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Title: Development of modal analysis methodologies for the identification of aerospace structures in operating conditions
Tutor: Coppotelli, Giuliano
Nitzsche, Fred
Keywords: System Identification
Operational Modal Analysis
Structural Dynamics
Experimental Modal Analysis
Rotating structures
Rotorcraft Technology
Environmental Testing
In-Flight Tests
Issue Date: 13-May-2013
Abstract: Operational modal analysis differs from traditional experimental modal analysis in that it only requires information of the output responses and the modal parameters (in terms of natural frequencies damping ratios and mode shapes) are estimated under the assumption of white noise excitation. It presents several advantages including the availability of modal properties of structure in operation thus representing a closer picture of the structure and its boundary conditions (which are not that easy to realize in laboratory conditions). However, lack of input excitation force information presents several challenges as well as the proper estimation of the frequency response functions and the accurate evaluation of modal parameters in presence of harmonic components in the excitation. Several methodologies have been developed in the last years as described in this thesis and the main purpose of the research is to assess their application in aerospace such as the rotorcraft technology and the environmental testing. Solutions to the main operational modal analysis limitations are suggested and the implementation of the related algorithms allows the application on several test cases after their validation. Taking advantages of this improving in the experimental analysis capabilities, a demanding application within the rotorcraft technology is carried out. Starting from the already developed Active Pitch Link prototype (based on the Smart Spring concept), its use for vibration reduction on rotating blade is numerically and experimentally investigated, thanks to the intensive use of the operational modal analysis for the identification of the real system properties that give the necessary information for the tuning of the numerical model, that in turn suggests the operative test conditions.
Description: The state-of-the-art in the field of vibration testing has been presented together with the developed methodologies. Within the Operational Modal Analysis the two major limitations regarding the application in presence of harmonic excitation and the scaling of the mode shapes have been addressed and the suggested solutions have been theoretically, numerically and experimentally validated. The mathematical formulations are described together with their implementation. Moreover, a numerical platform for the Operational Modal Analyses has been elaborated and it has been widely used. In particular, it has been shown as the variation of the spatial properties of a structure (in terms of mass or stiffness) induces a modal variation that allows the evaluation of the modal masses. The numerical investigations showed how two different methods can be proficiently used and the experimental activities gave the proof that they can be applied in environmental testing for space structures to increase the amount of information on the test specimen necessary to validate a numerical finite element model of the structure. A novel approach has been suggested and validated to identify the harmonic loadings acting on the structure by using the entropy statistical index. Moreover, a method on the application of the Operational Modal Analysis in case of colored noise (composed by these identified harmonic components and white noise) has been developed as a modified Hilbert Transform Method. Several applications with different sensors (accelerometers, strain gauges, Fiber Bragg Grating), different test articles (helicopters, blades, wind turbine) and several working conditions have been carried out with good results. The complex rotorcraft applications have been deeply investigated starting from the mathematical formulation until the manufacturing aspects, passing through the design and validation of the entire experimental setup. The problem of vibration reduction has been considered and, starting from the formulation of the Individual Blade Control problem for the Active Pitch Link, numerical and experimental results showed the effectiveness of the suggested control strategy. It should be stressed that all these results have been possible thanks to the application of the Operational Modal Analysis during the whole developing process because it provided the experimental parameters necessary to tune the numerical simulations. In conclusion, the importance of the Operational Modal Analysis for the aerospace applications has been proved and its capabilities to deal with different operating situations have been investigated.

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