自由液面附近的圓柱接面流分析

ANALYSIS OF JUNCTUR FLOW AROUND A CYLINDER NEAR THE FREE SURFACE

曾昱勛(Y.-H. Tseng)；陳建宏(J.-H. Chen)

接面流 ； 馬蹄渦 ； 自由液面 ； 有限平板 ； URANS ； Juncture flow ； Horseshoe vortex ； Free surface ； Finite flat plate ； URANS

中國造船暨輪機工程學刊

42卷4期（2023 / 11 / 01）

239 - 250

繁體中文;英文

接面流是邊界層與障礙物交互作用所形成的流場，且有明顯的三維特性，容易在障礙物前方產生馬蹄渦，對工程應用造成不利的效應，如在結構物的接和處附近造成淘刷或是流場產生噪音等，本論文針對自由液面附近的波流場對接面流的發展進行計算研究。本論文針對無限波流場通過有限長、寬平板上之圓柱體所造成的流場進行計算解析，其中平板係位在自由液面下有限深度處，波和入流方向與平板平行。我們使用商用軟體Fluent，並以URANS方法與SSTk-ω紊流模型進行相關的模擬研究，自由液面則以VOF模型來模擬。我們觀察在不同水深之平板與圓柱體組合下的情況下，所產生的接面流發展情況。本論文以均勻入流流速作為特徵速度、柱體直徑作為特徵長度，雷諾數約為5.5×10^5。計算結果顯示，柱體前的馬蹄渦會隨入射波呈現週期性的變化。當波峰接近柱體時，馬蹄渦主渦會靠近柱體，且會有較大的渦度；而當波谷接近柱體時，現象則相反，這是波浪之中的水質點速度變動造成的。平板沒水深度的則會影響波浪所造成效應，平板越靠近自由液面，效應越強，馬蹄渦位置與強度變化越明顯。其次，波浪同樣也會影響馬蹄渦延伸到下游的渦流渦度，不同的平板沒水深度不僅造成的圓柱下游渦流流洩週期產生變化，同時此等週期也與波浪的遭遇週期不同，因此衍生出整個尾流流場的非週期性渦流現象。

Juncture flow forms when a boundary layer passing an obstacle perpendicular to the flow. It exhibits distinct three-dimensional flow characteristics and tends to generate horseshoe vortices prominently in front of the obstacle. This phenomenon can have adverse effects in engineering applications, such as, for examples, causing scouring near the junctions of structures or generating noise in the flow field. This paper focuses on the development of juncture flow in a wave-current field near the free surface. The physical problem to be discussed in the present study is the juncture flow around a circular cylinder mounted on a finite flat plate which is located at a finite depth from the free surface in a wave-current field. The wave propagation and inflow are in the same direction normal to the flat plate. The commercial software FLUENT was employed, utilizing the URANS option and SST k- ω turbulence model for computational investigation, while the free surface was simulated using the VOF model. Throughout the pre-sent study, the Reynolds number was fixed at 5.5×10^5 with the current speed and cylinder diameter being the characteristic velocity and length, respectively. The development of the juncture flow for the plate at three water depths from the free surface was computed and analyzed. The results show that the horseshoe vortex in front of the cylinder moves periodically forwards and backwards with the wave propagation. When the wave crest approaches at the leading edge, the horseshoe vortex moves toward the cylinder and vice versa. The horseshoe vortex moves more significantly if the plate is closer to the free surface. Furthermore, we also found that the vortex shedding period due to the cylinder varies as the depth of the flat plate from the free surface changes and is different from the encounter period of the wave. This results in a non-periodic wake flow behind the cylinder, quite different from that in the uniform flow past a cylinder without free surface.