Development of Validated Blade Structural Models

The focus of this paper is on the development of validated models for wind turbine blades. Validation of these models is a comprehensive undertaking which requires carefully designing and executing experiments, proposing appropriate physics-based models, and applying correlation techniques to improve these models based on the test data. This paper will cover each of these three aspects of model validation, although the focus is on the third – model calibration. The result of the validation process is an understanding of the credibility of the model when used to make analytical predictions. These general ideas will be applied to a wind turbine blade designed, tested, and modeled at Sandia National Laboratories. The key points of the paper include discussions of the tests which are needed, the required level of detail in these tests to validate models of varying detail, and mathematical techniques for improving blade models. Results from investigations into calibrating simplified blade models are presented. I. Introduction HERE are a number of reasons why one desires to develop models of wind turbine blades, and in each case one wants to ensure that these models are useful for the intended purpose. For example, correctly predicting failure in blades using a model can reduce the need for numerous costly tests including the fabrication of additional blades for a test-based failure prediction approach. An additional benefit of modeling and simulation is that the time required to complete the design and fabrication cycle can be reduced significantly when validated models are used to evaluate key aspects of the design that would otherwise require testing. Additionally, modern blades are large and costly – a validated predictive tool would be useful for assessing larger blades of the future. An important step in ensuring that a model is useful for the purpose of the analysis, that is ensuring that a model accurately predicts the behavior of interest, is a process called model validation. A validated model is one in which an analyst or designer can place a great deal of confidence – one can use this model to accurately predict performance. The validation process incorporates both testing and analysis. A set of calibration experiments are designed which provide enough data to improve the model so that the observations from the test and the corresponding predictions from the analysis are suitably correlated. In the next step, additional “validation experiments” are conducted in order to ensure that the model is predictive for the conditions of the validation experiments. If the validation experiments can be predicted, then the model is considered validated, otherwise additional experiments must be performed to provide data for further improvement of the model. It is important to note that a model which has been calibrated to match the test data is not necessarily a valid model. The process of calibrating a model is called model updating, while model validation includes the additional step of performing validation experiments. The main objective of the paper is to detail a general model validation process applied to wind turbine blades designed and tested at Sandia National Laboratories. The key points to be covered include those related to 1) testing (experiment design), 2) analysis (model development), and 3) comparison of test-analysis data for use in model calibration. Key points are covered in each of these areas; however, the focus of this paper is on model calibration. Optimization of the model parameters incorporating various types of blade test observations, including modal testing and static testing, is a novel development. As an example encompassing the key points, a program aimed at improving current modeling capabilities for a research-sized wind turbine blade is discussed in detail.