針對方形建築物以近導流結構減少阻力及其振盪之研究

Investigation of reducing drag and its oscillation for a square building by introducing nearby flow-guided structures

呂晟瑋(Cheng-Wei Lu)

指導教授 ： 朱錦洲

共同指導教授 ： 張建成(Chien-Cheng Chang)

國立臺灣大學/工學院/應用力學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202102762

摘要

本研究利用數值模擬與水洞實驗，探討邊長D = 0.03 m、柱高H = 0.15 m的正方柱體，受到上游置於不同位置但與主方柱等高度的導流控制物之影響，所產生的降低阻力與渦流震盪的效果。研究所使用的導流控制物有兩種型式：分別是邊長5 mm的正方柱以及8 mm x 2 mm的長方柱體。流場基於正方柱體邊長的雷諾數為7530與16740。正方柱體的一端固定在平板上，另一端為自由端；控制物中心至主方柱中心的距離為S，S/D分別為1、1.5…4.5及 5。藉此模型探討一方形建築物受到上游近物導流結構之控制物的影響，所產生之降低風阻及渦流震盪的效果。
分析方始將使用商用數值計算軟體(CFD ANSYS FLUENT)建立3D的流場，
首先先進行網格的獨立性研究，對Re = 16740的流場確定網格數目與Cd結果無相依關係後，在主方柱上游加入控制物(次方柱或薄板)進行模擬。結果發現次方柱的模擬在S/D = 2時有最佳的減阻效果；在薄板模擬時，則在S/D = 3時有最高的減阻比例。水洞實驗將安置柱體模型的平板倒置於液面上方，模擬自由流通過立於地面的方形柱體。吾人並利用測力規(load cell)量測主方柱的瞬時及平均阻力，藉此探討兩種控制物在不同位置所造成主方柱減阻的效果。實驗的結果驗證了模擬趨勢的正確性，且發生最佳的減阻之近物導流結構的位置皆一致。
配合比較模擬的截面流線圖以及實驗的PIV分析而推測減阻的原因，吾人發現在S/D = 1時，流體將會視控制物及主方柱為一大型物體，流線將直接繞其而過，僅少許流體通過中間的空間；至於在最佳減阻位置時，控制物後方的空間不足以產生完整的渦流脫落，使流體平順通過主方柱四周及柱頂，並使主方柱尾流範圍縮小，達到有效的抑制，為減阻的主要原因；在S/D ≥ 4時，此時的空間已足夠使控制物產生完整的渦流脫落，且流體在脫落後直奔主方柱並在邊緣分離產生渦流，如同單一主方柱般產生大範圍的尾流渦流，此時兩者將呈現獨立的狀態。

關鍵字

渦流脫離 ； 實驗量測 ； 有限方形建物 ； 數值模擬 ； 抑制震動

並列摘要

This study uses numerical simulation and water tunnel experiments to explore the influence of a square column with side length D = 0.03 m and column height H = 0.15 m, which is affected by the diversion sub-square column that is placed at a different position upstream but has the same height as the main square column, causing the effect of reducing drag and vortex oscillation. There are two types of diversion control bodies used in the research. They are square columns with a side length of 5 mm and rectangular columns with a side length of 8 mm x 2 mm. The Reynolds numbers of the flow field based on the side length of the square column are 7530 and 16740. One end of the square column is fixed on the flat plate, and the other end is a free end. The distance from the center of the secondary square column to the center of the main square column is S, and S/D is 1, 1.5...4.5 and 5 respectively. This model is used to explore the effect of reducing wind resistance and eddy currents caused by the influence of the secondary square column of the upstream diversion structure of a square building.
The analyst will use the commercial numerical calculation software (CFD ANSYS FLUENT) to establish a 3D flow field. First, grid independence is studied, and the number of meshes in the flow field of Re = 16740 is determined to be incompatible with the result of Cd. Start to simulate after adding a control body (secondary square column or thin plate) upstream of the main square column. It is found that the simulation of the second square column upstream of the main column has the best drag reduction effect when S/D = 2, in the other one simulation, it has the highest drag reduction ratio when S/D = 3.
In the water tunnel experiment, the flat plate on which the column model is set is placed upside down above the liquid surface to simulate free flow through the square column standing on the ground. We also use a load cell to measure the instantaneous and average resistance of the main square column, discussing the drag reduction effect of the main square column caused by the two control bodies at different positions. The result of the experiment verifies the simulated.
With the comparison of the original streamline diagram of the simulation and the PIV analysis of the experiment, the reason for the drag reduction is inferred. Found that when S/D = 1, the fluid will flow past the control body, regarding the two objects as a larger object. There is little fluid passing through the gap between them. As for the best drag reduction position, the space behind the control body is not enough to generate a complete vortex shedding, which allow the fluid to smoothly pass around the main square column and the top of the column, and reduce the wake of the main square column. When S/D ≥ 4, the space is enough to cause the control body to produce a complete vortex shedding, and the fluid goes straight to the main square column after shedding. The separation on the edges of the main square column generates eddy current, causing a wide range of wake vortices, just like the situation with only one main square column. At this time, the two will be independent.

並列關鍵字

vortex shedding ； experimental measurement ； finite square building ； numerical simulation ； vibration suppression

參考文獻

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