Print Email Facebook Twitter Operational Modal Analysis in the Presence of Harmonic Excitations Title Operational Modal Analysis in the Presence of Harmonic Excitations Author Mohanty, P. Contributor Rixen, Daniel J. (promotor) Faculty Design, Engineering and Production Date 2005-01-10 Abstract The dynamic behavior of structures is a primordial factor determining its structural stability, reliability and lifespan. Therefore, in many advanced engineering applications, it has become essential to predict the dynamics of new designs through simulations, to identify the dynamic characteristics of prototypes and finally to monitor systems in operation. Operational Modal Analysis (OMA) provides an interesting manner to investigate the dynamic properties the structure while in operation. Indeed it allows identifying eigenparameters of structures excited randomly and for which the force inputs cannot be measured, as is often the case in operation. However OMA methods have limitations when applied to practical cases. One limiting constraint of OMA is that the non-measured excitation to the system in operation must a stochastic white noise. This implies that if harmonic excitations are present in addition to random forces, the standard OMA procedures cannot be applied in a straightforward way. The harmonic response components can sometimes be considered as virtual modes in the identification, but when the harmonic excitation frequencies are close to eigenfrequencies the standard OMA approaches may break down. In this thesis, three new procedures are proposed to extend the applicability of the standard OMA methods. Based on the principle of Natural Excitation Technique (NExT), all the procedures can be used to extract modal parameters of structures in operation condition when the unknown input forces can be assimilated to a combination of stochastic white noise and harmonic excitation. Such combination of random and harmonic excitations are quite common in real life: wind excitations and acoustic noise for instance can be considered as stochastic and harmonic excitation forces can arise due for instance to the presence of rotating components. This thesis discusses three identification algorithms adapted from standard methods in order to explicitly take into account harmonic response components of known frequency. In particular we have investigated the efficiency of the new procedures to properly identify eigenfrequencies, modal damping and mode shapes. The major and original contributions of this thesis are: * In chapter 3, a modified Least Square Complex Exponential (LSCE) method is proposed, which handles as input the auto- and cross-correlation of the response signals to perform OMA in the presence of harmonic excitations. * Similar to the modified LSCE, in chapter 4, a modified Ibrahim Time Domain (ITD) method, based on the Single Station Time Domain (SSTD) method is proposed which also performs OMA in the presence of harmonic excitations. * In chapter 5, a modified Eigensystem Realization Algorithm (ERA) method is presented to perform eñciently OMA in the presence of harmonic excitations. Unlike the first two methods, this third approach does not directlyidentify modal parameters but it first identifies the linear system matrix from which eigenparameters are then found. * In chapter 6, a method has been proposed to identify mode shapes by OMA in the presence of harmonic excitations. In this chapter modal parameters computed by the methods presented in the previous chapters are used as starting point to derive mode shapes. The examples used in this thesis work (a simply supported beam and a free plate) have shown that the proposed methods significantly improve the accuracy of the identification compared to standard OMA techniques and therefore applying them to complex structures is expected to yield similar improvements. Nevertheless application to industrial case studies was beyond the scope of the present thesis. Subject operational modal analysisharmonicexcitationsdynamic testingmodal analysisexperimental modal analysis To reference this document use: http://resolver.tudelft.nl/uuid:74990606-3798-45d7-afae-d16227f2576b Part of collection Institutional Repository Document type doctoral thesis Rights (c) 2005 P. Mohanty Files PDF dep_mohanty_20050110.pdf 2.96 MB Close viewer /islandora/object/uuid:74990606-3798-45d7-afae-d16227f2576b/datastream/OBJ/view