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Study on Characteristics of Vortices Generated by a Solitary Wave Propagation Over a Symmetric Cavity
Chang, Chin Shiang
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本研究係以先進之質點影像測速儀(PIV)搭配高速攝影機，對於孤立波通過對稱穴槽時，穴槽周邊流場的渦流運動進行定性的觀察及定量的分析。在水深h = 7 cm、穴槽深度D = 3 cm的條件下，針對孤立波通過(波高H = 1.4 cm)不同無因次間距G/D = ∞、2.5、2.0、1.0、0.75、0.5及0.33之對稱穴槽時，因流體分離所產生之渦流流場特性，包括主渦渦Av之渦心軌跡、渦Av渦心渦度值及射流軌跡等進行比較分析。
定性分析方面，在穴槽間距為G/D = ∞、2.5、2.0時，射流軌跡呈一弧線往水面移動。然而於穴槽間距G/D = 1.0及G/D = 0.75時之射流軌跡，因受到穴槽尾端之逆向速度的影響而往尾端偏移。穴槽間距調整至G/D = 0.5及G/D = 0.33時，射流軌跡則往穴槽入射端的方向移動。渦Av渦心軌跡在穴槽間距為G/D = ∞、2.5、2.0時，渦流自入射端脫離後往水面移動而後下降，於穴槽間距G/D = 1.0及G/D = 0.75時，則在穴槽中順時針環繞一圈後往尾端移動，且並無到達水面。穴槽間距調整至G/D = 0.5及G/D = 0.33時，渦心軌跡則在穴槽中順時針環繞一圈後往入射端移動，亦無到達水面。
本文亦利用相位平均法針對各不同無因次間距加以定量分析探討，發現渦流強度隨著穴槽間距的大小不同而有所差異。當穴槽間距G/D = ∞時，渦Av之渦心渦度絕對值及環流量絕對值與其它間距相比相對較大，渦流消散時間亦較長。當穴槽間距G/D = 0.33時，渦Av之渦心渦度絕對值及環流量絕對值則為最小，渦流消散時間亦最短。且渦Av渦心渦度絕對值及環流量絕對值隨時間有先增加而後減少的趨勢。
The vortex evolutions induced by a solitary wave propagating over a symmetric cavity were investigated experimentally. The vortex shedding processes were observed qualitatively and measured quantitatively using Phantom Camera and PIV, respectively. The target solitary wave height, H, is 1.4 cm, which gives the dimensionless height ratio (H/h) of 0.15 with h being the depth of water. The depth of cavity D is 3 cm, which gives H/D = 0.47. The characteristics of vortex evolution including trajectory of jet and position of major vortex were discussed in various cases with different gap ratios (G/D = ∞, 2.5, 2.0, 1.0, 0.75, 0.5 and 0.33).
Based on the observation results using flow visualization technique, it is found that the vertical jet ejects in a curvilinear trajectory from the edge to the water surface while the G/D = ∞, 2.5 and 2.0. The vertical jet shifts to the lee side due to the effect of backward velocities generated at the downstream edge of cavity when G/D = 1.0 and 0.75. With the gap ratios of G/D = 0.5 and 0.33, the corresponding vertical jet moves from the cavity to the upstream side. The flow pattern of vortical movement can be categorized to obtain the corresponding trajectory of major vortex Av according to three different sets of gap ratios.
In the analysis of circulation and vorticity for the core of major vortex Av, the phase-average method was utilized in order to precisely obtain the phase-dependent distribution of flow field for vortex shedding process. The results reveal that the magnitudes of vortex strength vary with the gap ratios. When G/D = ∞, both of the corresponding vorticity at the core of major vortex Av and the absolute value of circulation become the largest and have the longest time of energy dissipation. Relatively, the vorticity at center of major vortex Av and the absolute value of circulation are minimum for all the dimensionless time at G/D = 0.33. Moreover, it is found that the vorticity at core and the absolute value of circulation of major vortex Av increase in the beginning and then decrease gradually during dissipation processes.
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