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  • 1. Borghei, Seyed Mahmood
    et al.
    Nekooie, Mohammad Ali
    Sadeghian, Hadi
    Ghazizadeh, Mohammad Reza Jalili
    Parvaneh, Ali
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Javaheri, Amir
    Kabiri-Samani, Abdorreza
    Discussion: Triangular labyrinth side weirs with one and two cycles2016In: Proceedings of the Institution of Civil Engineers: Water Management, ISSN 1741-7589, E-ISSN 1751-7729, Vol. 169, no 3, p. 111-114Article in journal (Other academic)
  • 2.
    Chen, Wenchuang
    et al.
    School of Civil Engineering, Sun Yat-Sen University, Zhuhai, 519082, China.
    Zhang, Yongliang
    State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing, 100084, China.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Yu, Huifeng
    State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing, 100084, China.
    Liang, Sendong
    State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing, 100084, China.
    Experiments and CFD modeling of a dual-raft wave energy dissipator2021In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 237, article id 109648Article in journal (Refereed)
    Abstract [en]

    A wave energy dissipator (WED) is proposed in this paper, composed of two asymmetric hinged floaters and two hydraulic cylinders, with the purpose to resist wave-induced relative rotation around the hinge. Experimental studies are performed to explore its performance under a wide range of regular waves. Based on Reynolds-averaged Navier-Stokes equations, a 3D numerical model is set up, considering the non-linear interaction of wave and WED. The agreement between the experimental and numerical results is good. With the validated numerical model, the influence of the WED geometrical factors on energy conversion and wave transmission is examined. For a given raft length, the difference in energy conversion between the two multiplicative inverse fore-and-aft raft lengths is significant, which is dependent on the wave frequency. However, as for the wave transmission, the difference in between is insignificantly small, almost independent of the wave frequency. The findings demonstrate that, for energy conversion, its maximum value per unit length corresponds to a large fore-and-aft length ratio but a small total raft length. The combination of experiments and simulations provides reference for both understanding of hydrodynamic behaviours and design including parameter selection of the dissipator.

  • 3.
    Chen, Yuejun
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098, China.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yu, J.
    Fu, Z.
    Chen, Q.
    Flow Expansion and Deflection Downstream of a Symmetric Multi-gate Sluice Structure2020In: KSCE Journal of Civil Engineering, ISSN 1226-7988, E-ISSN 1976-3808, Vol. 24, no 2, p. 471-482Article in journal (Refereed)
    Abstract [en]

    A sluice structure with multiple gates is often followed by an enlarging channel downstream. Experiments are conducted for different enlargement ratios in cross-section and Froude numbers at the gates. A large-scale PIV system is adopted to capture the surface flow field for examination of the flow features. The study shows that, despite the centrally placed sluice structure with symmetric outflow, a hydraulic jump occurs and the main flow downstream exhibits, in terms of expansion and deflection, a high degree of asymmetry and formation of large circulation zones. The degree of deflection increases significantly along the longitudinal direction for all the enlargement ratios except the smallest. The toe of the hydraulic jump is controlled at the upper edge of the sloping surface downstream of the gates. In light of outflow width, Froude number and enlargement ratio, the flow is classified into three regions. Immediately downstream, the effect of outflow width gradually decreases with increasing Froude number. Further downstream, the degree of deflection augments with increasing enlargement ratio, with circulation zones. At a given cross-section, the degree of expansion is positively related to the enlargement ratio. The study is expected to provide guidance for examination of similar issues of flow pattern and erosion protection design.

  • 4.
    Ekström, Ingvar
    et al.
    Sweco.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Berg, Mikael
    Improving spillway discharge safety at Bergeforsen2011Conference paper (Refereed)
    Abstract [en]

    Bergeforsen is a 35 m high embankment dam. The technical discharge capacity of the existing spillway is 2300 m3/s at full reservoir retention level (FRRL). To meet new design flood criteria a first major rehabilitation was carried out 2003, which allowed discharge of an extreme flood at a water level higher than the FRRL. Recent underwater inspections of the spillway and energy converter have however revealed a need for rehabilitation and adaptation of the concrete structures to the design flood flow conditions. To make it possible to take the spillway out of operation for the time period required for adequate repairs it has been decided to rebuild the dam, adding an alternative spillway structure that will serve as discharge facility during the upgrade of the existing spillway. For this reason, a new 25-m gated spillway, with a stilling basin followed by a curved tunnel section, will be constructed in the left river bank. Once the existing spillway is rehabilitated the combined discharge capacity will make it possible to safely pass the revised design flood at a reservoir level not exceeding the FRRL. A physical model was built to asssit in the design decision, the purpose of which is to verify and optimize engineering solutions for increased discharge capacity, so that the design flood is released without jeopardizing the stuctural integrity of the dam.

  • 5.
    Ekström, Ingvar
    et al.
    Sweco.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Mören, Lena
    Cederström, Malte
    Vattenfall AB.
    Adapting Ligga to higher design flood, spillway channel modification through physical & numerical modeling2007Conference paper (Refereed)
    Abstract [en]

    As part of Vattenfall’s dam-safety program, Ligga dam is to be rebuilt for higher safety level. Besides diverse measures, the spillway channel will be modified. The channel is bounded by natural bedrock. Spillway operations during the past years have evidenced severe erosion and damages in the fractured rock. Rock instability becomes visible even at moderate spillway discharges 800 – 900 m3/s. The updated design-flood of the dam is in the order of the magnitude of 2000 m3/s. To adapt the dam to the higher flood, the spillway channel has to be re-shaped, so that the flood is safety released and the structural integrity of the embankment dam is maintained. As a cost-effective alternative, excavation is recommended, as a large volume of rock material is required for stabilizing the dam and for replacing the slope protection upstream. To achieve satisfactory flow behaviors, hydraulic model tests are made to examine and optimize excavation possibilities. The re-shaping of the waterway is complicated by the fact that the existing channel bends in plan and the excavation must follow roughly the same curvature. Considerations are also given to exemption of the dam toe from erosion and the forest on the left riverside from being flooded. Computer simulations of water-rock interaction are made with the commercial code UDEC. Against previous flood release records, the numerical model of rock erosion is calibrated and uses measured water level, flow velocity and dynamic water pressure in the model as input data. Results show that the new waterway configuration provides proper flow patterns and the rock foundation after reinforcement is not subjected to unacceptable damage at spillway discharges up to the design flood.

  • 6.
    Ferdos, Farzad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Wörman, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Characterization of hydraulic behaviours of coarse rock materials in a large permeameter2013In: Journal of Geoscience and Environment Protection, ISSN 2327-4336, Vol. 1, no 3, p. 1-6Article in journal (Refereed)
    Abstract [en]

    The hydraulic behavior of a rock material structure is a major feature for its design and safety assessment. Similar to all other physical prob-lems, in order to enclose the governing equations systems and achieve a solution, the hydraulic characteristics of these materials need to be determined experimentally and implemented then into adopted thermo-dynamical models. This paper covers the process of the design, con-struction and operation of an experimental rig built for this specific purpose. Using the constructed large-scale permeameter, tests have been conducted. The non-linear hydraulic behavior of various materials under extreme turbulent conditions, where Reynolds number reaches un-precedented values, has not been studied before. Preliminary results are presented and discussed.

  • 7.
    Guo, Wencheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering. Huazhong Univ Sci & Technol, Sch Hydropower & Informat Engn, Wuhan 430074, Peoples R China..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering. KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Labs, SE-81426 Alvkarleby, Sweden..
    Yang, J. B.
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Peoples R China..
    Effect of throttling orifice head loss on dynamic behavior of hydro-turbine governing system with air cushion surge chamber2019In: 29th iahr symposium on hydraulic machinery and systems, IOP PUBLISHING LTD , 2019, article id 052018Conference paper (Refereed)
    Abstract [en]

    This paper aims to study the effect of throttling orifice head loss on the dynamic behavior of the hydro-turbine governing system with air cushion surge chamber. Firstly, a nonlinear mathematical model of hydro-turbine governing system considering the nonlinear head loss of throttling orifice is established. Then, the nonlinear dynamic behavior of the hydro-turbine governing system is investigated by using the Hopf bifurcation theory and direct numerical integration. The stability and dynamic response of the system are detailed described. Finally, the effect mechanism of throttling orifice head loss on the dynamic behavior of system is revealed based on the comparison between the cases with nonlinear head loss and no head loss. The results indicate that the mathematical model considering the nonlinear head loss of throttling orifice for the hydro-turbine governing system is a fifth-order nonlinear state equation. The system goes through Hopf bifurcation at the bifurcation points and the type of Hopf bifurcation is supercritical. The domain at the bottom of the bifurcation line is the stable domain The throttling orifice cannot affect the system stability through its head loss. The throttling orifice can affect the system dynamic response through its head loss. The throttling orifice head loss is favorable for the damping of the water level oscillation in the surge chamber.

  • 8. Kung, C. S.
    et al.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Energy interpretation of hydraulic transients in pipelines1993In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 31, no 6Article in journal (Refereed)
    Abstract [en]

    The energy concept sheds light upon the hydraulic system that comprises the throttled surge tank, the tunnel and the penstock. The mathematical manipulation of the governing differential equations of water hammer yields an energy (power) equation in point form. By integration with respect to pipe distance, this relation is extended first to an individual pipe, then to the surge-tank system with branching connection. The system energy (power) equation states the relationship between the total (internal plus kinetic) energy, the convective energy, the work done on the system and the energy dissipation. A case study identifies the conversion between the internal and kinetic energy, and quantifies the behaviour and magnitudes of related energy components of the system.

