Simulation study on hydraulic characteristics of Changzhou fishway based on FLUENT

Guangxi Changzhou Fishway is the first significant water conservancy project above the mouth of the Pearl River and also the first large fishway independently designed in China. It is the primary migration channel for six kinds of state-level protected fish, such as Chinese sturgeon and a typical vertical slit fishway. This paper takes the Changzhou fishway as the research object, based on the theory of fluid mechanics, and adopts the method of combining theoretical analysis, experimental research, and numerical simulation to study the flow field of different water depths in the pond. The analysis results show that the upper water flow has a larger resting area than the lower water flow, and the fishway has a rich flow field distribution and can produce “S” shaped meandering flow characteristics. The research results provide some reference for the design of vertical fishways.


Introduction
To promote the rapid economic development of our country, water conservancy workers have built many DAMS and embankments on the rivers by utilizing their hydropower resources [1].On the one hand, these water conservancy projects play a massive role in flood control, power generation, shipping, irrigation, etc.; on the other hand, they also cause changes in the water environment (such as changes in water level, discharge, and water temperature, etc.) [2], which are manifested explicitly in changing the flow conditions of river water before and after the dam, blocking the cave swimming routes of fish with foraging, spawning, and overwinter migration habits.The two-way exchange of matter and energy between upstream and downstream has been cut off [3].Many countries have built channels to help fish migration on DAMS and other hydraulic structures that have been built to ensure that fish with migratory habits can migrate to feed and reproduce [4].To better protect fish and other aquatic animals, experts and scholars have proposed measures to add fish passage [5].
Vertical slit fishway is the most widely used form of fishway in current international projects.It mainly uses dividers to reduce the flow rate, and fish pass through the vertical slit to complete the migration activities by crossing the dam and other water retaining structures [6].Most researchers analyze and study the flow field in the fishway from the structural changes of the fishway tank chamber (length-to-width ratio, slope, vertical seam width, vertical seam guide Angle, etc.).For example, foreign scholars N. Rajaratnam [7], Laurent Tarrade [8], and domestic scholars Xu Tibing [9] used physical tests and computer simulation techniques to study the influence of the ratio of fishway length to width on the change of fishway hydraulic characteristics at all levels.They concluded that the aspect ratio will have a significant influence on the flow pattern in the vertical slit fishway, and the flow pattern is better when the aspect ratio is about 10:8.Xu et al. also pointed out that although the baffle pier head will have a particular influence on the flow, the effect is not apparent.Puertas et al. [10] systematically studied the correlation between hydraulic characteristics and biological variables in the vertical slit fishway, analyzed the vertical slit fishway with computer software, and came to the conclusion that the design of the fishway should not only consider the factors of hydraulic characteristics but also quantify the relevant parameters of biological characteristics of target fish species.
In China, the Changzhou fishway is the most representative vertical slit fishway with high efficiency and good engineering practice.The Changzhou Fish Road is the first large-scale water conservancy project on the Pearl River Estuary, and its geographical location is critical.This section's central target fish populations are six species of Chinese sturgeon, shad, anchovy, Anguilla cilia, and white musculus, among which Chinese sturgeon and Anguilla Anguilla are important protected fish in our country [11].
Based on the measured data and the Computational Fluid Dynamics (CFD) numerical method, the influence of the orifice size of the baffle on the flow velocity distribution in the tank is systematically analyzed in this paper, which provides a scientific theoretical basis for the future design and optimization of the vertical fishway.

Mathematical model theory
The numerical simulation of fishway adopts CFD software Fluent and standard k-ε.The turbulence model and the volumetric fraction method are used to track the free liquid surface, which can accurately describe the flow field in the fishway [12][13].
The governing equation is as follows: (4) in the equation, μ and μt are the viscosity coefficient and turbulent eddy viscosity coefficient (m 2 /s).k is turbulence kinetic energy (m 2 /s 2 ); Gε is the turbulent kinetic energy generation term.Turbulence constant σk=1.0.

