Experimental Study On Flow Structure Of Open Channel With Flexible Submerged Plants

In this study,flow structure of open channel with flexible submerged plants was studied by flume experiment.The bottom of the laboratory flume was covered with silicone tubes to simulate flexible submerged plants.The flow field of the open channel with flexible submerged plants was measured by particle image velocimeter(PIV)under different working conditions including inflow,plant density and submergence degree.Based on the measured and processed flow field data,the mean velocity,turbulence characteristics and turbulence structure were investigated.Through the analysis of the velocity data obtained from the experiment,the variational law of flow velocity distribution in open channel with flexible submerged plants under different working conditions was found.When the flow rate increases,the velocity in the whole range of water depth also increases,the inflexion point of the velocity curve at the top of the plant decreases,and the velocity gradient increases;under the condition of different plant density,the velocity of the plant layer changes inversely with that of the non-plant layer.the greater the plant density,the greater the velocity gradient at the top of the plant,and the closer the position where the velocity reaches the maximum is to the water surface.Under different conditions of submergence degree,the smaller the submergence degree is,the closer the position of the maximum velocity is to the water surface.Through the analysis of the flow velocity at different locations,it is also found that in the middle section of the plant,the flow velocity in different positions of the same height changes a little bit;in the plant section,the flow velocity in front of the plant is greater than that after the plant.By comparing the flow velocity distribution of flexible submerged plants with that of rigid submerged plants,it is found that the internal velocity gradient of rigid submerged plants is smaller,the inflexion point of velocity at the top of plants is higher,and the influence range of rigid submerged plants on the water above them is wider than that of flexible submerged plants.Under different working conditions,flexible submerged plants have different effects on the turbulence characteristics of open channel flow.When the flow rate increases,the peak of turbulence intensity also increases,and the turbulence at the top of the plant becomes more intense;the greater the plant density,the greater the maximum turbulence intensity near the top of the plant;the smaller the degree of submergence,the greater the range of influence of water turbulence caused by plants.On the whole,the distribution of Reynolds stress is similar to the turbulent intensity.The study on the penetration depth of Reynolds stress shows that compared with flexible submerged plants,rigid submerged plants can destroy the shear vortex structure near the plant canopy more effectively,thus weakening the penetration of Reynolds stress.The increase of flow rate and plant density aggravates the shear vortex motion near the top of the plant,so that the reverse maximum of vorticity increases accordingly.Quadrant analysis,spectrum analysis and detections of coherent vortex are used to study the turbulent structure of open channel flow with flexible submerged plants.Through the quadrant analysis of the instantaneous velocity,it is found that the sweeping flow pattern is the dominant flow pattern in the plant layer,the critical point of ejecting and sweeping flow transition at the top of the plant layer,and the ejecting flow pattern above the plant layer.With the increase of plant density,the sweeping effect of high-speed flow and the ejecting effect of low-speed flow are more intense in the plant interior and the plant tip.Spectral analysis shows that there is a shear vortex at the top of dense plants,and the frequency of the peak of the power spectral density curve corresponds to the frequency of Kelvin-Helmholtz instability,while the peak of the power spectral density curve can not be observed at the top of sparse vegetation,indicating that the plant resistance is small and is not enough to form a shear vortex at the top of the plant.The detection of coherent vortex shows the periodic phenomenon of high-speed fluid sweeping and low-speed fluid ejection in the open channel flow field with flexible submerged plants,which is compared with the results of rigid submerged plants.It is found that the top of rigid submerged plants will produce more intense eject and sweep,and its momentum and material exchange is more intense than that of flexible submerged plants.