  • 9.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Li, Qiulin
    Chongqing Jiaotong Univ, Southwest Sci Res Inst Water Transport Engn, Chongqing 400016, Peoples R China..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    CFD Modelling of a Stepped Spillway with Various Step Layouts2019In: Mathematical problems in engineering (Print), ISSN 1024-123X, E-ISSN 1563-5147, Vol. 2019, article id 6215739Article in journal (Refereed)
    Abstract [en]

    A traditional stepped spillway is prone to cavitation risks. To improve its hydraulic behaviors, distorted step faces and pool weirs are devised. By numerical modelling, comparative studies are conducted to look into the flow features. The pressures on step surfaces of the unconventional layouts exhibit 3D distributions. Pool weirs are essential in increasing both the min. and max. pressure loads. Pressures on the downstream bed show a unique pattern for V- and inverted V-shaped models, with the extreme pressures at the sidewalls for the former and at the central plane for the latter. Symmetrical secondary flows are formed in V- and inverted V-shaped cases with different patterns. Distributions of turbulent kinetic energy suggest differences in flow motions in all cases. Furthermore, the relative energy loss of flat setups is similar to 5.4% lower than that of the pooled ones with the same step face angle; inverting the face angle does not give rise to noticeable change. The results provide reference for relevant projects.

  • 10.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Xie, Qiancheng
    Luleå Univ Technol, Div Fluid & Expt Mech, Luleå, Sweden..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering. Vattenfall AB, R&D Hydraul Lab, Alvkarleby, Sweden..
    Daily suspended sediment forecast by an integrated dynamic neural network2022In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 604, p. 127258-, article id 127258Article in journal (Refereed)
    Abstract [en]

    Suspended sediment is of importance in river and dam engineering. Due to its high nonlinearity and stochasticity, sediment prediction by conventional methods is a challenging task. Consequently, this paper establishes a new hybrid model for an improved forecast of suspended sediment concentration (SSC). It is a nonlinear autoregressive network with exogenous inputs (NARX) integrated with a data pre-processing framework (thereafter INARX). In this model, wavelet transformation (WT) is used for time series decomposition and multigene genetic programing (MGGP) for details scaling. The two incorporated modules improve time and frequency domain analysis, allowing the network to unveil the embedded characteristics and capture the non-stationarity. At a hydrological station on the upper reaches of the Yangtze River, the records of daily water stage, flow discharge and suspended sediment are collected and refer to a nine-year period during 2004-2012. The data are used to evaluate the models. Several wavelets are explored, showing that the Coif3 leads to the most accurate prediction. Compared to the sediment rating curve (SRC), the conventional MGGP, multilayer perceptron neural network (MLPNN) and NARX, the INARX demonstrates the best forecast performance. Its mean coefficient of determination (CD) increases by 7.7%-38.6% and the root mean squared error (RMSE) reduces by 15.1%-54.5%. The INARX with the Coif3 wavelet is further evaluated for flood events and multistep forecasts. Under flood conditions, the model generates satisfactory results, with CD > 0.83 and 84.7% of the simulated data falling within the +/- 0.1 kg/m3 error. For the multistep forecast, at a one-week lead time, the network also yields predictions with acceptable accuracy (mean CD = 0.78). The model performance deteriorates if the lead time becomes larger. The established framework is robust and reliable for real-time and multistep SSC forecasts and provides reference for time series modeling, e.g. streamflow, river temperature and salinity.

  • 11.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering. Vattenfall AB, R&D Hydraulic Laboratory, Älvkarleby, Sweden.
    A hybrid gene expression programming model for discharge prediction2021In: Proceedings of the Institution of Civil Engineers: Water Management, ISSN 1741-7589, E-ISSN 1751-7729, p. 1-12Article in journal (Refereed)
    Abstract [en]

    The head-discharge relationship of an overflow weir is a prerequisite for flow measurement. Conventionally, it is determined by regression methods. With machine learning techniques, data-driven modelling becomes an alternative. However, a standalone model may be inadequate to generate satisfactory results, particularly for a complex system. With the intention to improve the performance of the standard gene expression programming (GEP), this paper proposes a hybrid evolutionary scheme, which is coupled with grey system theory and probabilistic technique. As a gene filter, the grey relational analysis (GRA) eliminates the noise and the simulated annealing (SA) reduces overfitting by optimising the gene weights. The GEP-based model is developed and validated using the experimental data of a submerged pivot weir. Compared with the standalone GEP, the GRA-GEP-SA model generates more accurate results. Its coefficients of determination and correlation are improved by 3.6% and 1.7%, respectively. The root mean square error is lowered by 24.8%, which is significant. The number of datasets with an error below 10% and 20% becomes 15% and 12% more, respectively. The proposed approach outperforms the classic genetic programming (GP) and shows a comparative error level with the empirical formula. The hybrid procedure also provides reference for applications in other hydraulic issues.

  • 12.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering. Vattenfall AB, R&D Hydraul Lab, S-81426 Alvkarleby, Sweden..
    Effects of Inclination Angles on Stepped Chute Flows2020In: Applied Sciences, E-ISSN 2076-3417, Vol. 10, no 18, article id 6202Article in journal (Refereed)
    Abstract [en]

    Owing to its effective energy dissipation and aeration, a stepped spillway is commonly used for flood release in hydraulic projects. Its conventional design features horizontal step surfaces. Designed for certain flow rates, it does not function satisfactorily at larger discharges. To improve this, layouts with inclined step surfaces, both downward and upward, are proposed. Computational fluid dynamics (CFD) modelling in 3D is performed to examine and compare their flow properties in the skimming flow. The results suggest that a shift from a downward to an upward layout leads to a gradual decrease in the flow velocity at the chute end; the latter exhibit higher energy dissipation efficiency. Moreover, equations are developed to estimate the velocity and energy loss. The flow velocity in the developing zone, described by a power law, shows a decline with an increase in the angle of inclination. The downward layout is subjected to somewhat higher risk of cavitation if implemented in a prototype. The extreme pressure loads acting upon an upward layout are larger, and a correlation is proposed for its prediction. On an inclined surface, either upward or downward, the pressure demonstrates an S-shaped distribution. On a vertical surface, the flow pressure increases, after an initial decline over a short distance, towards the chute bottom.

  • 13.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. R&D Hydraulic Laboratory, Vattenfall AB, Älvkarleby, Sweden.
    Improved river water-stage forecasts by ensemble learning2022In: Engineering with Computers, ISSN 0177-0667, E-ISSN 1435-5663Article in journal (Refereed)
    Abstract [en]

    Forecasting water stages is of significance to river and reservoir management. However, conventional models sometimes fail to perform accurately, as water levels are characterized by high nonstationarity. To provide an improved estimation of water stages, this study develops a new prediction framework by coupling stand-alone machine learning models with ensemble algorithms. As base learners, the optimal regression tree (RT) and extreme learning machine (ELM) are integrated into four ensemble strategies, i.e., bagging (BA), boosting (BO), random forest (RF) and random subspace (RS), leading to eight ensemble models. They are then assessed using daily water-stage records at two hydrological stations on the Yangtze River. Their performance is evaluated by statistical criteria: coefficient of determination (CD), Nash–Sutcliffe efficiency (NSE), root mean square error (RMSE) and mean absolute error (MAE). The RT and the ELM generate satisfactory predictions with deficiency in capturing extreme values. The ensemble models generally enhance the prediction efficiency, with their mean CD and NSE augment by up to 6.9% and 7.0%, and mean RMSE and MAE reduction by up to 47.9% and 47.0%. The BO-based models, namely BO-RT and BO-ELM, result in the highest accuracy, with a mean absolute relative error (ARE) of 1.0% and 1.4%. Ensemble learning gains even in multi-step-ahead forecasts, which satisfactorily extends the lead time up to 14 days. This study illustrates the capability of ensemble learning for improved water-level forecasts, which provides reference for modeling related issues such as sediment load and rainfall-runoff.

  • 14.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Hydraul Lab, Alvkarleby, Sweden..
    Modelling of suspended sediment load by Bayesian optimized machine learning methods with seasonal adjustment2022In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 16, no 1, p. 1883-1901Article in journal (Refereed)
    Abstract [en]

    Suspended sediment load (SSL) is essential to river and dam engineering. Due to the complexity and stochastic nature of sedimentation, SSL prediction is a challenging task and conventional methods often fail to generate accurate results. Aiming to provide an improved estimation, this paper contributes to a new forecasting framework by integrating the seasonal adjustment (SA) and Bayesian optimization (BOP) into a machine learning (ML) model (denoted as BMS). The SA is used for de-seasonalisation and trend extraction; the BOP is to optimize the ML architecture. The BMS is evaluated using the daily SSL records from the Yangtze River. Its performance is appraised by statistical criteria of Nash-Sutcliffe efficiency (NSE), correlation coefficient (CC), root mean squared error (RMSE) and mean absolute error (MAE). With the de-noising and hyper-parameter tuning modules, the BMS effectively heightens the accuracy of the standard ML models. The most significant improvement occurs in the Boosting model, with its augments in NSE and CC by 4.3% and 1.6%, and reductions in RMSE and MAE by 24.9% and 24.2%. The BMS gains by comparison even under flood conditions, where it remarkably reduces the errors of the constituent models by up to 47.9% for RMSE and 48.3% for MAE.

