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In practice, it is usually difficult to estimate the characteristic parameters of a system due to the system nonlinearity and uncertainty. To address this problem, in this study, the characteristic parameters of a nonlinear system are identified by using a data driven method based on the best discretization of the Nonlinear Differential Equation (NDE) model of the system. The best discretization of a NDE model is firstly determined, and then the discretized model, known as the Nonlinear Auto Regressive with eXegenous input (NARX) model, is determined by using the Least Squares (LS) algorithm from the input and output data of system. A case study is discussed to validate the proposed system parameter identification method, where the characteristic parameters of a rotating blade-casing system are evaluated under a bandwidth rub impact in the horizontal direction with noise. The result shows that the identified model can be used to describe the characteristics of the underlying system accurately, which provides a reliable model for the dynamic analysis, control of rotating blade-casing system. Used the input and output datum of system and the functional relationship between parameters and coefficients of NARX model to confirm these key parameters. Image 1 • Four different discrete methods are compared and the best discretization of a NDE model is confirmed. • The function relationship between characteristic parameters of systems and coefficients of NARX model is discussed. • The error of each discrete method is analyzed theoretically. • A novel identification method, referred as the DDM, is utilized to identify the parameters of a rotating blade-casing system. [ABSTRACT FROM AUTHOR]

Abstract Data acquisition is one of the key steps in near-field acoustic holography. Most of the existing measurement methods become time-consuming or even impractical when the total number of measurement points is excessively large, for example for the sound field reconstruction of a large-scale source or for a mid-high frequency problem. Alternatively, a three-dimensional scanning near-field acoustic holography (3DSNAH) method is hereby proposed to overcome such problems providing the sound field is time stationary. Data measurement at a large number of points can be accomplished in a matter of minutes by using scanning measurements to acquire the holographic data. The data is then used in an inverse boundary element method to obtain a detailed reconstruction of the sound field radiated by the sound source. Comparison with the traditional point-by-point measurement method demonstrates that the proposed method can acquire holographic data of good quality and reconstruct the sound field with a reasonable accuracy, which verifies the feasibility of the 3DSNAH. [ABSTRACT FROM AUTHOR]

• This paper proposes an unbalance identification method for full-size rotor system. • The unbalance value is identified by using the scaling laws and scaled model. • The unbalance phase is identified by the proposed method. • The distortion in geometrical dimension is taken into account. • Testing for both prototype and model is carried out. • The proposed method is verified by numerical and experimental cases. Perfect balance of rotor systems is difficult to achieved in practical engineering and some amount of unbalance is unavoidable. However, identifying the unbalance value of full-size rotor system is difficult, time-consuming and sometimes dangerous. Aiming at this issue, this paper proposes an unbalance identification method for full-size rotor systems. The unbalance value can be identified through the scaling laws and scaled model, where the scaling laws are developed by incorporating the least squares method and sensitivity analysis. The effectiveness of the proposed method is verified by the numerical and experimental case studies, where the distortion in geometrical dimension is taken into account. The numerical case identifies the unbalance value of a rotor system and evaluate the identification errors. Furthermore, the experimental testing for the prototype and model test rigs is carried out. The results show that the unbalance values can be accurately identified by the proposed method. This allows one to identify the unbalance value of full-size rotor system through a small scaled model. [Display omitted] [ABSTRACT FROM AUTHOR]

• A new system identification approach for the rotating machinery is proposed. • The corresponding modeling framework for the proposed approach is developed. • The method of building the frequency-domain version modeling dictionary is given. • The effect of the data sequence on the identification result is discussed theoretically. In this study, the dynamic modeling of rotating machinery, which is a harmonic excitation system, is investigated based on a nonlinear autoregressive (NARX) model with external inputs. Generally, NARX model-based techniques require Gaussian (white) noise, and thus these methods are not suitable for rotating machinery. Although there have been some reports on the modeling of harmonic excitation systems, the existing methods cannot establish a single-input single-output (SISO) NARX model to represent the rotating machinery over a wide range of rotational speeds. An improved modeling method called the frequency sweep system identification approach is proposed in this study to solve this issue. A discrete-time Fourier transform (DTFT) is performed on the system input and output datasets over a wide speed range to obtain the resulting spectra, and the amplitudes corresponding to the rotational frequencies are extracted and spliced together to convert multiple time-domain signals into one input data set and one output data set composed of frequency-domain data. Then, the modeling process can be carried out using the orthogonal forward search algorithm. Moreover, the effect of the data sequence on the identification results is discussed theoretically. A key feature of the proposed method is that the model structure detection and coefficient calculation are conducted with spliced frequency-domain vectors. The feasibility of the proposed modeling approach is validated through numerical and experimental cases. This work is a supplement to existing modeling methods based on the NARX model and provides a modeling basis for the analysis and design of rotating machinery in combination with the NARX model. [ABSTRACT FROM AUTHOR]

• A dynamic model of bolted joint rotor system was established by considering bolted joints and pedestals. • The effect of the pedestal looseness fault on the bending stiffness of bolted joint structure was evaluated. • The feature of the pedestal looseness fault that happens in a bolted joint rotor system was investigated. • A rotor-bearing test rig with bolted joint and pedestal was set up to verify the numerical simulation results. Pedestal looseness is a common fault in rotating machines. To enhance the economy and convenience of the assembly of the aero-engine rotor system, the bolted joint structures are a useful tool for fastening multiple parts of different materials into an integrated system. To our knowledge, the time-varying bending stiffness and looseness fault at the pedestal significantly influence rotor dynamics. The present work aims to gain insight into the dynamic behaviors and mechanical behaviors of a rotor-bearing system with a bolted joint structure subjected to a pedestal looseness fault. Firstly, the dynamic model of a bolted joint rotor-bearing system supported by pedestals was set up taking into account the time-varying bending stiffness of bolted joint and the pedestal looseness fault. The feature of the pedestal looseness fault that occurs in a bolted joint rotor system was investigated by comparing the variation ranges of bending stiffness and dynamic responses. An experimental study was then performed using a rotor-bearing test rig equipped with a bolted joint and pedestals. Finally, dynamic phenomena such as offset of the equilibrium position of the vibration waveform, appearance of continuous frequency spectra, and irregular shaft orbits can be known as the dynamic features for the pedestal looseness fault. [ABSTRACT FROM AUTHOR]