D. Todd Griffith

Modal Testing for Validation of Blade Models

The focus of this paper is a test program designed for wind turbine blades. Model validation 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. Structural models are useful for making decisions when designing a new blade or assessing blade performance, and the process of model validation is needed to ensure the quality of these models. Blade modal testing is essential for validation of blade structural models, and this report discusses modal test techniques required to achieve validation. Choices made in the design of a modal test can significantly affect the final test result. This study aims to demonstrate the importance of the proper pre-test design and test technique for validating blade structural models.

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.

Modeling and Testing of 9m Research Blades

Wind turbines and their blades continue to grow in size. The resulting increase in blade mass and cost requires the implementation of new design concepts. Among these is the selective use of carbon fiber. In 2002, Sandia National Laboratories (SNL) initiated a research program to investigate the use of carbon fiber in 9m subscale blades. Two sets of blades were designed, one with a carbon spar-cap and the other with off-axis carbon in the skin which produces bend twist coupling. Blades of each design have recently undergone modal and structural testing. In addition, finite element analysis (FEA) of both blades has been performed. This paper describes the design, testing, and analysis work that have been completed.

An evaluation of wind turbine blade cross section analysis techniques.

The blades of a modern wind turbine are critical components central to capturing and transmitting most of the load experienced by the system. They are complex structural items composed of many layers of fiber and resin composite material and typically, one or more shear webs. Large turbine blades being developed today are beyond the point of effective trial-and-error design of the past and design for reliability is always extremely important. Section analysis tools are used to reduce the three-dimensional continuum blade structure to a simpler beam representation for use in system response calculations to support full system design and certification. One model simplification approach is to analyze the two- dimensional blade cross sections to determine the properties for the beam. Another technique is to determine beam properties using static deflections of a full three-dimensional finite element model of a blade. This paper provides insight into discrepancies observed in outputs from each approach. Simple two-dimensional geometries and three-dimensional blade models are analyzed in this investigation. Finally, a subset of computational and experimental section properties for a full turbine blade are compared.

Experimental Modal Analysis of 9-meter Research-sized Wind Turbine Blades

The dominant and persistent trend with wind turbine technology, particularly in the past three decades, has been growth in the length of the blades. In order to investigate design choices which reduce blade weight, Sandia Labs initiated a study, which is near completion, to evaluate innovative concepts for large blades. The innovations include strategic use of carbon fiber in the spar caps, bend-twist coupling in the composite layup, and thick, flatback airfoils. Several large blades were designed and then built at a down-scaled 9-meter length. Each blade design has undergone a full series of structural tests including modal tests, static tests, and fatigue tests. The modal tests performed for evaluation of these blades is the focus of this paper. Major findings from these tests are summarized, and they include: (1) techniques for experimental quantification of uncertainty in the modal parameters, (2) insight into model calibration using both static load-deflection data and the modal parameters, (3) novel test techniques for reducing the uncertainty in the root boundary condition, and (4) the development of validated structural models. This paper will provide a summary of blade modal testing and structural model validation, and will emphasize recent validation tests using a seismic-mass-on-airbags boundary condition.

Structural Dynamics Testing and Analysis for Design Evaluation and Monitoring of Heliostats

Heliostat vibrations can degrade optical pointing accuracy while fatiguing the structural components. This paper reports the use of structural dynamic measurements for design evaluation and monitoring of heliostat vibrations. A heliostat located at the National Solar Thermal Testing Facility (NSTTF) at Sandia Labs in Albuquerque, New Mexico, has been instrumented to measure its modes of vibration, strain and displacements under wind loading. The information gained from these tests will be used to evaluate and improve structural models that predict the motions/deformations of the heliostat due to gravitational and dynamic wind loadings. These deformations can cause optical errors and motions that degrade the performance of the heliostat. The main contributions of this work include: (1) demonstration of the role of structural dynamic tests (also known as modal tests) to provide a characterization of the important dynamics of the heliostat structure as they relate to durability and optical accuracy, (2) the use of structural dynamic tests to provide data to evaluate and improve the accuracy of computer-based design models, and (3) the selection of sensors and data-processing techniques that are appropriate for long-term monitoring of heliostat motions.Copyright

Attitude and interlock angle estimation using split-field-of-view star tracker

An efficient Kalman filter based algorithm has been proposed for the spacecraft attitude estimation problem using a novel split-field-of-view star camera and three-axis rate gyros. The conventional spacecraft attitude algorithm has been modified for on-orbit estimation of interlock angles between the two fields of view of star camera, gyro axis, and the spacecraft body frame. Real time estimation of the interlock angles makes the attitude estimates more robust to thermal and environmental effects than in-ground estimation, and makes the overall system more tolerant of off-nominal structural, mechanical, and optical assembly anomalies.

Higher Order Sensitivities for Solving Nonlinear Two -Point Boundary -Value Problems

In this paper, we consider new computational approaches for solving nonlinear Two - Point Boundary -Value Problems. The sensitivity calculations required in the solution utilize the automatic differentiation too l OCEA (Object Oriented Coordinate Embedding Method). OCEA has broad potential in this area and many other areas since the partial derivative calculations required for solving these problems are automatically computed and evaluated freeing the analyst fro m deriving and coding them. In this paper, we demonstrate solving nonlinear Two -Point Boundary Value Problems by shooting and direct methods using automatic differentiation. We demonstrate standard first -order algorithms and higher - order extensions. Add itionally, automatic generation of co -state differential equations and second - and higher -order midcourse corrections are considered. Optimization of a sample Low -thrust, Mars -Earth trajectory is considered as an example. Computational issues related to domain of convergence and rate of convergence will be detailed.

Large Offshore Rotor Development: Design and Analysis of the Sandia 100-meter Wind Turbine Blade

Sandia National Laboratories’ (SNL) Wind & Water Power Technologies Department, as part of its ongoing R&D efforts, creates and evaluates innovative large blade concepts for horizontal axis wind turbines to promote designs that are more efficient aerodynamically, structurally, and economically. Recent work has focused on the development of a 100-meter blade for a 13.2 MW horizontal axis wind turbine, a blade that is significantly longer than the largest commercial blades of today (approximately 60 meters long). This paper summarizes the design development of the Sandia 100-meter All-glass Baseline Wind Turbine Blade, termed as “SNL100-00”, which employs conventional architecture and fiberglass-only composite materials. The paper provides a summary of performance margins from a series of analyses that demonstrate changes in various design drivers for large blade technology. Recommendations for improvements to large blade design and future research investment needs are discussed.

The Hamel Representation: A Diagonalized Poincaré Form

The Poincare equations, also known as Lagranges equations in quasicoordinates, are revisited with special attention focused on a diagonal form. The diagonal form stems from a special choice of generalized speeds that were first introduced by Hamel (Hamel, G., 1967, Theorctische Mechanik, Springer-Verlag, Berlin, Secs. 235 and 236) nearly a century ago. The form has been largely ignored because the generalized speeds create so-called Hamel coefficients that appear in the governing equations and are based on the partial derivative of a mass-matrix factorization. Consequently, closed-form expressions for the Hamel coefficients can be difficult to obtain. In this paper, a newly developed operator overloading technique is used within a simulation code to automatically generate the Hamel coefficients through an exact partial differentiation together with a numerical evaluation. This allows the diagonal form of Poincares equations to be numerically integrated for system simulation. The diagonal form and the techniques used to generate the Hamel coefficients are applicable to general systems, including systems with closed kinematic chains. Because of Hamels original influence, these special Poincare equations are called the Hamel representations and their usefulness in dynamic simulation and control is investigated.