  • 15.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering. Vattenfall AB, R&D Hydraul Lab, Alvkarleby, Sweden..
    Two-phase flow modelling by an error-corrected population balance model2023In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 17, no 1, article id 2178512Article in journal (Refereed)
    Abstract [en]

    High-velocity aerated flow is a common phenomenon in spillways. Its accurate modelling is challenging, mainly due to the lack of realistic physics in the conventional two-phase models. To this end, this study establishes a population balance model (PBM) approach to account for the evolutionary process of air bubbles. The air-water flow in a stepped chute is examined. The model performance is evaluated by statistical metrics: correlation coefficient (CC), root mean squared error (RMSE), and mean absolute error (MAE). Compared with conventional models, the PBM generates improved air-water predictions. However, the flow parameters are still underestimated, particularly in areas with intense air-water interactions. For further development, an error-corrected PBM (EPBM) is proposed by incorporating machine learning (ML) techniques into the PBM. Compared with the PBM, the EPBM leads to a mean augmentation in velocity prediction by 19.8% for the CC, 73.0% for the RMSE, and 77.1% for the MAE. The gains in air concentration estimation are 2.0%, 67.6% and 73.5%, respectively. The EPBM generates the most accurate results, with 99.6% and 89.6% of the velocity and air concentration predictions within a 20% relative error range. The main contributions are establishing a PBM for air-water flows and developing an error-corrected PBM using ML.

  • 16.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. R&D Hydraulic Laboratory, Vattenfall AB, 81426, Alvkarleby, Sweden.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Data-driven reduced-order simulation of dam-break flows in a wetted channel with obstacles2023In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 287, article id 115826Article in journal (Refereed)
    Abstract [en]

    Accurate and timely information on dam-break waves is essential for risk assessment and disaster mitigation. The unsteady flow interacting with in-channel obstacles renders numerical simulations computationally costly. This study establishes a machine learning (ML)-enhanced reduced-order model (ROM), which provides accelerated and accurate flow predictions. The model consists of three phases: dimensionality reduction, long short-term memory (LSTM) optimization and forecasting, and flow field reconstruction. The proper orthogonal decomposition (POD) first reduces the complexity of the physical system while maintaining the dominant flow dynamics. Subsequently, an LSTM fine-tuned by the grey wolf optimization (GWO) predicts the evolution of the POD coefficients in the reduced-order space. Lastly, the flow field is reconstructed using the high-energy POD modes and the estimated amplitudes. The proposed GWO-LSTM-ROM is evaluated for time-dependent dam-break flows in a wetted channel with obstacles. Based on the comparison of millions of data samples, the approach is highly consistent with the high-fidelity full-order model, with a coefficient of determination over 0.99. Meanwhile, the average computational efficiency is improved by 86%. The main contribution of this work is to develop an improved method for fast and accurate modeling of complex flows, benefiting a wide range of applications, e.g., multiphase flows and fluid-structure interactions.

  • 17.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Discharge prediction for rectangular sharp-crested weirs by machine learning techniquesManuscript (preprint) (Other academic)
    Abstract [en]

    The stage-discharge relationship of a weir is essential for a posteriori calculations of flow discharges. Conventionally, it is determined by regression methods, which is time-consuming and may subject to limited prediction accuracy. With the intention to provide better estimate, the machine learning models, artificial neural network (ANN), support vector machine (SVM) and extreme learning machine (ELM), are assessed for prediction of discharges of rectangular sharp-crested weirs. A large number of experimental data sets are adopted to develop and calibrate these models. Different input scenarios and data management strategies are employed for optimization of the models, for which performance is evaluated in the light of statistical criteria. The results show that all three models are capable of predicting the discharge coefficient with high accuracy, but the SVM exhibits somewhat better performance. Its maximum and mean relative error are respectively 5.44 and 0.99%, and 99% of the predicted data show an error below 5%. The coefficient of determination and root mean square error are 0.95 and 0.01, respectively. The model sensitivity is examined, indicative of the dominant roles of weir Reynolds number and contraction ratio in discharge estimation. The existing empirical formulas are assessed and compared against the machine learning models. It is found that the relationship proposed by Vatankhah exhibits a highest accuracy. However, it is still less accurate than the machine learning approaches. The study is intended to provide reference for discharge determination of overflow structures including spillways.

  • 18.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering. Vattenfall AB, R&D Hydraulic Laboratory, 81426, Älvkarleby, Sweden.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Discharge prediction for rectangular sharp-crested weirs by machine learning techniques2021In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 79, article id 101931Article in journal (Refereed)
    Abstract [en]

    The stage-discharge relationship of a weir is essential for posteriori calculations of flow discharges. Conventionally, it is determined by regression methods, which is time-consuming and may subject to limited prediction accuracy. To provide a better estimate, the machine learning models, artificial neural network (ANN), support vector machine (SVM) and extreme learning machine (ELM), are assessed for the prediction of discharges of rectangular sharp-crested weirs. A large number of experimental data sets are adopted to develop and calibrate these models. Different input scenarios and data management strategies are employed to optimize the models, for which performance is evaluated in the light of statistical criteria. The results show that all three models are capable of predicting the discharge coefficient with high accuracy, but the SVM exhibits somewhat better performance. Its maximum and mean relative error are respectively 5.44 and 0.99%, and 99% of the predicted data show an error below 5%. The coefficient of determination and root mean square error are 0.95 and 0.01, respectively. The model sensitivity is examined, indicative of the dominant roles of weir Reynolds number and contraction ratio in discharge estimation. The existing empirical formulas are assessed and compared against the machine learning models. It is found that the relationship proposed by Vatankhah exhibits the highest accuracy. However, it is still less accurate than the machine learning approaches. The study is intended to provide reference for discharge determination of overflow structures including spillways. 

  • 19.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Hydraul Lab, S-81426 Alvkarleby, Sweden..
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Evaluation of Pool-Type Fish Passage with Labyrinth Weirs2022In: Sustainability, E-ISSN 2071-1050, Vol. 14, no 3, article id 1098Article in journal (Refereed)
    Abstract [en]

    Due to the construction of dams and sluices, habitat fragmentation of riverine species is an issue of concern in hydropower projects. An artificial fishway aims to restore flow connectivity and facilitate migration. As an alternative to the conventional layout with straight walls, labyrinth layouts are introduced, consisting of either triangular, trapezoidal, or rectangular weirs. Three-dimensional CFD simulations are performed to examine their influences on the fishway hydraulics. The labyrinth layouts exhibit an appreciable three-dimensional flow structure in a pool, featuring flow fluctuations in both cross- and along-channel directions, which is indicative of the potential for multiple species with distinct swimming preferences. The streamwise reduction in mean flow velocity is correlated with the Froude number, with the labyrinth weirs leading to a somewhat lower reduction than the conventional one. The dimensionless pool water depth declines with specific flow discharge. The turbulence kinetic energy is the lowest in the triangular layout and the vorticity in the conventional one. Among the labyrinth layouts, the differences in volumetric power dissipation are small. An equation is developed for dissipation estimation. All the labyrinth layouts generate higher energy loss than the conventional one. With optional weir configurations, the findings provide reference for engineering design.

  • 20.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Enmark, Gustav
    Fortum AB.
    Air entrainment and transport in a bottom outlet controlled by a cylindrical gate2022Conference paper (Refereed)
    Abstract [en]

    This study deals with a bottom outlet that causes significant surges at both the inlet and exit. The bottom outlet comprises an intake, a vertical shaft and a tunnel underneath the rockfill dam and a submerged exit. In the intake tower, a cylinder gate controls hexagonal openings near the reservoir bottom. Operations during the dam construction show that discharges at a low reservoir water level result in upsurges of air-water mixture within the hollow gate cylinder and blowouts in the tailwater. To understand the flow behaviors, 3D CFD modeling is performed to examine air entrainment at the intake and transport down the waterway. Both low and high water levels are simulated, and the air-water flow phenomenon is reproduced. In both cases, air pockets are generated, which undergo accumulation and breakup process. When moving downstream, they could cause severe surface fluctuations, even blowouts. Consequently, engineering solutions are required to address this issue. The aim of this study is to provide basis for risk assessment of outlet operations and potential rehabilitation measures.

