本研究以三維數值模式探討海嘯湧潮作用下圓柱結構物周圍之流場變化、自由液面抬升與局部沖刷特性，並分析前置犧牲結構對主圓柱周圍水流與床面沖淤型態之影響。研究內容分為兩部分：首先以穩態流作用下之單一圓柱作為基礎案例，分析圓柱迎水面壅高、滯流區、壓力集中與下游尾流等流場特徵；其次以潰壩流型態模擬海嘯湧潮入侵，建立可動床條件下之單一圓柱基準案例，並進一步比較不同前置犧牲結構配置之影響。 本研究採用改良型 Truchas / Splash3D 三維數值模式，結合 VOF（Volume of Fluid）方法追蹤水體與空氣間之自由液面變化，並以非連續雙黏性模型（Discontinuous Bi-viscous Model， DBM）描述底床材料受高速水流作用後之變形與位移。潰壩流案例中，除單一圓柱基準案例外，另配置前置犧牲圓柱高度 0.3H_0、0.5H_0、0.8H_0，以及半徑 1.0D之單一前置圓球與雙前置圓球並排案例，以比較不同前置結構形式對主圓柱入流條件、近底床速度、壓力分布與床面沖淤之影響。 數值結果顯示，潰壩流波前撞擊單一圓柱後，圓柱迎水面形成局部滯流區與壓力集中，使部分水平動量轉換為壓力能與垂向動量，進而造成自由液面壅高與非定常流場變化。配置前置犧牲結構後，前置結構物會優先承受波前衝擊並於其周圍形成局部沖刷；同時，其後方產生之遮蔽、分流與尾流作用亦會改變主圓柱前方入流條件，使主圓柱周圍床面沖淤型態產生變化。 量化結果顯示，單一圓柱案例中主圓柱上游側平均床面變化為負值，呈現平均沖刷趨勢；加入前置犧牲圓柱後，該區域平均床面變化轉為正值，表示主圓柱上游側由平均沖刷趨勢轉為平均堆積趨勢。其中，前置圓柱高度由 0.3H_0增加至 0.5H_0時，平均堆積量增加；當高度進一步增加至 0.8H_0時，平均堆積量未再增加，而呈現略微下降或趨於平緩之趨勢，顯示前置圓柱高度增加所帶來之床面改善效果並非線性增強。 整體而言，前置犧牲結構並非單純消除沖刷，而是透過改變入流條件、重新分配水流動量與轉移局部沖刷位置，進而影響主圓柱周圍床面沖淤型態。本研究以 S/D、Δz_b/D 與 h_s/H_0作為主要無因次量化指標，研究成果可作為潰壩流型海嘯湧潮條件下，理解前置犧牲結構對主圓柱周圍沖淤型態、流場調控與沖刷轉移機制影響之初步數值依據，並可供後續水槽實驗、模式驗證及參數化配置研究參考。;This study employs a three-dimensional numerical model to investigate flow-field variations， free-surface rise， and local scour characteristics around a circular cylinder under tsunami bore conditions， with emphasis on the influence of front sacrificial structures on the flow field and bed morphological changes around the main cylinder. The study consists of two parts. First， a single cylinder under steady flow conditions is used as a baseline case to examine upstream water-level rise， stagnation region， pressure concentration， and downstream wake characteristics. Second， a dam-break flow is adopted to simulate tsunami bore inundation， and a movable-bed single-cylinder case is established as the reference case for comparison with different front sacrificial structure configurations. The simulations are conducted using a modified Truchas / Splash3D three-dimensional numerical model. The Volume of Fluid (VOF) method is used to track the free-surface variation between water and air， while the Discontinuous Bi-viscous Model (DBM) is applied to describe bed deformation and displacement under high-speed flow conditions. In the dam-break cases， in addition to the single-cylinder baseline case， front sacrificial cylinders with heights of 0.3H_0， 0.5H_0， and 0.8H_0， as well as a single front sphere and a double-sphere side-by-side configuration with a radius of 1.0D， are considered to evaluate their effects on the inflow condition， near-bed velocity， pressure distribution， and bed morphological changes around the main cylinder. The numerical results show that when the dam-break bore impacts the single cylinder， a local stagnation region and pressure concentration form on the upstream face of the cylinder. Part of the horizontal momentum is converted into pressure energy and vertical momentum， resulting in local free-surface rise and unsteady flow-field variations. After front sacrificial structures are installed， the sacrificial structures directly receive the incoming bore impact and experience local scour around themselves. Meanwhile， the shielding， flow diversion， and wake effects formed behind the sacrificial structures modify the inflow conditions in front of the main cylinder and change the scour and deposition patterns around it. The quantitative results indicate that the mean bed elevation change upstream of the main cylinder is negative in the single-cylinder case， showing an average scouring trend. After installing front sacrificial cylinders， the mean bed elevation change in this region becomes positive， indicating a transition from average scour to average deposition. The mean deposition increases as the front-cylinder height increases from 0.3H_0 to 0.5H_0. However， when the height further increases to 0.8H_0， the mean deposition does not continue to increase and instead shows a slight decrease or a tendency to level off， suggesting that the bed-improvement effect of increasing front-cylinder height is not linear. Overall， the front sacrificial structures do not simply eliminate scour. Instead， they modify the inflow conditions， redistribute flow momentum， and transfer local scour locations， thereby influencing the bed morphological pattern around the main cylinder. In this study， S/D， Δz_b/D， and h_s/H_(0 )are used as the main dimensionless parameters. The findings provide preliminary numerical evidence for understanding how front sacrificial structures influence bed morphological patterns， local flow regulation， and scour-transfer mechanisms around the main cylinder under dam-break-type tsunami bore conditions， and may serve as a reference for future flume experiments， model validation， and parametric configuration studies.
