Journal of Hydroelectric Engineering

水力发电学报

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2023 Vol. 42, No. 9 Published: 2023-09-25

	1	Study on numerical model of water droplet generation caused by water jet plunging into scour pool ^Hot!
		HE Guicheng, ZHANG Hua, PENG Yanxiang
		DOI: 10.11660/slfdxb.20230901
		Atomization of jet nappes in flood discharge is closely related to the moving behaviors of splashed water droplets, and its droplet generation is a key issue. This paper develops a numerical model of water droplet formation by coupling the Eulerian and Lagrangian methods. First, the volume of fluid method is used to simulate flow patterns, flow velocity, and turbulent kinetic energy of the flow in the scour pool. Then, by the energy balance theory of droplet breakup, characteristic parameters are constructed to calculate the position and quantity of water droplets splashed out from the jet nappe plunging into the pool water body. Finally, the Lagrange method is used to track the trajectories of droplet movement, and the results are compared with the experimental data of a hydraulic model. The results show that the simulations－of water jet distance, water droplet generation position, water droplet group distribution, and rainfall intensity distribution－are in good agreement with the measurements, verifying the accuracy of the model and a new idea for further studying jet atomization in flood discharge.
		2023 Vol. 42 (9): 1-10 [Abstract] ( 66 ) PDF (4440 KB) ( 251 )

	11	Experimental study on energy consumption and entrainment in stable hydraulic jumps of sediment density current
		WEN Zhichao, HUANG Zhe, XU Haijue, BAI Yuchuan, ZHANG Jinliang
		DOI: 10.11660/slfdxb.20230902
		Stable hydraulic jump is a typical flow regime in discontinuous density current, which switches the flow from supercritical to subcritical. It usually features energy consumption and entrainment, but previous research lacks a good explanation of the corresponding relationship between the both processes. In this study, the hydraulic jumps of sediment density current are observed in a flume experiment, and their energy consumption and local entrainment are examined by deriving a head loss formula. We find that an increase in sediment concentration or initial water depth at the inlet or a decrease in inlet velocity will lead to the weakening of the jump and a decrease in conjugate depth ratio. And the increase in density current layer thickness will be more limited. Such inlet flow changes will also lead to less head loss, a reduced volume of clear water entrained from the upper layer, and a reduced density of the turbid layer. An analysis of the energy consumption process of the jump reveals that the decrease in energy consumption indicates the weakened turbulence intensity near the interface of the two flow layers, thereby effectively reducing the entrainment from the upper layer. Using regression analysis, we find that either conjugate depth ratio versus head loss ratio ?hf/hf1 or ?hf/hf1 versus entrainment coefficient KQ shows a positive linear relationship. The energy index derived in this paper is a feasible method to analyze the hydraulic jumps of density current and reveal their local entrainment mechanism.
		2023 Vol. 42 (9): 11-21 [Abstract] ( 59 ) PDF (1488 KB) ( 293 )

	22	Medium and long-term joint forecast of power outputs for hydro-wind-photovoltaic complementary energy system
		LEI Hongxuan, LIU Pan, MA Li, WU Di, GONG Lanqiang, ZHANG Yang, LIN Dongsheng
		DOI: 10.11660/slfdxb.20230903
		The outputs of hydropower, wind power and photovoltaic in a hydro-wind-photovoltaic complementary energy system (HWPCES) are integrated into the power grid. The traditional method forecasts these three outputs separately and then sums them up as the system’s total power capacity, but such a method suffers from error accumulation and lacks consideration of spatiotemporal complementarity. To improve the forecasting accuracy, first we consider spatiotemporal correlation and complementarity, and select certain predictors from the teleconnection factors and power factors. Then, a point forecasting model and an interval forecasting model are constructed based on the Long Short-term Memory network and the Lower Upper Bound Estimation method. Finally, joint forecasting of the power outputs is achieved. This study selects the Ertan HWPCES as the case study. The results show that in the verification period of total power forecasting, its Nash-Sutcliffe efficiency coefficient reaches 0.908 by the joint forecasting method, or an increase of 0.016 compared to the accumulation method. The interval forecasting method achieves a reduction of 0.352 in the coverage width-based criterion. Our new method is useful for complementary operation of hydropower, wind power, and photovoltaic.
		2023 Vol. 42 (9): 22-33 [Abstract] ( 137 ) PDF (2675 KB) ( 355 )

