Output-only Identification Methods Research Articles

On the Effect of Nonlinear Damping Sources in Output-Only Identification Methods Applied to Floating Wind Turbines

Output-only methods for modal identification are only strictly valid if a set of requirements are fulfilled regarding both structural and environmental conditions. A particularly challenging effect in wind turbine dynamics is the significant presence of nonlinear damping sources coming from aerodynamic forces and, in offshore applications, hydrodynamic forces on the substructure. In this work, the impact of these terms is firstly discussed in analytical terms, and then the corresponding effect on the performance of the covariance-driven stochastic subspace identification is evaluated on a single-degree-of-freedom model. The analysis is then extended to a full hydro-aeroelastic simulation of a 5 MW floating wind turbine using the open source software OpenFAST, mimicking the structural response in free decay tests and in parked conditions with turbulent wind fields. The results show that output-only identification methods are applicable in these challenging scenarios, but the results obtained must be carefully interpreted, since their dependence on the environmental conditions and motion amplitude imply that they are not directly translated into the structure properties, although still closely related to them.

Low order modelling of thermoacoustic instabilities and intermittency: Flame response delay and nonlinearity

This paper investigates the simplifying assumptions used for deriving low order models of thermoacoustic instabilities and thermoacoustic intermittency in turbulent combustor. These models consists in self-sustained oscillators subject to parametric and additive stochastic forcing. The main aspects of the thermoacoustic dynamics investigated in the present work are the phase between acoustic pressure and heat release rate oscillations, and the nonlinearity ruling the saturation of the flame response. We propose a model of coupled acoustic oscillators subject to delayed nonlinear source term from the flame, with parametric and additive stochastic forcing. This model qualitatively reproduces the experimentally observed thermoacoustic dynamics, including the intermittency in the vicinity of supercritical Hopf bifurcations, as well as the full spectral signature of the acoustic pressure. It is also shown that a simpler model, consisting of a single Van der Pol oscillator, is capable of capturing the essential aspects of the thermoacoustic coupling at the fundamental oscillation frequency and can serve as a basis for an effective estimation of the system’s linear growth rate via output-only identification methods.

VULNERABILITY ASSESSMENT OF A CIVIC TOWER USING AMBIENT VIBRATION TESTS

This paper focuses on the vulnerability assessment of a civic tower built in 1512, which is now considered a national monument. It is the original bell tower of S. Ambrogio church that was destroyed in 1809. Experimental investigations have been carried out on this historical tower. First, detailed investigations have been carried out to identify the geometry of the tower as well as the mechanical features of the constituting materials. Then, ambient vibration tests have been applied using five Micro Electro-Mechanical Systems (MEMS) sensors to detect of the main dynamic features, e.g., modal parameters and damping. Two output-only identification methods, including Frequency Domain Decomposition and Random Decrement Techniques, have been used. The outcomes of the modal identification have been employed to inform the FE model. The numerical analysis can be used for vulnerability assessment, providing a valuable picture of possible damage evolution, tower collapse mechanism, and subsequently, useful hints for the execution of structural retrofitting strategies.

System Identification Based on Output-Only Decomposition and Subspace Appropriation

Output-only identification methods have been developed on a stochastic framework, but for the first time, a subspace-based approach is proposed without using geometric and statistical tools. This aids the computational efforts to be significantly reduced and the range of input sources to be extended in a much realistic manner for future output-only analyses. The approach encompasses any input type and can properly work for systems excited by inputs with finite periods. It is demonstrated that the row space of the output sequences spanned by column vectors of the decomposed orthonormal matrix is sufficient to reconstruct the observations. The transient and steady-state portions of the output row space, afterward, can be captured to reconstruct an integrated innovation model. The advantages of the algorithm are highlighted through several numerical and experimental examples comparing with the traditional subspace identification algorithms.

