Thomas G. Carne

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.

Comparison of Inverse Structural Filter (ISF) and Sum of Weighted Accelerations Technique (SWAT) Time Domain Force Identification Methods

Two time domain force identification methods are compared to the standard frequency domain technique in terms of accuracy and sensitivity to errors and a number of extensions are presented which improve their accuracy. Much of the previous in research force reconstruction has focused on frequency domain methods, yet there are applications in which a real time estimate of the input forces is desired or when time data is available over such a short duration that frequency domain methods cannot be applied effectively. Furthermore, the challenges inherent to the inverse problem are manifested differently in the time domain, so it is possible that accuracy and robustness could improve by considering both time and frequency domains. This work reviews two time domain force identification methods, the Inverse Structural Filter (ISF), which is based on a discrete time, state space representation of the dynamics, and the Sum of Weighted Accelerations Technique (SWAT), which is based upon modal filtering. Both of these techniques make use of a modal description of the structural dynamics, so particular attention is given to identifying an adequate model. Actual test data from a free-free beam is used to compare the methods. The application reveals some of the deficiencies of the methods and a number of extensions of the ISF method are presented which greatly improve its performance at certain frequencies and are perhaps easier to apply than the original ISF method. The results of a Monte Carlo simulation are also presented, illustrating the sensitivity of the methods to errors in the modal parameters of the forward system. The results suggest that an accurate description of the forces can be found using the structural response in many important cases, especially when the forces have short duration or relatively smooth spectra in the frequency band of interest.

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.

Health monitoring of operational structures -- Initial results

Two techniques for damage localization (Structural Translational and Rotational Error Checking -- STRECH and MAtriX COmpletioN -- MAXCON) are described and applied to operational structures. The structures include a Horizontal Axis Wind Turbine (HAWT) blade undergoing a fatigue test and a highway bridge undergoing an induced damage test. STRECH is seen to provide a global damage indicator to assess the global damage state of a structure. STRECH is also seen to provide damage localization for static flexibility shapes or the first mode of simple structures. MAXCON is a robust damage localization tool using the higher order dynamics of a structure. Several options arc available to allow the procedure to be tailored to a variety of structures.

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.

Precision truss structures from concept to hardware reality: application to the Micro-Precision Interferometer Testbed

This paper describes the development of the truss structure at the Jet Propulsion Laboratory that forms the backbone of JPLs Micro-Precision Interferometer (MPI) Testbed. The Micro- Precision Interferometer (MPI) Testbed is the third generation of Control Structure Interaction (CSI) Testbeds constructed by JPL aimed at developing and validating control concepts. The MPI testbed is essentially a space-based Michelson interferometer suspended in a ground- based laboratory. This instrument, mounted to the flexible truss, requires nanometer level precision alignment and positioning of its optical elements to achieve science objectives. A layered control architecture, utilizing isolation, structural control, and active optical control technologies, allow the system to meet its vibration attenuation goals. Success of the structural control design, which involves replacement of truss struts with active and/or passive elements, depends heavily on high fidelity models of the structure to evaluate strut placement locations. The first step in obtaining an accurate structure model is to build a structure which is linear.

Characterization of aluminum honeycomb and experimentation for model development and validation :volume II, honeycomb experimentation for model development and validation.

The crush of aluminum honeycomb is a very attractive shock mitigation concept for dissipating large amounts of kinetic energy in laydown weapon systems such as the B61-7 and for shipping container applications. This report is the second of a three-volume set describing aluminum honeycomb crush behavior and model validation. Volume I documents an experimental study of the crush behavior of high-density aluminum honeycombs. Volume III is yet to be published. It will cover the execution of the validation plan described in Volume II. This report, Volume II, describes the need for an improved constitutive model for the large deformation of aluminum honeycomb and is intended to document the procedure that was followed to provide data to calibrate and validate a new constitutive model for large deformation of aluminum honeycomb. The emphasis is on the experimental procedures, but sufficient model description is given to motivate the experiments that were documented herein. The model is first discussed along with the metric, or measuring stick, that will be used to quantify the model’s fit with test data. Next, a description of the necessary constitutive tests and the associated test data are shown that are being used to calibrate the model parameters for the new Honeycomb Crush Model. Parameters for the linear elastic portion of the model are described first, followed by the nonlinear crush parameters. Next, a description of the dynamic experiments used to quantify strain rate sensitivity of the honeycomb are given. The final three chapters cover the basic model (single physics or Tier 1) validation and the combined physics or Tier II model validation steps. Finally, all the calibration and validation data are presented.