Discharge Tunnel Research Articles

Characteristics of aeration in the flow downstream of a radial gate with a sudden fall-expansion aerator in a discharge tunnel

Abstract The mechanisms of cavitation damage in flood releasing tunnels remain unclear. In this study a series of physical experiments and numerical calculations were conducted to investigate the flow pattern, pressure distribution and cavitation downstream of a sudden fall-expansion aerator in a discharge tunnel. When the radial gate was partly open, the bottom cavity length reduced drastically, the lateral cavity disappeared, and the flow cavitation index near the sidewalls was less than 0.2. The pressure on the floor and sidewalls can be divided into four regimes: the cavity regime, the impact regime, the reflective regime and the stable regime. The time-average pressure is subject to a unimodal distribution when the gate is fully open, whereas a bimodal distribution is presented when the gate is partly open. The negative pressure regime presented an elliptic shape. Cavitation erosion occurred easily on lateral expansion sidewalls in the tunnel with the radial gate partly open.

Experimental research on hydraulics of flood discharge tunnel and improving schemes for choking

Due to the wide range of water levels in reservoir operation, the hydraulic functionality of free surface tunnels in supercritical flow conditions, while discharging beyond design conditions, deserve further studies. To investigate the safe discharging of low drop tunnels, a scale model of a spillway system was built and tested under different flow conditions and structural configurations. Water depth, velocity and flow regime in tunnel and flip bucket reach were measured in experiments. In emptying reservoir operations, a surprising phenomenon was observed; specifically, at low discharges, supercritical flow occurred inside the tunnel while subcritical flow characterized the exit section, with a hydraulic jump forming and moving back into tunnel, forcing it to work under pressure. Critical discharge conditions below which hydraulic jump occurs in the tunnel is theoretically derived by 1D hydraulic analytical computation and experimentally confirmed as q c =0.485q d . To eliminate risk of choked tunnel, a modification is envisioned by setting a torsion angle and a side-weir along the flip bucket section. Experiment results indicate that the combination of a skewed flip bucket and side weir can effectively decrease the outlet water depth, and mitigate the hydraulic jump, avoiding tunnel choking thanks to redirecting the flow into the side weir and to centrifugal force effects. Ski jump flow through the skewed flip bucket and the downstream river regime were also improved.

Research on Energy Dissipation in a Discharge Tunnel with a Plug Energy Dissipater

Research on Energy Dissipation in a Discharge Tunnel with a Plug Energy Dissipater

Manufacturing of an earth pressure balanced shield TBM cutterhead for a subsea discharge tunnel and its field performance analysis

Manufacturing of an earth pressure balanced shield TBM cutterhead for a subsea discharge tunnel and its field performance analysis

Manufacturing of an earth pressure balanced shield TBM cutterhead for a subsea discharge tunnel and its field performance analysis

본 연구에서는 해저 배출관로 건설을 위한 직경 4.4 m 토압식 쉴드TBM을 설계 제작하였다. 압축강도가 가장 큰 구간을 대상으로 커터헤드를 설계하고 쉴드TBM의 핵심 사양을 검토하였다. 쉴드TBM의 최대 용량을 만족하기 위해서는 디스크커터의 관입깊이를 7 mm/rev 이하로 적용해야 하는 것으로 나타났다. 설계 제작된 재활용 쉴드TBM의 현장 굴진자료를 분석한 결과, 약 95%의 현장 관입깊이가 7 mm/rev이하였으며, 디스크커터와 TBM 최대 용량 이내로 굴진이 이루어졌음을 확인하였다. 또한 CSM모델에 의한 커터 작용력과 현장의 커터 작용력을 비교한 결과, CSM모델로 예측한 커터 적용력이 약 22~25% 크게 나타났으나 대체로 현장의 커터 작용력과 유사한 경향을 나타내었다. An earth pressure balanced shield TBM with the diameter of 4.4 m was designed and manufactured for a subsea discharge tunnel excavation. Its cutterhead was designed to be optimized for the strongest rock mass condition in the tunnel alignment, and then the applicability of the refurbished shield TBM was validated for its maximum capacity. Especially, the maximum cutter penetration depth for the strongest rock mass condition should be kept to be below 7 mm/rev in order to satisfy the allowable capacities of the shield TBM. From the analysis of TBM advance data, approximately 95% of field data showed the cutter penetration depth below 7 mm/rev. In addition, it was certified that the acting forces of every disc cutter, TBM thrust and torque during TBM driving were within the allowable capacities of the shield TBM and its disc cutters. When real acting forces of the disc cutters in the field were compared with those predicted by the CSM model, they showed the close relationships with each other even though the predictions by the CSM model were approximately 22~25% higher than field data.

