| 摘要: | 本研究使用三維大渦模式和流體體積法來探討靠近水面之橋面板的水力負載以及波浪/結構物/紊流之互制作用。模擬得之波高和橋面板所受的壓力、總力皆與實驗結果比對，以驗證數值模式的正確性。再使用此數值模式研究各種狀況下，矩形橋面板周圍的波浪/紊流相互作用與矩形橋面板所受的波浪力，流況包括週期波、孤立波和波流合併流。
週期波的模擬結果顯示：橋面板後方的波高小於橋面板前方的波高，且因為碎波和橋面板引起的紊流會影響橋面板的表面壓力。而橋面板上的阻力、升力和彎矩皆與波高H成線性正比關係，故可採用波高H標準化橋體所受之阻力、升力。而無因次之波浪力係數與雷諾數(橋體的縮尺比)無關，亦即阻力、升力係數可適用於不同的波高、橋體大小。由於波浪引起的壓力在隨著水深增加而遞減，有最大波浪力發生於橋體靠近水面(潛沒比S/D = 0 ~ 1.0)時，隨著潛沒比的增加，橋面板的波浪負載減小。
本研究並探討波流交互作用下，矩形橋面板所受的水動力負載。依據模擬結果，本研究提出一個修正莫里森方程式來預測橋面板的水力負載，將分為同向流造成之穩態項和波浪所造成之加速度項。而穩態項正比於流速，加速度項與波高H成線性正比關係。且由於橋面板長度大於橋面板厚度，因此造成橋面板破壞的主要外力為作用在橋面板上方的垂向力。此外，阻力係數與波高H、縱橫比L/D和Keulegan-Carpenter (KC)數無關，而升力係數取決於潛沒比S/D。工程設計可以使用莫里森方程式和矩形橋面板的最大阻力係數CD = 2.73，格子樑橋面板的阻力係數CD = 2.51，升力係數CL = -2.05，慣性係數CMx = 0.95和CMz = 2.53來計算波流交互作用的最大負載。
孤立波施予橋面板的負載則與波高A的成正比，可用參考速度Ur = [gA(A+h)/h]1/2 來標準化橋體所受波浪力，而無因次化之阻力係數、升力係數與波高無關。但阻力、升力係數皆隨著長深比及阻滯比的增加而增加。對於相同的波高，孤立波的波浪力大於週期波之波浪力。而最大正阻力係數CD = 1.40，負阻力係數CD = -0.95，正升力係數CL = 0.57，負升力係數CL = -0.86可用於計算孤立波之波浪力。
;This study uses a Large Eddy Simulation (LES) model and the Volume of Fluid (VOF) method to examine the wave/turbulence interactions and hydrodynamic loadings on submerged bridge decks. The flow condition includes periodic waves, solitary waves, and wave-current combined flows. The surface waves in the numerical model were generated by an internal source function. The simulated wave heights and surface pressures on the rectangular deck are compared with the experimental results to validate the accuracy of the present numerical model. The numerical model was then used to examine the wave loads of different wave conditions.
For periodic wave flows, the influences of wave height, submergence ratio, scale ratio, and blockage ratio on the wave loads of the submerged deck are studied. The simulation results point out that the drag, lift, and pitching moment on the deck are linearly proportional to the wave height H. The dimensionless force coefficients are functions of submergence depth S, but are independent of Reynolds number of the bridge deck. The maximum force coefficients occur when the deck is near the water surface (submergence ratio S/D = 0 ~ 1.0) and decrease with the increasing submergence ratio. This results from the wave-induced pressure being the largest close to the water surfaces. Moreover, the turbulence induced by the wave breaking affects the leeward pressures and hydrodynamic forces on the bridge deck.
For wave-current combined flows, the influences of current velocity, wave height, deck length, and blockage ratio on the wave loads are examined. The simulation results concluded that the wave loads are linearly proportional to wave height H when H 0.4h, h is the water depth. The hydrodynamic load mainly comes from the surface pressures on the upper side of the decks due to the deck length being much larger than the deck thickness. A modified Morison equation is proposed to predict the hydrodynamic loadings on the deck. By adopting the reference velocity Ur = (gH)1/2 for wave-induced flow, the hydrodynamic loads can be separated into a steady term (current-induced force) and an acceleration term (wave-induced force). Furthermore, the drag coefficient is independent of the wave height H, aspect ratio L/D, and the Keulegan-Carpenter (KC) number, while the lift coefficient depends on the submergence ratio S/D. The maximum drag coefficient CD = 2.73 for a rectangular deck, drag coefficient CD = 2.51 for a girder deck, lift coefficient CL = -2.05, the inertia coefficients CMx = 0.95 and CMz = 2.53 could be used to design bridge decks against wave-current combined flows.
For solitary wave flows, the influences of current velocity, wave height, deck length, and blockage ratio on the wave loads of a rectangular deck are investigated. The simulation results indicate that the wave loads of solitary waves are larger than those of the periodic waves of the same wave amplitude. In addition, the resulting force coefficients are independent of the wave heights when the reference velocity Ur = [gA(A+h)/h]1/2 is used to normalize the hydrodynamic loads, and A is the amplitude of the solitary wave. Nonetheless, the drag and lift coefficients increase nonlinearly with increasing the aspect ratio and blockage ratio. For coastal engineers, the maximum drag coefficient CD = 1.40 and -0.95, and the lift coefficient CL = 0.57 and -0.86 can be utilized to compute the wave loads of solitary waves. |