System Identification of a Base-Isolated Bridge by Ambient and Forced Vibration Tests

Abstract Sciarapotamo Bridge, located in Reggio Calabria Province, Italy, has been recently retrofitted by replacing its deteriorated concrete bridge deck with a composite steel- concrete deck, and isolating the new deck with eight high damping rubber, and four multi-directional sliding bearings. Full-scale ambient and forced vibration dynamic commissioning tests were performed on the bridge just before the bridge has become operational. Three different output-only system identification methods with ambient vibration measurements were used for modal parameter identification, namely: (1) multiple reference natural excitation technique in conjunction with eigen-system realization algorithm, (2) enhanced frequency domain decomposition, and (3) data driven stochastic subspace identification methods. Modal parameter estimation results using different output-only identification methods were presented and compared within the paper. In addition, relatively higher level forced vibration test data was used for estimating frequency response functions, identifying isolated natural frequencies, and damping ratios of the retrofitted bridge in as-built condition. Estimated damping coefficients under different excitation levels are of great value, showing the total damping in the system at low level ambient and relatively higher level forced vibration tests. Experimentally obtained modal parameters of the bridge set the undamaged benchmark state of the bridge for future potential condition assessment studies.

Blind modal identification of non-classically damped structures under non-stationary excitations

In the present paper, we extend a previously developed blind modal identification method to systems with non-classical damping, which have complex-valued mode shapes. Unlike conventional output-only identification methods, blind modal identification method can provide modal property estimates when the input excitations are unknown and non-stationary (e.g., for systems equipped with added dampers or soil–structure problems). In earlier work, we have developed a technique that can be used for non-classically damped systems, but it is only applicable to stationary (i.e., ambient) excitations. Herein, we present an extension to non-classically damped systems under non-stationary (e.g., seismic) excitations. The proposed method yields mode shapes, natural frequencies and damping ratios, sequentially. A critical ingredient in this new method is a combination of generalized eigen-decomposition and rough–fuzzy c-means clustering techniques, which are employed to identify the complex-valued mode shapes. The accuracy of the proposed method is verified through a simulated but realistic example. Copyright © 2016 John Wiley & Sons, Ltd.

Ambient vibration tests of XV century Renaissance Palace after 2012 Emilia earthquake in Northern Italy

This paper focuses on the dynamic behaviour of Mirandola City Hall (a XV century Renaissance Palace) that was severely damaged during May 2012 Emilia earthquake in Northern Italy. Experimental investigations have been carried out on this monumental building. Firstly, detailed investigations have been carried out to identify the identification of the geometry of the main constructional parts as well as the mechanical features of the constituting materials of the palace. Then, Ambient Vibration Tests (AVT) have been applied, for the detection of the main dynamic features. Three output-only identification methods have been compared: (i) the Frequency Domain Decomposition, (ii) the Random Decrement (RD) and the (iii) Eigensystem Realization Algorithm (ERA). The modal parameters of the Palace were difficult to be identified due to the severe structural damage; however the two bending modes in the perpendicular directions were identified. The comparison of the three experimental techniques showed a good agreement confirming the reliability of the three identification methods.

Observer Kalman Filter Identification for Output-Only Systems Using Interactive Structural Modal Identification Toolsuite

AbstractSeveral modal identification techniques have been developed in the past few decades, and their use is rapidly expanding due to new focus on the instrumentation of major structures. This paper focuses on the expansion of the eigenvalue realization algorithm (ERA)–observer Kalman filter identification (OKID) to identify modal parameters of output-only systems (OO) by splitting the state-space model into deterministic and stochastic subsystems (ERA-OKID-OO). The performance is then compared with other output-only identification methods in terms of the level of accuracy and efficiency. A newly developed software package [Structural Modal Identification Toolsuite (SMIT)] is used to provide a uniform and convenient way of utilizing several system identification (SID) methods, including variations of ERA, auto-regressive with exogenous terms (ARX) models, system realization using information matrix (SRIM), and numerical algorithms for subspace state space system identification (N4SID). The main purpose o...

Dynamic Characterization of Cercal Mechanosensory Hairs ofCrickets.