Stability analysis of the slope around flood discharge tunnel under inner water exosmosis at Yangqu hydropower station

The stability of the slope around a flood discharge tunnel is influenced by the space topography, the geological structure, the seepage of the flood discharge tunnel, the rainfall and so on, which introduce complexity and uncertainty to the problem of slope engineering. For slope stability analysis at the outlet of a flood discharge tunnel affected by high interior hydraulic pressure, the inner water exosmosis (IWE) phenomenon will become obvious, the rock’s mechanical properties will be changed, and the seepage effects of the flood discharge tunnel should be focused on. In this paper, a complicated three-dimensional (3D) numerical simulation and safety assessment of the slope around the flood discharge tunnel at Yangqu hydropower station is implemented in FLAC3D, and 3D slide arcs of good shape are obtained. When calculating the safety coefficient of slopes, the Shear Strength Reduction Technique (SSRT) is adopted, and a factor of safety (FOS) is then found. It is found that the FOS of the natural slope is 1.43 in its original condition, and in this case, the slope is in a stable state. The safety factor of the slope is 1.30 after the slope excavation without considering IWE. Under the condition of normal seepage from inside the tunnel to the outside, the safety factor is 1.29. For investigating the influence of IWE on the slope stability, we design three types of scenarios – minimal seepage, normal seepage and serious seepage – for the fluid–solid coupling calculation. Under the serious seepage condition, the safety factor of the slope is 1.26, and it is in a critical failure state. It should be pointed out that uncertainties in input parameters are not researched in this paper. There is not big difference among safety factors under different scenarios mainly because the maximum of inner water head of the flood discharge tunnel is only about 80m. It still can be found that seepage action has an effect on the stability of the whole slope from calculation results. The stress concentrated region (SCR) near the surrounding rock grows from inside to outside as the seepage intensity increases. The surrounding rock will experience more water pressure and seepage pressure, and, at the same time, the area of the plastic zone grows. Suitable treatments and suggestions are discussed to eliminate the adverse effects of IWE. The research results in this paper can provide a reference for construction, reinforcement and drainage design of the slope in similar hydropower slope engineering scenarios.

Cavity flow regime for spillway aerators

It is well known that the effect of air entrainment for cavitation damage controls is related not only to the air discharge into aerator devices but also the flow regime of the cavity below them. On the basis of the hydraulic characteristics of the flow, the aerator devices were for the first time classified. The theoretical considerations were performed about the jet length and cavity flow regime with the influencing factors. Comparing with the behavior of the flow through the aerator of discharge tunnels, the flow regimes of the cavity below spillway aerators were experimentally investigated, and the empirical expressions were presented to identify the conversions of the cavity flow regimes, including fully filled cavity, partially filled cavity, and net air cavity. Some issues of the design of the aerator devices were suggested in the present work.

Study on Hydraulic Characteristic of Aerator with Side-Walls Sudden Enlargement in Free-Flow Spillway Tunnel

Engineering practices showed side-walls in free-flow discharge tunnels with high speed flows and low pressure tended to be damaged by cavitation, especially behind concave reach. In this paper through experiment an aerator with side-walls sudden enlargement and soleplate vertical dropping was introduced on the concave reach bottom of the Vertical-Bend free-flow spillway tunnel side-walls. By applying this type of aerator, air entrainment increases significantly and un-airflow areas disappear. Experimental observations show that lateral cavity and bottom cavity are linked to each other and the forced aeration forms behind the aerator, which protects sidewalls and soleplates against cavitation damage.

Flow regimes below aerators for discharge tunnels

The flow regimes below an aerator influence directly the air entrainment and the cavitation damage control. Based on the theoretical considerations, the experiments of the aerator for a discharge tunnel were conducted, and the relationships between the flow regime and hydraulic and geometric parameters were investigated. The results showed that, there are two kinds of threshold values for the flow regime conversions. One is Fr1–2 standing for the conversion from the fully filled cavity to the partially filled cavity, and the other is Fr2–3 which shows the change from the partially filled cavity to the net air cavity. Two empirical expressions were obtained for the conversions of the flow regimes, which can be used in the designs of the aerators.

