平底光面明渠中紊流猝发的特性研究

Characteristics of turbulent bursting phenomenon in open channel with a flat and smooth bottom

在保持循环水槽底部平整和表面光滑仅改变流速的条件下进行了系统的实验研究 ,重点研究了雷诺数ReR 对紊流猝发的形态和特性的影响。采用流场综合分析法将流动显示与计算机图像处理相结合 ,与常用的点流速条件检测猝发现象相比有了明显改进 ,有利于加深对明渠紊流猝发的认识。PIV激光片光源频闪技术拍摄的猝发系列照片生动展示了明渠紊流流场的猝发特性 ,可用于直接分析 ;再经过计算机图像处理后可绘制流速矢量场图并进行相关计算分析。试验表明猝发喷射角约为 4 0°～ 6 0°,雷诺数ReR 增大会使喷射角略有增加 ;猝发喷射高度约可达到边界层名义厚度的 1/4～ 1/3高度。雷诺数ReR 增大明显增加了多头群体喷射现象 ,无量纲猝发周期加大 ,喷射尺度和清扫尺度也有增大趋势。对试验的分析表明雷诺数ReR 对明渠中紊流猝发的特性有重要影响 ,增大ReR

Serial experiments were carried out in a circulative open flume under the conditions of keeping the flat and smooth bottom but changing the flow velocity, the focus of which was placed on the effects of the Reynolds number Re R on the patterns and characteristics of turbulent bursting. By combining the flow visualization and the computer image processing the obtained flow field images were better than those with conventional methods based on the signal processing at one or few points in detecting the so called bursting phenomenon and improving the understanding of the turbulent coherent structure in open channel flow. Based on the PIV laser sheet technology, and data processing technique and hydrogen bubble visualization serial pictures, the turbulent bursting process was vividly depicted by frequency flash photo. These pictures were directly analyzed and used to plot the velocity vector field based on the computational correlation analysis concerned. It is shown by the analysis based on the experiments that the ejection angle of the bursting is about 40 to 60 degree,increasing with the Reynolds number more or less. The ejection height could be 1/4～1/3 of the nominal thickness of boundary. The increase of Re R causes the multi ejection phenomenon clearly, the non dimensional bursting period becomes larger; on the other hand, the non dimensional model lengths of an ejection and a sweep, Le + and  Ls + has the tendency to increase. It should emphasized by the above-mentioned analysis of the experiments that the Reynolds number Re R has some significant effects on the structure characteristics of bursting phenomena and the transition to turbulent state becomes more serious with the increase of Re R.