Multiphase Flow Simulations through Tarbela Dam Spillways and Tunnels

Tarbela dam is one of the largest earth filled dam in the world used for power generation and irrigation purposes. Like all reservoirs the sediments inflow in the Tarbela reservoir has resulted in reduction in water storage capacity and is also causing damage to the tunnels, power generating units and ultimately to the plant equipment. This numerical study was performed to predict the flow patterns and characteristics in Tarbela dam. Tunnel 3 and 4 inlets;originally on the bed level were raised in the 3-D model and meshed. Analysis was performed using multiphase flow (water and air) for maximum inflow in the reservoir, i.e., considering summer season and discharging water through different locations, i.e., tunnels and spillways. Pressure, velocities, flow rate and free surface height results obtained were found in good agreement with the analytical and existing results where available. Results show uneven discharge through each gate due to maximum velocity near exits and overall stagnant phenomena of water within the reservoir. Maximum velocity was observed along the spillways outlet. Strong vortex motion was observed near the spillways outlet and tunnel inlets. New design of Tunnel 3 and 4 were suggested to WAPDA in order to decrease the sediment inflow and improvements in design of the spillways were suggested.

Improved design of guide wall of bank spillway at Yutang Hydropower Station

Ensuring that water flows smoothly into spillways is the main challenge in spillway design. In order to help avoid the formation of vortices and separation of flow along the guide wall in front of the gates during overflow through the spillway, an experiment with a physical model of the Yutang Dam. bank spillway was carried out, The profile of the guide wall was redesigned to eliminate the formation of vortices and separation of flow. This involves opening up holes in the middle part of the guide wall. The test results show that the design is effective in improving the flow conditions of the inlet, and in ensuring the desired values of water head along the guide wall and discharge capacities of the spillway,

Effects of spillway types on debris flow trajectory and scour behind a sabo dam

Debris flows are one of the common natural hazards in mountainous areas.They often cause devastating damage to the lives and property of local people.The sabo dam construction along a debris flow valley is considered to be a useful method for hazard mitigation.Previous work has concentrated on the different types of sabo dams such as close-type sabo dam,open-type sabo dam.However,little attention has been paid to the spillway structure of sabo dam.In the paper,a new type of spillway structure with lateral contraction was proposed.Debris flow patterns under four different spillway structures were investigated.The projection theory was employed to predict trajectory of debris flow out from the spillway and to estimate the incident angle and terminal velocity before it plunged into the scour hole behind the sabo dam.The results indicated that the estimated data were in good agreement with the experimental ones.The discrepancy between the estimated and experimental values of main parameters remained below 21.82%(relative error).Additionally,the effects of debris flow scales under different spillway structures were considered to study the scour law.Although the debris flow pattern and scour law behind the sabo dam under different operating conditions was analyzed in this paper,further study on the scour mechanism andthe maximum scour depth estimation based on scour theory is still required in the future.

Physical modelling and scale effects of air-water flows on stepped spillways

During the last three decades, the introduction of new construction materials (e.g. RCC (Roller Compacted Concrete), strengthened gabions) has increased the interest for stepped channels and spillways. However stepped chute hydraulics is not simple, because of different flow regimes and importantly because of very-strong interactions between entrained air and turbu- lence. In this study, new air-water flow measurements were conducted in two large-size stepped chute facilities with two step heights in each facility to study experimental distortion caused by scale effects and the soundness of result extrapolation to pro- totypes. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and air-water flow turbulence intensity. For a Froude similitude, the results implied that scale effects were observed in both facilities, although the geometric scaling ratio was only Lr=2 in each case. The selection of the criterion for scale effects is a critical issue. For example, major differences (i.e. scale effects) were observed in terms of bubble chord sizes and turbulence levels al- though little scale effects were seen in terms of void fraction and velocity distributions. Overall the findings emphasize that physical modelling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies, and this is consistent with basic dimensional analysis developed herein.