Previous dynamic characterizations of the cercal mechanosensory hairs of crickets have generally been limited to the first resonant frequency and associated deflection shape. A more complete description of the mechanical dynamics of these structures could be obtained by an experimental modal analysis. This paper describes a method by which a full experimental modal analysis, giving natural frequency, mode shape, and modal damping ratio, of these sense organs can be performed. Results of this analysis, employing an unmeasured moving-air excitation and non-contact vibration measurement with an output-only identification method are presented. Two distinct types of behaviour were observed, one of which was a good match for the behaviour expected based on the literature, and one of which was quite different. These two behaviours had distinct patterns of modal parameters. The method described in this paper has been shown to be able to estimate the modal parameters, including natural frequency, modal damping ratio, and normalized mode shape, for the first mode of cercal mechanosensory hairs of crickets. The method could practically be extended to higher modes and a wide variety of other sound and vibration sense organs with the selection of appropriate excitation and specimen supports.

Fuzzy finite element model updating of bridges by considering the uncertainty of the measured modal parameters

It is significant to consider the effect of uncertainty of the measured modal parameters on the updated finite element (FE) model, especially for updating the FE model of practical bridges, since the uncertainty of the measured modal parameters cannot be ignored owing to the application of output-only identification method and the existence of the measured noise. A reasonable method is to define the objective of the FE model updating as the statistical property of the measured modal parameters obtained by conducting couples of identical modal tests, however, it is usually impossible to implement repeated modal test due to the limit of practical situation and economic reason. In this study, a method based on fuzzy finite element (FFM) was proposed in order to consider the effect of the uncertainty of the measured modal parameters on the updated FE model by using the results of a single modal test. The updating parameters of bridges were deemed as fuzzy variables, and then the fuzz-ification of objective of the FE model updating was proposed to consider the uncertainty of the measured modal parameters. Finally, the effectiveness of the proposed method was verified by updating the FE model of a practical bridge with the measured modal parameters.

Output-only modal identification of a cable-stayed bridge using wireless monitoring systems

The objective of this paper is to present two modal identification methods that extract dynamic characteristics from output-only data sets collected by a low-cost and rapid-to-deploy wireless structural monitoring system installed upon a long-span cable-stayed bridge. Specifically, an extensive program of full-scale ambient vibration testing has been conducted to measure the dynamic response of the 240 m Gi-Lu cable-stayed bridge located in Nantou County, Taiwan. Two different output-only identification methods are used to analyze the set of ambient vibration data: the stochastic subspace identification method (SSI) and the frequency domain decomposition method (FDD). A total of 10 modal frequencies and their associated mode shapes are identified from the dynamic interaction between the bridge’s cables and deck vibrations within the frequency range of 0–7 Hz. The majority of the modal frequencies observed from recording cable vibrations are also found to be associated with the deck vibrations, implying considerable interaction between the deck and cables.

On the Applicability of the Ho-Kalman Minimal Realization Theory

The reduced-order model of a time-invariant linear dynamical system, excited by a force of an impulsive type, may be readily obtained using the Ho-Kalman minimal-realization algorithm [1]. The method is based upon a particular factorization of the Hankel matrix in the Markovian representation of the discrete-time process. For stochastic systems, the applicability of the theory has been demonstrated by Akaike [2] on the assumption that the excitation is a zero-mean white noise of a gaussian type. Some of the most widely known output-only identification methods, such as Eigensystem Realization Algorithm (ERA), Canonical Variate Analysis (CVA), and Balanced Realization (BR)) are based upon the above-mentioned work, with the aid of a robust factorization technique, such as Singular-Value Decomposition (SVD). Notwithstanding the growing popularity of the above methods, some aspects of their applicability are not yet understood. Two points are of particular interest: the first regards the applicability of the theory in highly damped systems; and the second regards its applicability to systems driven by excitations different from the one hypothesized. The aim of the present work is to define a reliable test on the hypotheses. Some numerical and experimental results are presented.