| The key weir(PKW)based on upstream and downstream overhanging greatly improves the scope of application of traditional labyrinth weir,and can effectively improve the spillway discharge capacity and efficiency under small water head.Therefore,it is considered to be a very effective and economical solution for the lack of discharge capacity of the dam,which has broad application prospects in the optimization of effective storage capacity of new reservoirs and in the reinforcement and upgrading of dangerous reservoirs.However,due to the numerous geometric parameters,complex structure and short development history,the research results of PKW have not yet formed a system,the weir flow theory is not mature,and there is a lack of general design criteria and methods,which greatly restricts the engineering design and application of PKW.Based on the project of National Natural Science Foundation of China,more than 400 water tests were carried out on different PKW models in a 16 m long,0.5 m wide and 0.75 m high water tank in the Hydraulic Laboratory of Xi'an University of Technology to obtain the detailed flow infoemation of PKW under the free flow and submerged flow conditions.On this basis,based on VOF technology and RNG k-? turbulence model,the three-dimensional flow field of PKW under more than 280 water head conditions was numerically simulated.The flow characteristics,geometric parameters and discharge capacity of PKW are systematically studied and analyzed.The hydraulic mechanism affecting the discharge capacity of PKW is clarified,and the optimized shape parameter range of PKW is proposed.In addition,on the basis of a large number of measured and simulated data,the high-precision hydraulic design formulas of PKW based on multiple influencing factors and superposition principle are proposed respectively.The preliminary design of PKW in practical engineering application is discussed,and the general design criteria,design methods and optimization ideas of hydraulic and Engineering design are further given.The main contents and achievements of this paper are as follows:(1)Basic hydraulic characteristics and discharge mechanism of PKWUnder the free flow condition,the total discharge and discharge coefficient under different water head obtained by model test and numerical simulation are compared with the research results of other scholars.The research results show that the discharge coefficient of PKW is larger under low water head,and increases slightly and then decreases with the increase of water head;By using the numerical simulation method of surface integral,the discharge at the inlet,outlet and lateral overflow crest of PKW and its variation with water head are obtained respectively.Based on the detailed flow field information provided by the numerical simulation,the velocity vector field at the top and below of the weir,the Froude number at the inlet key and the turbulent kinetic energy distribution near the PKW are analyzed,It is revealed that the reduction of the efficiency of lateral overflow crest,effective overflow crest length and the effective cross-section area of inlet are the main factors that affect the discharge coefficient of PKW.(2)Free and submerged hydraulic characteristics of four types of PKWThe symmetrical overhanging(Type A),without downstream overhanging(Type B),without upstream overhanging(Type C)and without overhanging(Type D)are the four basic types(the latter three are the limit type)of PKW.Under the condition of free flow,the experimental and simulatinal results show that the order of discharge coefficient of the four kinds of PKWs is type B>type A>type C>type D,and the discharge efficiency of type B PKW is 13%higher than that of type A PKW,and 30%higher than that of type D.The variation of Froude number at the inlet section of four kinds of PKW and the discharge at the inlet,outlet and lateral overflow crest,the discharge per unit width,the discharge percentage and the discharge efficiency of four kinds of PKWs with the water head are analyzed.The results show that the discharge efficiency of the lateral overflow crest of type B PKW is the highest under medium and low water head,and its discharge of inlet overflow crest is also significantly higher than that of other types at high water head,so type B PKW shows the best hydraulic efficiency among the four types of PKWs.Although the downstream overhanging of type C PKW makes it lose the most length of effective overflow crest in the process of water head increasing,the discharge efficiency of its outlet chamber is the highest,so its total discharge is greater than that of type D PKW.Moreover,the sensitivity of four kinds of PKWs to submergence is analyzed by dimensionless head method and discharge reduction coefficient method.It is found that type B PKW is the most sensitive to submergence under the same upstream flow condition,followed by type A,type C and type D PKWs.By fitting the calculation formula of submerged discharge coefficient,the discharge capacity and hydraulic efficiency of four types of PKWs under submerged conditions are further compared.It is found that when the submerged coefficient S is raletively small,the discharge efficiency of type C and type D PKW is lower than that of typeA PKW,and the efficiency of type B is the highest.When S>0.7,the discharge efficiency of each type of PKW has a reverse law,and the submerged discharge efficiency depends on the comprehensive influence of "sensitivity to submergence" and free flow discharge efficiency.(3)Influence of key geometric parameters and auxiliary shape parameters of PKW on its discharge capacityFirstly,based on the results of physical model test and three-dimensional numerical simulation of PKW with different inlet and outlet width ratio,upstream and downstream overhanging ratio and weir height,the flow field structure and discharge mechanism of different key weir models are analyzed,the influence of main parameters on the discharge capacity of key weir is determined,and the optimal range of parameter ratio is proposed from the perspective of hydraulic performance.The results show that when the inlet/outlet width ratio Wi/Wo is 1.25 to 1.63,the the tangent value of overhanging angle Si is between 0.375-0.75,and the upstream/downstream overhang ratio Bo/Bi is 2.5,the PKW can provide the highest discharge efficiency.Secondly,based on the standard symmetrical type A PKW,the experimental study on the shape of weir nose and different height of parapet walls is carried out.The results show that the discharge of the PKW with weir nose is about 8.5%higher than that of PKW model without this structure when the water head is low,and the difference of the shape of weir nose(triangle or circle)has little effect on the flow pattern and discharge efficiency of PKW;For the PKW with fixed weir height,the discharge efficiency of the PKW model with 25mm-heigh of parapet wall is 14%higher than that of symmetrical type A PKW;The maximum discharge efficiency of the model with weir nose and 25mm-high parapet wall is about 16%higher than that of the standard PKW.(4)Hydraulic design and engineering application of PKWThe main dimensionless parameters affecting the PKW are determined by dimensional analysis method.Based on the orthogonal experiment and simulation data,the calculation method of discharge coefficient of PKW based on multiple influencing factors and the calculation method of unit discharge of PKW based on the superposition of inlet,lateral and outlet are proposed respectively.Furthermore,the applicability,accuracy and reliability of the two methods are verified by collecting the PKW existing laboratory research results,scale model and actual engineering measurement results.Analysis shows that the accuracy of the PKW discharge coefficient calculation formula based on multiple influencing factors has a probability of 97%within the range of ±6%,while the PKW discharge calculation formula based on the principle of superposition has a probability of more than 95%accuracy within the range of 10%.Based on the systematic study of the hydraulic characteristics,geometric parameters and discharge capacity of PKW,according to different engineering conditions,including the upgrading and reconstruction of existing projects and new projects,the overall design criteria of PKW in practical engineering application are formulated,the specific steps and methods of the preliminary design of PKW are put forward,and the hydraulic performance optimization suggestions of the final design scheme of PKW project are given. |
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