Experimental characterisation of complex structures using modal and wave approaches

Abstract

The experimental characterisation of the vibrational behaviour of complex structures is a fundamental step for the validation and troubleshooting of many industrial components. This thesis investigates both modal and wave approaches for the characterisation of such components, with a specific application being heat exchanger units. This study investigates and applies wave-based methods for such structures under operational environments and for the problem of localisation of structural changes.
For the modal methodology, the applicability and accuracy of operational modal analysis is assessed in comparison to traditional experimental modal analysis. A framework for testing heat exchangers is shown, which is also applicable to a wider range of industrial components. It allows for testing the effects of the operational conditions on the modal properties of these structures. The suitability of flow as excitation for the operational modal analysis is shown to depend on the heat exchanger characteristics: in specific scenarios additional excitation sources or reference data for a proper modal characterisation are required.
For the wave propagation analysis, the capability of existing experimental methods in determining wavenumbers of industrial components in the light of characteristic complexities is investigated. Initially, the correlation-based method and the method based on three-point measurements are outlined and discussed. For the correlation-based method, numerical simulations are used to verify its limitations, especially in cases where there are multiple simultaneous closely spaced wavenumbers. It is shown that the dependency between the wavenumber resolution of the correlation method and the spatial arrangement of the experiment varies with the relative properties of the multiple waves. An adjustment of the three-point method to make it applicable to signals with multiple waves forces it to incorporate more measurement points, and to adopt a numerical solver rather than the closed-form expression for the wavenumber estimation.
The integration of operational testing to wave-based methodologies for experimental characterisation of industrial parts led to a method subsequently presented to estimate wavenumbers in structures under operational conditions. It is based on the correlation method and employs cross spectral density functions estimated from the output-only data. Analytical, numerical and experimental analyses are provided where no loss of accuracy is observed in the proposed method in the light of multiple unknown excitations, noise and non-simultaneous measurements.
The last significant contribution is the investigation of the use of local wavenumber estimation to detect discontinuities in a structure. Two methods based on the analytic signal theory, the estimation of the Hilbert transform via frequency domain and the Direct Quadrature, are analysed and modified to suit the space-wavenumber application and the localisation purpose. Because of the proposed modifications, they are referred to as pre-truncated Hilbert transform and the modified Direct Quadrature. Numerical simulations and experiments on a
beam with anechoic terminations and local discontinuities are presented, validating the methods for the problem of localisation via local wavenumber estimation and detailing their limitations. A closing numerical study of these methods on a finite beam demonstrate the pre-truncated Hilbert transform as the most effective for the problem of localisation of a discontinuity in such a structure.

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Identifiers

ORCID for Milena Watanabe Bavaresco: orcid.org/0000-0003-4962-3527

ORCID for Neil Ferguson: orcid.org/0000-0001-5955-7477

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