Numerical investigation of hydraulic characteristics and prediction of cavitation number in Shahid Madani Dam’s Spillway

The prediction of the probability of cavitation occurrence to prevent serious damages in the spillways is the major concern for hydraulic engineers. In this research, the three-dimensional model of Shahid Madani Dam's spillway was simulated with the Flow 3D software and by the comparison of numerical model results with the experi- mental data, the probability of occurrence of the cavitation phenomenon has been investigated. The flow parameters including pressure, velocity, and water depth were calculated for three different flow rates of 495 m3/s, 705 m3/s and 2 205 m3/s respectively. The Renormalization Group (RNG) turbulence model was used to simulate current turbu- lence. Comparison of simulation results for pressure, velocity and water depth with the results of the experimental model with two statistical indices Root Mean Square Error (RMSE) and Coefficient of Determination (R2) showed that the numerical simulation results are in good consistency with experimental model. However, simulation results indicated that at any flow rates with a return period of 1 000 years, probable maximum flood and designed flow rates, the cavitation number is not lower than the critical cavitation number. Therefore, it is predicted that the cavitation phenomenon in Shahid Madani Dam's spillway will not happen.

Air Entrainment and Pressure Fields over Stepped Spillway in Skimming Flow Regime

This paper deals with some aspects of the air entrainment process along the chute of spillway and study of pressure fluctuations. The experimental study has been carried out using stepped spillway model located in the campus of Government College of Engineering, Amravati (India). It is observed that air concentration is increasing with discharge as well as with number of step. Air concentration is increasing along the length of spillway. It is also observed that the bottom mean air concentration increases with step height in the upstream reach of stepped spillway, which is prone to cavitation. The pressure profiles exhibit a wavy pattern down the stepped chute and pressure on each step increases with ratio of critical depth to step height (yc/h).

Numerical investigation of air-entrainment in skimming flow over stepped spillways

As a widely used flood energy dissipator, the stepped spillway can significantly dissipate the kinetic or hydraulic energy due to the air-entrainment in skimming flow over the steps. The free-surface aeration involves the sharp deformation of the free surface and the complex turbulent shear flows. In this study, the volume of fluid （VOF）, mixture, and Eulerian methods are utilized to simulate the air-entrainment by coupling with the Reynolds-averaged Navier-Stokes/large eddy simulation （RANS/LES） turbulence models. The free surface deformation, air volume fraction, pressure, and velocity are compared for the three different numerical methods. Only the Eulerian＋RANS method fails to capture the free-surface aeration. The air volume fraction predicted by the VOF＋LES method best matches the experimental measurement, while the mixture＋LES method predicts the inception point of the air entrainment more accurately.

Numerical Simulation of Flow over Stepped Spillways with Varying Step-Angle

In the present study,the flow over the stepped spillway was numerically investigated by using Flow3D model.The effect of step angle on different properties of Nappe flow regime such as the water surface profile,location of free-surface aeration inception,Froude number at the spillway’s toe,and pressure,flow velocity,air concentration and cavitation index were evaluated.The realizable k–εwas applied as the turbulence model,and Volume of Fluid(VOF)model was used to determine the free surface flow profiles of the spillway.The model was verified using experimental data.In order to investigate the different characteristics of Nappe flow regime,17 numerical runs was designed,in which,four step angles,four flow discharge were considered to investigate the flow characteristics over the stepped spillway.The results indicated that the numerical model is well suited with the experimental data over the stepped spillway(RMSE=0.147 and ARE=6.9%).In addition,with increasing the step angles,the aeration inception point is generally moved downstream.By increasing the step angles from zero to 10 degrees,the Froude number does not change significantly,however,at the angle of 15 degrees,the Froude number decreases by about 42 percent.

