Experimental Study And Level Set Method-based Numerical Simulation Of Steady Non-uniform Turbulent Flow In Open Channel

Open channel flow is a kind of flow which exists widely in nature and engineering technology.Although there are many researches on it through prototype observation,experimental research,theoretical analysis and numerical calculation,there is still a lack of understanding on the adjustment process of the non-uniform steady flow approaching to uniform in the long straight positive slop channel and the problem of flow connection under the control of sluice gate in the long straight positive slop channel.In addition,from the point of view of numerical calculation,the accuracy of free surface capture in open channel turbulence directly affects the accuracy of numerical results,so it is necessary to study the method of free surface capture in open channel turbulence more broadly.Based on the previous research work,this paper discusses the above three problems through experimental research and numerical simulation,and obtains the following results:(1)A vertical 2-D turbulent model of open channel based on level set method is constructed by using the level set method,which is proved to be effective in laminar flow research;the Finite Volume Method is applied to discretize the RANS;a program is developed,which is composed of two submodules: governing equation solving modules and the free surface solving module;and the key issue such as smoothness of physical parameters near interface and reconstruction of free surface are specially addressed.The results show that the calculated water surface lines are consistent with the measured results,and the simulation accuracy of the mean velocity is also high.Compared with the commercial software using VOF method,the calculation results show that: Both the numerical simulation technology of open channel turbulence based on Level set method and the numerical simulation technology of open channel turbulence based on VOF method in commercial software can be used to simulate open channel turbulence,but the free surface given by VOF method is obtained by interval band interpolation,However,the free surface given by the Level set method is clearer,and the capture accuracy of the free surface is higher in the verification calculation case.(2)In the open channel with a long straight bottom slope,the length of longitudinal recovery required for the flow from a steady non-uniform flow to a uniform flow is proportional to the degree of non-uniformity of the flow.When the flow reaches the state of uniform flow,the energy loss coefficient decreases with the increase of Reynolds number,but when the width-depth ratio is 5-10,the energy loss coefficient tends to increase slightly with the increase of width-depth ratio;and the energy loss in the non-uniform flow region is larger than that in the uniform flow region;the greater the degree of non-uniformity of the open channel flow,the longer the longitudinal length needed for the flow to reach the uniform flow state,and the longitudinal relative length is about 85～94 times of the non-uniform degree of the flow.(3)When the flow behind the gate in the open channel of the long straight positive slope connects with the uniform flow downstream in the state of subcritical flow,the flow not only has the connection mode of "supercritical flow-subcritical flow",but also can be connected in the way of "subcritical flow-subcritical flow";under the condition of the same flow rate,the longitudinal length needed to reach the uniform flow state from the supercritical flow to the subcritical flow is shorter than that between the slow flow and the slow flow.From the point of view of the longitudinal length required for the flow to jump to the highest point behind the gate to approach the uniform flow state,the greater the ratio of the water depth at the jump to the uniform flow depth,the longer the longitudinal length required to approach the uniform flow state.(4)By using the model established in this paper,the process of turbulent flow in open channel from non-uniform flow to uniform flow under general conditions is numerically simulated.The results show that in the process of adjusting the open channel from non-uniform flow to uniform flow,the recovery length of the uniform flow in the open channel is about 85～94 times of the initial non-uniformity of the flow,and there is a correlation between the energy loss of the open channel and the degree of non-uniformity of the flow.The greater the degree of non-uniformity of water flow,the greater the energy loss.For the dropdown flow of open channel,the water depth decreases,the bed shear stress increases,and the energy loss coefficient and roughness coefficient decrease.The specific changes of the overall characteristic parameters in the process of dropdown flow are as follows: the amplitude of the relative bed shear stress is 0.41～1 times,and the corresponding longitudinal relative length is 122～151;the amplitude of the relative energy loss coefficient is 1～1.32 times,and the corresponding longitudinal relative length is 113～158;the roughness coefficient is 1～1.18 times,and the corresponding longitudinal relative length is134～153.