The Operational Modal Analysis, OMA, allows the estimation of the dynamic properties of a structure, i.e., natural frequencies, damping ratios, and mode shapes, without measuring the input forces. One of the main advantages of this approach consists in the capability of identifying the system modal parameters in its operative conditions. According to the main hypothesis concerning the input excitation, that is stochastic with frequency independent spectra, it is not theoretically possible to apply the OMA procedures in structures characterized by the presence of harmonic components in the excitation loading. In this paper, an approach capable to identify the presence of harmonic excitations, acting together with a broad band stochastic loading, and then to remove their effects, in a further step, in the modal parameter estimate is presented. It is based on the joint use of the statistical parameter called "Entropy" together with the already developed Output-Only procedure based on the application of the Hilbert transformation properties to the output response signals. The capability to improve the OMA procedures is investigated through whirl tower experimental tests carried out at DLR Braunschweig (German Aerospace Center - DLR) in which the dynamic excitation of the rotating frame is provided by the perturbations arising from the operating conditions characterized by both stochastic and harmonic contributions. A sensitivity analysis is performed to evaluate the effects of the time filtering of the responses, required by the statistical characterization, on the estimate of the modal parameters. A comparison among the modal parameter estimates achieved by different methodologies that do not take into account the presence of harmonic excitation in the output responses, is performed for a final assessment of the proposed approach.
Operational Modal Analysis of a Rotating Helicopter Blade / Agneni, Alessandro; Coppotelli, Giuliano; Grappasonni, Chiara. - CD:(2010), pp. 3249-3262. (Intervento presentato al convegno International Conference on Noise and Vibration Engineering (ISMA)/Conference of USD tenutosi a Leuven, BELGIUM nel SEP 20-22, 2010).
Operational Modal Analysis of a Rotating Helicopter Blade
AGNENI, AlessandroUltimo
Membro del Collaboration Group
;COPPOTELLI, Giuliano
Primo
Membro del Collaboration Group
;GRAPPASONNI, CHIARASecondo
Membro del Collaboration Group
2010
Abstract
The Operational Modal Analysis, OMA, allows the estimation of the dynamic properties of a structure, i.e., natural frequencies, damping ratios, and mode shapes, without measuring the input forces. One of the main advantages of this approach consists in the capability of identifying the system modal parameters in its operative conditions. According to the main hypothesis concerning the input excitation, that is stochastic with frequency independent spectra, it is not theoretically possible to apply the OMA procedures in structures characterized by the presence of harmonic components in the excitation loading. In this paper, an approach capable to identify the presence of harmonic excitations, acting together with a broad band stochastic loading, and then to remove their effects, in a further step, in the modal parameter estimate is presented. It is based on the joint use of the statistical parameter called "Entropy" together with the already developed Output-Only procedure based on the application of the Hilbert transformation properties to the output response signals. The capability to improve the OMA procedures is investigated through whirl tower experimental tests carried out at DLR Braunschweig (German Aerospace Center - DLR) in which the dynamic excitation of the rotating frame is provided by the perturbations arising from the operating conditions characterized by both stochastic and harmonic contributions. A sensitivity analysis is performed to evaluate the effects of the time filtering of the responses, required by the statistical characterization, on the estimate of the modal parameters. A comparison among the modal parameter estimates achieved by different methodologies that do not take into account the presence of harmonic excitation in the output responses, is performed for a final assessment of the proposed approach.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.