Study On Vibration Characteristics And Mechanism Of Beat Vibration Induced By Flood Discharge

China has been the world’s top priority in the field of water conservancy and hydropower projects with high water head and large flow.In process of flood discharge,the high speed flow carrying the huge energy may easily inducted vibrate.When the vibration is too severe,it will threaten the safe of engineering operation,even leads to the failure of structures.In prototype testing,we had found that the Eetan arch dam and the Shuhe hydropower station sluice pier all appeared the vibration of “intermittency” and “shock”.The measured envelope of the response schedule lines increases or decreases substantially with time,and shows the form of "beat vibration".The pier of the sluice gate of the Shuhe hydropower station has a large vibration amplitude under some operating conditions,which has seriously affected the safety of the discharge structure.Therefore,it is of great significance to study the characteristics and internal formation mechanism of this type of vibration form,which is of great importance to the further improvement of the mechanism of structural vibration induced by flood discharge.Taking the project of Ertan arch dam and Shuhe hydropower station sluice pier as an example,the following four aspects are mainly carried out:(1)The concept of "beat vibration" in nature is introduced.Through the analysis of the influence of two waves overlapped initial phase difference,frequency ratio and amplitude ratio of the "beat",the frequency ratio and amplitude ratio conditions are obtained.Based on the above conditions,the conditions for beat vibration of several columns of wave(three or more)are analyzed and summarized by numerical experiment method,and the synthesis mechanism of beat vibration is further improved.(2)The characteristics and causes of the measured vibration phenomena are studied by the time domain and frequency domain analysis methods.Aiming at the phenomenon of hydraulic beat vibration in the prototype vibration test of the Ertan arch dam and Shuhe hydropower station sluice pier,first of all,the random subspace method is used to identify the modal parameters of the two projects,then,the frequency component composition of the measured vibration response is studied by the frequency domain analysis,finally,based on the condition of beat vibration formation,the causes of the beat vibration in the arch ring of the Ertan arch dam and the floodgate sluice pier of the Shuhe hydropower station are determined.(3)Numerical simulation of the hydraulic beat vibration of a cantilever beam.In view of the situation that the vibration response of the arch crown beam in the Ertan arch dam cannot be explained by the synthetic mechanism of "beat",three dimensional finite element models of two cantilever beams are established,the vibration response time course of the structure under the same flow load is extracted,the relationship among the dynamic load of the hydrodynamic force and the structure and the vibration response of the structure is studied by HHT time frequency analysis.It is found that when natural frequency of the structure coincides with the instantaneous frequedcy of the hydrodynamic load,the structure will experience a random resonance,and it’s vibration response will appear similar to beat phenomenon,this article describes this type of vibration response as random beat vibration.(4)Study on hydraulic beat vibration mechanism of discharge structure based on HHT time-frequency analysis method.First,the three-dimensional finite element model of the Ertan arch dam and the Shuhe hydropower station sluice pier is set up.The inherent characteristics of the structure are obtained by modal analysis and counted as the known conditions.Then the equivalent hydrodynamic loads of the Ertan arch dam and the Shuhe hydropower station sluice pier are obtained by introducing genetic algorithm and using the prototype measured vibration response as input.And the accuracy of the inversion is verified by the comparison of the inverse value of the dynamic displacement of the test point with the actual value.Finally,basing on hydrodynamic loads obtained by inversion,The time frequency distribution diagram of load and energy is obtained by using the HHT time frequency transformation method.The relationship among the hydrodynamic load and the inherent characteristics of the structure and the vibration response of the structure is studied,and the internal formation mechanism of the hydraulic beat vibration in the vibration response of the Ertan arch dam and Shuhe hydropower station sluice pier is revealed.