Velocity distribution and turbulence structure of open channel flow with floating-leaved vegetation

Abstract

Floating-leaved vegetation alters the flow structure through the combined action of floating leaves, stems, and the channel bed. This study investigated the vertical distribution of streamwise velocity and the turbulence structure of an open channel flow with floating-leaved vegetation through theoretical analysis and laboratory experiments. Bionic lotus leaves fixed on circular wooden cylinders were used to imitate floating-leaved vegetation, and the 3D velocity field was measured by using an acoustic Doppler velocimeter. The measured data showed that with the presence of the channel bed and vegetation cover, strong velocity gradients appear at the bottom and top layers, whereas the velocity is almost uniformly distributed in the internal layer. On the basis of the modified mixing length hypothesis, the vertical distribution of streamwise velocity was numerically predicted, and the prediction results agreed well with the experimental data. The model parameters were then determined, and the effect of mixing length was analyzed. Spectral analysis showed that the peak frequency in the streamwise velocity spectra is within the range of 0.1 Hz to 0.2 Hz and that the vortices are mainly affected by stem-scale turbulence. Quadrant analysis showed that the ejections and sweeps are the major contributors to Reynolds stress in the bottom layer, inward and outward interactions are prominent in the top layer, and the contribution of sweeps and ejections are approximately equivalent to those of outward and inward interactions in the internal layer. Different to open channel flow featured with emergent rigid vegetation, the velocity and wake production decrease while the Reynolds stress and shear production increase towards the floating leaf. The effect of stem diameter, vegetation density, stem flexibility and boundary roughness on flow structure of open channel flow with floating-leaved vegetation need to be examined in future research.