Proceedings of the 9th International Conference

on Hydrodynamics (ICHD 2010)

October 11 – 15, 2010,Shanghai, China
Editors: Y. S. Wu, S. Q. Dai, H. Liu, L. D. Zhou, X. C. Yang

Environmental Hydrodynamics (16)

F1-1 Turbulence structure of compound open-channel flows with one-line emergent vegetation

Michio Sanjou, Iehisa Nezu, Sohei Suzuki,Kota Itai

Department of Civil Engineering, Kyoto University, Katsura Campus, Kyoto, Japan

Abstract :

Vegetation such as trees and shrubs are often observed at the floodplain edge in natural rivers. Spanwise profiles of streamwise velocity component are influenced significantly by the drag force of trees. That is to say, the streamwise velocity decreases locally behind trees and near the junction between the main-channel and the floodplain. This property is quite different from that observed in typical compound open-channel flows, in which the velocity profile has a single inflection point and forms a mixing layer related to large-scale horizontal vortices. In contrast, in compound open channel flow with a one-line emergent vegetation, a “V“-shaped velocity profile appears with twin inflection points. It is thus very important to investigate these hydrodynamic properties and turbulence structure considering the emergent vegetation effects in river engineering and eco hydraulics. So, in the present study, turbulence measurements by 3-D acoustic Doppler anemometer (ADV) were conducted in 150cm wide laboratory flume, in which 2cm diameter and 25cm height cylinders are placed as vegetation models with 10cm span along the junction edge.

F1-2  Large eddy simulation of compound open-channel flows with emergent vegetation near the floodplain edge

Michio Sanjou, Iehisa Nezu

Department of Civil Engineering, Kyoto University, Katsura Campus, Kyoto, Japan

Abstract :

Trees and shrubs observed in floodplain edge in natural rivers have significant impacts on hydrodynamic characteristics. In particular, spanwise profile of streamwise velocity component is influenced by the drag force of the trees, and consequently, the streamwise velocity decreases locally behind trees and near the junction between the main-channel and the floodplain. Furthermore, concentration exchanges such as nutrients between the main-channel and the floodplain differ from those observed in the non-tree compound open-channel flows. It is of essential importance to reveal how emergent trees influences these mass transfers. However, it is very difficult to evaluate time and space variations of the concentration by laboratory measurements. So, in the present study, a 3-D LES is used to overcome this difficulty, and we reveal the relation between the tree-alignment density and the exchange-rate of the concentration.

F1-3 Study on hydrodynamic and sedimentationproblems in development of harbors located at offshore area with many islands and tidal channels

Shu-hua Zuo, Bei Li

Key laboratory of Engineering Sediment of Ministry of Communications,Tianjin Research Institute of Water Transport Engineering, Tianjin , China.

Abstract :

The Yangshan sea area which locates in the Qiqu Archipelago is a typical offshore area with muti-island and muti-channel. In this paper, taking Yangshan sea-area of the Qiqu archipelago as an example, a 2D numerical model of tidal currents and sediments is established to study the change of the current and sediment field. The validity of the model is verified by the observation data obtained in April, 2007 including the tidal flow and suspended sediment concentration in the vicinity of the harbor. The verification of calculation shows the calculated values are in good agreement with the measured data. The field of tidal currents, suspended sediment concentration and the seabed deformation can be successfully simulated. Finally, two layouts are simulated to study the changes of the current and suspended sediment field. And the seabed evolution is predicted with the model. Computational results show that the Branch Channel Layout is advantageous to the present situation.

F1-4 Numerical simulation of sediment erosion by submerged plane turbulent jets

Wan-yun Xue, Wen-xin Huai, Zhong-dong Qian

State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan , China.

Abstract :

Erosion of loose beds acted by submerged plane turbulent jets was simulated with the Eulerian two-phase model, which implements Euler-Euler coupled governing equations for fluid and solid phases. A modified κ-ε turbulence model was chosen to enclose the fluid phase. Both flow-particle and particle-particle interactions were considered in this model. The computational results were in a good agreement with previous laboratory measurements. The characteristics of the flow field in the two phases and the influences of hydraulic and geometric parameters on eroded bed profiles were analyzed based on the computational results. The calculational results reveal that: the Densimetric Froude number is the most important factor that influences the computational results of the eroded bed; sands may keep movable or still underthe interaction among particles gravity, seepage force of pore water and friction between particles. And sands on the upwind side of the dune may be promoted along the water-sand interface to the sand mound by the local shear stress, which mainly influences the scoured shape; by adjusting the computational steps and methods, two problems in the Eulerian two-phase simulation are solved.

F1-5  Prediction of sediment transportation in deep bay (Hong Kong) using genetic algorithm

F.X. Zhang, Onyx W.H. Wai, Y.W. Jiang

Department of Civil & Structural Engineering, The Hong Kong Polytechnic University, Hong Kong

Abstract :

The genetic algorithm (GA) is a powerful method which can be used to solve search and optimization problems. A genetic algorithm with tournament selection, uniform crossover and uniform mutation is used to optimize sediment transport parameters in this study. Two important parameters of sediment transport, the critical shear stress for deposition and resuspension, are optimized by GA. The results show that GA is efficient and robust for optimizing parameters of our sediment transport simulation of Deep Bay.

