魚類在水中與流場之間的互動，一直受到科學家們的注意，有相當多的研究指出，魚類能利用水流產生的渦旋來減少自身在推進時能量的耗損，達到節能的效果，進而提升自己的推進效率，因此，魚類在行進時與如何與渦流作用，是一個非常重要的研究課題。 本研究透過改變上游圓柱所產生之渦流到達魚身前端時與魚身擺動之相位，利用CFD-RC進行數值模擬分析，並觀察其流場特性及計算游動所需功率的變化。 本文以三種圓柱到魚身的距做相位差模擬，分別為2倍魚身長、3倍魚身長、4倍魚身長，並以模擬結果算其總功率、阻力功率、游動功率平均值。由結果可看出，當魚身上擺時若渦旋在下方，克服阻力所需功率會較小，魚尾的逆渦旋若沒被渦旋消耗，游動功率也會較小，因此總功率的消耗便會較小，但會因為渦旋的位置而有強度大小的差異，反之若魚身上擺時上方有渦旋，阻力功率變大，且產生的逆渦旋被消耗，使游動功率變大，總功率則會因此而增加。 由三種不同圓柱到魚身距離情況下模擬結果可發現，3倍魚身長與4倍魚身長其結果較為相近，相位差角度60~120度時則有其最佳的總功率產生，亦即魚類在此種相位下，其所消耗的功率最少，相位差角度240~300度則是產生最差的總率，魚類在此種相位下消耗的功較多，由以上可推測魚類游動時遇到不同相位渦旋時，其相位差在60~120度時魚類游動會最為省力。 This research simulated the flow field induced by a swinging fish downstream of a circular cylinder. The power required for swimming was also deduced. Three cases each with different distance between cylinder and the fish were studied to find the power required for the propulsive motion. The results showed that the power needed to overcome the drag force would be much smaller when fish tail swung up with the upstream vortex passed from the bottom and vice versa. The swimming power would be much smaller if the reverse vortex caused by fish tail undulation didn''t dissipated by the upstream vortex passing by. The total power consumption would increase otherwise. It is found that the simulations for distance of 3L and 4L were similar since the minimum powers required in both cases were observed to occur at about 60 degrees to 120 degrees of phase difference. The highest power consumptions were around phase angle of 240 degrees to 300 degrees. To summarize the above results, while a swimming fish encountered various phases of vortices, phases of 60 degrees to 120 degrees would be better for a fish to save the swimming energy.