- 學位論文
鈍體後方渦旋對魚擺動之影響的三維數值模擬
3D Numerical Simulation of The Effect of Shedding Vortices on a Fish Swimming Downstream
摘要
本研究主要是透過計算流體力學軟體(ANSYS-FLUENT)模擬三維流場中阻礙物後方的魚隻擺動情形。本研究分別以三種圓柱到魚身之距離(L=0.2m, 0.16m, 0.24m)讓魚在流場速度0.4m/s之自由流中游動，並觀察其流場特性及計算游動所需功率的變化。另外，本文也模擬在不同流速下(v=0.4m/s,0.04m/s)，固定距離(L=0.2m)的魚隻擺動及其流場特性與功率變化。 研究結果顯示，本研究之功率圖曲線較貼近二維研究結果中其擺動相位角204度之週期變化，而計算結果所坐落區間皆小於二維研究之數據區間，此處結果可由三維釋放效應(Three-dimension Relieving Effect)做解釋。在二維轉換到三維的過程中，流體流經魚身時，物理量被第三個維度給分散了，因此游動功率數值相較於二維流場來的小。另外，阻力功率則是因魚體表面無滑動邊界條件的影響加上無x方向運動，使得魚身x方向速度趨近於零，阻力功率因而趨近於零。綜合以上兩點，說明游動功率經三維釋放效應的影響，得到結果相較二維研究小，而阻力功率因無滑動邊界條件影響及魚游動的方式，使得阻力功率趨近於零。
並列摘要
This research simulated the characteristics of the flow field with an undulating fish behind a blunt body in three dimensions through a CFD software (ANSYS-FLUENT). Three cases with different distances (L=0.2m, 0.16m, 0.24m) between the cylinder and leading edge of fish were studied in a flow at speed of 0.4 m/s. A case with distance of 0.2m and speed of 0.04 m/s was also computed for comparison. The drag and swimming power generated in these cases were studied as well. The results demonstrated that the variation of drag and swimming power in the cycle were similar to those of phase angle 204° in our previous two dimensions study. However, the magnitude of the fluctuations were much smaller than those in two dimensions. Simulated results showed that the magnitude of swimming power were bigger than the drag power, it was opposite to those shown in previous study. However, these can be explained by three-dimension relieving effect. It’s mainly because the swimming power was calculated with the multiplication of z-direction velocity. With 3-D relieving effect, the velocity in z direction became much smaller than in 2-D, so does the swimming power. Furthermore, the drag power were computed using x-direction velocity. Since there were no-slip condition on the fish surface, and there were no actions in x-direction, the magnitude of x-direction velocity approached to zero, so does the drag power.