Alternate Techniques/New Approaches for Identification of Full Field Dynamic Stress Strain from Limited Sets of Measured Data for Wind Turbine Applications

Abstract

Current design analysis for wind turbine blade applications is based on an estimation of the forces that drive the system during operation. Much effort has been expended in the estimation of these forces from fluid structure interaction models as well as from measured operating responses. However, the existing approaches can only provide an approximate estimate of the forces for the structural modeling of the blades, primarily because of a lack of distributed sensing capability and because the assumed boundary conditions are not always accurate. Two relatively new techniques are currently under investigation that enable a much better overall prediction of the response of the system. Full-field optical measuring techniques and real time data expansion approaches have the potential to greatly improve these required predictions. Three-dimensional digital image correlation (DIC) and dynamic point tracking (3DPT) have recently been explored as possible approaches to collect data on various types of structures using noncontacting measurement while the system is operating (rotating). A basic discussion on these imaging techniques is provided to show their usefulness in making structural dynamic measurements. The sets of limited surface data that are collected can be used in conjunction with an expansion algorithm to obtain fullfield information. The finite element model mass and stiffness matrices are used to obtain the normal constitutive relations as well as the modal characteristics. This information is used to develop the expansion algorithm and for the stress recovery during the back substitution process typically employed in the finite element approach. A basic discussion of the process used is presented along with several different test cases to demonstrate the optical measurement process and the ability to achieve full-field results. Some thoughts on the exploitation of these new tools and the vision forward are also presented.