  • 21.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. R&D Hydraulic Laboratory, Vattenfall AB, Älvkarleby, 81426, Sweden.
    He, Xiaolong
    State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 610065, China.
    Modeling transient flow dynamics around a bluff body using deep learning techniques2024In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 295, p. 116880-116880, article id 116880Article in journal (Refereed)
    Abstract [en]

    The significance of understanding the flow past a bluff body (BB) lies in its relevance to ocean, structural, and environmental applications. Capturing the transient flow behaviors with fine details requires extensive computational power. To address this, the present study develops an improved method for modeling the complex flow dynamics around a BB under steady and unsteady conditions. It is a deep learning (DL)-enhanced reduced-order model (ROM) that leverages the strengths of proper orthogonal decomposition (POD) for model reduction, convolutional neural network-long short-term memory (CNN-LSTM) for feature extraction and temporal modeling, and Bayesian optimization for hyperparameter tuning. The model starts with dimensionality reduction, followed by DL optimization and forecasting, and terminates with flow field reconstruction by combining dominant POD modes and predicted amplitudes. The goal is to establish a DL-driven ROM for fast and accurate modeling of the flow evolution. Based on the comparison of millions of data samples, the predictions from the ROM and CFD are considerably consistent, with a coefficient of determination of 0.99. Furthermore, the ROM is ∼10 times faster than the CFD and exhibits a robust noise resistance capability. This study contributes a novel modeling approach for complex flows, enabling rapid decision-making and interactive visualization in various applications, e.g., digital twins and predictive maintenance.

  • 22.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Li, Qiulin
    Chongqing Jiaotong Univ, Southwest Sci Res Inst Water Transport Engn, Chongqing 400016, Peoples R China..
    Numerical Modelling of Air-Water Flows over a Stepped Spillway with Chamfers and Cavity Blockages2020In: KSCE Journal of Civil Engineering, ISSN 1226-7988, E-ISSN 1976-3808, Vol. 24, no 1, p. 99-109Article in journal (Refereed)
    Abstract [en]

    Owing to effective aeration and energy dissipation, a stepped spillway is commonly used in a roller-compacted concrete (RCC) dam. However, its complex air-water flow features are far from being fully understood. Roughness density, step and cavity shapes are essential parameters. Numerical simulations are carried out to investigate their effects on hydraulic properties. In combination with the realizable k-epsilon turbulence model, the two-phase Mixture Model is used. The results indicate higher air concentrations for the spillway with rounded steps than the ones with trapezoidal steps; the roughness density and cavity shape show no observable effects on the aeration performance with cavity blockages. The characteristic air-water velocity for the trapezoidal steps layout is larger than that for the rounded steps. However, neither layout is sensitive to the roughness density; the velocity results for trapezoidal cavity and rounded cavity cases are almost independent of the roughness density. The velocity for all cases exclusive of trapezoidal steps increase with an increase in roughness density. The min. and max. pressures on the trapezoidal steps are slightly larger than those on the rounded steps; they increase with an increasing roughness density. The cavity shape and roughness density do not evidently influence the extreme pressures. Compared with the conventional step layout, chamfering the step edges slightly enhance the energy dissipation; partially blocking the cavities do not lead to any substantial change. In addition, the energy loss is not clearly related to the roughness density and step edge/cavity shape.

  • 23.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. Vattenfall AB, R&D Hydraulic Laboratory, Älvkarleby, Sweden.
    Liu, Wei
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Sustainable Buildings.
    Estimation of aerator air demand by an embedded multi‐gene genetic programming2021In: Journal of Hydroinformatics, ISSN 1464-7141, E-ISSN 1465-1734, Vol. 23, no 5, p. 1000-1013Article in journal (Refereed)
    Abstract [en]

    A spillway discharging a high-speed flow is susceptible to cavitation damages. As a countermeasure, an aerator is often used to artificially entrain air into the flow. Its air demand is of relevance to cavitation reduction and requires accurate estimations. The main contribution of this study is to establish an embedded multi-gene genetic programming (EMGGP) model for improved prediction of air demand. It is an MGGPbased framework coupled with the gene expression programming acting as a pre-processing technique for input determination and the Pareto front serving as a post-processing measure for solution optimization. Experimental data from a spillway aerator are used to develop and validate the proposed technique. Its performance is statistically evaluated by the coefficient of determination (CD), Nash–Sutcliffe coefficient (NSC), root-mean-square error (RMSE) and mean absolute error (MAE). Satisfactory predictions are yielded with CD ¼ 0.95, NSC ¼ 0.94,RMSE ¼ 0.17 m3/s and MAE ¼ 0.12 m3/s. Compared with the best empirical formula, the EMGGP approach enhances the fitness (CD and NSC)by 23% and reduces the errors (RMSE and MAE) by 48%. It also exhibits higher prediction accuracy and a simpler expressional form than the genetic programming solution. This study provides a procedure for the establishment of parameter relationships for similar hydraulic issues

  • 24.
    Li, Shicheng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Hydraul Lab, Alvkarleby, Sweden..
    Ma, Xin
    Sichuan Univ, State Key Lab Hydraul & Mt River Engn, Chengdu, Peoples R China.;Nanjing Hydraul Res Inst, State Key Lab Hydrol Water Resources & Hydraul En, Nanjing, Peoples R China..
    Li, Xin
    Sichuan Univ, State Key Lab Hydraul & Mt River Engn, Chengdu, Peoples R China.;Univ Alberta, Dept Civil & Environm Engn, Edmonton, AB, Canada..
    Flow features in a pooled fishway with V-shaped weir formation2020In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 14, no 1, p. 1337-1350Article in journal (Refereed)
    Abstract [en]

    A pooled fishway is an artificial structure to improve the river flow connectivity and facilitate fish migration. Its conventional layout is characterized by straight weirs and suffers from limited passage rate. To improve its flow behaviors, an alternative design, featuring a V-shaped weir arrangement, is devised. CFD simulations are performed to examine its effects on the flow conditions. The governing geometrical parameters include weir angling, height, and spacing. Depending on weir arm angle, the flow is directed towards either the channel's central part or the sidewalls, leading to both across- and along-channel differences in flow velocity and turbulent kinetic energy. This offers conducive zones for multiple fish species with distinct swimming preferences. For a given location in a pool, the cross-sectional velocity exhibits a cosine-type variation; a relationship is established to predict the maximum longitudinal velocity. The pressure distribution exhibits a similar pattern, with high-pressure areas residing upstream of the weir. A power law describes the relationship between the energy dissipation rate and unit discharge with an exponent at approximately 2/3. With an increase in either weir spacing or height, the energy dissipation rate declines. A change in weir angle shows however a negligible influence on it.

  • 25. Liang, S. D.
    et al.
    Zhang, Y. L.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    An experimental study on pile scour mitigating measures under waves and currents2015In: Science China Technological Sciences, ISSN 1674-7321, Vol. 58, no 6, p. 1031-1045Article in journal (Refereed)
    Abstract [en]

    An experimental study has been carried out to investigate effects of four flow-altering scour countermeasures placed around a foundation pile under currents only as well as under coexisting waves and currents. The countermeasures are sacrificial piles, downstream bed sill, sleeve and slot in the pile. Their arrangements follow the suggested optimal configurations, with some of them slightly modified. In terms of the evolution of scour depth and bed topography around the pile, the scour countermeasures are evaluated, subjected to steady currents with or without irregular waves. A comparison between maximum scour depth with the countermeasures and one without the countermeasures demonstrates the efficiency of countermeasures. All the tested scour measures reduce the scour depth by 17.6%–42.6% under the action of currents only and 5.8%–24.0% under the combined action of both currents and waves. The results also show that it takes a shorter time for the scour depth to reach its equilibrium with the measures under either coexisting currents and waves or currents only than ones without the measures.

  • 26.
    Liang, SD
    et al.
    Tsinghua University.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    An experimental study of scour countermeasures for offshore piles in marine environment2013Conference paper (Refereed)
    Abstract [en]

    Four commonly used scour countermeasures, i.e. riprap, horizontal collar, tetrahedral frame and the combination of riprap and tetrahedral frames around a cylindrical pile are tested in a current-wave flume to examine their effects on scouring. The evolution of scour depth around the pile in regard of the scour countermeasures, subjected to the action of steady currents with or without irregular waves, and the bed topography change around the pile are evaluated. The range of scour and its erosion volume are estimated. A comparison between scour parameters with and without the countermeasures demonstrates the efficacy of the countermeasure in terms of the maximum scour depth. All of the tested countermeasures reduce the scour depth significantly. However, depending on the type of scour countermeasure adopted, the scour depth reduction efficacy varies considerably. The tests also show that it takes a longer time to reach the maximum scour depth in the current-wave conditions than only in the current conditions. For the majority of the cases, the scour range and volume subjected to the combined current-wave action is larger than only to the current action.

  • 27. Lin, C.
    et al.
    Kao, M. -J
    Wong, W. -Y
    Tsai, C. -P
    Chang, K. -T
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Characteristics of flow separation and hydraulic jump during run-down motion of shoaling solitary wave traveling over steep sloping bottom2016In: Proceedings of the International Offshore and Polar Engineering Conference, International Society of Offshore and Polar Engineers , 2016, p. 724-731Conference paper (Refereed)
    Abstract [en]

    The features of velocity fields for the evolution of shoaling solitary wave, having a wave-height to water-depth ratio of 0.363 and propagating over a 1:3 sloping bottom, are investigated experimentally. A flow visualization technique using particle trajectory method and a high-speed particle image velocimetry (HSPIV) system employing a high-speed digital camera were used. This study mainly focuses on the occurrence of separated shear layer from the sloping bottom, evolved vortex structure, subsequent hydraulic jump, and curling jet of the backward breaking wave impinging upon the free surface of retreated flow during the run-down motion of the shoaling solitary wave.

