Study Of Hydrodynamic Characteristics Of Vegetation Flow With Varying Vertical Porosity

Aquatic vegetation exists widely in nature which is a very important part of river ecosystem.Aquatic vegetation not only affects the health of river ecosystem,but also play an important role in maintaining the stability of river bed and protecting the slope of river bank.Therefore,the dynamic characteristics of vegetation flow have become a research focus in the field of environmental hydraulics.In the previous study of vegetation flow,scholars used to simulate aquatic plants which have uniform ecological characteristics with cylinders.But most aquatic plants have varying vertical ecological characteristics in nature.By combining the theoretical derivation with flume experiment and numerical calculation,this paper studies the effects of vegetation with vertical ecological characteristics on flow characteristics,which can be divided into the following several aspects.Based on the similarity of vegetation flow and porous media flow,the aquatic vegetation in river channel is regarded as solid skeleton.This paper studies on the vegetation flow by introducing the theory of porous media.First of all,by applying Biot's poroelastic theory into vegetation,this research obtains the governing equation for vegetation flow with vertical variation of vegetation density.In the process of research,three parameters of porous media(porosity,tortuosity and permeability)are introduced to describe the characteristics of vegetation flow.Porosity is assumed as a function of water depth,which is used to characterize the variation of vertical vegetation density caused by the thickness of stem.At the same time,tortuosity and permeability are used to reveal the effect of aquatic vegetation on water flow.Then,based on the dispersion coefficient,a new parabolic turbulence model is proposed and compared with experimental data,algebraic stress model and Reynolds stress model.The results show that the new turbulence model can accurately express the turbulent characteristics.By modifying the turbulent term and density variation,this research obtains the governing equations for vegetation flow under different conditions,which can indicate that the governing equation presented in this paper has a wide range of applicability.The governing equations are solved by finite analytic method.According to the boundary conditions in different situations,the finite analytic solutions of the velocity distributions of the emergent vegetation and the submerged vegetation flow are obtained.Submerged vegetation flow is divided into two layers,i.e.,vegetation flow and free surface layer.By comparing the calculated velocity distribution with measured velocity distribution,the research obtains the best fitting expression for permeability.This equation has a high precision in a certain range,and can satisfy the hydrodynamic characteristics of submerged vegetation flow under different conditions.In the flume experiment,truncated cones are selected to simulate aquatic vegetations with varying vertical ecological characteristics.By changing discharge and vegetation density,the measured velocity are obtained for both emergent and submerged vegetation flow.Then the experimental data are averaged over time and space to ensure the accuracy.At the same time,the experimental results are compared with the finite analytic solution of the velocity distribution.The results show that the experimental data and the calculated results are in good agreement which can indicate that the governing equations and calculation methods presented in this paper can be used to predict the velocity distribution of vegetation flow with vertical variation of vegetation density.In addition,based on the standard k-?model,OpenFOAM is adopted to numerical simulate the velocity distribution of vegetation flow with varying vertical porosity.The numerical simulation result has the same variation trend with the finite analytic solution and agrees with each other,which can further prove that the finite analytic solution for velocity distribution can describing vegetation flow characteristics accurately.In addition,comparing with numerical simulation,the finite analytic solution of velocity distribution in vegetation flow has higher calculation accuracy and simpler calculation.For porosity varies with water depth,the research also discusses the influence of discharge,vegetation density and turbulence coefficient on vegetation water flow.Results show that discharge only influences the value of velocity;the velocity distribution of vegetation flow with varying vertical porosity seems to move in parallel as discharge changes.Vegetation density influences the vertical variation of porosity.On one hand,vegetation density can change the value of velocity;on the other hand,it plays a domain role in impacting velocity distribution.By comparing with vegetation turbulent flow,the varying vertical vegetation density has a greater influence on vegetation laminar flow.