture tank, and this study focuses on these three angles. The best sewage collection effect in the circular aquaculture tank was achieved when the vertical single pipe multi-hole jet pipe was set up along the circular aquaculture tank with a flow rate Q of 7 L/s, a layout distance ratio of 1/4, and a layout angle θ = 45°. This is consistent with the results obtained by (Zhao et al., 2017)). in their study on the flow field characteristics and the sewage aggregation effect at the bottom of a square circular angle aquaculture tank by a control variable method to investigate the jet velocity of the jet pipe and the jet angle. It was found that in the dual jet pipe drive mode, when the jet angle of the tubular jet pipe was about 45°, the sewage aggregation was the highest.
When the flow rate Q was 9 L/s, the deployment angle was 45°, the deployment distance ratio was 1/4, in the vertical single-tube multi-aperture flow field distribution test, the average flow velocity reached 0.0653 cm/s, the standard deviation took the minimum value of 0.0213, and the flow field uniformity was the best. The average flow velocity and flow field uniformity are two key factors affecting the hydrodynamic characteristics of the aquaculture tank. This is almost consistent with the results obtained by (Oca et al., 2004), where the general trend of flow velocity at various points in the tank increased with jet velocity, with a large flow area at the wall and a small flow area at the center of the tank approaching 0 cm/s. This experimental study found that the large flow area of the circular aquaculture tank was located at the edge of the wall, and the flow velocity near the discharge outlet was close to 0 cm/s. (Venegas et al., 2014) investigated the effect of jet flow on the water in a circular aquaculture tank through field tests. The results showed that when the jet direction of the jets was set to 45°, the aquaculture tank had good results in terms of tangential velocity, uniformity, mixing time, and solid phase removal time from the tank. This is highly consistent with the flow field distribution results in the aquaculture tank under a high flow rate and at a deployment angle of 45° studied in this paper, verifying the accuracy of the results obtained in this paper. (GUI and ZHANG, 2020) investigated the effects of water tanker type aerator on the sewage accumulation and flow field characteristics in a square tangent tank. By employing a water tanker type aerator driven by a square tangent tank to perform wastewater dynamics test, they studied the effects of water tanker type aerator at different deployment angles, deployment distance ratios, and driving flow rate on the sewage accumulation and flow field characteristics in a square tangent tank. The results indicated that the sewage accumulation effect was better when the water tanker type aerator was deployed at 45° and the relative distance between the aerator and the wall of the tank was adjusted to about 1/4. A reasonable hydraulic drive device can drive the water body to form a better circulation pattern in the aquaculture tank (Davidson and Summerfelt, 2004) contribute to the optimal sewage aggregation effect. Therefore, considering the economic efficiency and actual operation of the single-tube jet pipe deployment, the jet pipe should be deployed with a vertical single-tube multi-hole inlet with a flow rate of 7 L/s, a deployment distance ratio of 1/4, and a deployment angle of about 45°, thus achieving better sewage aggregation effect and reducing energy consumption.