Model structure and boundary conditions
The following is the three-dimensional structure diagram of the fishway (Figure 1).GAMBIT software is used to model the model.The total length of the model is 36 m, the width B = 5 m, the baffle plate is staggered every 6 m, and the thickness of the baffle plate is 0.2 m.The size of the hidden hole is 1.5 m × 1.5 m, the width of the vertical seam is 15 m, and the slope of the fishway is 1% [14].The total number of meshes is 889896.A local encryption method is used to divide the grids for the structures

Grid division and boundary condition setting (1) Grid division
In this paper, the standard model is used to establish a three-dimensional model to simulate the range of the inlet section, the pool section, and the outlet section of the vertical slit fishway with different body types.The internal structure of the fishway is relatively regular, and all the structured grids with stable calculation, high precision, and simple data structure are used.The total number of grids is 901670.

Mathematical model verification
The calculated flow velocity was compared with the measured data in [15].To reduce the influence of upstream and downstream inlet and outlet boundaries on the water flow structure in the pool room, pool room No. 4 (Figure .2) was selected.On the mainstream curve, the depth h/2= 1.5 m, and the comparative analysis between simulation and measurement was shown as follows (Figure .3).As can be seen from the figure, the error between the simulated value and the measured value is less than 5%, and the results show that the numerical simulation results are in good agreement with the measured results, and the mathematical model can simulate the flow field structure in the fishway pond.

Analysis of the flow field in the indoor fishway of pool 4
The three-dimensional numerical simulation results of the vertical slit fishway were analyzed.In the mathematical model, the relative water depths of h = 1 m and h = 2 m were intercepted, and the flow structure and hydraulic characteristics of the bottom and surface of the conventional pool were studied based on two planes, respectively.To avoid the influence of the upstream and downstream boundary conditions on the studied water flow pattern, the pool chamber between baffle No. 3 and baffle No. 5 is taken as the research object, and the results are imported into Tecplot software for image processing.Finally, the flow field distribution in the middle layer of the reservoir is displayed in a two-dimensional form.Moreover, the two-dimensional numerical simulation results can represent the flow field, as is shown in the figure below.
As shown in the figure below, the distribution of the water flow field on the surface is similar to some extent, so the surface flow process is selected for analysis.The water flows through the baffle at a fast speed into the pool chamber, and the speed increases slightly at first and then decreases gradually.Near the baffle, the flow rate gradually increases along the direction of the flow, and the backwater will be generated in the front of the baffle, generally forming the maximum flow rate area at the baffle.Due to the design of the baffle, the maximum flow rate is effectively reduced, creating more favorable flow conditions for fish migration.The central flow zone is located in the upper part of the transverse middle of the pool, most of the flow velocity values in the central flow zone are between 0.8 and 1.4 m/s, and the maximum flow velocity at the vertical crack is less than 1.5 m/s.The vertical flow direction of the flow velocity value in the central flow zone gradually decreases from the center to both sides, and the downstream flow direction increases first and then decreases and then increases.At the lower part of the central flow zone, a return flow zone is distributed, and the return flow zone rotates slowly counterclockwise, and the speed gradually decreases from the edge to the center.The flow velocity at the edge of the return flow zone is below 0.4 m/s, and at the center, the speed is below 0.1 m/s.The width of the mainstream area is more expansive as the section is located at the bottom of the water depth, and there is a large area of still water at the bottom, which is suitable for fish to rest while swimming in the cave (Figure . 4).The bottom flow field cloud diagram below (Figure .5) shows that the water flow distribution is somewhat disordered.However, the staggered baffles play an excellent role in energy dissipation, making the maximum flow rate at the vertical joint near 1.5 m/s, which is suitable for migrating fish with solid swimming ability.Moreover, the symmetrical baffles arrangement enables the fish with strong swimming ability to quickly pass through the fishway and reduce the length of the migration route and save energy.

Conclusion
In this paper, the numerical simulation method is used to systematically study the hydraulic problems involved in the vertical slot fishway, and the main conclusions are drawn as follows: (1) The flow field characteristics of the vertical slot fishway were tested and determined.The results showed that the area ratio of the rest area of this type of fishway was more appropriate, which could produce "S" shaped meandering flow characteristics, and the distribution of the flow field was suitable for the upstream river swimming of the target fish.
(2) The staggered baffle arrangement not only provides a rich flow field distribution form of the fishway but also plays a good role in energy dissipation.The surface layer is suitable for fish with weak swimming ability, which can follow the mainstream curve of the "S" type and rest in the still water area; The bottom layer is suitable for fish with strong migratory ability and can pass through the fishway quickly.

Figure 3 .
Figure 3. Comparative analysis of simulation and measurement.

Figure 4 .
Figure 4. Surface water flow field distribution.