Fully automated (operational) modal analysis

Blind identification of full-field vibration modes from video measurements with phase-based video motion magnification

Analytical Methods in Vibrations

Automated output-only dynamic identification of civil engineering structures

Cross-wind modal properties of offshore wind turbines identified by full scale testing

Output-only modal analysis of linear time-periodic systems with application to wind turbine simulation data

A study and extension of second-order blind source separation to operational modal analysis

Operational Modal Analysis (OMA) is one of the branches of experimental modal analysis which allows extracting modal parameters based on measuring only the responses of a structure under ambient or operational excitation which is not needed to be measured. This makes OMA extremely attractive to modal analysis of big structures such as wind turbines where providing measured excitation force is an extremely difficult task. One of the main OMA assumption concerning the excitation is that it is distributed randomly both temporally and spatially. Obviously, closer the real excitation is to the assumed one, better modal parameter estimation one can expect. Traditionally, wind excitation is considered as a perfect excitation obeying the OMA assumptions. However, the present study shows that the aeroelastic phenomena due to rotor rotation dramatically changes the character of aerodynamic excitation and sets limitations on the applicability of OMA to operational wind turbines. The main purpose of the study is to warn the experimentalists about these limitations and discuss possible ways of dealing with them.

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Applicability Limits of Operational Modal Analysis to Operational Wind Turbines

Summary Output-only (Operational) Modal Analysis (OMA) is a modern branch of experimental modal analysis; the main advantage of OMA is its ability to extract modal model using only measured responses. This makes OMA extremely attractive for modal analysis of big structures such wind turbines. However, there are issues preventing straightforward application of OMA to operational turbines, e.g. structure invariance during the test. The effect of rotor rotation manifests itself in the equation of motion with time-dependent coefficients. Formulating and solving eigenvalue problem lead to time-dependent eigenvalues and eigenvectors which become meaningless as modal parameters. Fortunately, so-called Coleman coordinate transformation (also known as multi-blade coordinate transformation) allows one to eliminate time dependency of the system matrices, thus converting the original time-varying eigenvalue problem to a time-invariant one. This study extends this approach to experimental modal analysis. Forward Coleman transformation is applied to the data measured on the wind turbine blades, which is then combined with responses measured on the tower. The methods of Operational Modal Analysis are then applied to the transformed data, resulting in modal frequencies, damping and mode shapes. Backward Coleman transformation is finally employed for the mode shapes for their visualization. The study demonstrates the method using simulated vibrational responses of operational 3MW wind turbine. The responses of the tower and blades were obtained from the simulation of operational wind turbine dynamics under realistic wind load using commercial aeroelastic code.

Output-only modal analysis on operating wind turbines: Application to simulated data

Wind turbines are complex structures and their dynamics vary significantly in operation, in comparison to that under stand still (parked) conditions. Presence of rotational loads and considerable aeroelastic effects makes it difficult to apply conventional system identification techniques to wind turbines. Operational Modal Analysis (OMA) is a technique of determining dynamic characteristics of a structure based on the output responses only (Input excitation is assumed to be stochastic and its information is not required). Recently it was shown by means of analytical and experimental studies that Blind Source Separation and Independent Component Analysis (BSS/ICA) techniques, which are more common in bio-imaging, wireless communications etc, can also be utilized for system identification purposes in typical output only scenarios. This paper explores the possibility of applying these techniques (OMA and BSS/ICA) to wind turbines. These techniques lend themselves very well to this situation as wind turbine is a huge structure and also, stochastic excitation of turbine from the wind can be used as input. However, time variant nature of operating wind turbines and presence of harmonic components in excitation, still pose several challenge to application of OMA and BSS. To tackle these issues one at a time, this paper focuses in understanding only the aeroelastic effects, thus avoiding effect of rotational loads (and hence presence of harmonic components in excitation force). Aeroelastic effects are significant even in case of parked wind turbines, thus studies are conducted on simulated data obtained for a parked turbine. Ability of OMA and BSS/ICA techniques in determining damping, which is a Proceedings of the IMAC-XXVII

Shashank Chauhan

Papers

Output-only modal analysis of linear time-periodic systems with application to wind turbine simulation data

A study and extension of second-order blind source separation to operational modal analysis

Applicability Limits of Operational Modal Analysis to Operational Wind Turbines

Output-only modal analysis on operating wind turbines: Application to simulated data

Application of Operational Modal Analysis and Blind Source Separation / Independent Component Analysis Techniques to Wind Turbines