Air-vent layouts and water-air flow behaviors of a wide spillway aerator

A spillway aerator should guarantee favorable flow conditions in the coupled water-air system even if the aerator is unconventionally wide. Eight air-vent configurations are devised and incorporated into a 35-m wide chute aerator for a generalized study. Computational fluid dynamics(CFD) simulations are performed to explore their effects on water-jet and air-cavity features. The Re-normalisation group(RNG) k-ε turbulence model and the two-fluid model are combined to predict the two-phase flow field. The results demonstrate appreciable influences of the vent layouts on the water-air flow. The air vents stir the air motion and re-distribute the cavity air pressure. Once the vent layout is modified, reciprocal adjustments exist between the jet behavior and air-pressure field in the cavity, thus leading to considerable differences in air-flow rate, jet-trajectory length, vent air-flow distribution across the chute, etc. The large width plays a discernable role in affecting the aerated flow. Telling differences exist between the near-wall region and the central part of the chute. To improve the duct pressure propagation, a gradual augment of the vent area should be assigned towards the chute center. Relative to single-slot vents across the flow, the layouts with segregated vents gain by comparison. A designer should see to it that a vented aerator operates satisfactorily for a given range of flow discharges.

Computational investigation of hydraulic performance variation with geometry in gabion stepped spillways

Over recent years, there has been a clear increase in the frequency of reported flooding events around the world. Gabion structures offer one means of flood mitigation in dam spillways. These types of structures provide an additional challenge to the computational modeller in that flow through the porous gabions must be simulated. We have used a computational model to investigate the flow over gabion stepped spillways. The model was first validated against published experimental results. Then, gabion stepped spillways with four different step geometries were tested under the same conditions in order to facilitate inter-comparisons and to choose the best option in terms of energy dissipation. The results show that normal gabion steps can dissipate more energy than overlap, inclined, and pooled steps. An intensive set of tests with varying slope, stone size, and porosity were undertaken. The location of the inception point and the water depth at this point obtained from this study were compared with those from existing formulae. Two new empirical equations have been derived, on the basis of a regression analysis, to provide improved results for gabion stepped spillways.

Experimental study of drag reduction in flumes and spillway tunnels

Experiments in an open flume model and spillway tunnel model were carried out using drag reduction techniques. Two drag reduction techniques were adopted in the experiments： polymer addition and coating. The drag reduction effect of a polyacrylamide （PAM） solution and dimethyl silicone oil coating were studied in the flume model experiments, and the results were analyzed. Experiments were then carded out with a model of the Xilnodu Hydropower Station, the second largest dam in China. In order to reduce the resistance, the spillway tunnels were internally coated with dimethyl silicone oil. This is the first time that these drag reduction techniques have been applied to so large a hydraulic model. The experimental results show that the coating technique can effectively increase flood discharge. The outlet velocity and the jet trajectory distance are also increased, which enhances the energy dissipation of the spillway tunnel.

Modeling the Mangla Dam Spillway for Cavitation and Aerators Optimization

This study evaluated the effects of increased reservoir conservation level by 40 ft (12.2 m), on spillway velocities;it’s discharging capacity and associated cavitation risk. The study optimized the aerators size and shape to avoid cavitations. The mathematical model was used to estimate the flow velocities and cavitation risk, when scale model study assessed the spillway discharging capacity and optimized the performance of the aerators for modified conditions. The mathematical model simulations showed increased flow velocities and damage index for modified conditions. The damage potential was 2 - 3 times higher with modifications and falls within the major to catastrophic region. The scale model study showed that discharging capacity of the spillway can effectively be restricted to original design by raising spillway crest by 5.0 ft (1.52 m). The scale model study also showed that the two aerators near sluice and at the chute with an air duct pipe of 3.0 ft diameter can improve the free surface flow profile reducing the risks of cavitation. Simulations for several configurations demonstrated clearer affect of aerators ramps on flow trajectory and gate opening. It also depicted that the height of the ramp of sluice aerator has a positive effect on the flow performance to about 7.5 inches (19 cm), when further increase in the ramp height reduced the flow performance.

