Under Water Bed Form Deformation of Long Time Scale

• Main Authors: Jhong-Min Guo, 郭中閔
• Other Authors: Liang-Hsiung Huang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/98233776296705975870

碩士 === 國立臺灣大學 === 土木工程學研究所 === 101 === There are lots of hydraulic engineering buildings in rivers and the oceans, such as bridge pier and the seabed oil pipes, etc. Bed form deformation will destroy the hydraulic engineering buildings safety, like bridge pier will collapse and the seabed oil pipes will crack by bed form deformation. On the part of reservoir, Taiwan area is high gradient slope and large flow velocity, thus, there is serious impact of sediment deposition for Taiwan reservoirs, studying the problem of bed form deformation may have benefit for Taiwan reservoirs desilting. Taiwan is a small island surrounded by seas, bed form deformation relates to territory protection and plan. Through studying bed form deformation can predict that coastline grow and reduce. The aim of this research is to study the flow and water waves effects on bed form deformation of porous elastic media in the long time scale situation, and follow Kuo (2011)’s study. Because of Kuo (2011), the numerical simulation is only for simple situations of boundary conditions, there still have differences between actual physical problems. Furthermore, Kuo(2011) sets numerical sponge layer in water body on downstream for absorbing disturbance flow velocity to avoid water waves reflexing to inner water body which can lead to numerical divergence. However, there has no sponge layer mechanism in real problem; therefore, this process should be considered what effect for numerical simulation. In Kuo (2011), the bed form deformation time seems too short, and it can impute to the limitation of small time step; so that can’t be effective in simulating the actual bed form deformation problem. In this research, water body is based on potential theory to build up the governing equations and boundary conditions, and the soil body is based on Biot(1956a, 1956b, and 1962) which build up the porous elastic media theory to set up the governing equations. Utilizing the stream function to express the bed solid displacement, and using order analysis to simplify the soil body’s governing equations to Laplace equation and biharmonic equation. Bring up the significant difference between time scale of water waves and soil waves, and using boundary element method to simulate soft porous elastic media bed form deformation which is affected by flow and water waves. There are followings the main contribution of this study : (1.) Utilizing the dispersion relation of water waves and soil waves to plot angular frequency and wave number diagram, and investigating significant difference between time scales. (2.) Proposed a dimensionless parameter for the control of bed form deformation, the main controlling factors are flow velocity and shear modulus of the soil. (3.) Improved calculation of time step, that can be able to reflect soft porous elastic media bed form deformation trends on the flow and water waves affect. Using numerical skill to set up boundary condition which can let disturbance flow velocity pass in water body on upstream and downstream, in order to eliminate the impact of numerical sponge layer to make the results closer to actual physical phenomenon. And the calculation of its original boundary conditions of the process to make improvements to reduce the error of numerical simulation. (4.) Constructed different situations boundary conditions in this study, such as soil body permeable and impermeable boundary on upstream and downstream, adding suspended load, and adding bed load, so that numerical simulations are useful in reality. Finally, numerical simulation results of this study will verify with Shi (1998) and Li, et al (1990).