Research And Application On The Establishment And Efficient Analysis Of Transient Hydraulic Model For Urban Water Distribution Systems

The safety and reliability of urban water supply systems is one of the basic supporting conditions for the healthy and sustainable development of urban cities.As an important component of the water supply system,it is of great practical significance to ensure the safe operation of water distribution systems(WDS).In this paper,the transient flow which is closely related to the safe operation of WDS is taken as the research object,and the key problems encountered in the transient analysis of WDS,such as modeling difficulty,long solution time and lack of practical methods,are analyzed systematically.The research mainly involves the research and application on the establishment and efficient analysis of the transient hydraulic model for WDS,to guide and promote the practical application of transient hydraulic model in the design,operation and management of water supply network.First of all,the various aspects of the establishment of transient hydraulic model of WDS are systematically studied.The modeling theory and method of establishing transient hydraulic model from steady state hydraulic model are formed,including model input parameters,complex boundary conditions,solving method,time step selection and model calibration and so on.Aiming at the complex boundary conditions in WDS,an identification and classification method and a general solution method of boundary conditions are established,which greatly simplifies the model processing and programming development.In order to solve the problem of minor timestep for complex networks,the traditional timestep selection method of wave speed adjustment is improved by determining the pipe with minor travelling time as the short pipe to increase the timestep.The example shows that this method can significantly increase the time step when dividing a small number of short pipes,which greatly improves the efficiency of transient analysis.For example,dividing 5 short pipes in 115 pipes can obtain 90% improvement of analysis efficiency and imposes little effect on the accuracy of transient analysis.In addition,the complexity and particularity of the transient model calibration relative to the steady-state model are studied and discussed and a step-by-step calibration method for the transient hydraulic model is proposed.Based on the efficiency requirement of transient analysis for WDS,a Lagrangian-based method for transient analysis is developed,which can obtain a quick estimation of transient state.The transient theoretical system of the Lagrangianbased method is studied and expanded,including the pressure wave propagation mechanism,the general solution methods of various boundary conditions and the line friction approximation.On this basis,the solution method of efficient Lagrangian-based model(ELM)and the applicable efficiency control strategies(the time interval and the minor wave truncation)are established.Results of the case study show that the computational efficiency of ELM is greatly superior to that of the traditional MOC under reasonable efficiency control strategies,and the computational accuracy is within acceptable ranges with only the consideration of steady-state friction.For example,when the time interval is close to the time step of MOC(ie,0.01 s),the computational error is little but the computational efficiency is increased by nearly 60%.Then the transient skeletonization methods are established to reduce the complexity of the hydraulic model and improve the efficiency of transient analysis.(1)The traditional pipe merging methods including the series and parallel merging are improved to be suitable for transient model skeletonization based on the pressure wave propagation mechanism.(2)A probabilistic analysis and evaluation method of nodal demand effect is developed to evaluate the influence of nodal demand on transient analysis.This method can evaluate the nodal demand effect by the probabilistic analysis based on Monte-Carlo simulation(MCS)and global sensitivity analysis(GSA)methods,thus can be utilized to determine whether the demand node is suitable for skeletonization.Results of the case study show that the accuracy of transient model skeletonization is effectively maintained by this method.On the basis of the above research,the efficient and practical application methods of transient hydraulic model are established to solve some problems in the application of transient hydraulic model.(1)The discrimination method of transient analysis scenario is formed by combining the system energy analysis with the ELM.The method can clearly and quantitatively determine whether a dynamic event needs to be analyzed as transient flow,which is more useful than the empirical judgment.(2)The transient influence zone based transient analysis method is proposed,which makes the transient analysis not limited to the full size model,thus reducing the computational requirement and time consuming.This method takes full advantage of the computational efficiency of the ELM and the computational accuracy of the traditional MOC,which provides a general and efficient application method for the transient analysis of complex WDS.Results of case study show that the computational efficiency of different transient events are improved by 20~50% and the computational accuracy of the partial model is still within acceptable ranges(the errors of nodal pressure extreme are generally below 1.0 m).(3)The former two parts are combined with the traditional transient analysis and application methods to form a systematic method for efficient application of transient hydraulic models of complex WDS,which is utilized for the efficient transient analysis and surge protection design in complex WDSs.At last,a practical study on the establishment and application of transient hydraulic model for a realistic complex WDS is carried out,including the establishment and skeletonization of transient hydraulic model and a comprehensive analysis of different transient events.The practical work of model skeletonization shows that the proposed skeletonization method for transient hydraulic model can effectively ensure the accuracy and reliability of model skeletonization,while the traditional skeletonization technologies lead to obvious errors.The comprehensive analysis of various transient events demonstrates that the daily scheduling pump operation may case significant transient phenomena;the pump trip of pump stations can cause serious transient events,and reasonable protective strategies(such as air vessels,air valves,etc.)should be determined for safety protection;accidental water abrupt changes,such as main burst,leaks,fire demand operation,etc.,can induce severe transients(e.g.,overpressure,negative pressure or cavitation),but may also result in only minor pressure fluctuations,thus should be specifically analyzed to determine its effect on the operation of WDS.Therefore,the establishment and application of transient hydraulic model in realistic WDS can effectively guide the safe operation of the system under the influence of transient flow.Through the above research work,this paper has formed a comprehensive system including the establishment,efficient analysis and application of transient hydraulic model for WDS and provided theoretical and technical support for the establishment and application of transient hydraulic model of WDS.These work possess great theoretic and practical significance for the safety operation of WDS,such as protection design,safe operation scheduling,accident prevention and prediction,etc.