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.