  • 28. Lin, C.
    et al.
    Kao, M. -J
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Teng, P. -H
    Raikar, R. V.
    Study on probabilistic mean features of lower and upper free-surface profiles and velocity fields of a highly fluctuating free jet over a chute2018In: Journal of Marine Science and Technology (Taiwan), ISSN 1023-2796, Vol. 26, no 3, p. 309-326Article in journal (Refereed)
    Abstract [en]

    An optic-based method that utilizes the particle-laden images captured during high-speed particle image velocimetry (HSPIV) measurements is presented, aiming to explore the probabilistic mean characteristics of the free surface profiles and velocity fields of a free jet with high-frequency random fluctuations over a chute. The technique based on the gray-level gradients in the smoothed gray level distribution of the contrast-enhanced images is used to determine the probabilistic mean features of the free jet, right beneath and above which the water-air interfaces have I%/(100 I)% intermittent appearance of air/water phase and (100 I)% /I% fitful show-up of water/air phase. Further, the cross-correlation calculation for HSPIV measurements is employed to obtain the instantaneous and probabilistic mean velocity fields of the free jet. A target experiment of the free jet having a mean water-depth of 2.76 cm and a Froude number of 3.92 over a 17 chute model is performed in a re-circulating water channel to demonstrate the application of this method. The entire process for obtaining the probabilistic mean positions of the free surface profiles is elucidated step-by step. The lower/upper part of the free surface changing from the height at which the possibility of intermittent appearance of water phase is 3%/97%, via the counterpart for 50%/50%, to that for 97%/ 3% is identified precisely. In addition, the probabilistic mean velocity field is further categorized into the conditionally and overall time-averaged ones. Each streamwise velocity profile in the conditionally time-averaged velocity field is fairly uniform. However, the counterpart in the overall time-averaged velocity field evidently shows the non-uniform feature with prominent velocity gradient in the lower/upper part between the height at which the possibility of intermittent appearance of water phase is 3%/97% and the counterpart is 97%/3%.

  • 29. Lin, C
    et al.
    Kao, MJ
    Lo, LF
    Yeh, PH
    Raikar, RV
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Huang, H
    Similarity velocity profile of vortex flow in the cavities between girders under a partially inundated bridge deck2014Conference paper (Refereed)
    Abstract [en]

    This paper presents the experimental results on similarity velocity profile of vortices inside the cavities, formed between two neighboring girders, under a partially inundated bridge deck. Particle image velocimetry (PIV) and flow visualization technique are both employed to explore the flow field. The approaching flow is subcritical with Froude number varying in the range 0.137–0.381. The velocity characteristics of vortex structure inside the cavities under a partially inundated bridge deck, where water is fully occupied without air-pocket, are mainly investigated. The similarity profile of the azimuthal velocity along an arbitrary line, passing through the vortex core, is uniquely obtained using the measured azimuthal velocities for two different flow types. The selection of the characteristic length and velocity scales used for obtaining the similarity profile is discussed in this study.

  • 30. Lin, C.
    et al.
    Lu, C. H.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Liu, Ting
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Study on water flow field around a stationary air bubble attached at the top wall of a Circular Pipe2013In: Computational methods in multiphase flow VII, WIT Press, 2013, p. 323-338Conference paper (Refereed)
    Abstract [en]

    The presence of bubbles in a pipeline is thought to be one of the reasons to cause the hydraulic-electrical and hydraulic-mechanical facility systems to lose their efficiency. From previous research, the bubble also reduces the effective pipe cross section, which results in a reduction in pipe capacity. The efficiency and service life of pumps and turbines are reduced and shortened consequently. It may even create the interruption of the flow field within blowout phenomenon. As a result, the presence of a bubble in the pipeline is anticipated to create potential hazards. Therefore, it is very interesting to make clear the corresponding variation of a water flow field around a stationary air bubble attached at the top inner-wall of pipe due to the surface problems in contact mechanism of these three phases among the solid wall of pipe, stationary air bubble, and ambient water flow. This study applied flow visualization techniques and high time-resolved PIV to investigate the characteristics of a flow field around a stationary bubble in a fully-developed horizontal pipe flow. Experiments were carried out in a pipe having a constant inner diameter of 9.60 cm and a length of 260.0 cm, yet varied with different bubble volumes (or lengths). Two settling water chambers with different still water levels were connected to both ends of the circular pipe. Titanium dioxide powder being uniformly dispersed in the pipe flow was used as a tracer both for flow visualization tests and for PIV measurements. The results show that a horseshoe vortex and reverse flow at the upstream and downstream of the bubble respectively are commonly seen in all test cases. The experimental results also show that the shape and volume of a bubble highly affect the flow field in the surroundings of the stationary air bubble. Since the bubble surface is slippery, flow velocity exists on the surface of a bubble. As a result, the reverse flow at the end of a long-flat bubble would not affect the velocity on the bubble surface.

  • 31. Lin, C.
    et al.
    Yu, S. -M
    Wong, W. -Y
    Tzeng, G. -W
    Kao, M. -J
    Yeh, P. -H
    Raikar, R. V.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Tsai, C. -P
    Velocity characteristics in boundary layer flow caused by solitary wave traveling over horizontal bottom2016In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 76, p. 238-252Article in journal (Refereed)
    Abstract [en]

    The characteristics of horizontal velocity in the bottom boundary-layer flow induced by a solitary wave propagating over a horizontal bottom are presented experimentally, using high-speed particle image velocimetry (HSPIV). The ratio of wave height to water depth varies from 0.096 to 0.386 and the flow inside the boundary layer is laminar. The results show that the horizontal velocity profiles can be mainly classified into two categories with respect to the passing of the solitary wave-crest at the measuring section: the pre-passing (or acceleration) phases under favorable pressure gradient and post-passing (or deceleration) phases under adverse pressure gradient. For the velocity distributions obtained during the pre-passing phases, a nonlinear regression analysis was used to precisely determine the time-dependent characteristic length and velocity scales underlying these profiles. A similarity profile of the horizontal velocity is established first using the time-dependent free-stream velocity and boundary layer thickness as the characteristic velocity and length scales, respectively. In addition, the displacement thickness, the momentum thickness, and the energy thickness are also considered as alternative length scales. All these four representative thicknesses are closely related to each other, demonstrating that any one amongst them can be regarded as the characteristic length scale. The forms of similarity profiles for the non-dimensional velocity distributions are somewhat analogous to the results of steady boundary layer flow over a thin plate under with pressure gradient, but with different coefficients or powers. While during the post-passing phases, flow reversal which acts like an unsteady wall jet and moves in the opposite direction against the wave propagation occurs after the passage of solitary wave-crest. The thickness of flow reversal layer increases with time. A similarity profile is proposed for the velocity distributions corresponding to occurrence of the extreme value in the maximum negative velocity of flow reversal. Variations of the maximum negative velocity and the thickness of flow reversal with the time right after the start of flow reversal are also discussed in detail. Moreover, the non-dimensional time leads of the horizontal velocities at different heights in the boundary layer over the free-stream velocity are evidenced to be more noticeable toward the bottom, and also in lower ratio of wave height to water depth. A similarity profile for the non-dimensional time lead versus the non-dimensional height above the bottom surface is also presented.

  • 32. Lin, Chang
    et al.
    Hsieh, SC
    Lo, LF
    Chang, SY
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Green aerator using periodically oscillatory flow generated inside a vertical dropping pool2012Conference paper (Refereed)
    Abstract [en]

    The flow conditions for generating periodically oscillatory flows and thus entraining a large amount of air bubbles into the water mass in a vertical dropping pool, which can be used potentially as a green aerator, were investigated experimentally. The approaching flows passing through upstream reach of a vertical dropping pool were studied under sub-critical condition. A wave gauge was used to measure the free surface fluctuations in the pool, and flow visualization technique was employed to reveal the flow structure of the dropping flows qualitatively. Under certain conditions, the falling flow over a vertical dropping pool forms a switching jet that oscillates up and down periodically and impinges on the bottom and the downstream corner of the pool alternately. The switching jet switches between an impinging jet (or napped flow) and a sliding jet (or skimming flow), causing it to oscillate periodically with a unique period and to entrain a large number of air bubbles into the switching jet, thus enhancing the dissolved oxygen quantity and turbulent mixing. The primary frequency of the periodic oscillation was determined by applying spectral analysis to the time history of wave-gauge measurements for the free surface elevation of the dropping flows. Variables influencing the fundamental oscillation frequency were carefully checked, and an empirical relation between a weighted Strouhal number and a grouped non-dimensional parameter was proposed to predict the primary frequency of the periodically oscillatory flow.

