Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements
Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements
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使用同步高速视频和热膜测量来表征湍流明渠流中的近床相干结构
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
Y. Niño
中科院分区：
文献类型：
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作者：
M. Garcia;F. López;Y. Niño
在近壁区域的湍流明渠流的流动可视化的高速视频记录（500 Hz）与流速和床剪切应力的热膜测量同步。对视频图像的分析提供了与床附近低速条纹喷射的发生有关的相干流结构的主要特征的信息。这些结构主要由振荡剪切层组成，这些剪切层向下游方向转换并被提升离开床层。一个视觉检测标准的开发，以获得合奏平均的速度和剪切应力的数据，在喷射事件期间，允许相关的流场的特性在相干结构的发生。有条件的平均表明，这种相干模式的发生主要影响在壁区的湍流结构，所观察到的事件揭示了一个合理的机制，通过该机制，能量从大尺度湍流波动的平均流中提取，然后进一步转移到较小的涡流，而结构失去了它们的连贯性。湍流能量的产生和耗散的间歇性变得明显，发生在大约21%的时间。获得的结果还提供了证据，似乎将负责动量湍流垂直输送和维持湍流状态的结构与触发沉积物夹带进入悬浮液的机制联系起来。与其他实验结果进行了比较，在不同类型的流动强烈证实了一个普遍的结构，相干事件的壁有界流。
High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.