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|標題:||Flow Types Around and Vortex Structure beneath Inundated Bridge Deck|
|關鍵字:||partially inundated bridge|
fully inundated bridge
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).
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