  • 33. Lin, Chang
    et al.
    Kao, Ming-Jer
    Raikar, Rajkumar Venkatesh
    Yuan, Juan-Ming
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Chuang, Po-Yu
    Syu, Jie-Ming
    Pan, Wei-Chih
    Novel similarities in the free-surface profiles and velocities of solitary waves traveling over a very steep beach2020In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 32, no 8, article id 083601Article in journal (Refereed)
    Abstract [en]

    This study investigates experimentally similarity and Froude number similitude (FNS) in the dimensionless flow features of three solitary waves traveling on a 1:3 sloping beach. These three waves, designated as cases A, B, and C, respectively, have different heights H-0 (=5.8 cm, 2.9 cm, and 1.815 cm) and still water depths h(0) (=16.0 cm, 8.0 cm, and 5.0 cm), but identical ratios H-0/h(0) (=0.363). A high-speed particle image velocimetry system is employed to obtain the free surface profiles (FSPs) and velocity fields/profiles. These features include the free surface elevation (FSE)/FSP time series; velocity fields and profiles, positions, and propagation speeds of flow demarcation curves; times and maximum onshore distances of the maximum run-up heights (MRHs); and times and onshore distances of hydraulic jumps for cases A and B. When the swash tip of a solitary wave reaches the MRH, the contact point becomes almost immobile for a short time interval, with the contact angle changing from obtuse, via right, to acute angle. For cases A and B, the similarities in the dimensionless MRHs and times, at which the run-down motions of the wave tips start, are affirmed. These facts highlight that the swash tips and contact points are subject to complex interactions among gravity force, viscous friction, and surface tension of fluid. Case C, having the smallest length scale, is only focused on the arrival or starting time of the MRH or run-down motion and the MRH and used as a counterexample to demonstrate the absence of similarity or FNS due to the relatively prominent effects of viscous friction and surface tension.

  • 34.
    Lin, Chang
    et al.
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Kao, Ming-Jer
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Raikar, Rajkumar Venkatesh
    KLE Dr MS Sheshgiri Coll Engn & Technol, Dept Civil Engn, Belagavi 590008, Karnataka, India..
    Yuan, Juan-Ming
    Providence Univ, Dept Data Sci & Big Data Analyt, Taichung 43301, Taiwan..
    Hsieh, Shih-Chun
    Axesea Engn Technol Co Ltd, Taichung 407034, Taiwan..
    Particle acceleration and pressure gradient in a solitary wave traveling over a horizontal bed2020In: AIP Advances, E-ISSN 2158-3226, Vol. 10, no 11, article id 115210Article in journal (Refereed)
    Abstract [en]

    This study presents experimental data on the hydrodynamic features of a nonlinear solitary wave of height H-0 = 2.9 cm propagating over a horizontal bed in otherwise still water depth h(0) = 8.0 cm. High-speed particle image velocimetry is used to quantify the local and convective particle accelerations and pressure gradients under the solitary wave. It is found that the magnitudes of the horizontal and vertical particle local accelerations are invariably larger than their convective counterparts, highlighting the important contribution of local acceleration to the pressure gradient in a solitary wave. The dimensionless pressure gradient in the horizontal direction is favorable, zero, and adverse for time before, at, and after the wave crest passes the section of interest. The dimensionless adverse pressure gradient exhibits a maximum for the dimensionless time T[=t(g/h(0))(1/2), where g is the gravitational acceleration and t is the time with respect to the crest passing the section] = 1.39, at which the dimensionless local acceleration has a negative maximum. Subsequently, flow reversal takes place above the bed surface. The relationship is elucidated between flow reversal at the bed surface and the evolution of the adverse pressure gradient in the near-bed zone, where uniform horizontal/free stream velocity exists.

  • 35.
    Lin, Chang
    et al.
    Department of Civil Engineering, National Chung Hsing University, Taichung 40227, Taiwan.
    Kao, Ming-Jer
    Department of Civil Engineering, National Chung Hsing University, Taichung 40227, Taiwan.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Raikar, Rajkumar
    Department of Civil Engineering, KLE Dr. M.S. Sheshgiri College of Engineering and Technology, Udyambag, Belagavi 590008, Karnataka, India.
    Yuan, Juan-Ming
    Department of Data Science and Big Data Analytics, Providence University, Taichung 43301, Taiwan.
    Hsieh, Shih-Chun
    Axesea Engineering Technology Co., Ltd., Taichung 407034, Taiwan.
    Similarity and Froude Number Similitude in Kinematic and Hydrodynamic Features of Solitary Waves over Horizontal Bed2021In: Processes, ISSN 2227-9717, Vol. 9, no 8, article id 1420Article in journal (Refereed)
    Abstract [en]

    This study presents, experimentally, similarity and Froude number similitude (FNS) in the dimensionless features of two solitary waves propagating over a horizontal bed, using two wave gauges and a high-speed particle image velocimetry (HSPIV). The two waves have distinct wave heights H0 (2.9 and 5.8 cm) and still water depths h0 (8.0 and 16.0 cm) but identical H0/h0 (0.363). Together with the geometric features of free surface elevation and wavelength, the kinematic characteristics of horizontal and vertical velocities, as well as wave celerity, are elucidated. Illustration of the hydrodynamic features of local and convective accelerations are also made in this study. Both similarity and FNS hold true for the dimensionless free surface elevation (FSE), wavelength and celerity, horizontal and vertical velocities, and local and convective accelerations in the horizontal and vertical directions. The similarities and FNSs indicate that gravity dominates and governs the wave kinematics and hydrodynamics.

  • 36.
    Lin, Chang
    et al.
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Kao, Ming-Jer
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Hydraulic Laboratory, Älvkarleby, 81426, Sweden.
    Yuan, Juan-Ming
    Providence Univ, Dept Data Sci & Big Data Analyt, Taichung 43301, Taiwan..
    Hsieh, Shih-Chun
    Axesea Engn Technol Co Ltd, Taichung 407034, Taiwan..
    Effects of Nonlinearity on Velocity, Acceleration and Pressure Gradient in Free-Stream Zone of Solitary Wave over Horizontal Bed-An Experimental Study2022In: Water, E-ISSN 2073-4441, Vol. 14, no 22, article id 3609Article in journal (Refereed)
    Abstract [en]

    For solitary waves on a horizontal bed, the study deals experimentally with the high ratio of wave height (H-0) to still water depth (h(0)) that amplifies the wave nonlinearity. The value of H-0/h(0) tested in a wave flume ranges from 0.050 to 0.550, indicating the shift from a quasi-linear solitary wave to a highly nonlinear one. A high-speed particle image velocimetry (HSPIV) and a flow visualization technique of particle-trajectory tracking method are utilized to measure velocity fields and identify near-bed flow structures. The unsteady free-stream velocities with equal magnitude take place in a free-stream zone, FSZ). The FSZ underlies the internal flow zone, over which the external free surface of solitary wave exists and is situated beyond the boundary layer. The spatio-temporal variation of free-stream velocity, moving in phase with the free surface elevation, characterizes the pattern of pressure gradient in the FSZ and thus dominates the behavior of boundary layer flow. Accordingly, nonlinear effects on the time series as well as the maximum values of horizontal velocity, particle acceleration, and pressure gradient in the FSZs of solitary waves are presented. Before, at, and after the wave crest's intersection with a given measurement location, favorable, zero, and adverse pressure gradients occur in the FSZ, respectively. For H-0/h(0) = 0.179, 0.363, and 0.550, the values of the dimensionless maximum free-stream velocity are about 3.10, 5.32, and 6.20 times that (= 0.0473) for H-0/h(0) = 0.050; and the corresponding values of the dimensionless maximum adverse pressure gradient are about 5.74, 14.54 and 19.84 times that (= 0.0061) for H-0/h(0) = 0.050. This evidence highlights the nonlinear effect on the kinematic and hydrodynamic features of solitary waves. Finally, the effect of nonlinearity on the relationship between the dimensionless time for the maximum adverse pressure gradient in the FSZ and that for the incipient flow reversal in the bottom boundary layer is explored for the first time. It is found that the incipient flow reversal takes place immediately after the maximum adverse pressure gradient, together with a decrease in the dimensionless time for flow reversal if H-0/h(0) increases. The fact accentuates the nonlinear effect on the incipient flow reversal right above the bed.

  • 37.
    Lin, Chang
    et al.
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Kao, Ming-Jer
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Yuan, Juan-Ming
    Providence Univ, Dept Data Sci & Big Data Analyt, Taichung 43301, Taiwan..
    Raikar, Rajkumar Venkatesh
    KLE Dr MS Sheshgiri Coll Engn & Technol, Dept Civil Engn, Belagavi 590008, Karnataka, India..
    Wong, Wei-Ying
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Yang, Ray-Yeng
    Natl Cheng Kung Univ, Dept Hydraul & Ocean Engn, Tainan 701301, Taiwan.;Natl Cheng Kung Univ, Tainan Hydraul Lab, Tainan 709015, Taiwan..
    Features of the flow velocity and pressure gradient of an undular bore on a horizontal bed2020In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 32, no 4Article in journal (Refereed)
    Abstract [en]

    Experimental results are presented regarding the free-surface elevations, velocity fields, and horizontal pressure gradients of dambreak-generated undular bores propagating over a horizontal bottom. Ultrasonic wave gauges, high-speed particle image velocimetry, and a flow visualization method are used to investigate the flow fields. Features of the full-depth velocity field and pressure gradient (equal to minus the sum of the local and convective accelerations) in the free stream over the boundary layer are elucidated with respect to the phase of the free-surface elevation and are categorized into four temporal stages. For stage I with rising free-surface elevations, a successive increase in the magnitude of the pressure gradient (with a negative value) corresponds to a favorable pressure gradient in the free stream. Around the zero-up/down-crossing phase of each leading wave evolving in stage II, the pressure gradient has a negative/positive maximum, revealing the maximum favorable/adverse pressure gradient in the free stream. However, the pressure gradient is zero at each crest or trough phase, showing an instantaneous zero pressure gradient. Within stage III characterized by a constant free-surface elevation, the pressure gradient is almost zero. In stage IV with descending free-surface elevations, the pressure gradient first increases from nearly zero to a positive maximum (representing the maximum adverse pressure gradient), then keeps this value for a period of time, and eventually decreases to zero. Subsequently, flow reversal with an increase in thickness over the bottom and free-stream velocity equal to zero takes place.

