受卡門渦街影響之仿生撓性尾鰭泳動之流場實驗分析

Abstract

大自然中魚類以群游的方式游動主要是為了抵禦大型掠食者的侵略與利用外在環境產生的動能達到較高的游動效率。根據Liao(2007)觀察對於群游中的單一魚體受到上游魚群尾流所產生的影響，與其受到卡門渦街的影響相似，故本實驗以半圓柱產生的卡門渦街流場，在其下游，(1)放置小圓柱體，(2)擺放仿生撓性尾鰭，觀察並量測物體受到卡門渦街的影響，以驗證上述過程。 前人在此方面的研究主要以計算模擬並且在流場雷諾數的設定上略低、魚尾自主的擺動居多。以實驗為主的文獻方面，以半圓柱產生卡門渦漩並將活體魚隻放入流場中，以影像拍攝的結果在魚尾擺動頻率、魚隻大小、魚隻所停留的最佳位置做統計上的計算。我們以近似BCF撓性尾鰭的模型放入流場中量測受力及流場顯影及PIV量測，且提高流場雷諾數操作，不同於文獻中實驗活體魚無法量測受力及計算模擬流場雷諾數設定略低的缺點。 在流場雷諾數為14400、19200、24000的實驗結果中，從撓性尾鰭在產生卡門渦街下游1.5D位置，能受到外在流場提供額外的推力與最大史卓荷數的結果，得知魚體在卡門渦街下游1.5D為最佳游動位置。

In nature, the function behind the schooling behavior of fishes mainly is to defend themselves from large predators, and to use kinetic energy generated by the external environment to achieve a better swimming efficiency. For an individual among the schooling fish, the wake flow generated from the fishes that are ahead and upstream is affected by the Karman vortex. As a result, this experiment uses the Karman vortex flow field generated by a semicircular cylinder, and places a flexible fish model downstream, to observe the effect of the Karman vortex on the force and swing condition of the fish model. Predecessors of this research principally used computer simulation to conduct the experiment. They used slightly lower Reynolds number of the flow field, and had more free oscillation of the fish’s tails. Semicircular cylinders were used to generate the Karman vortex, and live fishes were placed within the flow field. The results provided by filming the fish's tail swing frequency, size of the fish and the ideal resting location of the fish provided the predecessors of the experiment with their statistical calculations. We placed similar fish models in the flow field for force measurement and increase the Reynolds number of the operation. The experimental result differs from the citations in that the usage of live fish models failed to measure force and failed to calculate the slightly lower Reynolds number of flow fields detected by computer simulation. Within the resulting Reynolds number of 14400, 19200, 24000; it is found that 1.5D downstreams from the Karman vortex producing fish model, additional thrust and a maximal Strouhal number provided by the external flow field is detected. Therefore, we conclude that the ideal location for fishes to navigate is at 1.5D downstreams from the Karman vortex.