請用此 Handle URI 來引用此文件: http://hdl.handle.net/11455/87966
標題: Flow Types Around and Vortex Structure beneath Inundated Bridge Deck
淹沒橋樑之流況類型及橋面版下方漩渦結構特性
作者: Cheng Lin
Ming-Jer Kao
林呈
高明哲
關鍵字: partially inundated bridge
fully inundated bridge
PIV
flow visualization
vortex structure
部分淹沒橋梁
完全淹沒橋梁
質點影像測速儀
流場可視化
漩渦結構
摘要: 一般非淹沒橋面條件下之河川水流乃屬重力流;惟當洪水位高過梁底、達至橋梁上部結構而形成橋面部分淹沒或完全淹沒流況時,橋面版下方區域則逐漸形成壓力流。此時對水流而言,上部結構形同一障礙物而窄縮通水斷面,造成上游側之水位抬升,且自上游流入橋面版下方之水流流速逐漸增強,形成下射式水流引致河床鉛垂向的束縮沖刷,進而促使橋基更加裸露。本研究採用質點影像測速儀(PIV)與流場可視化法,以穩態均勻流之定床水工模型實驗,模擬橋梁上部結構被洪水淹沒之情境,研究橋梁為洪流淹沒時之流況類型、及橋面版下方流場之漩渦特性。依據橋梁淹沒狀況與流場特性,淹沒橋梁之流況類型可分為兩大類及六種流況:一、部分淹沒橋梁;與二、完全淹沒橋梁。其中前者可再細分為:(一)潛沒孔口流況(TypeI、TypeII);(二)過渡帶流況(TypeIII);(三)閘孔流況(TypeIV);而後者則亦可細分為:(一)自由堰流流況(TypeV);(二)潛沒越流流況(TypeVI)。再者,對於梁間穴槽內漩渦之運動特性,漩渦呈現逆時針或順時針旋轉方向(若來流方向係由左方流至右方)的決定機制,係取決於橋下梁底區域之自第一根大梁前緣分離流之剪力層與最末根大梁下游端之尾跡流兩者的相對強度關係。若流況以前緣之分離流剪力層為主時,則漩渦呈現逆時針方向旋轉為主;而當流場受最末根大梁下游端之尾跡流影響時,因部分水流逆流,漩渦乃以逆時針方向旋轉為主。
During the rising stage of a certain flood in Taiwan, the water level in a river generally rises. The bridges located in the upstream or middle reach often experience gradual increase in water level, and then become submerged either partially or totally. Such inundated bridges are subjected to a combined effect of pressure flow below the bridge deck and the weir flow over it, depending upon the water level of the approaching flow relative to the headroom clearance below the bridge deck. To investigate the flow field under a relatively idealized situation without the effect of bed scour, hydraulic model tests with rigid bed were carried out in a re-circulating flume with dimensions of 446 cm long, 50 cm high and 25 cm wide. The test section located at 150 cm from the inlet of the flume was fitted with glass-sided walls and glass bottom to facilitate optical access. The main bridge-deck model was made of acrylic with a scale of 1/100, and the overall width B and total depth D of bridge-deck model was 10.0 cm and 3.5 cm, respectively. It had four girders of 2.0 cm deep and 0.5 cm wide spaced at 2.4 cm center-to-center. Flow visualization and PIV techniques were used to observe and measure the flow field, respectively. The main parameters that dominate the flow field were Froude number F_r of the approaching flow, the proximity ratio P_r (=ratio of clearance below the bridge deck h to the total depth of the bridge deck D), the relative submergence ratio S_r (=ratio of the depth of water over the low chord of the bridge deck (H_1-h) to the total depth of bridge deck D), and the ratio of the depth of air-pocket to the depth of girder cavity, R_a. Depending upon the Froude number F_r, proximity ratio P_r, inundation ratio S_r, and relative cavity ratio R_a, different types of flow structures around the bridge decks can be classified. Six types of flow structures around the bridge deck were recognized. For partially inundated bridge, in flow Type I (submerged-orifice flow), the water surface elevation on the downstream side of bridge deck was slightly lower than the counterpart on the upstream side, and the shear layer formed at the bottom of upstream girder continuously fluctuated and touched the soffit of all girders. In the case of flow Type II (submerged-orifice flow), the water surface on downstream side of the bridge deck was lower than that on the upstream side and the shear layer originating from the upstream girder impinged near the third cavity between girders. However, in both cases, the cavities between the girders were completely occupied by vortices. On the contrary, in the cases of flow Type III and IV, the flow was separated from the upstream girder edge. However, in flow Type III (transitional flow), the separated flow impinges on successive girders and cavities were partially filled with water; while in flow Type IV, the flow was totally separated from the deck bottom like a sluice gate flow. When water level rose above the top of the parapets of bridge deck, which were if fully inundated situation (flows Type V and VI), a weir flow occurred and overtopped the bridge deck. In the case of flow Type V (free overflow), part of the approaching flow discharged through the bridge deck (pressure flow), and the other overtopped the bridge deck, which behaved like a broad-crested weir flow. In the case of Type VI (submerged overflow), the flow situation was identical to flow Type V. However, the flow depth above the bridge deck for flow Type VI was significantly larger than that of flow Type V. Moreover, the characteristics of vortex structure under partially and fully inundated bridge deck were investigated for the cases with cavities being full of water (i.e., without air-pocket).
URI: http://hdl.handle.net/11455/87966
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