Comparative Study of the Hydraulic Characteristics of Stratified Energy Dissipators in In-Chamber Longitudinal Culvert Systems

Abstract

The study of energy dissipation characteristics of stratified energy dissipators in the lock chamber is carried out by numerical simulation. Discussed the hydraulic characteristics of different energy dissipators in the lock chamber and open channel area (e.g., flow field in the lock chamber, flow distribution in the branch holes and energy dissipation mechanism). The results show that the stratified energy dissipation makes the water flow in the open ditch area more fully mixed; the flow distribution in the branch holes is more balanced; the maximum flow velocity into the bottom of the lock chamber is reduced. The in-chamber longitudinal culvert system with open channel + cover has the best flow conditions, followed by open channel + grating, both of the above are better than the traditional open ditch energy dissipator.

1 Introduction

With the rapid development of China’s economy, a large number of world-class high-head water conservancy projects have been completed and put into use one after another. The establishment of high headlocks to enable ships to travel unimpeded on the rivers has become another new national strategy. However, high headlocks are required to meet two conditions at the same time - cargo capacity and protection of locks and ship safety. The instantaneous large flow rate will endanger the safety of ships moored in the lock. To meet the safety of ships mooring in the lock, it is necessary to set up the energy dissipator in the filling and emptying system, so as to reduce the energy of the water around the ship and enhance the uniformity of water flow to achieve the purpose of reducing the mooring ship bollard force (Zong and Yang 1989).

In the 1930s, the United States in some locks used the open ditch energy dissipator type: Lower Granite, Bay Spring, New Bankhead and other locks had been applied. In addition, in the 1960s, the U.S. Army Corps of Engineers had also placed open ditch energy dissipators at ship locks experiment stations on the Columbia River and Snake River, and many experiments and studies had been done for this purpose. (Perkins and Chanda 1975; Richardson 1969; Richardson and Webster 1960). Similarly, Marmet Lock (Hite 1999) with the open ditch energy dissipator on both sides of the increased in force dissipation threshold to increase the smoothness of the flow to optimize mooring conditions. In China, the open ditch energy dissipation type was first used in Gezhouba 2 locks, after which the energy dissipation type has been widely used, such as Sichuan Fuzhou River Lotus Temple, Santai locks, Xijiang River Guiping a line of locks. Recently, China’s researchers on the basis of single ditch research, for this reason, the domestic energy dissipation on the basis of single ditch, proposed a double ditch, three ditch arrangement type (Li et al. 2021).

For higher heads, such as a 60 m single-stage ship lock, the open ditch energy dissipator will be difficult to reach the lock design requirements. By adding grilles or covers, the single layer open trench energy dissipator, is changed to a “double layer energy dissipator”. Whether the innovative types of ditch + grille and ditch + cover are better than single layer is to be demonstrated. This study investigates the hydraulic characteristics of single-layer open ditch energy dissipator and layered open ditch energy dissipation (ditch + grille and ditch + cover form) by numerical simulation, aiming to understand the effect of different energy dissipation arrangements on the flow regime of the lock chamber, the flow distribution of branch holes and energy dissipation effect.

2 Numerical Simulation

2.1 Calculation Area and Grid

Without affecting the calculation accuracy, reduce the calculation volume and improve the calculation efficiency. According to the characteristics of the arrangement of the outlet water transmission system in the second section of the open channel, half of the entire lock chamber area was selected for the simulation study in the calculation area. At the same time, to ensure that the flow velocity and flow direction on the inlet boundary is relatively smooth, the corridor inlet was extended horizontally, and the calculation model is shown in Fig. 1.

Fig. 1.

Calculation area schematic

The calculation area has meshed with a hexahedral structure. Mesh encryption is applied to the side branch holes, open trenches and their nearby areas. The total number of grid cells is about 2.54 million and the total number of nodes is about 1.65 million. The grid division diagram is shown in Fig. 2.

Fig. 2.

Grid profile diagram

2.2 Control Equations and Boundary Conditions

In terms of model selection, the volume of fluid method (VOF) was chosen for the free surface tracking (Hirt and Nichols 1981); the renormalization group (RNG) k - ε turbulence model was used for the turbulence model (Yakhot and Orszag 1986). According to the actual lock gate opening-closing flow variation under the lock chamber inlet, the inlet flow rate is adopted as variable flow rate that is, the incoming flow is non-constant. The highest head, the lowest initial head and the shortest opening time in the physical model test were selected (Design water head H = 60 m, Initial lock chamber water level h₀ = 6 m, Gate opening time t_v = 8 min, and the inlet flow velocity variation is shown in the following figure (Fig. 3).

