Determination of Wind Turbine Operating Deflection Shapes Using Full-field 3D Point-tracking

Abstract

The ability to collect operating data at a high number of points to monitor the structural dynamics of rotating structures will lead to a better physical understanding of these structures during operation. Today, however, structural dynamic measurements are typically obtained while the structure is in a static configuration. Differences are likely to exist in the structural dynamic behavior when comparing statically performed tests and the measurements while taken during operation. In order to evaluate a rotating system while in operation, slip-rings are typically used during testing with measurements made at only a few discrete points. Slip-ring configurations can be difficult to instrument, suffer from measurement noise, and the attached sensors can obscure the true dynamic response due to mass loading and aerodynamic effects. In past work, stereophotogrammetry techniques have been shown to measure the in- and out-of-plane motion of rotating structures. To extend these prior efforts, a 46 inch (1.17 m) diameter wind turbine was mounted to a commercial fan motor so controlled rotating conditions could be created in a laboratory environment. Displacements in three dimensions at dozens of measurement points were determined using 3D point-tracking while the turbine was spun at multiple speeds. Operating deflection shapes extracted from these data were compared to other rotating tests and to a static (non-rotating) modal test. The test results indicate a correlation between conventional static and optically measured mode shapes, however some spectral differences exist between the data sets and may be due to the changes in the structural boundary conditions present during operation at different speeds.