  • 38. Lin, Chang
    et al.
    Kao, MJ
    Tzing, GW
    Wong, WY
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Study on Flow Fields of Boundary-Layer Separation and Hydraulic Jump during Rundown Motion of Shoaling Solitary Wave2015In: Journal of Earthquake and Tsunami, ISSN 1793-7116, Vol. 9, no 5Article in journal (Refereed)
    Abstract [en]

    The characteristics of flow fields for a complete evolution of the non-breaking solitary wave, having a wave-height to water-depth ratio of 0.363 and propagating over a 1:5 sloping bottom, are investigated experimentally. This study mainly focuses on the occurrences of both flow separation on the boundary layer under an adverse pressure gradient and subsequent hydraulic jump with the abrupt rising of free surface during run-down motion of the shoaling wave, together with emphasis on the evolution of vortex structures underlying the separated shear layer and hydraulic jump. A flow visualization technique with particle trajectory method and a high-speed particle image velocimetry (HSPIV) system with a high-speed digital camera were used. Based on the instantaneous flow images visualized and/or the ensemble-averaged velocity fields measured, the following interesting features, which are unknown up to date, are presented and discussed in this study: (1) Flow bifurcation occurring on both offshore and onshore sides of the explicit demarcation curve and the stagnation point during run-up motion; (2) The dependence of the diffuser-like flow field, being changed from the supercritical flow in the shallower region to the subcritical flow in the deeper counterpart, on the Froude number during the early and middle stages of run-down motion; (3) The positions and times for the occurrences of the incipient flow separation and the sudden rising of free surface of the hydraulic jump; (4) The associated movement and evolution of vortex structures under the separated shear layer, the hydraulic jump and/or the high-speed external main stream of the retreated flow; and (5) The entrainment of air bubbles from the free surface into the external main stream of the retreated flow.

  • 39. Lin, Chang
    et al.
    Liu, Ting
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Lu, Chia-Hsun
    Visualizing Conduit Flows around Solitary Air Pockets2014In: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 141, no 5Article in journal (Refereed)
    Abstract [en]

    Understanding flow characteristics around air pockets is fundamental in the study of air entrainment and transport in pipelines. This study deals with the use of flow visualization technique (FVT) and high-speed particle image velocimetry (HSPIV) in exploration of the characteristics around stationary air pockets in horizontal-pipe flow. The air-pocket volume varies from 0 to 10.0 mL, and the air pocket is injected into a fully developed turbulent flow with Reynolds numbers between 17,000 and 18,400. In the plane of symmetry, the main flow features include (1) a horseshoe vortex upstream, (2) a stagnation point on the frontal interface, (3) a separation point and a separated shear layer beneath, (4) a reattached shear layer downstream of the reattachment point (for air-pocket volumes greater than 2.0 mL), and (5) a reverse-flow region downstream. The deformable air pocket in the turbulent flow causes streamwise random movements of both the stagnation and separation points around their mean positions. The flow pattern is categorized based on the occurrence of either separated flow or flow reattachment. Fully separated flow (Mode I) occurs at air-pocket volumes less than 2.0 mL. Intermittently reattached flow (Mode II) occurs if the volume is within 2.0–5.0 mL. Fully reattached flow (Mode III) is evident at volumes greater than 5.0 mL. Water particles on the air-pocket surface move with the adjacent flow, thus forming a slip boundary. The evolution of mean streamwise velocity beneath the air pocket demonstrates the formation of either a separated or a reattached shear layer. Using nonlinear regression analysis, appropriate characteristic velocity and length scales are determined to obtain similarity profiles in the separated shear layer beneath.

  • 40. Lin, Chang
    et al.
    Lu, CH
    Liu, Ting
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Characteristics of air-water interface of air pockets in a conduit2014Conference paper (Refereed)
    Abstract [en]

    The presence of air pockets in a pipeline system often causes reduction in its efficiency and shortens its service life. Potential safety hazards arise in some cases from air blowout or blowback. It is thus of interest to examine the water flow field at air pockets and the feature of water-air interface. This study applied flow visualization technique and high-speed particle image velocimetry (HSPIV) to investigate characteristics of flow fields at stationary solitary air pockets in a fully-developed horizontal pipe flow. Experiments were performed in a Plexiglas pipe having an inner diameter of 9.6 cm, with Titanium dioxide powder as tracer for measurements. The results show that a horseshoe vortex and reverse flow pattern existed both up- and downstream of the air pockets. A deformable air pocket in the turbulent flow caused streamwisely a random movement of both stagnation and separation points around their mean positions. An intermittent flow re-attachment occurred also downstream of the mean separation point. The air-water interface was not stationary but moved with the adjacent water flow.

  • 41.
    Lin, Chang
    et al.
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Wong, Wei-Ying
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Kao, Ming-Jer
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. Älvkarleby Lab, Vattenfall AB Res & Dev, SE-81426 Älvkarleby, Sweden..
    Raikar, Rajkumar V.
    KLE Dr MS Sheshgiri Coll Engn & Technol, Dept Civil Engn, Angol Main Rd, Belgaum 590008, Karnataka, India..
    Yuan, Juan-Ming
    Providence Univ, Dept Data Sci & Big Data Analyt, Taichung 43301, Taiwan..
    Hydrodynamic Features of an Undular Bore Traveling on a 1:20 Sloping Beach2019In: Water, E-ISSN 2073-4441, Vol. 11, no 8, article id 1556Article in journal (Refereed)
    Abstract [en]

    The hydrodynamic characteristics, including local and convective accelerations as well as pressure gradient in the horizontal direction, of the external stream of an undular bore propagating on a 1:20 sloping beach are experimentally studied. A bore with the water depth ratio of 1.70 was generated downstream of a suddenly lifted gate. A high-speed particle image velocimetry was employed to measure the velocity fields during the run-up and run-down motions. The time series of free surface elevation and velocity field/profile of the generated bore, comprising a pure bore accompanied by a series of dispersive leading waves, are first demonstrated. Based on the fast Fourier transform (FFT) and inverse FFT (IFFT) techniques, the free surface elevation of leading waves and the counterpart of pure bore are acquired separately at a specified measuring section (SMS), together with the uniform horizontal velocity of the pure bore. The effect of leading-wave-induced velocity shift on the velocity profiles of the generated bore are then evaluated at the SMS. To understand the calculation procedure of accelerations and pressure gradient, three tabulated forms are provided as illustrative examples. Accordingly, the relationships among the partially depth-averaged values of the non-dimensional local acceleration, convective acceleration, total acceleration and pressure gradient of the generated/pure bore acquired at the SMS versus the non-dimensional time are elucidated. The trends in the non-dimensional accelerations and pressure gradient of the external stream of generated bore are compared with those of the pure bore. During the run-up motion from the instant of arrival of the bore front to the moment of the peak level at the SMS, continuous decrease in the onshore uniform horizontal velocity, and successive deceleration of the pure bore in the onshore direction are evidenced, exhibiting the pure bore under the adverse pressure gradient with decreasing magnitude. However, the pure bore once ridden by the leading waves is decelerated/accelerated spatially and accelerated/decelerated temporally in the onshore direction during the rising/descending free surface of each leading wave. This fact highlights the effect of pre-passing/post-passing of the leading wave crest on the velocity distribution of generated bore. It is also found that, although the leading waves have minor contribution on the power spectrum of the free surface elevation as compared with that of the pure bore, the leading waves do play an important role on the magnitudes of both accelerations and pressure gradient. The largest magnitude of the acceleration contributed by the leading waves is around 26 times the counterpart contributed by the pure bore. Further, during the run-down motion right after the moment for the peak level of the bore, a linear increase in the magnitude of the offshore uniform horizontal velocity and a constant local acceleration with increasing time are both identified. The partially depth-averaged value of the non-dimensional pressure gradient is equal to a small negative constant (-0.0115) in the offshore direction, indicating that the bore is subject to a constant favorable pressure gradient.

  • 42. Lindblom, E.
    et al.
    Arnell, M.
    Flores-Alsina, X.
    Stenström, F.
    Gustavsson, D. J. I.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Jeppsson, U.
    Dynamic modelling of nitrous oxide emissions from three Swedish sludge liquor treatment systems2016In: Water Science and Technology, ISSN 0273-1223, E-ISSN 1996-9732, Vol. 73, no 4, p. 798-806Article in journal (Refereed)
    Abstract [en]

    The objective of this paper is to model the dynamics and validate the results of nitrous oxide (N2O) emissions from three Swedish nitrifying/denitrifying, nitritation and anammox systems treating real anaerobic digester sludge liquor. The Activated Sludge Model No. 1 is extended to describe N2O production by both heterotrophic and autotrophic denitrification. In addition, mass transfer equations are implemented to characterize the dynamics of N2O in the water and the gas phases. The biochemical model is simulated and validated for two hydraulic patterns: (1) a sequencing batch reactor; and (2) a moving-bed biofilm reactor. Results show that the calibrated model is partly capable of reproducing the behaviour of N2O as well as the nitritation/nitrification/denitrification dynamics. However, the results emphasize that additional work is required before N2O emissions from sludge liquor treatment plants can be generally predicted with high certainty by simulations. Continued efforts should focus on determining the switching conditions for different N2O formation pathways and, if full-scale data are used, more detailed modelling of the measurement devices might improve the conclusions that can be drawn.

