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Title: Experimental and numerical studies of open-channel turbulent flow over rough bed
Other Titles: Cu cao bi mian tiao jian xia ming qu tuan liu de shi yan yan jiu yu shu zhi mo ni
Authors: Wang, Xianye (王宪業)
Department: Department of Building and Construction
Degree: Doctor of Philosophy
Issue Date: 2010
Publisher: City University of Hong Kong
Subjects: Turbulence.
Channels (Hydraulic engineering)
Frictional resistance (Hydrodynamics)
Notes: CityU Call Number: TA357.5.T87 W36 2010
xx, 194 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2010.
Includes bibliographical references (leaves [167]-194)
Type: thesis
Abstract: Mean and turbulence characteristics of open-channel flows over rough bed are greatly influenced by the irregular surface roughness. The protrusions of roughness elements lead to complicated near-wall flow structure, and the sediment particles transported by the flow can also change the geometry of the river channel by means of erosion and deposition. Due to the significance of industrial and engineering applications, flow characteristics in turbulent flow over gravel beds merit much research. In this thesis, experimental investigations and the corresponding statistical analysis on the issue are carried out together. Mean flow characteristics are investigated in both fixed and mobile gravel beds using particle image velocimetry (PIV) technique. In addition, Reynolds stresses and high-order turbulence moments are studied using the acoustic Doppler velocimeter (ADV) technique over a wide range of roughnesses and Reynolds numbers. Under large roughness conditions, the difficulty of numerical simulation in the near wall region arises. Via analysis of the experimental data, the standard wall-function (WF) near-wall treatment has been assessed and a corresponding modification has also been made to improve its accuracy. With the PIV technique, the plane flow-field in a cross section can be investigated from the captured images. The fixed gravel beds employed in experiments are hydraulically rough. Mean velocity profiles and flow resistance factor are studied in various hydraulic conditions. An improved expression is established to represent the friction factor. In mobile beds, two types of grains with different median diameters are adopted. Both flow field and sediment-motion images are obtained. A mass of sediment particles - namely, a sand-wave, is found to be moving on the bed during the experiment. The sand-wave motion is studied through image analysis. The mean velocity profiles and flow resistance factors are also analyzed in a similar manner to those of the fixed bed. The ADV technique is used for monitoring three-dimensional velocity at high-frequency sample rates. The median diameters of bed material span a wide range of roughnesses. The log-law, skin friction, and Reynolds stress are experimentally investigated. The results are used to further investigate the roughness function and wall similarity hypothesis. From a statistical analysis of the experimental data, the higher-order turbulence moment and the sweep and ejection motions have also been examined. The quadrant analysis and turbulence moment distribution reveal that the turbulence isotropy changes with the increase of roughness. In order to numerically predict the wall-bounded flows, the near-wall region should be carefully treated. A numerical simulation is performed using the Reynolds stress model (RSM) turbulence model and a high quality unstructured mesh with non-equilibrium wall function. Though the low-Reynolds-number (LRN) turbulence models are proposed with fine meshes, the standard WF method can still achieve a balance between accuracy and efficiency. It is a convenient and robust method in industrial applications, and it is also vital to the accurate prediction of turbulent flow. The theory that lies behind WF method is the log-law in the near-wall region. The protrusions of roughness elements cause flow separation and vortices in the lees. In order to better represent the roughness effects, an assessment of the WF method in fixed gravel beds is conducted. Turbulence in the outer layer is modeled through solving the Reynolds-averaged Navier-Stokes (RANS) equations. In the inner layer, the grid sensitivity and the valid range of the WF method are discussed. The standard WF method has been assessed in the application of flow over large roughness walls. According to the experimental results and considering the size of the roughness elements and the hydraulic conditions, an improvement to the function based on semi-empirical theory related to shear velocity and roughness function is presented. Subsequently, a series of numerical simulations in rough beds are employed to show its capabilities. Through validation, the extension of the WF allows this method to be utilized in a wide range of practical projects.
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