| 1. |
- Liu, Ting, et al.
(författare)
-
Experiments of Air-pocket Movement in an 18.2 degrees downward 240-mm Conduit
- 2012
-
Ingår i: 2012 International Conference On Modern Hydraulic Engineering. - : Elsevier. ; 28, s. 791-795
-
Konferensbidrag (refereegranskat)abstract
- 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.
|
|
| 2. |
- Teng, Penghua
(författare)
-
CFD MODELLING AND EXPERIMENTS ON AERATOR FLOW IN CHUTE SPILLWAYS
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A chute spillway is a typical component of large dams for discharging floods. Because of the high water head, the flow velocity in the chute is often in excess of 20 m/s. Consequently, the structure is usually prone to cavitation damages. Flow aeration is evidenced to efficiently eliminate or to mitigate the damages. An aerator is a device that entrains air into the water flows and is an effective technical measure to counter the cavitation damages.Aerator flow includes intense air-water exchange and involves a process of air entrainment, transport, and detrainment. Because of the complex phenomena, it is still a challenge to investigate the behaviors of interaction between air and water. It is fundamental to understand the flow behaviors downstream of the aerator. This thesis investigates the aerator flow features using both the Computational Fluid Dynamics (CFD) and advanced measurement techniques.The CFD method presents three two-phase flow models to describe the aerator flows, namely, the Volume of Fluid Model, the Mixture Model, and the Two-Fluid Model. They are applied and evaluated via practical engineering projects and experimental data. The Volume of Fluid model leads to reasonable results regarding the water flow discharge and flow fields. For predicting the air concentration distribution and air bubble transport processes, the Two-Fluid Model is superior to others because it includes forces acting on the air bubbles. However, the model still overestimates the air content near the chute bottom. Based on the aerator flow from a chute spillway in Sweden, three two-phase flow models are applied and compared.Physical model tests are commonly conducted to investigate aerator flow features. Because of the scale effects, the results may lead to a discrepancy in the flow behaviors compared with the prototype. Thus, CFD modeling becomes an alternative tool when seeking the reason for the difference. Based on the aerator flow in a real spillway, CFD is applied to reproduce the flow; the discrepancy between the model tests and prototype observations is evidenced. The results show similar flow features with the prototype but differ from those of the model tests. An explanation for the discrepancy is discussed in terms of flow features, effect of surface tension in model tests, and the prerequisite for air entrainment of the free-surface flow.Laboratory experiments are conducted to study the aerator flow in a chute. Four image-based measurement techniques－i.e., high-speed particle image velocimetry (HSPIV), shadowgraphic image method (SIM), bubble tracking method (BTM), and bubble image velocimetry (BIV)－are employed. The study focuses on issues of exploring characteristic positions of water-air interfaces, interpreting the evaluation process of air bubbles shed from the tip of the air cavity, identifying the probabilistic means for characteristic positions near the fluctuating free surface, and obtaining the flow field both water flow and air bubbles features of the aerator flow. The application of these techniques leads to a better understanding of two-phase flow characteristics of the chute aerator.
|
|
| 3. |
- Yang, James, adj prof, et al.
(författare)
-
Refurbishment for Effective Energy Dissipation in a Chute Spillway – Physical and Numerical Modeling
- 2023
-
Ingår i: Proceedings of the 40th IAHR World Congress. - : Research Publishing Services. ; , s. 2037-2044
-
Konferensbidrag (refereegranskat)abstract
- Many existing spillways in Sweden are now refurbished for higher discharge capacity than originally constructed for, which calls for improved energy dissipation functions to reduce downstream impacts. The spillway examined belongs to this category, with an increase in design flood by nearly 45%. To attain effective energy dissipation and curtail scouring potential in the tailwater, a new stilling basin is excavated immediately downstream of the spillway piers. Acting as roughness elements, cross-sectional bottom ribs covering the whole width are added to the chute. The element width and height chosen are 0.50 and 0.40 m. Both hydraulic model tests and CFD simulations are performed to explore and optimize the chute flow conditions and assist in the engineering solution. Upon project completion, spillway flood release has seen satisfactory energy dissipation results.
|
|
| 4. |
- Li, Shicheng, et al.
(författare)
-
Air entrainment and transport in a bottom outlet controlled by a cylindrical gate
- 2022
-
Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315.
-
Konferensbidrag (refereegranskat)abstract
- 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.
|
|
| 5. |
- Li, Shicheng, et al.
(författare)
-
Daily suspended sediment forecast by an integrated dynamic neural network
- 2022
-
Ingår i: Journal of Hydrology. - : Elsevier. - 0022-1694 .- 1879-2707. ; 604
-
Tidskriftsartikel (refereegranskat)abstract
- Suspended sediment is of importance in river and dam engineering. While, 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 (denoted as 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 its 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 forecast. 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 forecast and provides reference for time series modeling, e.g. streamflow, river temperature and salinity.
|
|
| 6. |
- Li, Shicheng, et al.
(författare)
-
Estimation of aerator air demand by an embedded multi‐gene genetic programming
- 2021
-
Ingår i: Journal of Hydroinformatics. - : IWA Publishing. - 1464-7141 .- 1465-1734. ; 23:5, s. 1000-1013
-
Tidskriftsartikel (refereegranskat)abstract
- 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
|
|
| 7. |
- Li, Shicheng, et al.
(författare)
-
Modeling transient flow dynamics around a bluff body using deep learning techniques
- 2024
-
Ingår i: Ocean Engineering. - : Elsevier BV. - 0029-8018 .- 1873-5258. ; 295
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 8. |
- Lin, C., et al.
(författare)
-
Characteristics of flow separation and hydraulic jump during run-down motion of shoaling solitary wave traveling over steep sloping bottom
- 2016
-
Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - : International Society of Offshore and Polar Engineers. - 9781880653883 ; , s. 724-731
-
Konferensbidrag (refereegranskat)abstract
- 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.
|
|
| 9. |
- Lin, Chang, et al.
(författare)
-
Visualizing Conduit Flows around Solitary Air Pockets
- 2014
-
Ingår i: Journal of engineering mechanics. - : American Society of Civil Engineers (ASCE). - 0733-9399 .- 1943-7889. ; 141:5
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 10. |
- Lindblom, Erik, et al.
(författare)
-
Dynamic modelling of nitrous oxide emissions from three Swedish sludge liquor treatment systems
- 2016
-
Ingår i: Water Science and Technology. - : IWA Publishing. - 0273-1223 .- 1996-9732. ; 73:4, s. 798-806
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
