Giuliano Coppotelli

Development of a FBG based distributed strain sensor system for wind turbine structural health monitoring

The development of a fiber Bragg grating (FBG) based distributed strain sensor system for real time structural health monitoring of a wind turbine rotor and its validation under a laboratory scale test setup is discussed in this paper. A 1 kW, 1.6 m diameter rotor, horizontal axis wind turbine with three instrumented blades is used in this study. The sensor system consists of strain sensors, surface mounted at various locations on the blade. At first the sensors are calibrated under static loading conditions to validate the FBG mounting and the proposed data collection techniques. Then, the capability of the sensor system coupled with the operational modal analysis (OMA) methods to capture natural frequencies and corresponding mode shapes in terms of distributed strains are validated under various non-rotating dynamic loading conditions. Finally, the sensor system is tested under rotating conditions using the wind flow from an open-jet wind tunnel, for both a baseline wind turbine and a wind turbine with a structurally modified blade. The blade was modified by attaching a lumped mass at the blade tip simulating structural damage or ice accretion. The dynamic characteristics of the baseline (healthy) blade and modified (altered) blade are compared to validate the sensor systems ability for real time structural health monitoring of the rotor.

Aeroelastic Sensitivity Analyses for Flutter Speed and Gust Response

Two methods for the aeroelastic eigensensitivity analysis and the sensitivity analysis of an aeroelastic discrete-gust response have been developed. Finite state modeling of the unsteady aerodynamics allows one to determine explicitly the aeroelastic sensitivity with respect to a structural design variable and the aeroelastic behavior with respect to other design variables such as fuel weight, wing stiffness, and engine location. An analytical method based on the matched filter theory has been developed that allows one to estimate the sensitivity, with respect to the same design variable, of the maximum peak reached by the gust response due to a discrete gust. This approach allows one to evaluate the maximum value of the response corresponding to a discrete-gust input once the energy level of the input has been established. The sensitivity of this maximum value with respect to an aeroelastic-design variable can be evaluated too. The structural and aerodynamic contributions to the sensitivity have been separately identified following several levels of approximation. Numerical results, in the case of an ultrahigh capacity aircraft wing, are presented. Because of the large flexibility of the wing, the aeroelastic behavior has been included in the stability margin estimate and in the gust response. The application limits of the sensitivity approximations are discussed. The proposed approach, which uses structural and aerodynamic data by standard codes, could be useful in the preliminary design to evaluate and preestimate the aeroelastic performances.

Experimental Investigation on Modal Signature of Smart Spring/Helicopter Blade System

To achieve efficient attenuation of noise and vibration characteristics within the helicopter environment, solid-state actuators are seen as one of the most promising smart technologies. The Smart Hybrid Active Rotor Control System project is expected to demonstrate the ability of smart structure systems, employing multiple active material actuators, sensors, and closed-loop controllers, to reduce simultaneously rotorcraft vibration and noise. Within this project, piezoceramic elements were used as actuators to vary the dry friction and the stiffness of the whole helicopter blade system. This active control concept, named Smart Spring, originated a prototype used to demonstrate the ability to attenuate vibrations. Before testing the Smart Hybrid Active Rotor Control System in operative conditions, the dynamic properties of the Smart Spring installed on a nonrotating helicopter blade are investigated. The effects of the Smart Spring actuator on the modal properties are studied through experimental activities carried out at Carleton University. Furthermore, the capability of the Smart Spring to change the dynamic behavior of the helicopter blade is demonstrated by analyzing the shifts in the modal parameters. Finally, a beam finite element model of the blade, with stiffness and mass properties tuned to the experimental structure, is presented.

Output-Only Approach for Finite Element Model Updating of AB-204 Helicopter Blade

The recent developments in the operative modal analysis made possible a new approach in the estimate of the modal parameters. In fact, they could be evaluated considering only the responses of the structure when subjected to the “natural” excitation during its operative life, output‐only analysis. Therefore, this approach could reduce the costs needed for the experimental investigations, since no input measurements are required. Moreover, it takes into consideration the actual loadings and boundary conditions acting on the structure leading to a more accurate identification of the modal parameters. The experimental model so far identified could be considered as reference for a further updating of the structural model. The developed updating procedure, is based on the minimization of an error function, representing the dierences between the numerical and experimental model, by means of design variables associated to the finite element model. The error function considered is built from the evaluation of the sensitivity of correlation functions of the Frequency Response Functions, FRFs, to design parameters. In this paper the eectiveness of output-only experimental analysis in both the estimate of the frequency response functions and, then, in the structural updating of the finite element model of an AB-204 helicopter blade is investigated. Specifically, a comparison between the updated finite element models obtained both from the FRFs, gained from the well established experimental modal analysis, and from the output only analysis will be performed.

Advanced shaker excitation signals for aerospace testing

The need to reduce testing time without diminishing the quality of the data is an important driver for innovation in the aerospace testing industry. In this paper, the use of advanced, flexible shaker excitation signals will be investigated with the aim (1) to obtain improved Frequency Response Function (FRF) estimations and (2) to assess the non-linearities of the excited system / structure. Pseudo-random and more general multisine signals, rather than the more traditional pure or burst random signals, will be used to increase the accuracy of the FRF estimate. Moreover, special multisine data acquisition and processing methods to identify the level of non-linearity will be illustrated by means of Ground Vibration Testing data of an F-16 aircraft. The presented methods allow assessing the non-linearities at a single excitation level, which is in contrast to the more traditional method of repeating the test at multiple excitation levels and observing the FRF differences. In addition, a new perspective will be given on the post-processing of stepped sine FRFs. Stepped sine shaker excitation signals are traditionally used to highlight and study non-linear behaviour. In this paper, a curve-fitting method based on FRF data at fixed response levels is applied to identify and quantify the non-linearities of the structure. Again, the approach will be illustrated by means of F-16 aircraft data.

