三維海嘯湧潮對近岸結構物之影響; Analysis on the 3D bores and their interaction with structures

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Title:	三維海嘯湧潮對近岸結構物之影響;Analysis on the 3D bores and their interaction with structures
Authors:	魏妙珊;Miao-Shan Wei
Contributors:	水文所
Keywords:	海嘯;潰壩;浪與結構物之交互作用;移動固體法;湧潮;大渦模擬;體積分率法;wave-structure interaction;bore;dam-break;Tsunami;moving-solid;VOF;LES
Date:	2009-07-01
Issue Date:	2009-09-22 09:52:13 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	本文以潰壩湧潮 (dam-break bore) 研究海嘯波抵達近岸時之海嘯湧潮行為，以及其與近岸結構物之交互作用。在上溯期間，海嘯湧潮在近岸地區之行為與潰壩湧潮相似。為了研究湧潮的特性，使用數值和實驗室試驗產生一個潰壩湧潮，於壩體下游5.26m處設置一垂直方柱12*12cm。本研究求解不可壓縮流之三維 Navier-Stokes 方程式及大渦模擬 (Large-eddy Simulation, LES) 紊流模式，以體積分率法 (Volume of Fluid, VOF) 描述自由液面之運動，以有限體積法來離散網格，並以移動固體法 (Moving Solid Method, MSM) 模擬潰壩實驗時擋板抽取之運動。潰壩湧潮於長達14.72m之渠道內前進，以模擬海嘯波抵達近岸地區後所造成之類均勻 (Quasi-uniform) 海嘯湧潮。討論在潰壩湧潮產生時之初始波形、撞擊於結構物之力、波形、速度場、以及壓力場。結果顯示在潰壩湧潮所施於結構物之力主要與壩後水深 (impoundment)以及結構物面積有關，並敏感於壩前薄水墊厚度 (waterbed thickness) 以及壩體閘門之抽取速度。分析結果得知，要取得良好之數值模擬結果，閘門抽取速度以及薄水墊厚度需要精確描述。本研究發現，壩前薄水墊扮演潤滑之角色，有薄水墊之湧潮將產生較大之初期撞擊力。本研究亦發現潰壩湧潮撞擊至結構物時，將於結構物前方產生上下兩個方向相反之漩渦，其尺度與湧潮深度相仿。其飛濺之碎波高度亦相仿於湧潮深度。構物受力最大之區域並非落於結構物底端，而是位於二分之ㄧ平均湧潮高度，證實初期主破壞力為水體之動壓力，而非靜壓力。 When a tsunami wave propagates to the near-shore region, it transforms from a long wave to a tsunami bore and dissipates the energy in the coastal region. This is the most destructive period during the whole tsunami life. In order to have better understanding on the tsunami bore, this paper investigates the physical processes of a dam-break bore which has similar behavior as the tsunami bore. The bore-structure interaction is studied by placing a square cylinder at the downstream region. The numerical approach is adopted. We solve the 3D incompressible Navier-Stokes (NS) equations with large-eddy simulation (LES) turbulence model. The free-surface kinematics and wave breaking are tracked by the volume-of-fluid (VOF) method. The domain is discretized by the finite volume method (FVM) with an irregular mesh domain. The broken dam is modeled by the moving-solid algorithm (MSM) to detailed describe the lifting motion of the gate. The dam-break bore is marching in a channel with a length of 14.72 m to mimic the quasi-uniform tsunami bore. The numerical results are validated with the laboratory experiments in terms of the wave force acting on the square structure. We focuses the discussions on the effect of the impoundment height, waterbed lubrication, gate motion, free-surface profiles around the broken-dam and square cylinder, pressure profile around the cylinder. The results show that the impinging force on the cylinder is mainly determined by the impoundment, width of the channel, and the projected area of the structure. However, it also sensitive to the thickness of the waterbed and the lifting speed of the gate. When the bore is impinging with the cylinder structure, two eddies with inversed rotation direction will be generated right in front of the structure. Their length scales are similar to each other and close to the thickness of the incoming bore. The largest wave fore acting on the cylinder is not located at the bottom but at half of the incoming bore height. This indicates that the maximum impinging force is dominated by the hydrodynamic pressure. The waterbed plays a lubrication role which accelerates the bore speed and increases the wave force.
Appears in Collections:	[Graduate Institute of Hydrological and Oceanic Sciences] Electronic Thesis & Dissertation

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