淹没植物条件下明渠KH涡的垂向几何尺度和频率

含淹没植物明渠中存在的Kelvin-Helmholtz（KH）涡，对水流能量交换和物质输移具有直接的影响。基于淹没刚性植物条件下的水槽实验，利用粒子图像测速技术（PIV）得到瞬时流速场和涡量场，定量刻画KH涡的垂向几何尺度（上下边界和垂向作用范围），结合三维声学多普勒流速仪（ADV）测量的流速时间序列和小波变换方法，分析KH涡的频率，研究植物淹没度对KH涡上下边界位置、作用范围和频率的影响。研究发现:本实验确定的KH涡上、下边界与根据能量分布计算得到的混合层上、下边界基本一致；在本实验条件下，KH涡上边界为0.7~0.8倍水深，下边界在植物密度一定时围绕一常数波动，KH涡的频率在0.14~0.20 Hz之间；随着淹没度的增大，KH涡的上边界升高，下边界变化不明显，作用范围和频率增大。

Vertical geometry scale and frequency of KH vortex in open channel flow through submerged vegetation

Kelvin-Helmholtz (KH) vortices, which exist in submerged vegetated open channels, have direct impacts on the energy exchange and mass transport in the flow. In laboratory experiments carried out in a flume whose bed is covered with rigid model vegetation, we use Particle Image Velocimetry (PIV) to measure the instantaneous velocity and vortex fields and examine the characteristic geometry of KH vortices, including their positions and heights. Meanwhile, the frequency of KH vortices is analyzed using the wavelet transform method and the velocity time series obtained with 3-D Acoustic Doppler Velocimetry (ADV). The influence of submergence on the position, height and frequency of the KH vortex is explored as well. Experimental results indicate that the upper and lower boundaries of the KH vortex are in agreement with those of the mixing layer which can be computed according to energy distribution. The upper boundary is about 0.7—0.8 times the water depth, while the lower boundary fluctuates around a value dependent on vegetation density. The frequency of KH vortices is between 0.14—0.20 Hz in the present experiments. With the increase of submergence, the upper boundary of the KH vortex moves upward, the lower boundary does not change much, while the height and frequency increase.