Velocity and pressure distributions in discharge tunnel of rotary-obstruction composite inner energy dissipation

On the basis of model test and theoretical analysis of velocity and pressure distributions, an hypothesis is presented that the distribution of tangential velocity in radial direction seems to be a combinational distribution of a quasi-free vortex and a quasiforced vortex for the discharge tunnel of rotary-obstruction composite inner energy dissipation. The variations of corresponding parameters about the hypothesis are obtained under test conditions in this paper. The formula of pressure distribution in radial direction is deduced theoretically, and the theoretical values of pressure distribution computed by the formula are well consistent with the measured ones, showing that the formula is correct and can be applied to the computation and analysis of pressure distribution of this discharge tunnel.

Hydraulics of discharge tunnel service gate: the flow regimes

During experiments which were conducted on a discharge tunnel laboratory model, bottom cavity beneath the free-falling nappe immediate downstream of the service gate was focused on. Three different flow regimes were detected each of which with particular features. The results provide a fair characterization of each flow regime. They highlight some differences between the limits of flow regimes in terms of dimensionless bottom cavity length and Froude number values under various step heights. The results also demonstrate that increase in step height at service gate provides more aeration in lower range of flow rates.

Theoretical and Experimental Studies of Hydraulic Characteristics of Discharge Tunnel with Vortex Drop

The velocities at given points in the volute chamber, the contracted section and the vertical dropshaft of a discharge tunnel with vortex drop were measured by a small specially designed L-shaped tube, as Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) would not work there due to the special structure of the discharge tunnel with vortex drop. Hydraulic empirical formulas were proposed to predict the velocities and the angles of the velocities made with the vertical direction θ. The theoretical analysis results were in good agreement with experimental data. Therefore, the method proposed in this paper can be used to analyze related characteristics of discharge tunnels with vortex drop. Additionally, different model scales were considered to predict the cavitation characteristics on the wall of a dropshaft in practical engineering.

Head loss coefficient of orifice plate energy dissipator

Orifice plates as an effective energy dissipator have been used in hydropower projects. The head loss coefficient, relating directly to the energy dissipation ratio, is an important index of these elements. This coefficient and parameters such as the contraction ratio of the orifice plate diameter and the flood discharge tunnel diameter, the ratio of the orifice plate thickness to the tunnel diameter, the dimensionless recirculation length and the Reynolds number were theoretically analysed, and relationships were obtained by numerical simulations and physical model experiments. The head loss coefficient is mainly dominated by the contraction ratio and the ratio of the orifice plate thickness by the effects of the recirculation region. The less the contraction ratio of the orifice plate, the larger the recirculation length and height, and the larger the head loss coefficient. An empirical expression allows us to determine the head loss coefficient.

Effects of hydraulic and geometric parameters on downstream cavity length of discharge tunnel service gate

Aerators on discharge tunnel outlets may be regarded as an effective protection against cavitation erosion. The air entrainment of aerators is governed by a number of independent parameters, including the bottom slope of releasing free-surface flow tunnel downstream of service gate, the end top slope of pressure tunnel, the height of step, and the Froude number at take-off. During eight phases of experiments, the effects of above-mentioned parameters were observed on the cavity length downstream of the fully open operating service gate of a discharge tunnel. The results show that, the bottom slope of releasing free-surface flow tunnel has obvious effect on the cavity length as well as the Froude number at gate take-off. The effect of the step height variations on the cavity length could be considered for higher discharges and steeper tunnel top slope, particularly in higher discharges, resulting in shorter cavity length downstream of service gate.

Hydraulic characteristics of plug energy dissipater in flood discharge tunnel

The hydraulic characteristics of three types of plugs, namely, the single plug, the stepped plug and the gradually contracted plug were studied by means of experimental and numerical simulations. Main research findings are as follows. For the single plug, the pressure recovery lengths inside and after the plug range from 0.63–1.05 times and 2.02–2.84 times of the tunnel diameter, respectively. For the stepped plug, the lengths are 0.24–0.32 times and 1.62–2.84 times of the tunnel diameter, respectively. The best ratio of the inlet diameter to the outlet diameter of the gradually contracted plug can be expressed by a linear function. The relationship between the head loss coefficient and the area contraction ratio is obtained. The incipient cavitation numbers of different plugs are experimentally and numerically determined, and the incipient cavitation numbers are expressed by a formula. Model experiment with scale of 1:50 was carried out on a pressure tunnel with three-stage gradually contracted plugs. The results show that this type of energy dissipater is suitable for spill tunnels of high head (nearly 200 m) and large flow rate (nearly 2500 m 3/s).

