| This dissertation mainly focuses on the movement characteristics of open channel flow within vegetation, i.e., the hydrodynamic characteristics of vegetated flow. The main contents of this research include the analytical solution of the velocity distribution for different types of vegetated flow and resistance characteristic of vegetation in flow. Analytical solutions of the streamwise velocity distribution were deduced for flexible vegetation and double-layered rigid vegetation in steady uniform open channel flow. And the resistance characteristics of non-submerged vegetation in steady non-uniform flow were shown in this dissertation, i.e., the relationship between drag coefficients with the Reynolds number was studied in this dissertation. Combining mathematical model with laboratory test, this research obtains hydrodynamic characteristics of different types of vegetated flow, which can be divided into the following several aspects.Bending deformation of flexible vegetation occurs in open channel flow. This dissertation studied bending properties of flexible vegetation and deduced the vertical distribution of longitudinal velocity by analytic solution in the case of small bending. Vegetated flow was divided into two layers, i.e, vegetation layer and free surface layer. For vegetation layer, theory of cantilever beam is applied for flexible vegetation to deduce the bending angle of each point and the height of the after bending. Then vegetation resistance and the expression of Reynolds stress were substituted into the momentum equation to get the analytical solution of the velocity. Vegetation resistance of flexible vegetation is different from rigid one. The resistance force of flexible vegetation is the resultant force considering both drag and friction. When bending angle is small, the Reynolds stress can be expressed in the form of exponential distribution in vegetation layer. So this velocity solution only can be used for small bending condition. In a free water, in order to satisfy the condition of zero-velocity gradient, polynomial expression was adopted for velocity distribution. Compared with the traditional logarithmic velocity expressions, this polynomial expression performs better in actual situation.When the flow velocity is larger or flexibility of vegetation is higher, large deflection bending deformation occurs. In this case, exponential distribution is not suitable for this condition. And the experiment results show that when the bending degree of vegetation is larger, the resistance to water flow will be smaller, and the relationship between vegetation resistance and longitudinal velocity shows linear relationship. Based on this phenomenon, this dissertation puts forward a new formula of vegetation resistance, and analytical solution of the longitudinal velocity distribution in flow was obtained through newly proposed Karman coefficient in the vegetation layer and free surface layer under the condition of large bending deformation for vegetation. Good agreements between experimental results and calculated velocities prove the validtidy of this model.In natural environment, the vegetation height in open channel is not the same. In this case, the flow characteristics are more complicated. Take double-layered rigid vegetation as an example, momentum equations were solved by adopting the method of power series. And analytical solution of longitudinal velocity distribution for each layer was obtained for double-layered vegetated flow. At the same time, experiments were conducted in the laboratory, and velocity characteristics were obtained by Particle Image Velocimetry System. It shows that the velocity at the lower of the vegetation layer almost constant, and increase gradually in the upper part of vegetation layer. Good agreements between the experimental data and the analytical solutions prove that this analytical velocity model can predict streamwise velocity in double-layered vegetated flow. The formula of penetration length of shear vortex under this condition was given in this dissertation.Above studies are focused on steady uniform flow conditions. However, in the natural river system, the flow tends to be nonuniform. Previous studies have obtained the relationship between drag coefficient and Reynolds number under the condition of steady uniform flow. When using this drag coefficient relationship into the Saint-Venant equations to calculate flow surface line under the condition of steady non-uniform flow, large deviation between calculation and measurement occurs. This implies the drag coefficient is not only related to the Reynolds number, but also associated with the non-uniformity of water flow surface. For this situation, research was conduted under the condition of steady non-uniform flow, and the relationship betweent drag coefficient and Reynolds number shows parabolic feature, which related to non-uniformity of the flow and vegetation propertie. Also, empirical formula for drag coefficient was present in this dissertation.In the natural environment, the depth of shallow flow will change due to rainfall. So, rainfall factor was considered to investigate the influence on vegetation drag coefficient under the condition of steady non-uniform flow through vegetation. First of all, the expression of drag coefficient was deduced by solving Saint-Venant equations with 'rainfall item'. Then, flow surface profiles for different vegetation density and rainfall intensity were obtained through experiments. And logarithmic model of flow surface line was adopted to get best-fitting curve. Finally, the relationship between drag coefficient and Reynolds number was obtained for differnet cases. The rainfall has a great influence on drag coefficient. The larger the rainfall is, the larger the influence becomes. When the rainfall intensity is larger, the drag coefficient will decrease with the increase of Reynolds number, showing the monotone decreasing feature, which is completely different from the vegetation drag coefficient features without rainfall. |
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