An Experimental Study for Construction of Emergency Spillway in Daechung Dam

Recently, the flood frequently happened by several effects, such as El Nino et al., is the meteorological accident to attack the stability of dam. Hence, it was necessary to increase the stability of existing dam and dam design criteria were reinforced with probable maximum flood. This study was conducted in order to install the emergency spillway in Daechung Dam. Daechung Dam is located in the upper course of Guem River, in the middle of Korea, near a Sobak mountain range. The stream length and area of Daechung Dam are each about 401 km, about 9886 km2. According to the Report of new training project in Daechung Dam published in 2004 year, Maximum flood water level of Daechung Dam was calculated EL. 84.18 m increased of previous maximum flood water level (EL. 83 m). The aim of this study is to make an alternative plan on problems predicted by analysis of experimental data which is the results on the hydraulic model on dam structures. Analysis of experimental results was classified dam structures, such as approach channel, spillway, energy dissipater and stream, by design conditions.

Variation of Discharge Coefficient of Spillway and Broad Crested Weir Due to the Effectiveness of the Longitudinal Slope in a Non-horizontal Channel

In this work, experiment efforts were devoted to study the effect of the longitudinal slope of channel on the discharge coefficient for ogee spillway and broad crested weir. A comprehensive laboratory study including 17 tests was conducted to estimate the variation of the discharge coefficient due to variation of the longitudinal slope. It was shown that the discharge coefficient is significantly increasing with the increase of the slope by more than 90% or 75% and 80% or 70% for weir and spillway in case of excluding or including the approach velocity head, respectively. Also, CFD （computational fluid dynamics） with a help of Comsol-multyphsics program was used to simulate the problem. The program explained that the linear distribution of the hydraulic pressure changes to a non-linear distribution as the longitudinal slope is considered. Consequently, the values of the discharge coefficient are also affected.

Numerical Modeling of Sediment Transport and Its Effect on Algal Biomass Distribution in Lake Pontchartrain Due to Flood Release from Bonnet CarréSpillway

In order to protect the city of New Orleans from the Mississippi River flooding, the Bonnet Carré Spillway (BCS) was constructed from 1929 to 1936 to divert flood water from the river into Lake Pontchartrain and then into the Gulf of Mexico. During the BCS opening for flood release, large amounts of freshwater, nutrients, sediment, etc. were discharged into Lake Pontchartrain, and caused a lot of environmental problems. To evaluate the environmental impacts of the flood water on lake ecosystems, a two-dimensional numerical model was developed based on CCHE2D and applied to simulate the flow circulation, sediment transport and algal biomass distribution in Lake Pontchartrain. The effect of sediment concentration on the growth of algae was considered in the model. The numerical model was calibrated using field measured data provided by USGS, and then it was validated by the BCS Opening Event in 1997. The simulated results were generally in good agreement with filed data and satellite imagery. The field observation and numerical model show that during the spillway opening for flood release, the sediment concentration is very high, which greatly restricts the growth of algae, so there is no algal bloom observed in the lake. After the closure of BCS, the sediment concentration in the lake reduces gradually, and the nutrient concentration of the lake is still high. Under these conditions, numerical results and satellite imagery showed that the chlorophyll concentration was high and algal bloom might occur.

Unsteady Spillway Flows by Singular Integral Operators Method

The Singular Integral Operators Method (S.I.O.M.) is applied to the determination of the free-surface profile of an un-steady flow over a spillway, which defines a classical hydraulics problem in open channel flow. Thus, with a known flow rate Q, then the velocities and the elevations are computed on the free surface of the spillway flow. For the numerical evaluation of the singular integral equations both constant and linear elements are used. An application is finally given to the determination of the free-surface profile of a special spillway and comparing the numerical results with corresponding results by the Boundary Integral Equation Method (B.I.E.M.) and by using experiments.