F2-1 Stochastic diffusion by progressive waves in turbulence

Adrian Wing-Keung Law, Siu-Kui Au, Jie Song

School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Ave., Singapore.

Abstract :

Pollutants that are chemically inert flow with the carrier fluid passively while diffuse at the same time. In this study, the diffusion behavior of passive pollutant in a progressive wave field with strong turbulence is examined with analytical means. The focus is on the nonlinear interactions between the stochastic diffusion and the deterministic wave-induced oscillatory advection. We limit our scope to cases whereby a small parameter, ε, exists between the advective and diffusive displacements, which then allows a perturbation analysis to be performed. With the sinusoidal progressive wave, the results show that the oscillating wave motion can either increase or decrease the stochastic diffusion depending on the wave characteristics. Longer wave lengths and shorter wave periods tend to promote diffusion, while shorter wave lengths and longer wave periods act in the opposite manner. The reinforcing effect is however much stronger than the diminishing effect.

F2-2 Study on Bohai sea tidal dynamics under extratropical storm surge

Xin Li, Wei-sheng Zhang, Jin-shan Zhang

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing, China.

Abstract :

Bohai Bay is one of the areas affected by by extratropical storm surges and the economy loss there due to this is remarkable. A numerical model is established to couple the astronomic tide and the storm surge for Bohai Sea and its vicinity. Based on validation, it is applied to analyse the influence of the extratropical storm surge in Oct. 2003 on the water level set-up/down and on the spatial-temporal variations of the flow fields. Due to the extratropical storm surge, the water level setup is obvious along the Bohai Bay coast; the water level fluctuation varies greatly. The tidal flow of Bohai Sea is changed from the local reversing tidal current to unidirectional flow.

F2-3 Velocity distribution of secondarycurrents in curved channels

Il Won Seo , Young Jai Jung

Dept. of Civil and Environmental Engineering, Seoul National Univ. Gwanak-Gu, Seoul, Korea

Abstract :

This research concerns steady secondary currents developing in open-channel bends and reviews the theoretical equations of the transverse velocity profile which have been developed by previous researcher. Rozovskii’s equation in which fully developed flow was defined at first contains function which is difficult to integrate analytically. Kikkawa et al.’s equation presents the characteristics of the section well. Baek et al’s equation reveals the stream-wise variations and the vertical profile of the transverse velocity. These existing equations are generally in good agreement with experimental results. However, equations didn't include exactly effects of flow resistance at water surface and bottom. Furthermore, no-slip condition was not considered..

F2-4 Numerical simulation of the plug discharge under aerated condition

Ze-gao Yin, Xian-wei Cao, Jin-xiong Zhang, Hong-da Shi

Ocean engineering key laboratory of Shandong province, Ocean University of China, Qingdao,China.

Abstract :

In Fluent, 3-D RNG ε − k mathematical model of water and air mixtures is employed to compute plug discharge under aerated condition. When flow enters the plug section, the gradient of air volume fraction increases much and varies drastically, and cavitation number decreases sharply and soon reaches the minimum. When flow enters the sudden expansion, air volume fraction and cavitation number increases respectively. With the increase of sudden expansion length, air volume fraction appears large at the top and small at the bottom and cavitation number increases to recover eventually. Cavitation number increases with the increase of air volume fraction.

F2-5 Vertical 2-d mathematical model of sediment silting in dredged channel

Teng Wu, Xiu-xia Li

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing, China

Abstract :

Channel excavation is an important project measure in channel dredging, which is widely applied in channel regulations. Since channel excavation changes the original channel’s bed morphology, the pattern of flow becomes more complicated and the relative balance between water flow and sediment is broken. The paper constructed a vertical 2-D numerical model of excavated channel focusing on the sediment deposition and flow distribution, which takes the hydrodynamic pressure into account and adopts auto-mesh technique. The model can effectively reduce the error caused by calculating meshes not completely matching with the calculating region. By introducing the hydrodynamic pressure, the mode can overcome the difficulty in modeling the severely changing velocity caused by the channel excavation. To verify the reliability of the model, Alfrin’s experimental datum is adopted in the paper, and the calculating results agree well with the experimental datum.

F2-6 Specification of wall boundary conditions and transverse velocity profile conditions in finite element modeling

Il Won Seo, Chang Geun Song

Department of Civil and Environmental Engineering, Seoul National University, Seoul, Korea

Abstract :

Present work mainly focuses on the influence of wall boundary conditions and imposition of transverse velocity boundary conditions on the flow field of a bluff body. Uniform velocity over the node string is specified in slip condition mode. Three types of transverse velocity profile are adopted for the no slip situation; top hat is same as slip case except for no velocities at wall; parabola for the simple and typical approximation of transverse velocity shape in open channel flow; beta distribution for the general expansion. Assignment of top hat and parabola velocity profiles yields maximum velocity nearby the cylinder while uniform one has maximum magnitude at side walls. Maximum velocity of parabola input is about 44 % higher than that of uniform assignment.

