以數值模式研究受水流衝擊之橋面版

A Numerical Study of Hydrodynamic Loading on Fully Submerged Bridge Decks

李承鴻(Cheng-Hung Li)

指導教授 ： 朱佳仁

國立中央大學/工學院/土木工程學系/碩士(2011年)

摘要

近年來全球氣候變遷，世界各地水災、風災頻傳。尤其台灣位處西太平洋颱風帶，颱風期間暴雨造成河川流量、水位在短時間暴漲，經常發生橋梁被沖毀之事件。本研究使用計算流體動力學模式，計算橋面版周圍之流場與橋面版所受到阻力與升力。本模式使用大渦流模式與VOF方法來探討橋體周圍之自由水面及水流的運動情形。模擬結果先與Chang (1998)、Malavasi & Guadagnini (2007)之實驗結果及Bruno et al. (2010)之數值結果比較、驗證，以增加數值模式的可信度。
本研究再利用數值模式針對矩形斷面之橋面版進行一系列的模擬，研究雷諾數、沉沒比、阻滯比、接近率等參數對橋體的阻力與升力之影響。研究結果顯示：當雷諾數大於45000時，橋面版阻力係數不隨雷諾數而變，但會隨阻滯比增加而變大，這是因為阻滯效應造成水流加速通過橋面版，導致水流施予橋面版的阻力增加。但升力係數則會隨橋面版接近水面(沉沒比下降)而變小，這是因為橋面版上方的壓力隨之變大，使得橋面版受到向下的總力。本研究並探討底床效應對橋體受力之影響，研究結果發現當接近率5.0 ? Pr > 2.0時，橋面版愈接近底床(接近率愈小)，升力係數逐漸愈大，因為橋面版下方流速小於上方流速，橋面版下方壓力大於上方，導致升力係數變大。但Pr < 2.0時，接近率減小，升力係數逐漸變小，這是因為橋面版接近底床時，渦流逸散會隨之減弱，使得橋面版上方壓力逐漸變大，導致升力係數逐漸變小。

關鍵字：阻力係數、升力係數、沉沒比、阻滯比、接近率

關鍵字

接近率 ； 阻滯比 ； 沉沒比 ； 阻力係數 ； 升力係數

並列摘要

In recent years, the number of natural disasters have increased due to the global warming. The typhoon struck Taiwan and brought heavy rain, which caused the the flow rate and water level rise in a short time and destroy the bridge. This study used Computational Fluid Dynamics (CFD) model to investigate the hydrodynamic forces on rectangular section, fully submerged bridge decks. The turbulent flow and pressure distribution around the deck were computed by the Large Eddy Simulation (LES) model with the Volume of fluid (VOF) method to handle the water surface. The experimental results of Chang (1988), Malavasi & Guadagnini (2007), and Bruno et al. (2010) were compared with the simulation results to verify the numerical results.
Then the verified LES model was utilized to examine the influences of Reynolds number, submergence ratio, blockage ratio and proximity ratio on the drag and lift coefficients. The simulation results demonstrated that the drag coefficient remain a constant increased as the blockage ratio increased. This can be explained by the velocity and pressure around the deck increased due to the blockage effect. In addition, the proximity effect was examined by changing the distance between the deck and channel bed while keeping other parameters constant. When proximity ratio 5.0 ? Pr > 2.0, the results indicated that the lift coefficient increased as the proximity ratio decreased. This is because the velocity beneath the deck is smaller than the velocity above the deck. According to Bernoulli equation that the pressure on the lower side of the deck is larger than the pressure above the deck, and produce a positive lift force.

Keywords: Drag coefficient, Lift coefficient, Submergence ratio, Blockage effect, Proximity effect.

並列關鍵字

Proximity effect ； Submergence ratio ； Blockage effect ； Lift coefficient ； Drag coefficient

參考文獻

Apelt, C. J. (1968). “Bridge piers – Hydrodynamic force coefficients.” J. Hydraulics Div., ASCE, 94(1), 17–30.

Bruno, L. Fransos, D., Coste, N., Bosco, A. (2010). “3D flow around a rectangular cylinder: A computational study.” J. Wind Engineering and Industrial Aerodynamics. 98: 263-276.

Cigada, A. Malavasi, S. and Vanali, M. (2006). “Effects of an asymmetrical confined flow on a rectangular cylinder.” J. of Fluids and Structures, 22, 213–227.

Hirt, C. W. and Nichols, B. D. (1981). “Volume of fluid (VOF) method for the dynamics of free boundaries”, J. of Computational Physics, 39(1), 201-225.

Jempson, M. A. and Apelt, C. J. (2005). Discussion of “Hydrodynamic loading on river bridges.” J. Hydraulic Eng., 131(7), 621–622.