Structural Health Monitoring Techniques for Aerospace Applications

Recently, structural health monitoring and damage detection has reemerged as an area of interest for the mechanical, automotive, and aerospace industry. Increasingly, there is a need to develop in-service and on-line health monitoring techniques. Such techniques allow systems and structures to monitor their own structural integrity while in operation and throughout their life and are useful not only to improve reliability, but also to reduce maintenance and inspection costs. Although, there has been an extensive amount of work performed in the area of structural health monitoring and damage sensing technologies, most of the existing methodologies suer from defects that include low sensitivities, complex FE models that may take long periods of time to calibrate, and modal truncations that may or may not lead to accurate predictions. In this paper, the possibility of addressing this problem with dierent techniques is proposed and applied to both numerical and experimental test cases. This work constitutes a first contribution toward the goal of integrating these techniques for an ecient and robust real-time health monitoring technique. The first approach is a novel application of an adaptive control technique for non-destructive monitoring and evaluation for structural integrity of aerospace structures, while the second method is based on a structural updating technique predicting the evaluation of the changes of the dynamic characteristics of the structures in the frequency domain. A numerical and experimental investigation of the above methodologies will be carried out taking into account dierent problems in system dynamics.

Development of an Aerodynamic and Aeroelastic Tool for Wind Turbine Design

The work focuses on the unsteady aerodynamics and aeroelastic properties of a small-medium sized wind-turbine blade operating under ideal conditions. A tapered/twisted blade representative of commercial blades used in an experiment setup at the National Renewable Energy Laboratory is considered. The aerodynamic loads are computed using Computational Fluid Dynamics (CFD) techniques. A commercial finite-volume code, FLUENT, that solves the Navier-Stokes and the Reynolds-Averaged Navier-Stokes (RANS) equations is used. Turbulence effects in the 2-D simulations are modeled using Wilcox k-ω model for validation of the CFD approach. For the 3-D aerodynamic simulations, the unsteady laminar Navier-Stokes equations were used to determine the unsteady loads acting on the blades. Five different blade pitch angles were considered and their aerodynamic performance compared. The structural dynamics of the flexible wind-turbine blade undergoing significant elastic displacements has be en described by a nonlinear flap-lag-torsion slender-beam differential model. The aerodynamic quasi-steady forcing terms needed for the aeroelastic governing equations have been predicted through a strip-theory based on a simple 2-D model, and the pertinent aerodynamic coefficients a nd the distribution over the blade span of the induced velocity derived using CFD. The resulting unsteady hub loads are achieved by a first space integration of the aeroelastic equations by applyin g the Galerkins approach - using as shape functions the bending and torsion free vibration mo des of the structure - and by a time integration using a harmonic balance scheme. Comparison among two- and three- dimensional computations for the unsteady aerodynamic load, the flap, lag and to rsional deflections, forces and moments are presented in the paper. Results, discussions and pe rtinent conclusions are outlined.

Correlation and updating of an unmanned aerial vehicle finite element model

In this paper the numerical models of the structural components of an UAV are separately correlated with their experimental counterparts and then updated, in order to improve the correlation. A sensitivity-based updating method is considered and its capabilities to identify changes of the design parameters that are physically acceptable investigated. The method iteratively minimizes a residual vector defined on the dynamic properties of the considered structure, natural frequencies and mode shapes, in order to find the unknown vector of updating parameters. The structure under investigation is the Golden Eagle UAV, designed, manufactured and operated by Clarkson University to perform a variety of scientific research flight campaigns including air-quality and wind profiling measurements. Individual components (wings, fuselage, horizontal tail, vertical tails and tail booms) have been tested, correlated and updated.

Finite Element Structural Updating using FRFs

In the field of the structural dynamics the accuracy of the finite-element model of a structure can be verified via experimental modal analysis. The discrepancies between the two models can be reduced using structural updating techniques. In this paper a sensitivitybased updating method is considered. This method iteratively minimizes a residual vector of correlation functions, defined on the Frequency Response Functions (FRFs), in order to find the unknown vector of the updating parameters. The solution generally relies on a least-square Bayesian technique that, in turn, requires the use of weighting matrices to reduce the effects of noisy data. The aim of the paper is the enhancement of a solution technique by providing a formulation for the definition of the weighting matrices, thus improving the overall numerical efficiency and accuracy. Both numerical analyses and experimental investigations on simple structures are carried out to validate the proposed approach.

Whirl-tower Open-loop Experiments and Simulations with an Adaptive Pitch Link Device for Helicopter Rotor Vibration Control

In the present work are presented both experimental and numerical simulation results obtained with an open-loop control law applied to an individual blade control system that incorporates a mechanism for blade active impedance adaptation at the root, the active pitch link or “smart spring”. It is demonstrated that the active pitch link provides parametric excitation of the blade in the rotating frame and, with it, alters the vibration spectra of the vibration loads both in the blade structure and transmitted to the nonrotating frame by the hub. The experimental results were obtained in whirl tower tests where blade periodic excitation was provided by a fan located at the base of the rotor and generated a transversal flow at a range of blade azimuth angles.