	34	Machine learning method for monthly runoff prediction based on improved Grey Wolf algorithm
		ZHAO Yingyu, PENG Huichun, LI Jiqing
		DOI: 10.11660/slfdxb.20230904
		To improve the accuracy of runoff forecast, we construct combined prediction models by integrating gradient lifting tree regression (GBDT), back propagation algorithm (BP), and with differential evolution Grey Wolf algorithm (HGWO) support vector regression (SVR) algorithm optimized using the variational mode decomposition (VMD) and the extreme-point symmetric mode decomposition (ESMD), and apply them to the monthly runoff series measured at the Tangnaihai station and Lanzhou station of the Yellow River. The results show the combined model VMD-HGWO-SVR gives the best predictions compared with other models. Its average absolute error in predicting monthly runoff at the two stations is decreased by 53.38%, 14.27% and 6.8% compared with ESMD-HGWO-SVR, VMD-BP and VMD-GBDT, respectively. On average, its root-mean-square error is decreased by 53.66%, 22.0% and 11.54%, average relative error by 54.92%, 12.0% and 3.67%; its Nash efficiency coefficient is increased by 17.09%, 3.26% and 1.36%, respectively. This verifies our new method achieves satisfactory effects in predicting monthly runoff time series.
		2023 Vol. 42 (9): 34-45 [Abstract] ( 94 ) PDF (1035 KB) ( 448 )

	46	Change trends of runoffs in major river basins in Tibetan Autonomous Region and their causes
		YE Ting, YANG Hanbo, HUO Junjun
		DOI: 10.11660/slfdxb.20230905
		The Qinghai-Tibet Plateau is a region impacted most dramatically by climate change. The trends of changes in the river runoff and their causes have attracted much attention. In this study, the Mann-Kendall trend test and the Pettitt test are used to detect the runoff variations from 1980 to 2016 at 11 hydrological stations in the Tibetan Autonomous Region, and the climate elasticity method based on the Choudhury-Yang equation is used to quantify the effects of precipitation, potential evaporation, and underlying surface changes on annual runoff changes. The results show that the annual runoff in the study area presents an insignificant upward trend, and has an abrupt point around 1997. The sensitivity of the runoff to climate change and underlying surface change is the lowest in the lower reaches of Yarlung Zangbo River. Precipitation is the main factor responsible for annual runoff change and its increase leads to a runoff increase in all basins, but this impact exhibits an obvious regional difference with a spike in the Lhasa River basin.
		2023 Vol. 42 (9): 46-57 [Abstract] ( 78 ) PDF (3159 KB) ( 389 )

	58	Study on spatiotemporal characteristics of snow cover depth in Yarlung Zangbo River basin in 1979-2021
		XU Xiaorong, TIAN Yuanshi, SUN Qicheng, JIN Feng, ZHANG Shanghong
		DOI: 10.11660/slfdxb.20230906
		The Yarlung Zangbo River (YZR) basin is located in a cold plateau area. Understanding the spatiotemporal characteristics of its snow cover is of great significance to the construction of projects and the prevention and control of snow disasters. In this study, the characteristics of snow depth in this basin are examined using the Mann-Kendall trend analysis and Pearson correlation analysis, based on the snow cover depth data set of China (1979-2021) and the monthly precipitation and temperature data set of China. The results show the snow cover depth presents a spatial trend of higher at two ends and lower in the middle, with a deep snow cover over the river sources in the west and over the northeastern margin. The snow cover is distributed mostly at an elevation of 4000-5200 m with a slope milder than 35°, and the deepest snow is mostly located around the elevation of 4850 m. During 1979-2021, the annual average snow depth decreased at a rate of 0.032 cm/a, taking its lowest value in the year of 2017. In recent years, the snow depth has gradually rebounded to the multi-year average level. The annual mean temperature is increasing at a rate of about 0.024 ℃/a, and it has a significant negative correlation with snow cover depth.
		2023 Vol. 42 (9): 58-69 [Abstract] ( 97 ) PDF (4861 KB) ( 222 )