Fig. 3.

Variation of inlet flow rate and inlet flow rate

The boundary settings are shown below:

• (1) Inlet boundary: the velocity inlet boundary condition is used, and the UDF is applied for the input of variable flow velocity.

• (2) Outlet boundary: the pressure outlet boundary condition is adopted, and the normal gradient of all variables is 0.

• (3) Wall boundary: using no-slip boundary conditions and standard wall functions for the viscous bottom layer.

• (4) Free water surface: the free water surface is treated as pressure boundary, and the relative pressure value on the pressure boundary is P = 0.

2.3 Model Validation

In the physical model experiments, the pressure measurement points of the side branch holes and the monitoring points of the water level change in the lock chamber are compared respectively (Fig. 4).

Fig. 4.

Comparison of numerical simulation results with physical experimental data: a. Water level change in the lock chamber; b. Pressure change at the top of the main corridor.

According to the change of water level in the lock chamber and the change of pressure in the branch hole, we could get that the simulation was validated against the experimental data which were within a reasonable range.

3 Results and Analysis

3.1 Lock Chamber Flow Field

It can be seen from Fig. 5, at the maximum flow rate of Q = 330 m³/s (T = 400 s), the jet velocity of the traditional open channel branch hole reaches 12.21 m/s; the flow velocity of the open channel + grille branch hole is 9.42 m/s; the flow velocity of the open channel + cover plate branch hole is 11.32 m/s. From the Fig. 5, it can be seen that after the water flows through the open ditch, there is still a high-speed water flow of more than 3 m/s into the lock chamber. And no larger flow velocity is found in the open ditch + grille and open ditch + cover. It means that the open ditch cannot fully kill the energy of water flow in the branch hole.

Fig. 5.

Three different energy dissipator transverse flow velocity diagram and vector diagram

From the vector distribution diagram (Fig. 6), it can be seen that, compared with the conventional open channel energy dissipators, the addition of the grille makes the water in the open ditch dissipator area mixed more strongly, the spin roll is enhanced, energy dissipation is more adequate. In addition, the addition of the grille also adjusts the direction of the main jet of the lock chamber flow, so that the speed into the lock chamber is further reduced, and the lateral diffusion is more rapid. The upper branch hole and the addition of the cover plate (ditch + cover energy dissipator), so that the bright ditch area whirlpool intensity more and more intense and in the cover plate out of the flow formed after the second spin roll, into the lock chamber inside the maximum flow velocity of about 2.24 m/s, only 47% of the traditional bright ditch energy dissipators, bright ditch + cover plate layered energy dissipation water transfer system.

Fig. 6.

Longitudinal chamber flow velocity distribution at the moment of maximum water transfer (with ditch energy dissipator)

From the longitudinal flow field diagram at the time of the maximum flow rate (x = 6.75 m cross section), it can be seen that the maximum inclination of the water flow in the vertical direction at the time of the maximum flow rate is about 45°, of which the inclination of the upstream side is about 52°, that is, part of the jet is shot obliquely to the water body of the lock chamber and the maximum flow velocity into the lock chamber is 4.97 m/s. This makes the whole lock chamber form a huge transverse vortex, and the main jet decays along the course to maintain a certain velocity near the water surface. The flow velocity is maintained near the water surface. This indicates that when the design head reaches 60 m, the dissipation effect in the open channel area slows down significantly, and more energy needs to be dissipated by mixing and diffusion in the water body of the lock chamber (Fig. 7).

Fig. 7.

Longitudinal chamber flow velocity distribution at the moment of maximum water transfer (with ditch + grille energy dissipator)

The velocity of water entering the lock chamber on the upstream side is obviously reduced, but at the end, there is still a large flow velocity (maximum velocity out of the lock chamber is 2.64 m/s) due to the blockage at the end of the branch corridor and the inertia of the water. However, after the water flows through the grille, the water flows make a local impact due to the sudden narrowing of the outlet aperture, which restrains the flow direction, and the flow direction after the lock chamber is almost vertical into the lock chamber. This leads to the lock chamber without a wide range of vortex, and because of the grille barrier, the water flows out of the lock chamber and the water body inside the lock chamber through the role of volume suction, rapid mixing and elimination of energy, combined with a faster reduction in velocity (Fig. 8).

Fig. 8.

