| The floating vegetation,floating-leaved vegetation and floating treatment island are typical type of suspended canopy,which is widely found in rivers,lakes,wetlands,can affect the hydraulic characteristics of the flow and provide a wide range of ecosystem functions in the aquatic ecosystem.This study investigated the hydraulic characteristics of an open channel flow covered with different suspended canopy through theoretical analysis and laboratory experiments.The main study contents and conclusions are as follows:Experiments were conducted in a flume to investigated the hydraulic characteristics of an open channel flow covered by floating vegetation with thin cover and undeveloped roots,the characteristics of the vertical distribution of streamwise velocity,Reynolds stress and turbulence intensity were quantified,and the theoretical models were then established.Floating vegetation alters flow structure due to its combined action with channel bed roughness and generates vertically asymmetric flow.The flow region can be vertically divided into two parts,namely,the bed and vegetation layers,on the basis of the vertical distribution of streamwise velocity.The vertical profile of streamwise velocity obeys a two-power law expression.Reynolds stresses are negative for the upper flow region,whereas they are positive for the lower flow region,and Reynolds shear stress shows a linear distribution along the vertical axis.The locations of zero shear stress and maximum velocity do not coincide in a vertical asymmetric channel flow with two different roughness boundaries,and the former is constantly located closer to the smoother surface relative to the latter.The turbulence intensity firstly decreases and then increases along the vertical direction,the minimum value of turbulence intensity is located at the plane of zero shear stress,and its value is equal to the total shear velocity.The vertical profile of turbulence intensity can be divided into two parts via the plane of zero shear stress,and can be described as exponential expressions.The roughness of channel bed and vegetation cover directly affect the distribution of streamwise velocity and Reynolds stress,and the locations of zero shear stress and maximum velocity locate closer to the smoother surface with a larger ratio of vegetation boundary roughness and the channel bed roughness.Bionic lotus leaves fixed on circular wooden cylinders were used to imitate floating-leaved vegetation,and the laboratory experiments were conducted to investigated the open channel flow with floating-leaved vegetation,and the turbulence structures were investigated through spectral analysis and quadrant analysis.The flow region was divided into the leaf-dominated region and bed-dominated region,then a numerical model was built based on the modified mixing length hypothesis to solve the vertical distribution of streamwise velocity,and the prediction results agreed well with the experimental data.Floating-leaved vegetation alters the flow structure through the combined action of floating leaves,stems,and the channel bed,large velocity gradients appears at the bottom and top layers,the velocity and velocity gradient are all decrease with the increase of critical distance L_m,and the velocity remains approximately constant in the internal region and not affected by critical distance L_m.The turbulence production is dominated by stem wake and that the vortices are mainly affected by the stem-scale turbulence.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,while there are no dominant events in the internal layer.Laboratory experiments of open channel flow with fully and partially floating treatment island were conducted,the influence of the floating treatment island(FTI)on the open channel flow were investigated through theoretical analysis.For the flow covered fully by floating treatment island,the flow region was divided into an FTI layer and a gap layer,an then theoretical models for the vertical distribution of streamwise velocity and Reynolds stress were established.In the gap layer,Reynolds shear stress shows a linear distribution along the vertical axis,then the velocity profile can be obtained by the modified eddy viscosity model.In the FTI layer,the vertical profile of Reynolds shear stress obeys an exponential law,and the velocity can be obtained by solve the momentum balance equation.The prediction results agreed well with the experimental data.The turbulence structures were investigated through spectral analysis and quadrant analysis.The thickness of the mixing layer and penetration depth are all increase with the increase of FTI submergence ratio.In the junction of the FTI layer and gap layer,Reynolds shear stress and turbulence intensity reach to their maximum values.The shear-scale vortices are obvious at the interface,while the stem-scale vortices dominate the FTI layer.The ejections and sweeps are the major contributors to Reynolds stress in the region close to channel bed,whereas inward and outward interactions are prominent in the other region.For the flow covered partially by floating treatment island,the flow region was divided into an FTI domain and a non-FTI domain,then theoretical models to predict the lateral distribution of depth-averaged velocity was established,and the comparison of the measured and predicted data confirmed the reliability and authenticity of the proposed theoretical model.The sensitivity analysis of the dimensionless eddy viscosity,the secondary flow coefficient and the velocity coefficient indicated that the parameters have significant impact on the lateral velocity profile.The vertical profile of streamwise velocity approximately obeys the logarithmic law in the central non-FTI domain,while it shows the same distribution as fully developed flow with FTI in the central FTI domain,and the transitional form appears in the interface of non-FTI domain and FTI domain.In the non-FTI domain,the depth-averaged turbulence intensity of the FTI layer is larger than that of the gap layer,whereas the depth-averaged turbulence intensity of the FTI layer is smaller than that of the gap layer in the FTI domain.The longitudinal,transversal and vertical depth-averaged turbulence intensity all firstly increase and then decrease along the transversal direction,and the maximum value of turbulence intensity is located at the interface of the non-FTI domain and FTI domain.Laboratory experiments of open channel flow with a single FTI and FTIs in series were conducted,and the time-averaged velocity and turbulence structure along open channel flow with discontinuous FTI patches were investigated.For the flow with a single FTI,the influence of the FTI on the velocity,Reynolds stress and turbulence intensity of upstream and downstream were discussed.Three regions are divided along the flow according to the flow development process:upstream region,FTI region and downstream region.The vertical profile of streamwise velocity approximately obeys the logarithmic law in the upstream far away from the FTI region,the velocity firstly increases and then decreases along the vertical direction in the FTI region,and velocity profile gradually tend to logarithmic distribution in the downstream region.The depth-averaged velocity of the FTI layer firstly decreases and then increases along the longitudinal direction,whereas the depth-averaged velocity of the gap layer firstly increases and then decreases.The vertical distribution of shear stress is approximately linear in the upstream region,while it is similar to the fully developed flow with FTI in the downstream region,and the transitional form appears in the FTI region.The turbulence intensity firstly decreases and then increases along the vertical direction in the upstream region,whereas the turbulence intensity firstly decreases and then increases and remain constant finally along the vertical direction in the FTI region,while the turbulence intensity profile gradually tends to decreases firstly and then remain constant in the downstream region.For the flow with FTIs in series,the time-averaged velocity and turbulence structure along the flow were discussed,and the differences of hydraulic characteristics between the FTI region and the gap region were analyzed.The depth-averaged velocity firstly increases and then decreases along the longitudinal direction in the FTI region,while it firstly decreases and then increases in the gap region,and the depth-averaged velocity shows a wavy distribution along the mainstream.The velocity is clearly slower while the fluctuation is larger in the FTI layer as compared to the gap zones.The depth-averaged turbulence intensity and turbulence kinetic energy all vary wavily along the flow.The ejections and sweeps dominate the region near channel bed,whereas inward and outward interactions are prominent in other regions,and the contributions of inward and outward interactions firstly increase and then decrease along the vertical direction.The velocity,turbulence intensity and turbulence kinetic energy are all smaller in the gap region than in the FTI layer,which indicates that the gap regions are beneficial for aquatic organisms inhabitation and sediment deposition.Smaller fluctuations of time-averaged velocity and turbulence are observed for the cases with larger spacing between the FTI,indicates that the longer gap region is beneficial for the flow stability. |