Numerical Investigation of Upstream Face Slope and Curvature of Ogee Spillway on Flow Pattern

The major object of this research is the numerical analysis of flow fields over ogee spillway with elliptical crest and the USBR （United States Bureau of Reclamation） in both vertical upstream face state and inclined one with slope 3H：3V. And it also researched the effect of geometry, slopes of curvature of spillway upstream face and water head on the flow field and pressure distribution. In fact Navier-Stokes was used for analyzing flow fields, and VOF （volume of fluid） method was utilized for assigning the free surface, k-e turbulence model RNG was used for analyzing the turbulence field too. Studies show the geometric changes of the spillway pressure distribution and discharge. The spillway with the elliptical crest due to existence of coherence at the curvature of crest created less negative pressure on the crest than USBR crest. So it increased efficiency of spillway discharge about 2% than USBR spillway. Inclining upstream face of spillway affects discharge coefficients and discharge. Hence slope 3H：3V in the upstream face of the crest causes 3.4% more negative pressure in tangent surface and discharges more 1.62% discharge than one of spillway including vertical face. Effect of water head on pressure distribution is simulated on both spillways with elliptical and USBR crest. Numerical results compared to experimental data are satisfactory. The resulted errors of computed discharge compared with experimental data were 0.83%-2% in USBR spillway and 0.29%-2.87% in the elliptical spillway.

Research on the optimal design of spillway tunnel

In order to meet the needs of various tasks of water conservancy project, tunnel excavation under the ground, called hydraulic tunnel. Hydraulic tunnel according to the functions of flood discharge, water diversion, irrigation, water supply, and the construction diversion tunnel of desilting emptying. Nowadays, water conservancy project on flood discharge runnel design requirements more and more high, so the factors taken into account the amount of spillway tunnel engineering, engineering cost, structure design and other factors to evaluate nature of the spillway tunnel is very important. Therefore, in actual construction, of spillway tunnel of optimization design, whether from the economic point of view, or from the point of view of safety and applicability are necessary. This article mainly from the overall layout of flood discharge tunnel, structure design and so on, and then discuss the optimization scheme.

Design and Performance of a Novel Spillway Turbine

The high-speed supercritical flow in steeply sloped channels contains a significant amount of hydro-kinetic energy. A novel, horizontal axis, spillway turbine as presented in this paper attempts to convert that energy into electricity. We report on the turbine’s design and experimental testing. Its intended use is in low-head, low-flow, manmade, concrete-lined channels such as chutes, spillways and other similar steeply sloped open-channels. The design lends itself from an impulse turbine runner but without a pipe or a nozzle. The spillway turbine consists of 2 main components: 1) the runner and 2) an accelerator channel that directs the water towards the runner’s blades. The runner, once fitted with Pelton-inspired “cup inserts” shows performance improvements both in terms of efficiency and specific speeds. The specific speed and the speed factors calculated confirm that this novel spillway turbine runner can be categorized as an impulse turbine. The maximum efficiency obtained during laboratory testing is 43.4% and hence competes well with standard hydrokinetic turbines.

狭窄河道小交角溢洪道燕尾坎挑流消能研究

狭窄河道小交角溢洪道挑流空间有限,流量变化大时挑流落点不稳定。为研究这种特殊地形下燕尾挑坎消能效果,依托某实际工程,采用数值模拟的方法拟定多种燕尾挑坎方案,研究了在狭窄河道小交角溢洪道条件下燕尾桃坎的消能效果。结果表明,方案2消能效果最优,当开口与燕尾宽度比为1且开口与溢洪道宽度比为0.5时水舌掺气效果最好,水流入水前能削减更大流速,在满足起挑前提下增大跌坎角度有利于水舌纵向拉伸,提高消能利用空间。最后复核了该方案在不同梯级流量时的消能冲刷情况,发现方案2挑坎在小流量和大流量泄洪时水舌落点依然稳定,冲坑对建筑物不构成安全威胁。