  • 43.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Air-pocket movement in an 18.2 degree downward 240 mm conduit, experimental studies2012In: Procedia Engineering, ISSN 1877-7058, E-ISSN 1877-7058, Vol. 28, p. 791-795Article in journal (Refereed)
    Abstract [en]

    Experiments are carried out in a test rig, consisting of a Plexiglas pipe with an inner diameter of 240 mm and an inclination of 18.2o, to investigate air-water two-phase flows in conjunction with bottom spillways. Results show that the critical velocity, which is the minimal water velocity to start moving an air pocket, in the rough pipe, is independent of the air-pocket volume; in the smooth pipe it doesn’t increase with increasing diameter as much as the previous researchers indicated. Pipe roughness doesn’t affect the velocity of the air-pocket when it moves upstream in the downward inclined pipe.

  • 44.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    CFD Modeling of Air Pocket Transport in Conjunction with Spillway Conduits2011In: 11th International Conferenceon Fluid Control, Measutements and Visualization, Keeling, Taiwan, December 2-9 2011, 2011Conference paper (Refereed)
    Abstract [en]

    This paper focuses on simulations of enclosed air pocket movements in conjunction with bottom outlet operations. The critical velocity of water for air pocket transport in pipe is the minimal flow velocity for the air pocket start to move downstream. A numerical model is developed to simulate the critical velocity of air pocket transport in pipe flow and to discuss the impacts of tunnel slope, size of the air pocket and wall roughness. The computations are performed in FLUENT using Volume of Fraction (VOF) model combined with k-epsilon model. Parallel computing is adopted for high computational performance.

    The modeled critical velocity is compared with experimental results and they increase with increasing slopes. However, as the roughness height defined in the model is not big enough to represent the reality and no wall shear stress is applied in the upper wall where air pocket and wall contact, the modeled critical velocity is smaller than the experimental ones. Therefore, wall roughness contributes to keep the air pocket from moving downstream which is important in modeling critical velocity. However, by assuming a constant wall shear stress for the air phase the same as the water phase will overestimate the shear stress on the air pocket.

    Two air pocket volumes are simulated at the slope 0.8 degrees which shows the bigger the air pocket is the higher the critical velocity is. Modeling results also show that the critical velocity is non-zero in horizontal pipe and there is a limit for the carrying capacity at all slopes. The simulations of air pockets with different volumes in the bottom tunnel of Letten dam in North of Sweden is shown in this paper as well.

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  • 45.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Experimental studies of air pocket movement in a pressurized spillway conduit2013In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 51, no 3, p. 265-272Article in journal (Refereed)
    Abstract [en]

    Undesired air entrainment in a bottom outlet conduit causes pressure transients, leading to conduit vibrations, blowbacks and discharge pulsations and thus endangers operational safety. In this study, the propagation velocity of a solitary air pocket and the characteristics of its critical velocity were examined in experiments conducted using a 240-mm-diameter pipe. Air pocket movement depends on the pipe diameter, slope, roughness and air pocket size. The critical pipe Froude number for initiating downstream movement of an air pocket is smaller in a larger pipe, most likely due to the scale effect and/or to a smaller reduction in the effective cross-sectional area. The critical velocity in rough pipes was found to be independent of the air pocket size. A minimum Froude number was suggested for a rough pipe instead of a critical pipe Froude number because the air removal process was found to involve successive air losses from the air pocket caused by turbulence.

  • 46.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Experiments of Air-pocket Movement in an 18.2 degrees downward 240-mm Conduit2012In: 2012 International Conference On Modern Hydraulic Engineering, Elsevier, 2012, p. 791-795Conference paper (Refereed)
    Abstract [en]

    Experiments are carried out in a test rig, consisting of a Plexiglas pipe with an inner diameter of 240 mm and an inclination of 18.2o, to investigate air-water two-phase flows in conjunction with bottom spillways. Results show that the critical velocity, which is the minimal water velocity to start moving an air pocket, in the rough pipe, is independent of the air-pocket volume; in the smooth pipe it doesn't increase with increasing diameter as much as the previous researchers indicated. Pipe roughness doesn't affect the velocity of the air-pocket when it moves upstream in the downward inclined pipe.

  • 47.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Incipient motion of solitary air pockets in a rectangular pipe2013In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 1, no 1, p. 58-68Article in journal (Refereed)
    Abstract [en]

    The operation of bottom-outlet gates often gives rise to entrained air in the form of air pockets in the conduit under full-flow conditions. If unexpectedly released, it would cause problems for both personnel security and operational function. The present study addresses, through experimentation, the incipient movement of solitary air pockets in a rectangular pipe. A horizontal pipe and a 9.6° downward-inclined pipe are examined. The cross-section of the pipe measures 200 mm (width) by 250 mm (height). As distinct from a circular pipe, an air pocket in the rectangular pipe exhibits, at its incipient motion, a shape that depends mainly on factors such as the sloping angle of the pipe, cross-sectional location of the air pocket and its volume. These factors also determine the critical velocity of the air pocket. The experiments have shown that only small air pockets can exist under the roof. The corner is a cross-sectionally equilibrium position for larger air pockets. The air pocket in the corner position takes the shape of an elongated rectangular prism in the horizontal pipe and a triangular prism in the sloping one. When compared with a circular pipe, the critical velocity of air pockets in the rectangular pipe is lower if the pipe is horizontal and higher if it has a downward inclination.

  • 48.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Three-Dimensional Computations Of Water-Air Flow In A Bottom Spillway During Gate Opening2014In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 8, no 1, p. 104-115Article in journal (Refereed)
    Abstract [en]

    Undesired entrainment of air in a bottom spillway often leads to problems in both safety and operational functions. A numerical analysis of a transient process of air entrainment into bottom spillway flows when a spillway gate is opened was conducted in this study. The Volume of Fluid (VOF) model was used. The 3D computational domain consisted of a spillway conduit, a moving bulkhead gate, a gate shaft, an upstream reservoir and a downstream outlet. The large number of cells, together with the dynamic mesh modelling of the moving gate, required substantial computational resources, which necessitated parallel computing on a mainframe computer. The simulations captured the changes in the flow patterns and predicted the amount of air entrainment in the gate shaft and the detrainment downstream, which help in the understanding of the system behaviour during opening of the spillway gate. The initial conduit water level and the gate opening procedure affect the degree of air entrainment in the gate shaft. To release the undesired air, a de-aeration chamber with a tube leading to the atmosphere was added to the conduit. Despite the incomplete air release, the de-aeration chamber was found to be effective in reducing water surface fluctuations in the downstream outlet.

  • 49. Lundell, Dan
    et al.
    Jin, Feng
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Some aspects of rock-filled concrete in hydraulic structures2011Conference paper (Refereed)
    Abstract [en]

    The study addresses several aspects of Rock-Filled Concrete (RFC) – a new construction material developed for use in hydraulic structures. Some of the latest results of compressive strength, tensile strength and compaction level of the material are presented. In RFC, Self-Compacting Concrete (SCC) is used together with large aggregates (minimum of 30 cm in size), to produce concrete with a high content of aggregate material and a low content of cement. The large aggregates in RFC complicate the material testing. In this study, the aggregate size is limited to 150 mm in size and the strength parameters have been evaluated using cubic samples with a surface area of 500 * 500 mm. The results imply that RFC does not lead to reduced material strength as compared to SCC. Based on the latest performance results of the material, together with results from earlier studies of the production costs and environmental impacts of RFC, the conclusion is that RFC has advantages of lower material cost, faster construction and less environmental impacts as compared to both conventional concrete (CC) and Roller-Compacted Concrete (RCC) when used in large concrete structures such as dams.

  • 50.
    Molinder, Gabriella
    et al.
    Sweco.
    Ekström, Ingvar
    Sweco.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Reliability-based safety factors for earth dam stability calculations2019In: Sustainable and Safe Dams Around the World / [ed] Jean-Pierre Tournier, Tony Bennett & Johanne Bibeau, Canadian Dam Association , 2019, Vol. Vol 2, p. 229-237Conference paper (Refereed)
    Abstract [en]

    Probability based stability analysis is of rising prevalence in dam safety aroundthe world, though it is most commonly used for concrete structures. The stability of a Swedishrockfill dam has been analyzed with deterministic and probabilistic stability analyses. The probabilisticanalysis was made using the Rosenblueth method (point estimation method), where thesafety factor and reliability index β has been calculated for circular slip surfaces. A comparisonbetween deterministic and probabilistic stability analysis concludes that reliability-based stabilityanalysis is a useful method for integrating uncertainties into a stability analysis. However, itin our opinion is more suitable as a complement to deterministic analysis rather than as areplacement. High quality material data and a deep understanding of both geotechnical engineeringand probability theory is essential for reliability index β to be useful.

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