Research On Flow Field And Total Flow Characteristics Of Open Channel Flow With Different Dense Roughness Scales And Vegetated Beds

Channel flow in natural systems and engineering practices commonly have beds with varied roughness scales.For example,the bed material might be constituted with fine sediments,large scale cobbles and various vegetation.Research on their flow field(mean velocity distribution,mean pressure,Reynolds stress,turbulent kinetic energy,et al.)and total flow characteristics(cross sectional averaged velocity,flow capacity,total flow energy loss,et al.)are both of great significance.In former research,there exists no general relationship between the description of flow field and total flow characteristics.Meanwhile,as the flow conditions vary significantly with the roughness scales,different methods are needed to describe the field and total flow characteristics.In this thesis,open channel flows with various roughness scales have been studied through theoretical analysis,numerical simulation and flume experiments.The results are as follows:1)A total flow integral model for steady uniform open channel flow was built from the basic theory of incompressible viscous fluid flow.A new total flow energy equation was derived,together with explicit formulas relating the total flow energy loss and the Manning roughness coefficient to the field statistical variables(mean velocity,Reynolds stress,et al.).The total flow energy loss was further divided into two components based on its mechanism:losses due to molecular viscous dissipation and maintaining turbulence,respectively.2)The field and total flow characteristics of open channels with small scale roughness in both laminar and turbulent conditions were studied.Through theoretical analysis,an analytical resolution was obtained from the kinematic equation for laminar flow in open channel with rectangular cross section,and both the velocity distribution and the calculation formula of total flow energy loss were obtained.The results show that both the velocity distribution and energy loss coefficient are functions of the Reynolds number and the aspect ratio(the depth to width ratio of the cross section),and tend to be invariable as the aspect ratio increases.The turbulence model was used to simulate the turbulent open channel flow with wall roughness.From the obtained flow field statistics above,the corresponding total flow statistics were further calculated through the proposed total flow integral model.The results show that the total flow energy loss in an open channel is a function of the Reynolds number,the aspect ratio,and the relative wall roughness.When the Reynolds number and the aspect ratio are sufficiently large,the flow develops into high Reynolds number turbulence in wide open channel,the total flow characteristics become independent of the Reynolds number and aspect ratio,and the proposed total flow integral model is consistent with the existing formulas in hydraulics.When compared with laminar flow condition,the turbulent open channel flow can be considered as wide open channel flow in a much smaller aspect ratio,and the total flow energy loss is mostly due to maintaining turbulence rather than molecular viscous dissipation.Meanwhile,the distribution of energy loss density in turbulent conditions is more concentrated at the channel bed and get even closer to bed as the Reynolds number increases.3)By considering the subsurface flow,a coupled three layer analytical model was built to describe the flow characteristics of steady uniform 2D open channel flow with large scale roughness.Meanwhile,a series of flume experiments with packed cobble beds were conducted.The results show that both the field and total flow characteristics over large scale roughness are quite different from that over small scale roughness.The mean velocity profile converge to the log law far away from the bed,while inflecting significantly to the non-zero subsurface velocity at bed;the near bed velocity fluctuations are analogous with that of the vegetation flow instead of the flow over small scale roughness;the total flow energy loss approximates the existing formulas in hydraulics when the relative flow depth is large,while increases significantly as the flow goes shallower.The proposed model considers the subsurface flow and couples the roughness length scale into the near wall mixing length,and the relation between the subsurface velocity and the frictional velocity was illustrated.As a result,it is able to predict velocity profiles and energy loss in a wider flow conditions.4)Turbulence in open channel flow with vegetation is generated both at the scale of vertical shear and at the scale of vegetation stem wakes.The two scales are different and there exists circuiting from the former scale turbulence to the later.In this thesis,based on the double averaging(both temporal and spatial)principle,a turbulence closure model for flow over vegetation patch scale was built from the theory of viscous fluid flow.The typical 3D flow in a vegetation patch scale was simplified into a steady uniform 2D problem.This model considers the budget of various scales of turbulence.The double averaged downstream velocity,Reynolds stress and turbulent kinetic energy were calculated and then compared with flumes measurements.The results show that there exists an obvious inflectional point in the velocity profile at the canopy top,and the velocity gradient in the vegetation layer is very small.The Reynolds stress and turbulent kinetic energy reach their maximum at the canopy top and decrease sharply into the vegetation layer,which is in consistent with the mean velocity inflection.In the total flow description of a reach scale vegetated open channel flow,a calculation formula relating the Manning roughness coefficient to the vegetation blockage ratio(the area of vegetation patch to the channel cross section area)was obtained through integral derivation and simplification from the double averaged kinematic equation.The formula was then calibrated with field observations in natural vegetated rivers.