Turbulent Structure of Backward-Facing Step Flow and Coherent Vortex Shedding from Reattachment in Open-Channel Flows
Turbulent Structure of Backward-Facing Step Flow and Coherent Vortex Shedding from Reattachment in Open-Channel Flows
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明渠流中后向阶梯流的湍流结构和相干涡旋脱落
DOI：
10.1007/978-3-642-73948-4_26
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发表时间：
1989
期刊：
影响因子：
--
通讯作者：
H. Nakagawa
中科院分区：
文献类型：
--
作者：
I. Nezu;H. Nakagawa
利用两套双分量激光多谱勒测速仪(LDA)研究了明渠流动中包括逆流区在内的后向台阶流的湍流结构。揭示了其湍流特性和再附着特性。在明渠流动中，时间平均再附着长度同时取决于雷诺数和弗劳德数。特别值得注意的是，超临界流动中的再附着长度比亚临界流动中的短。瞬时重附着点在时间平均重附着长度的距离上移动。这强烈地表明，由于再附着点的非定常和低频运动，可能会产生Kolk-Boil涡的拟序结构。这些计算值与目前LDA系统测得的实验值吻合得很好。明渠流动中的分离阶梯流及其再循环与边界层和管道阶梯流相似。但是，前者比后者更复杂，因为自由面的存在使压力更加松弛。其次，对压力和速度波动的频谱分析表明，在台阶的再附着附近存在相干的低频运动。为了揭示台阶流分离剪切层和再附着点下游的拟序结构，利用两套激光多谱勒风速仪和压力传感器进行了速度-压力和速度-速度组合的时空相关测量。Kelvin-Helmholtz不稳定性以与混合层中相同的方式产生了涡旋结构的准周期序列，它们被对流到再附着点。讨论了这种混合层型拟序旋涡在明渠流动中触发大尺度马蹄形旋涡从再附着点脱落的可能性。然后，这种马蹄形涡旋可能已经发展到自由表面，并变成沸腾涡旋，这是在实际的河流和河口观察到的。
The turbulent structure of the backward-facing step flow, including the reverse flow region, in open-channel flows was investigated by making use of two sets of two-component Laser Doppler anemometers (LDA). The turbulence characteristics and reattachment properties were revealed. The time-averaged reattachment length was dependent on both the Reynolds number and Froude number in open-channel flows. In particular, it should be noted that the reattachment length became smaller in supercritical flows than in subcritical flows. The instantaneous reattachment point moved over a distance of the time-averaged reattachment length. This suggested strongly that a coherent structure of the kolk-boil vortex might be generated due to the unsteady and low-frequency motions of the reattachment point.The structures of dynamic pressure and shear stress were analyzed on the basis of the momentum equation. These calculated values coincided well with the experimental values measured by the present LDA system. The separated step flow and its recirculation in open-channel flows were similar to that in boundary-layer and duct step flows. But, the former was more complicated than the latter, because the pressure was more relaxed by the existence of the free surface.Next, the spectral analyses of pressure and velocity fluctuations revealed that a coherent low-frequency motion existed near the reattachment of the step. In order to reveal the coherent structures in the separated shear layer of the step flows and also in the downstream of the reattachment point, the space-time correlation measurements of the velocity-pressure and velocity-velocity combinations were conducted by using two sets of the Laser Doppler anemometers and the pressure transducer. Quasi-pe- riodic trains of vortical structure were generated due to the Kelvin-Helmholtz instability in the same manner as in the mixing layer, and they were convected toward the reattachment point. The possibility was discussed that these mixing-layer type coherent vortices might trigger a shedding of the large-scale horseshoe vortex from the reattachment point in open-channel flows. Then, this horseshoe vortex could have developed up to the free surface and become boil vortex, which was observed in actual rivers and estuaries.