Water quality problems and potential for wetlands as treatment systems in the Yangtze River Delta, China

This paper summarizes water pollution problems in the Yangtze River Delta, China, caused by rapid economic growth and lack of wastewater treatment. Although precipitation and river runoff are abundant, surface waters in and around the delta are seriously polluted because of the dense population that contributes domestic sewage, and non-point source drainage from intensive agriculture, aquaculture, animal husbandry, and numerous small enterprises. On the other hand, the delta itself is a huge wetland with many artificial and natural wetlands, such as paddy fields, reed marsh, lakes, and rivers. How to make full use of these wetlands to improve the water quality in the Yangtze River Delta is an important challenge for wetland scientists. Potential solutions include establishment of constructed wetlands near outlets of sewage discharge tunnels, and developing floating mats on open water surfaces. Pilot experiments in the Yangtze Delta have shown improvements to water quality. In the meantime, clean industrial production lines, recycling water usage, and large scale application of water purification techniques should be implemented to mitigate the sources of water pollution.

Hydraulic characteristics of bottom underlay-type pier for water-wing control

The pier, placed at the outlet of the pressure flow section, is an effective design to deal with the problems of the manufacture and operation of the gates for a discharge tunnel with high water head. It is crucial to control the water-wing, induced by this type of the pier. Through observing the phenomena of the water-wing, the reason of the water-wing inception, i.e., the concave of the flow surface, was presented, and a type of the new pier with bottom underlay was designed. The hydraulic characteristics of the pier, including the variations of the water-wing features, both length and height, with the water head, as well as with the concave depth, and with the height of the bottom underlay, and the variations of the concave depth with the height of the bottom underlay, were investigated by physical model experiments. The results show that the approach of the modification of the concave through the structures with the bottom underlay-type pier is remarkably effective in the water-wing control.

Hydraulic characteristics of partial opening of the working gate for a discharge tunnel

The partial opening of the working gate for a discharge tunnel is used more and more widely in the various conditions, such as releasing discharge to meet the needs of the various stages under construction, or of the operations at low reservoir levels. In this article the hydraulic characteristics were investigated of the partial opening of the working gate based on the physical model experiments of the discharge tunnel for the Longtan Hydropower Project, including the discharge coefficient, the surface profile of the free flow section, the bottom pressure distribution of the tunnel with an aerator, the cavity length just from the ramp, and the wall pressure of the curve section of the outlet. Some new issues were discussed and the properties were presented of the partial opening of the gate.

On necessity of placing an aerator in the bottom discharge tunnel at the Longtan Hydropower Station

The air entrainment for avoiding cavitation damage has been widely used in long free flow tunnels. It is crucial to determine whether an aerator is needed for shorter tunnels. In this article, the bottom discharge tunnel at the Longtan Hydropower Station was involved, for which the free flow tunnel section was only 50.00 m long. The cavitation in the tunnel with and without the aerator was investigated using the physical models of the scale 1/30, through the measurements of cavitation noise. The experimental results show that it is necessary to place the aerator at the inlet of the free flow section for higher reservoir level to protect this tunnel from cavitation damage.

Case Study: Dam Safety during Construction, Lessons of the Overtopping Diversion Works at Aguamilpa Dam

A study on risk analysis by overtopping of the diversion works of Aguamilpa Dam in Mexico was carried out to examine the overtopping event of January, 1992. Specifically, this study focuses on the upstream water surface elevation during the flood considering the tunnel discharge actual width and roughness. During this maximum flow event, the upstream cofferdam was overtopped and the discharge tunnels exceeded their maximum hydraulic capacity. The risk analysis was made considering the original hydrologic data (until 1992), the final constructed conditions, to establish a performance function that compares the risk of original deterministic analysis with a probabilistic analysis made in 1992. The study also focuses on practical improvements in the construction stage of tunnels; like lining the floor with hydraulic concrete and shotcrete in vaults and walls, that improves the safety of dams during construction and increases the real return period. The work here is presented as a case study because of the unique large-scale flow conditions that are registered in diversion works of dams.