The Natural Excitation Technique (NExT) for modal parameter extraction from operating wind turbines
01 Feb 1993-Vol. 93, pp 28603
TL;DR: The Natural Excitation Technique (NExT) is a method of modal testing that allows structures to be tested in their ambient environments as mentioned in this paper, and has been used extensively in the literature.
Abstract: The Natural Excitation Technique (NExT) is a method of modal testing that allows structures to be tested in their ambient environments This report is a compilation of developments and results since 1990, and contains a new theoretical derivation of NExT, as well as a verification using analytically generated data In addition, we compare results from NExT with conventional modal testing for a parked, vertical-axis wind turbine, and, for a rotating turbine, NExT is used to calculate the model parameters as functions of the rotation speed, since substantial damping is derived from the aeroelastic interactions during operation Finally, we compare experimental results calculated using NExT with analytical predictions of damping using aeroelastic theory
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TL;DR: In this paper, a novel approach of stochastic subspace identification is presented that incorporates the idea of the reference sensors already in the identification step: the row space of future outputs is projected into the rowspace of past reference outputs.
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TL;DR: In this article, a review of stochastic system identification methods that have been used to estimate the modal parameters of vibrating structures in operational conditions is presented. But it is not shown that many of these methods have an output-only counterpart.
Abstract: This paper reviews stochastic system identification methods that have been used to estimate the modal parameters of vibrating structures in operational conditions. It is found that many classical input-output methods havean output-only counterpart. For instance, the Complex Mode Indication Function (CMIF) can be applied both to Frequency Response Functions and output power and cross spectra. The Polyreference Time Domain (PTD) method applied to impulse responses is similar to the Instrumental Variable (IV) method applied to output covariances. The Eigensystem Realization Algorithm (ERA) is equivalent to stochastic subspace identification.
849 citations
TL;DR: In this article, an ambient vibration survey of the Humber Bridge was carried out in July 2008 by a combined team from the UK, Portugal and Hong Kong, where ten stand-alone tri-axial acceleration recorders were deployed at locations along all three spans and in all four pylons during five days of consecutive one-hour recordings.
278 citations
01 Nov 2007
TL;DR: This research highlights the need to understand more fully the role of phosphorous in the response of the immune system to disease and its role in promoting wound healing.
Abstract: Financial support for this research was provided in part by the National Science Foundation (NSF) under NSF grants CMS 03-01140 and CMS 06-00433 (Dr. S. C. Liu, Program Manager). The first author was supported by a Vodafone-U.S. Foundation Graduate Fellowship. This support is gratefully acknowledged.
256 citations
TL;DR: In this article, a short-span bridge taking a busy main road across a coastal inlet near a major port facility was assessed before and after highway bridge upgrading works including strengthening, and the utility of the combined experimental and analytical process in direct identification of structural properties has been proven.
Abstract: The Land Transport Authority of Singapore has a continuing program of highway bridge upgrading for refurbishing and strengthening bridges to allow for increasing vehicle traffic and increasing axle loads. One subject of this program has been a short-span bridge taking a busy main road across a coastal inlet near a major port facility. Experiment-based structural assessments of the bridge were conducted before and after upgrading works including strengthening. Each assessment exercise comprised three separate components: ~1! a strain and acceleration monitoring exercise lasting approximately one month; ~2! a full-scale dynamic test carried out in a single day without closing the bridge; and ~3! a finite-element model updating exercise to identify structural parameters and mechanisms. This paper presents the dynamic testing and the modal analysis used to identify the vibration properties and the quantification of the effectiveness of the upgrading through the subsequent model updating. Before and after upgrade, similar sets of vibration modes were identified, resem- bling those of an orthotropic plate with relatively weak transverse bending stiffness. Conversion of bearings from nominal simple supports to nominal full fixity was shown via model updating to be the principal cause of natural frequency increases of up to 50%. The utility of the combined experimental and analytical process in direct identification of structural properties has been proven, and the procedure can be applied to other structures and their capacity assessments.
256 citations
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