Longitudinal chamber flow velocity distribution at the moment of maximum water transfer (with ditch + cover energy dissipator)

At the time of the maximum water transfer of the open channel + cover, there is still a lower velocity water body around the branch hole outflow in the middle section for absorption and mixing. It means that the setting of the open channel + cover plate has caused some influence on the outflow of the branch hole. It can be found that, due to the value-added cover plate, the dissipation caused by the impact of water flow on the cover plate is very large, plus the skirt of the cover plate to increase the outflow length, adjust the uniformity, the outflow are more uniform and flow velocity is not large (maximum velocity out of the lock chamber is 1.67 m/s). The middle main shot in the low water depth for the emergence of convergence, increasing the respective lateral diffusion space, while the upstream and downstream ends on both sides also bear a certain dissipation. Resulting in the entire flow is very uniform, near the surface combined velocity does not exceed 0.28 m/s.

3.2 Branch Hole Flow Distribution

The energy dissipator are different, but the outflow law is roughly similar: at the beginning of the lock transmission, the outflow velocity of each branch hole is decreasingly distributed along the flow direction of the branch corridor, that is, the flow velocity of the branch hole near the inlet of the branch corridor is large (4#, 10#), and the flow velocity of the branch hole at the end of the branch corridor is small (16#, 19#); over time, the outflow velocity of each branch hole is increasingly distributed along the branch corridor, that is, the flow velocity of the branch hole near the inlet of the branch corridor is small, and the flow velocity of the branch hole at the end of the branch corridor is large. The flow velocity of each branch is increasing along the branch corridor (Fig. 9).

Fig. 9.

Three different energy dissipators side branch hole flow velocity distribution: a) ditch dissipator; b) ditch + grille dissipator; c) ditch + cover dissipator.

The difference is that, when the flow rate is larger, the uniformity of the traditional open channel energy dissipator branch hole outflow is poor (Q = 330 m³/s moment each branch hole flow rate statistics standard deviation $\sigma$ is 0.75). Adding the equal spacing grille, the maximum flow rate moment (Q = 330 m³/s) standard deviation $\sigma$ reduced to 0.47, the flow peak is obviously reduced. The addition of the cover, the water flow can be more fully in the open ditch energy dissipation area mixing, so that the water pressure near each branch hole to get re-equilibrium, resulting in the flow of water outflow flow rate does not appear large differences, regardless of the initial filling or maximum water transfer time, branch hole outflow uniformity is the best among the three (the maximum flow time standard deviation $\sigma$ only 0.44).

3.3 Energy Dissipation Analysis

The magnitude of turbulence energy can reflect the dissipation state of the open ditch area:

Fig. 10.

Turbulent energy values corresponding to the internal flow structures of the three body types (T = 400 s, cross-section at the longitudinal symmetry center of the lock chamber)

From the Fig. 10, the turbulence is most intense in the open trench area of the open ditch + cover, with large turbulence energy values near the orifice, in the middle of the open trench and near the cover. The addition of the grille allows the more turbulent flow to be concentrated in the middle of the orifice and open trench. The turbulent energy values in the ditch energy dissipator are relatively discrete except at the orifice, and there is still some mixing at the bottom of the lock chamber (marked in red). It shows that the effect of energy dissipation by stratified-energy dissipation is obvious, and the energy dissipation is concentrated in the open ditch area.

4 Conclusions

In this study, the hydraulic characteristics of the stratified energy dissipator open channel + grate and open channel + cover are studied by numerical simulation. The flow characteristics, branch hole flow distribution and energy dissipation mechanism of the two new energy dissipators are discussed in detail. Based on these studies, the following conclusions were obtained.

• 1 Single-layer open channel energy dissipators due to the uneven flow of branch holes, as well as into the open channel, the lock chamber after the diffusion and mixing efficiency are low, resulting in the lock chamber flow pattern is poor.

• 2 Ditch + grille energy dissipator although the side branch hole flow and out of the hole flow pattern and open ditch energy dissipator similar, but due to the addition of grille, the water in the open ditch mixes more fully and adjust the flow of water into the lock chamber flow pattern, so that with the surrounding water diffusion more fully, the main jet to the bottom of the lock chamber near the energy is lower.

• 3 Ditch + cover because the upper branch hole area is smaller so that the entire branch corridor side branch hole inlet pressure distribution is more uniform. The cover greatly enhances the strength of the water turbulence in the open ditch area. The flow pattern of water entering the lock chamber is the best among the three.

In summary, for 60 m head, stratified energy dissipators is more superior than traditional energy dissipators.