Chapter35 Effects of Boundary Conditions on the Structural Dynamics of Wind Turbine Blades. Part 2: Edgewise Modes Javad Baqersad, Christopher Niezrecki, and Peter Avitabile Abstract Mode shapes and resonant frequencies of an individual wind turbine blade can be readily determined in either a laboratory or a blade test facility using experimental modal analysis. However, performing a modal test on a utility-scale wind turbine with several blades attached to a tower can be a challenge due to the number of sensors required, the size of these structures, and cost. Therefore, understanding the influence of the coupled three-bladed turbine/tower system and identifying a correlation between the dynamic behavior of an assembled wind turbine to an individual blade or three-bladed turbine on a hub is desirable. In the current paper, which is the second part of a two-part paper, variation in natural frequencies and mode shapes of a three-bladed turbine due to the increase in stiffness of the support is numerically studied using a finite element beam analysis (a similar study was performed on flapwise modes of a turbine and is presented in the first part of this paper). The results of the study reveals that differential edgewise modes of the turbine that represent approximately 2/3 of edgewise modes are independent of the rotational stiffness of the support; thus, these modes can be predicted when the modes of cantilevered single blade are known. However, collective edgewise modes change significantly by changing rotational stiffness of the support. The mode contribution matrix of the wind turbine attached to a tower indicates the necessary set of modal vectors of the tower and three-bladed turbine to accurately obtain the edgewise dynamics of the final assembly. Keywords Blade • Wind turbine • Boundary condition • Tower • Mode contribution 35.1 Introduction Recently, wind power has been emerging as one of the major resources of clean renewable energy. Modern wind turbines are manufactured significant in size to reduce the cost of energy compared to fossil fuels and to meet increasing energy demands. As the size of the wind turbines scales up, the capital investment for these machines is escalating. Therefore, periodic damage prognosis and condition based monitoring of wind turbines have become of particular interest. A validated numerical model of a wind turbine is an essential part of most of the health monitoring algorithms. Likewise, an expansion algorithm in conjunction with a finite element model can be used to augment the limited set of operating measured data at sensor locations and predict the full-field dynamic strain in rotating wind turbines. However, performing experimental modal tests on utility-scale wind turbines attached to towers to validate the finite element models is very challenging. Therefore, the work performed in the current paper is an attempt to understand how the edgewise mode shapes of a single cantilever blade or a three-bladed wind turbine correlate with the mode shapes of the wind turbine attached to a tower. Numerous researches have tried to model the dynamic behavior of wind turbines using the finite element (FE) method. Beam elements were traditionally used for modeling rotating blades [1, 2]. Furthermore, three-dimensional (3D) solid element models require a high computation time; hence, they are typically used for modeling single blades or towers [3–6]. On the other hand, the finite element models need to be validated and/or updated using experimental measurements. The sheer size of three-bladed wind turbines makes the modal analysis of entire structures very challenging. Thus, experimental measurements to validate numerical models are typically performed on single blades [7–12]. The number of studies found in the published literature in which modal tests were performed on a group of blades is limited [13]. J. Baqersad ( ) • C. Niezrecki • P. Avitabile Structural Dynamics and Acoustic Systems Laboratory, One University Avenue, Lowell, MA 01854, USA e-mail: javad_baqersad@student.uml.edu M. Allen et al. (eds.), Dynamics of Coupled Structures, Volume 1: Proceedings of the 32nd IMAC, A Conference and Exposition on Structural Dynamics, 2014, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-04501-6__35, © The Society for Experimental Mechanics, Inc. 2014 369
RkJQdWJsaXNoZXIy MTMzNzEzMQ==