Study on observation system of seismic forward prospecting in tunnel: A case on tailrace tunnel of Wudongde hydropower station

Seismic method is a major approach for detecting the seismic geological features ahead of the tunnel, understanding the distribution of unfavorable geology, and ensuring the safety of tunnel construction. Observation system is the key for seismic detection, many studies have been conducted to optimize the observation system; however, most of them focused on the surface seismic investigation and numerical simulation rather than in tunnel field environment (limited aperture and full space environment). How to obtain better wavefield information with limited observation aperture is a great challenge. In this study numerical simulation and instrumental techniques (GPR, DC, etc.) were implemented to further check the result of seismic detection at the 1# tailrace tunnel at the Wudongde hydropower station. In the field case, observation detectors were arranged spatially in the tunnel and source points were placed in four ways: linearly along a single side, on the tunnel face, in front of the detectors, and behind the detectors. Then, after data acquisition, the data processing is conducted to carry out the migration results. The imaging results indicate that the observation system with sources and detectors in liner arrangement (with equal interval) helps to suppress artifacts, further supporting the advantages of spatial observation system with liner observation line (detectors). Moreover, the study provides suggestions for geological prospecting in similar tunnel projects.

Optimization Control on the Mixed Free-Surface-Pressurized Flow in a Hydropower Station

The mixed free-surface-pressurized flow in the tailrace tunnel of a hydropower station has a great impact on the pressure, velocity, and operation stability of the power station. In the present work, a characteristic implicit method based on the upwind differencing and implicit finite difference scheme is used to solve the mixed free-surface-pressurized flow. The results of the characteristic implicit method agree well with the experimental results, which validates the accuracy of the method. Four factors that influence the amplitude of pressure fluctuation are analyzed and optimized, and the results show that the relative roughness can influence the maximum pressure in the tailrace tunnel. Additionally, the maximum pressure decreases with the increase of the tunnel’s relative roughness. When the surface relative roughness increases from 0.010 to 0.018, the maximum pressure can decrease by 4.33%. The maximum pressure in the tailrace tunnel can be effectively restrained by setting vent holes in the flat-topped tunnel section (tunnel (4)) and a vent hole at 81.25%L (L is the length of tunnel (4)), which can reduce the maximum pressure by 56.72%. Increasing the vent hole number can also reduce the maximum pressure of the mixed free-surface-pressurized flow in the tailrace tunnel. An optimal set of two ventilation holes 10 m in diameter at 93.75%L and 56.25%L is proposed, which can reduce the maximum pressure by 15.30% in comparison with the single vent case.

A Review of the Hydraulic Transient and Dynamic Behavior of Hydropower Plants with Sloping Ceiling Tailrace Tunnels

The sloping ceiling tailrace tunnel is a novel tailrace tunnel system for hydropower plants. The design, operation, and maintenance of hydropower plants with sloping ceiling tailrace tunnels are based on the calculation and analysis of hydraulic transients and dynamic behavior. Research achievements have provided guidance and a basis for the safe, stable, and efficient operation of hydropower plants with sloping ceiling tailrace tunnels. Based on research achievements, sloping ceiling tailrace tunnels have been applied to more and more hydropower plants. This review paper gives a systematic literature investigation on the hydraulic transient and dynamic behaviors of hydropower plants with sloping ceiling tailrace tunnels. First, the appearance and development of sloping ceiling tailrace tunnels are stated. Key issues in the hydraulic transient and dynamic behaviors of hydropower plants with sloping ceiling tailrace tunnels are illuminated. Then, research achievements on six issues (i.e., the working principles of sloping ceiling tailrace tunnels, the shape design of sloping ceiling tailrace tunnels, the free surface pressurized flow characteristics in sloping ceiling tailrace tunnels, numerical simulations of transient processes for hydro-turbine governing systems with sloping ceiling tailrace tunnels, the stability of hydro-turbine governing systems with sloping ceiling tailrace tunnels, and the transient process control of hydro-turbine governing systems with sloping ceiling tailrace tunnels) are elaborated. Finally, future research trends are presented. In future research, fluid–solid coupling of the tunnel wall and free surface pressurized flow in sloping ceiling tailrace tunnels is worth studying. For hydropower plants with sloping ceiling tailrace tunnels, a combined operating scheme with thermal power and wind power should be explored.