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Influences of different hydraulic gradients on flow structures near submerged spur dike
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摘要: 采用超声波水位和PIV流速测量技术，在U形水槽中试验研究了不同水力坡度下淹没单丁坝对水流的影响。同一水力坡度，丁坝上游横比降沿程逐渐增大，丁坝下游横比降沿程逐渐减小。不同水力坡度，随着水力坡度的增大：① 丁坝附近的水位、横比降、淹没程度和丁坝迎水面与背水面的水位差逐渐减小；② 丁坝上游最高水位位置向下移动，丁坝下游水位恢复点位置向上移动；③ 断面平均流速及最大流速逐渐增大，但最大流速出现的位置基本不变；④ 漩涡流速分布较相似，但漩涡强度增加，坝后漩涡中心逐渐向下游对岸移动。当水槽坡度为-1‰~1‰时，淹没丁坝对丁坝上方5 cm处的剖面水流影响较小；当水槽坡度为2‰时，淹没丁坝对丁坝上方5 cm处的剖面水流影响较大；当水槽坡度为1‰~2‰时，淹没丁坝对水流结构的影响可能存在1个临界变化点。研究结果有助于对河床演变及海岸工程等复杂水环境问题的深入探索。Abstract: Using ultrasonic wave and PIV technical methods for measuring water level and velocity respectively, the influences of different hydraulic gradients on the flow structures near a submerged spur dike are studied by laboratory experiments in a U-shape flume. Experimental results show that under the same hydraulic gradients, the transverse slope of water surface along the longitudinal direction increases in the upstream of the dike, but it decreases in the downstream of the dike. Under different hydraulic gradients, with the increase of hydraulic gradients, it is found that: ① the water level, transverse slope of water surface, relative flooding depth in the vicinity of the dike and the water level difference between the upstream face and downstream face of the dike will gradually decrease; ② the position with the highest water level in the upstream of the dike moves downward, while the water level recovery point in the downstream moves upward; ③ the transverse mean flow velocity and the maximum flow velocity gradually increase, but the position of the maximum flow velocity almost does not change; ④ the vortex velocity distribution is relatively similar with an increase of vortex intensity, while the center of the main vortex behind the submerged spur dike moves toward the opposite bank of the downstream. When the flume slope changes from -1‰ to 1‰, the submerged spur dike has a little effect on the flow patterns in the area of 5 cm above the dike. However, when the flume slope reaches 2‰, the submerged spur dike has an obvious effect on the flow patterns in the same area. It implies that there may exist a critical value between 1‰ and 2‰ for the flume slope that affects the submerged spur dike and flow patterns. The research results will be favourable to the deep exploration of the complicated environmental hydraulic problems in the fields of fluvial processes and coastal engineering.
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Key words:
- hydraulic gradient /
- submerged spur dike /
- flow structure
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图 3 流速测量区域及观测点布置(单位：cm)
Figure 3. Layout of flow velocity measurement areas and observation points (unit: cm)
图 5 不同水槽坡度沿程水面横比降变化
Figure 5. Longitudinal changes of water surface transverse gradient under different flume slopes
图 6 横比降均值与水槽坡度的关系
Figure 6. Relationship between averaged water surface transverse gradient and flume slope
图 7 不同水槽坡度距水槽底25 cm处观测点流速沿程分布
Figure 7. Flow velocity distributions at observation points with 25 cm distance from flume bottom under different slopes
图 8 不同水槽坡度距水槽底10 cm处水平剖面流场
Figure 8. Flow fields at horizontal plane with 10 cm distance from flume bottom under different flume slopes
表 1 不同水槽坡度淹没丁坝上下游水面比降测量值
Table 1. Measured values of water surface gradient in vicinity of submerged dike under different flume slopes
水槽坡度/‰ 不同位置水面比降/‰ 丁坝对岸侧 丁坝侧 -1 8.204/-8.119 8.670/-9.320 0 3.356/-3.379 5.232/-5.800 1 3.290/-2.608 4.415/-4.509 2 1.080/-2.208 2.560/-3.388 注：“/”前为上游水面比降，“/”后为下游水面比降。 
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表 2 淹没丁坝水流特征参数
Table 2. Characteristic parameters of submerged spur dike
水槽坡度/‰ △H/H 丁坝两侧水位差/mm 弗劳德数 -1 0.351 1.925 0.060 0 0.347 1.320 0.054 1 0.341 1.121 0.059 2 0.337 0.782 0.065
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表 3 不同水槽坡度距水槽底10 cm处漩涡中心位置
Table 3. Vortex center positions at plane with 10 cm distance from flume bottom under different flume slopes
水槽坡度/‰ 与丁坝距离/ cm 与丁坝侧边壁距离/ cm -1 15.7 11.5 0 13.2 10.8 1 13.4 11.3 2 14.8 11.7
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表 4 不同水槽坡度距水槽底10 cm处沿漩涡中心横向观测点流速
Table 4. Flow velocity of observation points along vortex center transversal line with 10 cm distance from flume bottom under different flume slopes
(m·s-1) 水槽坡度/‰ 横向各观测点流速 平均流速 1 2 3 4 5 6 7 -1 0.148 0.151 0.136 0.060 -0.002 -0.032 -0.044 0.059 0 0.149 0.154 0.148 0.079 -0.002 -0.026 -0.036 0.066 1 0.152 0.156 0.147 0.074 -0.002 -0.030 -0.048 0.065 2 0.153 0.157 0.147 0.073 -0.002 -0.041 -0.052 0.062
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