Velocity distribution of flow with submerged flexible vegetations based on mixing-length approach

By choosing a PVC slice to simulate flexible vegetation, we carried out experiments in an open channel with submerged flexible vegetation. A 3D acoustic Doppler velocimeter （micro ADV） was used to measure local flow velocities and Reynolds stress. The results show that hydraulic characteristics in non-vegetation and vegetation layers are totally different. In a region above the vegetation, Reynolds stress distribution is linear, and the measured velocity profile is a classical logarithmic one. Based on the concept of new-riverbed, the river compression parameter representing the impact of vegetation on river is given, and a new assumption of mixing length expression is made. The formula for time-averaged velocity derived from the expression requires less parameters and simple calculation, and is useful in applications.

Interaction of Streamwise and Wall-Normal Velocities in Combined Wave-Current Motion

The aim of this paper is to present an analytical expression for the streamwise velocity distribution in a non-uniform flow in the presence of waves; the correlation between the horizontal and vertical velocity components has been compreheusively examined. Different from previous researches which attributed the deviation of velocity from the classical log-law to the wave Reynolds stress, i.e. - ρ uv^- only, this study demonstrates that the momentum flux caused by mean velocities, i.e., u^- and v^-, is also responsible for the velocity deviation, and it is found that the streamwise velocity for a flow in the presence of non-zero wall-normal velocity does not follow the classical log-law, but the modified log-law proposed in this study based on simplified mixing-length theorem. The validity of the modified log-law has been verified by use of available experimental data from published sources for combined wave-current flows, and good agreement between the predicted and observed velocity profiles has been achieved.

Mean flow and turbulence structure of open channel flow with suspended vegetation

In combination with a channel bed,suspended vegetationin an open channel can alter flow structure and generate vertically asymmetric flow.This study investigated the mean flow and turbulence structure of an open channel with suspended vegetation through theoretical analysis and laboratory experiments.Three patterns of bionic leaves with different roughness were adopted to imitate suspended vegetation,and three-dimensional velocity field was measured by using an acoustic Doppler velocimeter.The measured data showed that the vertical profile of streamwise velocity obeys a two-power law and that the maximum velocity at the middle depth is close to the smooth boundary(i.e.,the channel bed in the experiment)under the combined action of vegetation cover and channel bed.Shear stress is linearly distributed along the vertical axis,and the vertical profile of turbulence intensity obeys an exponential law.Then,a two-power law expression was adopted to predict the vertical profile of streamwise velocity.Theoretical models for the vertical distribution of shear stress and turbulence intensity were also established.The predicted results validated by measurements showed that the different magnitudes of vegetation cover and channel bed boundary roughness exert an obvious impact on flow structure.