跌坎下游流场的时均流速分布和脉动特性

本文采用颗粒示踪的方法试验研究了跌坎下游主流重新接触床面流区的时均流速分布，流速的脉动强度及各点脉动流速的概率密度分布。试验结果表明，跌坎下游水流分离，具有稳定的漩涡发展，漩涡产生的频率约为每２秒３次。重新接触点离跌坎的距离约在３￣６倍坎高之间。在主流重新接触床面的流区，紊动剪切应力最大，在量测的流区，脉动流速的概率密度接近正态分布。

The Distributions of Velocity and Turbulence Characteristics Downstream of a Backward-facing Step

The formation of hydraulic drop due to the rise of upstream water level is frequently encountered in hydraulic engineering, and it is of significant importance to get a correct knowledge of the flow behavior and the characteristics of the reattachment point in such situations. An experimental investigation for this purpose has been carried out by using PIV technique. Results are summarized as follows:(1) The separation of flow takes place just downstream of the backward-facing step and a stable eddy forms and moves downstream. A new eddy develop soon afterwards. The average distance between two sequential eddies falls between 2 and 4 times of the step height, while the time interval stays at about 0. 7 second.(2) The poistion of reattachment is usually downstream of the second eddy, 3-6 times of the step height away from the step. Reattachment point is periodically changed at a rate about 3 times every 2 seconds.(3) Turbulent shear stress reaches its peak in the reattachment region. So this section of river bed should be protected with emphasis.(4) The probability density distribution of the fluctuation velocity in the studied flow field can be approximately characterized by Gaussian distribution.