Study On The Characteristics And Total Flow Description Of Turbulent Flow In Open Channel

Open channel turbulent flow widely exists in nature and fields of engineering such as water conservancy,hydropower,transportation and environment.Researchers have studied a lot on open channel turbulent flow from flow characteristics analysis,floating mechanism study and simulation method construction,and have obtained abundant achievement.However,most of the experimental studies on open channel focused on steady flow,and the knowledge about unsteady turbulent flow in open channel,especially the deformation characteristics of free surface,was insufficient.Although the energy loss equation has been constructed,there was no research on the classification of energy loss,nor the energy loss density function.The motion equation in the total flow differential model was to regard fluid as a rigid body,and the momentum change caused by the free surface deformation couldn't be considered.The research on the differential model of the total flow description of unsteady gradual turbulent flow in open channel under the framework of viscous fluid motion was not enough.This paper discussed the above questions through theoretical analysis,flume experiments,and numerical simulation calculations,based on previous studies.The main conclusions are as follows:(1)The energy loss of steady turbulent flow in open channel was studied through theoretical analysis.The results showed that the energy loss of steady turbulent flow in open channel came from the two effects of mean flow viscosity dissipation and turbulence maintenance according to the generation mechanism.For the uniform turbulent flow in wide and shallow open channels,the viscous dissipation effect of mean flow decreased with the increase of Reynolds number,while the turbulence maintenance effect increased with the increase of Reynolds number.When the Reynolds number was close to 9000,the contributions of these two effects to energy loss were approximately equal.From the spatial region of source,the energy loss of steady and uniform turbulent flow in open channel was mainly concentrated near the channel bottom,and the concentration degree of energy loss generation region to the channel bottom increased continuously with the increase of Reynolds number.(2)Through the open channel unsteady turbulence experiment,the spatiotemporal variation characteristics of the free surface were studied.The results showed as followed: the generated unsteady flow in open channel had the characteristics of unsteady gradually varied flow,but at the same time,the longitudinal change of water level was much gentler.The water surface gradient of each measure point showed cyclical change trend,and its period was the same as that of water level change.But the time when the water surface gradient reaches the peak was earlier than the time when the water level reaches the peak.The stage-discharge relation of each measuring point was characterized by loop curve,and it had the characteristic that the flow reaches the maximum first,and then the water level reaches the maximum in time.(3)Based on the theory of viscous fluid motion,the control equations of the turbulent total flow differential model of unsteady gradually varied flow in open channel could be obtained.The control equations were composed of the continuous equation and the motion equation.Of which the continuous equation was completely consistent with the existing continuous equation in hydraulics,but the motion equation had an additional surface deformation term caused by the free surface deformation.And the energy slope term in the motion equation also had an explicit expression,which could achieve the consistency between the description of total flow and the description of flow field.(4)Through theoretical analysis and numerical simulation,the model parameters in the control equation of the total flow differential model of unsteady gradually varied turbulent flow in open channel were studied.The results showed as followed:for the unsteady gradually varied turbulent flow in open channels,the roughness coefficient could be used as a parameter to simulate the energy slope term after neglecting the contribution of the potential difference between the dynamic and static surface forces to the energy slope term.For the open channel unsteady flow in the tidal reach,the roughness coefficient had the characteristics of non-constant change with the change of water level,and the relative variation of the roughness coefficient could reach 40% with in a tidal wave period.For the turbulent flow in wide and shallow open channel under the condition of high Reynolds number,it is appropriate to take the momentum correction coefficient and kinetic energy correction coefficient as 1.In the surface deformation term,the surface deformation coefficient decreases with the increase of Reynolds number.For the turbulent flow with high Reynolds number,the value can be approximately taken as 0.61.The value of the surface deformation term is related to the ratio of water level amplitude and equilibrium water depth.The smaller the ratio is,the smaller the impact of the surface deformation term is.(5)Based on the new total flow differential model,a mathematical model that uniformly describes the main stream flood and lake flow was constructed to simulate the flow movement of the river and lake connection section.The results showed that the flow movement of connection section between a river and a lake had the characteristics of unsteady gradually varied flow,in which the main stream was dominated by the runoff effect and the lake flow was dominated by the storage effect.Although the motion characteristics of the river and lake were different,the mathematical model of the total flow description could be established based on the concept of the total flow description of turbulent flow in the open channel.The application of typical examples verified the feasibility of this type of mathematical model,and showed that the jacking effect of lakes on main stream floods was only limited to a certain longitudinal range of the lake entry section.To reduce the peak water level of the main stream flood,the method of increasing the discharge of the lake was better than the method of simply increasing the capacity of the lake storage.