Spatially dense operating deflection shapes and mode shapes can be rapidly obtained by use of a continuously scanning laser Doppler vibrometer (CSLDV) system, which sweeps its laser spot over a vibrating structure surface. This paper introduces a new type of vibration shapes called a free response shape (FRS) that can be obtained by use of a CSLDV system, and a new damage
The strong drive to harness wind energy has recently led to rapid growth of wind farm construction. Wind turbine towers with increased sizes and flexibility experience large vibrations. Structural health monitoring of wind turbines is proposed in the wind energy industry to ensure their proper performance and save maintenance costs. This study proposes a system identification method for vibration-based structural assessment of wind turbine towers. This method developed based on the stochastic subspace identification method can identify modal parameters of structures in operating conditions with harmonic components in excitations. It benefits wind turbine tower structural health assessment because classical operational modal analysis methods can fail as periodic rotation excitation from a turbine introduces harmonic disturbance to tower structure response data. The effectiveness, accuracy and robustness of the proposed method were numerically investigated and verified through a lumped-mass system model. The method was then applied to an in-service utilityscale wind turbine tower. The field testing campaign and modal parameter identification as well as structural assessment results were presented.
A continuously scanning laser Doppler vibrometer (CSLDV) system is capable of rapidly obtaining spatially dense operating deflection shapes (ODSs) by continuously sweeping a laser spot from the system over a structure surface. This paper presents a new damage identification methodology for beams that uses their ODSs under sinusoidal excitation obtained by a CSLDV system, where baseline information of associated undamaged beams is not needed. A curvature damage index (CDI) is proposed to identify damage near a region with high values of the CDI at an excitation frequency. The CDI uses the difference between curvatures of ODSs (CODSs) associated with ODSs that are obtained by two different CSLDV measurement methods, i.e., demodulation and polynomial methods; the former provides rapid and spatially dense ODSs of beams, and the latter provides ODSs that can be considered as those of associated undamaged beams. Phase variables are introduced to the two methods for damage identification purposes. Effects of the order in the polynomial method on qualities of ODSs and CODSs are investigated. A convergence index and a criterion are proposed to determine a proper order in the polynomial method. Effects of scan and sampling frequencies of a CSLDV system on qualities of ODSs and CODSs from the two measurement methods are investigated. The proposed damage identification methodology was experimentally validated on a beam with damage in the form of machined thickness reduction. The damage and its region were successfully identified in neighborhoods of prominent peaks of CDIs at different excitation frequencies.
Abstract:The Hongze Lake serves as the first important transfer and storage path along the East Route of the South-to-North Water Diversion Project (SNWDP-ER). After the Project began operating in 2013, eutrophication in the lake has created increasing public concern regarding the direct effect on water quality from the project. In this paper, we used multivariate statistical methods to examine the variances between water quality variables at four site groups (inlets and outlets in the study area) and during three seasons (wet, normal, and dry seasons) in Eastern Hongze Lake. We also used redundancy analysis (RDA) to analyze environmental factors' impact on water quality. We found that (1) the concentrations of nitrogen and phosphorus were exceeding the standard values; (2) No significant spatial heterogeneity regarding nitrogen and phosphorus among the inlet/outlet of Eastern Hongze Lake existed in normal and wet seasons, and there was no evidence demonstrating that SNWDP-ER could improve eutrophication or even contribute to eutrophication in the dry season; (3) Environmental factors (land use, water diversion, and population) have limited influences on water quality, which was possibly caused by the overly-high river input and artificial input of nitrogen and phosphorus. Thus, it is critical to reinforce the control of pollution sources and enhance the ecological restoration.
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