	70	Multi-objective intelligent optimal drawdown operation of multiple reservoirs
		ZHOU Ying, ZHOU Yanlai, GUO Shenglian, LIN Fanqi
		DOI: 10.11660/slfdxb.20230907
		This study develops a multi-objective optimal drawdown operation model of multiple reservoirs by considering the demands of the river’s natural flow patterns and using three objective functions－the maximum hydropower output, maximum water supply reliability, and minimum flow alteration. This model is solved using the fast nondominated sorting genetic algorithm-II (NSGA-II), and applied in a case study of multiple reservoirs in the Xiangjiang River basin: the Dongjiang, Shuangpai, and Centianhe reservoirs. The results show that relative to conventional operation in the wet, normal and dry year cases, the maximum hydropower output scheme raises the power generation by 15.71 million kW?h (11.2%), 11.67 million kW?h (10.9%), and 8.11 million kW?h (10.0%), respectively; The minimum flow alteration scheme reduces river flow deviation by 1.56 (61.2%), 0.33 (6.3%), and 0.89 (13.6%), respectively; The maximum water supply reliability scheme promotes water supply satisfaction by 2.1%, 2.6%, and 3.4%, respectively. Our new model proves applicable and effective and would be useful for the drawdown operation of multi-reservoir systems in the Yangtze River basin.
		2023 Vol. 42 (9): 70-78 [Abstract] ( 79 ) PDF (1141 KB) ( 291 )

	79	Measurement and analysis on bolt stress of top cover in pump turbine
		ZHAO Qiang, CAO Jiali, CHEN Liu, LUO Yongyao, REN Shaocheng, LIU Yang
		DOI: 10.11660/slfdxb.20230908
		A pump turbine makes frequent startup and halt under rapid changes in its working conditions, and its internal flow patterns are usually complicated. During operation under different working conditions, the top cover is subjected to complex water thrusts; the high-strength bolts, tightening it on the seat ring, suffer a great variation in both stress level and its change rate. The bolt stress of top cover is directly related to operation safety, so it is necessary to understand its variations under typical operating conditions. This study conducts a prototype measurement of bolt stress for a unit at the Baoquan pumped storage power station. A resistance strain method is adopted to collect the bolt stress data through measuring micro-strain at four points evenly arranged on the middle section of the stud for various test conditions－bolt preloading process, full load turbine operation, load rejection process, zero flow pump condition, and pump normal operation. This will lay a basis for evaluating the top cover bolts and unit operation safety, and help improve the bolt design.
		2023 Vol. 42 (9): 79-87 [Abstract] ( 94 ) PDF (1293 KB) ( 364 )

	88	Electric power-energy balance in new-type power system based on power support
		PANG Feng, XIANG Huawei, WU Di, HUANG Wenbo
		DOI: 10.11660/slfdxb.20230909
		The reliable power supply is critical for the safe and steady operation of a power system. The reliable power support function of power supplies refers to the immediate full-load operation within its reliable capacity according to the load demand of a power system. This paper presents a calculation method of power-energy balance based on the reliable power support. The overall framework is to arrange the units on standby and in maintenance according to the power supply installation and system load demand. On this basis, the support capacity of each month (or week, or ten days) for various power sources was determined through power balance, and the annual 8760 h power-energy balance simulation was carried out. The simulation idea is to make room for wind and photovoltaic power as much as possible while meeting the minimum technical output or forced output of various power sources, so as to realize the overall power-energy balance at all time with the wind and photovoltaic power being absorbed as much as possible. This calculation method has been applied to an analysis of the Shanxi power grid with different scales of pumped storage plants, and the impact of wind power uncertainty on power-energy balance was evaluated. The results show the increase in the system’s pumped storage capacity will replace part of the thermal power capacity and increase the power for pumping, thus promoting the absorption of new energy sources. If the wind power committed too much as the power support role, in the case of wind power output is reduced by weather and other factors, the load demand could be difficult to be met even if the thermal power, hydropower and other kinds of power operate at full load, which suggests the harm to the safety and stability of power system.
		2023 Vol. 42 (9): 88-100 [Abstract] ( 140 ) PDF (2035 KB) ( 417 )

