Dynamic Characteristics Of Open Channel Flow With Vegetation

Such a research topic, the hydrodynamic characteristics of open channel flow with vegetation, was proposed. It was in use of flood management, river restoration and water resources for sustainable development. And it was stressed that the importance of this research topic has important practical value and academic significance.In this doctoral dissertation, a complex test system was established. It was made up of a slope alterable glass flume containing experimental vegetation. The accessorial monitoring instruments consists of a three-dimensional ultrasound Doppler Velocimeter (ADV), a standard two-dimensional particle image velocimetry (PIV), a two-dimensional high frame rate Mini-PIV, and some other traditional monitoring instruments, such as water level measurements needle, weir and pressure tubes, etc.. In use of the new established test system, the vegetation water flow resistance characteristics, turbulent flow characteristics, two-dimensional velocity field, two-dimensional vorticity field, the instantaneous vorticity field were studied. Then from the point of view of vortex dynamics study of vegetation flow with energy dissipation and the turbulent dissipation characteristics were studied by physical experiment. In this dissertation, such innovation and research findings were shown as follows:1. In constant gradient flow conditions, the two-dimensional flow velocity distribution of open channel flow was studied with submerged vegetation in different vegetation density and different discharge in use of the standard two-dimensional PIV.Research result shown that the existence of submerged vegetation had notable effect on the velocity distribution of open channel flow. And the distribution of velocity was the anti-S-shaped. The effect goes greater as the distance from the cross-section of the vegetation goes smaller. The largest velocity gradient occurs near the vegetation canopy. The velocity gradient increased with the flow discharge increasing, and the vegetation density increasing. The flow velocity gradient near the vegetation canopy with rigid submerged vegetation is greater than that near the flexible. The effect of vegetation to velocity gradient decreased in the area above the vegetation canopy.2. The Manning roughness coefficient of flexible vegetation changes with the flexible deflection was studied under the steady flow condition. The variation of Manning roughness coefficient was studied by comparative experimental of submerged rigid vegetation and flexible vegetation.Analysis with non-submerged rigid vegetation in open channel flow resistance factors, the drag coefficient and equivalent roughness coefficient of Manning formula was derived in a certain supposed condition. Fixed at the end of the slope, the experimental study of the different vegetation density and depth of flow under the conditions of the resistance mechanism to draw the drag coefficient and equivalent roughness coefficient of Manning with the different factors such as water depth, vegetation density and V/V_* such as changes in the law. 3. It was induced from incompressible fluid from the vortex equation of motion that the vorticity of the flow field depends on the velocity gradient and fluid viscosity. The vorticity mainly depends on the velocity gradient in open channel flow with vegetation. The fluid energy dissipation rate was directly related to entropy, that is, fluid vorticity and the square of the absolute value proportional. Therefore, the distribution of characteristic entropy was a direct reflection of the rate of energy dissipation distribution in flow field.The distribution of velocity field and vorticity field of the flow with submerged rigid and flexible vegetation were studied using a standard two-dimensional PIV measurement. The study shows that velocity gradient was a key element in vorticity of flow with vegetation. The water flow was disrupted near the vegetation canopy. And the larger the velocity gradient occurs the more close to the vegetation canopy. The maximum vorticity value appeared in the 30%～50% parts below the submerged vegetation canopy. The velocity gradient and absolute vorticity value near the vegetation canopy were affected by the stiffness of the vegetation.The existence of vegetation intended the characteristic energy entropy was much higher than entropy as no vegetation. Particularly, the peak value appeared near the vegetation canopy. And with the flow increased, the peak value went multiply increasing. The characteristic energy entropy of flow near canopy of submerged rigid vegetation was significantly higher than that of the flexible vegetation. The peak value was about 2 to 5 times of that near flexible vegetation at the same flow discharge. Which shown that vegetation canopy layer flow of energy dissipation was the main reason for the current energy loss, and was also one of the main reasons of resistance to flow. Compared with the flexible Vegetation, energy loss of water flow was obviously much higher near the canopy of rigid vegetation. That shown that the bigger elastic modulus value of vegetation the greater energy loss caused by flow.4. The turbulent flow characteristics with vegetation was studied to make a better understanding to the mechanism of vegetation resistance to water flow and turbulence structure. Under conditions of high Reynolds value, incompressible fluid turbulence dissipation rateεcan be approximately equal to . The data from turbulent entropy measured through the test could explore the law of turbulence dissipation.The velocity distribution and turbulent characteristics of open channel flow with submerged rigid and flexible vegetation were studied using the three-dimensional ADV. The results show that there had anisotropic characteristics of open channel turbulent flows with rigid and flexible submerged vegetation. The maximum value of the turbulence intensity, the turbulence and the Reynolds stress appeared near the plant canopy. And the exchange of turbulent flow was strongly. The turbulence intensity was higher obviously near the top-level part of vegetation which was rigid than flexible. The instantaneous flow velocity field and the instantaneous vorticity field were measured in the open channel flow test with submerged vegetation using the high frame rate two-dimensional Mini-PIV. And further analysis of calculating the turbulent entropy in open channel flows with submerged vegetation was done. A distribution law was proposed that was the turbulent entropy in open channel flows with submerged rigid and flexible vegetation.All the research results were shown that the turbulent entropy was small below the vegetation canopy. The vegetation canopy layer had significantly disturbance to the flow that the turbulent entropy and the turbulent energy dissipation near the vegetation canopy increased significantly. At the vegetation cross-section, the distribution of turbulent entropy became gradually homogenizing.