F3-1 Multiple tandem jet interaction in a crossflow

Adrian C.H. Lai, Joseph H.W. Lee

Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China

Abstract :

We formulate a general semi-analytical model for multiple tandem jet interaction in a crossflow. For an array of buoyant jets arranging in tandem, the rear jets experience a reduced effective crossflow velocity due to the blockage and sheltering effect of the leading jet. The jet entrainment is modelled by a distribution of point sinks, and the blockage and sheltering effect is modelled by a distribution of doublets - both along the jet axis. The reduction of rear jet effective velocity as observed in previous experiments is successfully predicted by the model.

F3-2 Transitional gravity flow ofsewers inappropriate entry into storm drainage of a separate system

Hai-long Yin, Zu-xin Xu

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai, China.

Abstract :

Transitional sewer flows into storm drainage were measured in Caohejing drainage system of Shanghai, a separate system with inappropriate sewage entry into storm drainage. Flowing measurement demonstrated the limitation of Hazzen-William equation on quantifying the gravity flows in sewers, so an empirical formula was established. The dimensionless coefficient ranges from 0.34 to 16.38, representing supercritical flow under river water surging into one underground sewer culvert, and subcritical flow under normal gravatational free fall. Based on field investigation, it was known that the dimensionless coefficient under normal free fall is in the range of 0.34-4.17, and correspondingly gravatational velocity of sewers inappropriate entry into storm drainage ranges from 0.06 m/s to0.71 m/s under investigated sewer water depth 1-5 cm.

 

F3-3 Lagrangian block hydrodynamics for environmental fluid mechanics simulations

Lai-wai Tan, Vincent H. Chu

Department of Civil Engineering and Applied Mechanics, McGill University,Montreal, Quebec, Canada

 

Abstract :

The Lagrangian block hydrodynamics is formulated based on the block advection of fluid. By enforcing the mass and momentum conservations on the Lagrangian mesh, the numerical oscillation problem encountered in the classical Eulerian computational methods is circumvented. A large number of the previously computationally difficult problems in environmental fluid mechanics are successfully simulated using the method. Examples of these simulations are described in this paper.

 

F3-4 Three-dimensional modeling of tidal circulation within the north and south passages of the partially-mixed Changjiang River Estuary, China

John Z. Shi*, Chen Li, Xi-ping Dou

Department of Harbour and Coastal Engineering, State Key Laboratory of Ocean Engineering, School of Naval Architecture, Ocean and Civil Engineering, The Shanghai Jiao Tong University, Shanghai,China

Abstract :

The COHERENS three-dimensional hydrodynamic model is used to simulate the barotropic tidal circulation within the North and South Passages of the partially-mixed Changjiang River estuary, China. Modeled surface elevation, current speed and direction have been validated against measured data during the spring, moderate, and neap tides in the dry season and flood season of 1996, respectively. Analyses of modeled results suggest that (i) tidal flow in the flood season showed stronger asymmetry than that in the dry season; and (ii) there was an apparent seasonal, spring/neap tidal and intratidal variability in tidal circulation within the North and South Passages.

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F3-5 The sensitivity of shallow mixing layers to upstream perturbations and its implication to numerical code validation

Man-yue Lam , Hong-wei Liu, Mohamed Ghidaoui

Department of Civil Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China

Abstract :

Unsteady numerical simulations (Unsteady RANS or Large-eddy simulation) of shallow mixing layers are gaining attention in researches. However, the fact that the development of shallow mixing layers is sensitive to upstream perturbations poses a challenge to the validation of numerical results against experiments. A valid question is that whether upstream perturbations should be simulated in detail so as to obtain numerical results directly comparable to experiments. In an attempt to answer the question, the paper studies the sensitivity of the downstream dynamics of shallow mixing layers to upstream perturbations. Simulations of temporal shallow mixing layers are performed with different types of initial perturbations, noting that initial perturbations in temporal simulations correspond to upstream perturbations in spatial simulations. Results suggest that although the initial development of vortices is known to be sensitive to initial perturbations, agreement is obtained for the energy spectra at large times when the difference in the perturbation forms is at large wavenumbers, because of the dissipation by both the bottom friction and the sub-depth scale eddy-viscosity. When the difference in the perturbation forms is at small wavenumbers, the difference in the initial development of large scale coherent structures is kept in the energy spectra at large times as the bottom friction causes the mixing layer stable after a certain time. It may imply that numerically simulated energy spectra at the downstream are comparable to experiments without simulating precisely the high wavenumber upstream perturbations, but low wavenumber perturbations should be well incorporated in modeling so as to obtain the correct dynamics of the large-scale coherent structures.