	101	Filtering method for data of pressure fluctuations in draft tube of hydropower units
		ZHANG Huan, ZENG Yun, ZHANG Hui, QIAN Jing, SUN Yanfei
		DOI: 10.11660/slfdxb.20230910
		Noise reduction processing of the draft tube pressure signals is of great significance for vibration fault identification of hydropower units; a key issue for a power plant is to process the interference component of such signals to eliminate its effect in subsequent signal analysis. To address the issue, this study used the Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMD-AN) to decompose the signal into several Intrinsic Mode Functions, and used the Improved Maximum Information Coefficient (IMIC) to calculate the correlation between each intrinsic mode function and the original signal. A threshold value is determined based on the correlation coefficient calculated, and finally the intrinsic mode functions were processed and superimposed to reconstruct the pressure signal. The efficacy of this method was verified using an example analysis, and compared with Empirical Mode Decomposition and the traditional Maximum Information Coefficient method. The results show our new method IMIC-CEEMDAN has higher reliability in the purification of pressure data for hydropower units.
		2023 Vol. 42 (9): 101-111 [Abstract] ( 81 ) PDF (1539 KB) ( 324 )

	112	Automatic modeling of high genus geological bodies at dam foundation based on Kmeans-ERT
		TAN Tianwen, WANG Jiajun, LYU Mingming, WANG Xiaoling, YU Jia
		DOI: 10.11660/slfdxb.20230911
		Geological bodies with high genus are complex structures that feature a variety of cavities, such as the interbedded strata and fracture zones at dam foundation. In handling these structures, the common geological modeling methods based on surface reconstruction have poor performance in automation and low accuracy, while methods using volume element representation can realize automatic modeling, but at a cost of redundant voxel data. Besides, previous algorithms can hardly suit the unevenly distributed borehole data or the multivariate shapes of high genus strata in hydropower engineering. This paper develops an automatic geological voxel modeling method based on the K-means-modified extremely randomized trees (Kmeans-ERT). First, to classify the ambiguous and complex high genus strata, ERT is selected as the base prediction model because of its robustness. Then, the K-means algorithm is adopted to modify ERT by adding a clustering analysis progress at each node to calculate dynamically the distribution of random split values. Moreover, a boundary recognition algorithm is constructed to optimize the model by hiding interior voxels. Engineering application shows our new model can automatically reconstruct high genus strata. Compared to SVC, KNN, random forest, deep forest, and BP neural network, the model improves the average accuracy by 17.4%, 19.1%, 4.7%, 6.5% and 17.1% respectively, and it sees a 69.3% decrease in memory cost. This verifies our new method has accuracy and efficiency superior to manual geological modeling or other automatic algorithms.
		2023 Vol. 42 (9): 112-124 [Abstract] ( 56 ) PDF (3544 KB) ( 294 )

	125	External water seepage into water conveyance tunnel across urban areas with composite lining structure
		WANG Hanhui, TAO Jun, ZHANG Cunhui, SU Kai
		DOI: 10.11660/slfdxb.20230912
		In recent years, a new type of composite lining has been adopted in the construction of diversion tunnels across urban areas in China. The joint of its outer segments may open under the internal water pressure transmitted by the back-filled self-compacting concrete, thereby causing a problem of external water seepage. In this paper, a three-dimensional finite element model for the seepage in a typical tunnel section is developed; the amount of seepage flow and the external water pressure of the lining under various external conditions are examined in a case study of the Luotian Reservoir to Tiegang Reservoir water conveyance tunnel project in Shenzhen. The results show that for the tunnel in the completion stage, its seepage flow decreases by about 90% if the seepage coefficient of the outer segments is decreased by one order of magnitude. Concrete backfilling reduces the seepage by 30% ~ 40%, and the grouting ring has little effect on the seepage. During operation, the lining joint is the main leakage channel, accounting for more than 90% of the total seepage; the grouting ring in this stage can reduce the seepage significantly. The peak seepage occurs in the operation stage, and it is thought to be a control condition of the tunnel seepage. The external water pressure determination should be based on the completion stage subsequent to the self-compacting concrete backfilling.
		2023 Vol. 42 (9): 125-134 [Abstract] ( 57 ) PDF (2418 KB) ( 327 )