Study On Vibration Inverse Problem Of Discharge Structures Based On Limited Measured Response

The construction of large-scale water conservancy and hydropower hubs in our country has made the vibration safety problem of the discharge structure induced by high-speed water flow very prominent.At present,research on the safety of flow-induced vibration is mostly focused on the engineering design stage,mainly using hydroelastic models for experimental simulation and prediction;For the engineering operation stage,the existing hydraulic monitoring mainly serves as feedback and verification of design,while the overall dynamic characteristic information contained in the measured vibration response of the prototype of the discharge structure is more complete and directly reflects the overall operational behavior of the structure.Currently,there are few reports on the use of the prototype measured vibration response for vibration back analysis.Therefore,this study focuses on the actual discharge structures of a arch dam and overflow dam piers,and adopts a combination of prototype observation,model experiments,theoretical analysis,and numerical simulation research methods to carry out vibration inverse problems of discharge structures based on limited measured response.The main content is as follows:(1)Based on the load characteristics and time-frequency analysis of the discharge structure,the flow-excited vibration modes of the discharge structure under different loads are clarified;for the vibration signals of discharge structures are easily affected by low-frequency water flow noise and background noise in complex environments,a conbined noise reduction method based on the automatic singular value decomposition and the improved variational modal decomposition is put forward,which solves the problems of the structure’s dense-frequency information being difficult to be separated,and the signal component modes being seriously overlapped,and achieves adaptive filtering and noise reduction of the vibration signals of the discharge structure.The adaptive filtering and noise reduction of the vibration signal of the structure is achieved,and the signal-to-noise ratio is improved by 5.013 times compared with the original signal,and the root-mean-square error is reduced by 93.5%.Aiming at the lack of accuracy and automation in the process of modal parameter identification for the discharge structure,a covariance-driven stochastic subspace modal parameter identification method based on the fixed-order of the system and the potential clustering is proposed.This method overcomes the difficulties of system order determination and automatic removal of false modes,and automatically and accurately obtains modal characteristic information of discharge structures under operating conditions.The relative error between the modal frequency identification result and the theoretical value is within 2%,and the relative error between the damping ratio identification result and the theoretical value is within15%.(2)Aiming at the insufficient computational efficiency and accuracy of the inversion of the power material parameters of the traditional discharge structure,a method of inversion of the dynamic material parameters of the discharge structure based on modal parameters and deep learning is developed;combining with the sensitivity analysis method,the influence laws of different material parameters and partitions on the modal characteristics of the discharge structure are clarified,and an improved multi-output deep neural network with long and short-term memory is constructed to accurately reflect the complex nonlinear mapping relationship between the discharge material parameters of the structure and the modal parameters.Using the improved multi-output long and short-term memory deep neural network,a direct inversion model is constructed to accurately reflect the complex nonlinear mapping relationship between the dynamic material parameters and modal parameters of the structure,and the modal parameters identified under the limited measured response are used as inputs to achieve efficient and accurate inversion of the dynamic material parameters of the discharge structure partitions,with the relative error between the frequency calculation value and the identification value based on the inversion results within 5%,and the relative error between the calculation value of the modal shape of the majority of the measurement points and the identification value within 10%.(3)Aiming at the problems that it is difficult to obtain the vibration source load of the discharge structure directly and the overall dynamic response field cannot be measured continuously and distributed,a time-domain identification of the vibration source of the discharge structure based on the limited measured response and the inversion method of the dynamic response field are proposed.The limited measured response is expressed as the convolution of the unit impulse response function and the excitation load,and then a mathematical inversion model for vibration source load identification is constructed.On the basis of exploring the key problems of load identification and the discomfort of qualitative analysis,a regularised pre-optimal double conjugate gradient algorithm is proposed to improve the stability and accuracy of the time domain identification of the load of the discharge structure,and to realise global control of the vibration source of the drainage structure and the overall dynamic response field under the condition of the limited measured response.Under the condition of limited measured response,the global control of the vibration source and the overall dynamic response field of the discharge structure is achieved,and the inverse dynamic displacement response matches well with the measured dynamic displacement response in terms of trend and law,with the correlation coefficient above 0.85,which is a very strong correlation.(4)Aiming at the problem that the formation mechanism of a new burst and impact type of flow-excited vibration mode(beat vibration)in the discharge structure is still unclear,combined with the beat vibration mathematical analysis model and numerical simulation of beat vibration,the relationship between hydrodynamic load-dynamic characteristics of the discharge structure-beat vibration response is clarified,and the formation conditions of beat vibration of the discharge structure are clarified;relying on the actual discharge structure,based on the results of vibration back-analysis under the conditions of the limited measured response,the internal and external mechanisms of beat vibration occurring in the discharge structure are clarified from the perspective of its own dynamic characteristics and hydrodynamic load characteristics.Based on the results of vibration inverse analysis under the limited measured response conditions,the internal and external mechanisms of the vibration of the discharge structure are clarified from the perspectives of the structure’s own dynamics and hydrodynamic loading characteristics,i.e.,when the hydrodynamic loading energy and the main intrinsic frequency of the discharge structure coincide randomly,and the frequency ratio ζ and amplitude ratio ?between the main intrinsic frequencies of the structure meets the requirements of 0.78 ≤ζ ≤1.29(ζ ≠1) and 0.33＜β＜3.0,the phenomenon of vibration of the structure will be more obvious,which is prone to lead to strong vibration of the discharge structure or even damage.Combined with the theory of fatigue analysis,the safety status of the discharge structure is systematically evaluated,which is of great significance for guaranteeing the operational safety of the discharge structure.