波浪通過海底管線之渦流場分析

Vortical flow in water waves passing over a submerged pipeline

廖國禎(Guo-Jhen Liao)

指導教授 ： 林孟郁

中原大學/工學院/土木工程研究所/碩士(2012年)

https://doi.org/10.6840/cycu201200865

摘要

本研究主要以應用Lin and Huang (2009) 所發展數值模式
GVM (Generalized Vortex Method)，進行孤立波通過底床上方
圓柱之二維流場模擬，此方法以邊界積分法處理自由液面邊界
及非旋性部分計算，並結合渦旋法進行渦度場之求解，在處理
大小尺度不一渦旋與自由液面具有計算上的優勢。

為了瞭解波浪作用下海底管線受力之行為表現，本研究著
重於探討在不同波高水深比下，圓柱底部與底床距離之差異所
生成的渦流尺度，以及此渦流受到底床效應之影響，分析兩者
間的交互作用，進一步得知其流體動力現象。

經由數值模擬成果得知，圓柱底部與底床之間距比
(gap-ratio)的改變，對於固體邊界的受力及渦流的發展有顯
著的影響；當比值上升，圓柱表面所受到的阻力逐漸下降，
當圓柱距離底床大於約1.5倍的圓柱直徑時，圓柱所產生渦流
與底床邊界層間之交互作用已甚微，可視為各自獨立發展，
阻力變化不再顯著。而底床剪應力最大值發生在孤立波波峰通
過圓柱時，且較不受gap-ratio 差距所影響，而隨波高水深比
增加，底床剪應力越大。當gap-ratio 為0.25 及0.5時，圓柱
與底床間同時分佈著正向與負向渦流，研判此間距比所產生的
渦流對於底床沖刷及輸砂具有一定的影響力。

關鍵字

孤立波 ； 圓柱 ； 渦流逸散 ； 間距比 ； 阻力

並列摘要

In this research, a numerical scheme that employs a generalizedvortex method is used to simulate the
two-dimensional flow around acircular cylinder placed at various heights above a plane boundary andexposed to a solitary wave. The vortical flow field is solved via a vortex method, the irrotational flow field and the motion of the free surface aresolved using a boundary integral technique.

In order to understand the hydrodynamic force exerted on the submarine pipeline under wave action, a series of simulations were performed with various gap-to-diameter ratios and wave heights to study their effects on vortex
generation as well as the drag exerted on the cylinder.

According to the numerical results, changes in
gap-to-diameter ratios have a significant impact on the evolution of the hydrodynamicforce and vortices. As the value of gap-to-diameter ratio increases, the drag force exerted by the circular cylinder decreases. When the value
of the gap-to-diameter ratio is greater than 1.5, the gap height is large enough so that there is no interplay between the separating shear layers generated from the cylinder and from the seabed; there force, the variation
of the drag and to the gap height is insignificant. When the solitary wave crest is passing through the cylinder, strong shear stress is generated on the seabed, and is
not affected by the gap ratio. As the wave height increases, seabed shear stress is more and more strong. At the gap ratio is the 0.25 and 0.5, in the gap between the cylinder
and the flat bottom, positive and negative vorticity appear at the same time. This condition may have significant influence on seabed erosion and sediment transport.

並列關鍵字

Solitary wave ； circular cylinder ； vortex shedding ； drag ； gap ratio

參考文獻

1. An, Hongwei., Cheng, Liang., and Zhao, Ming. (2011). “Steady

Streaming around a Circular Cylinder near a Plane Boundary due to

circular cylinder near a plane boundary.” Journal of Fluid Mechanics;

3. Chiew, Yee-Meng. (1990). “Mechanics of local scour around

submarine pipelines.” Journal of Hydraulic Engineering; 116: 515 –