On Vortex Trails over Ocean Islands

島嶼後方渦旋列之研究

鮑咸平(Hsien-Ping Pao) ； 高鋙(Timothy W. Kao)

《大氣科學》 3卷1期 (1976/05) Pp. 28-38

摘要

在本研究中，若于實驗結果經行作出，說明流體密度分層（stratification）為海洋島嶼後方渦旋列之出現之主要原因。有甚多證據顯示，由於其一般流紋線（streak-line）型式近似致使這些渦旋列被認為范卡門（Von Kármán）渦旋街。實驗顯示，當一球體以定速在層流中拖移時存在有一三維狀之渦旋列逸出（vortex shedding）結構。當流體垂直密度變化不大及欒氏數（Reynolds number）在10^3至10^4範圍中時。渦旋係作三維狀之逸出。但流體中密度分層，迅速並有效地遏止其垂直向之運動，原始之亂流性尾流（wake）崩潰，並顯示出渦旋結構豎向部份。當自上往下觀察時，似若圓柱後方二維（或水平）之渦旋列型式。有關此三維狀物體層流中渦旋列逸出及其結構，本文並作有頗詳細之論述。

並列摘要

In this study some experimental results are given to demonstrate that the density stratification is the main reason for the appearance of cloud vortex trails over ocean islands. Evidences are given that the similarity of these vortex trails to von Kàrmàn streets may not go beyond the general streak-line pattern. The experimental results reveal the three-dimensional vortex shedding structure when a sphere is towed at a constant velocity through a stratified fluid. It is found that for small or moderate stratification and Reynolds numbers in the range from 10^3 to 10^4 the vortex is shed three-dimensionally. The stratification however quickly and effectively inhibits the vertical motion and the initially turbulent wake collapses and reveals the vertically oriented portion of the vortex structure, reminiscent of the three-dimensional vortex street behind a circular cylinder when viewed from above. Considerable insights are given concerning the vortex shedding and its structure in the wake of a three-dimensional body.