Marcin Luczak

Full-scale modal wind turbine tests: comparing shaker excitation with wind excitation

The test facilities at the National Wind Technology Center (NWTC) of the National Renewable Energy Laboratory (NREL) include a three-bladed Controls Advanced Research Turbine (CART3). The CART3 is used to test new control schemes and equipment for reducing loads on wind turbine components. As wind turbines become lighter and more flexible to reduce costs, novel control mechanisms are necessary to stop high winds from damaging the turbine. However, wind turbines must also be designed to capture the maximum amount of energy from the wind, so engineers must devise new ways of achieving this while controlling wind loads that would cause the turbines to fatigue quickly. New control mechanisms and computer codes can help the wind turbine shed some loads in extreme or very turbulent winds. The special configuration of the CART3 allows researchers to analyze these diverse control schemes. This paper reports on the initial results of a major full-scale modal testing campaign to validate and refine simulation models. One model is a tailored multi-body dynamic simulation model that will be used to develop an advanced controller designed to optimize power and minimize structural loads. Researchers would also like to tune Finite Element Models of the blades, nacelle and tower assembly to predict the higher order rotating modes of the wind turbine for a range of inflow conditions. The paper will discuss an Experimental Modal Analysis approach where the wind turbine in parked condition is excited by shakers connected with cables. This approach will be compared to Operational Modal Analysis where the same structure is subjected to wind excitation without the shakers activated. These tests and data analyses will provide experience and increase confidence in the approach used for future tests in rotating conditions.

Updating Finite Element Model of a Wind Turbine Blade Section Using Experimental Modal Analysis Results

This paper presents selected results and aspects of the multidisciplinary and interdisciplinary research oriented for the experimental and numerical study of the structural dynamics of a bend-twist coupled full scale section of a wind turbine blade structure. The main goal of the conducted research is to validate finite element model of the modified wind turbine blade section mounted in the flexible support structure accordingly to the experimental results. Bend-twist coupling was implemented by adding angled unidirectional layers on the suction and pressure side of the blade. Dynamic test and simulations were performed on a section of a full scale wind turbine blade provided by Vestas Wind Systems A/S. The numerical results are compared to the experimental measurements and the discrepancies are assessed by natural frequency difference and modal assurance criterion. Based on sensitivity analysis, set of model parameters was selected for the model updating process. Design of experiment and response surface method was implemented to find values of model parameters yielding results closest to the experimental. The updated finite element model is producing results more consistent with the measurement outcomes.

Uncertain Parameter Numerical Model Updating According to Variable Modal Test Data in Application of Large Composite Fuselage Panel

This paper presents a novel approach in the field of experimental and numerical investigation of mechanical properties of composite structures. It takes into account test data variability resulting from structural dynamic properties measurement and uses them to quantify uncertainties in model parameters updating. The main goal of the conducted research is to investigate the dynamic properties of fibre reinforced composite structures. Non-destructive experimental and numerical simulation methods are used hereto. In the experimental part, different test configurations were taken into account. The excitation was performed by means of random and harmonic, single and multi point stimuli while the response measurement was done through contact and non-contact acceleration, velocity and dynamic strain sensing. The test results are applied in two ways: for the structural identification of the object and for non-deterministic updating of the numerical model according to a range of experimental models obtained from test. The sources of the test data variabilities were related to the excitation and measurement technique applied for the investigated object. Non - deterministic model updating and verification & validation included uncertainties of its parameters by means of interval and stochastic methods. A number of variable test modal models were statistically assessed to investigate impact of variability source onto modal model parameters. The presented research was conducted in the context of the FP6 Marie Curie project UNVICO-2.

Damage Detection in Wind Turbine Blade Panels Using Three Different SHM Techniques.

A comparison of three different damage detection methods is made on three nominally identical glass reinforced composite panels, similar to the load carrying laminate in a wind turbine blade. Sensor data were recorded in the healthy state and after the introduction of damage by means of a four–point bending quasi–static test. Acceleration sensors, PZT transducers and the piezoelectric excited Lamb waves were used for the measurements of the panels. All three methods are based on the comparison of the healthy and damaged structure. The first method is statistical covariance–driven damage detection using a subspace–based algorithm, where one damage indicator for all three panels was computed. The second method is based on PZT transducers and the A0 mode of Lamb waves propagating in the panel, making use of the reflection of the signal at damage in the panel. The third method is based on the estimation of modal parameters of the intact and damaged panel using pLSCF and following their deviations. The results from these three damage detection methods are compared and discussed.

In-Service Measurement of the Small Wind Turbine Test Stand for Structural Health Monitoring

This paper presents the research activity performed on a Small Wind Turbine (SWT) test stand. Commercially available turbine was modi fie towards incorporation of the sensors system for condition monitoring. Installed sensors measure angular shaft position, torque applied from the wind loads, vibration accelerations and la st but not least rotational speed. All gathered dat a are then transferred and processed in Test.Lab by m eans of automatic in house developed Visual Basic application which afterwards converts TDF fil es to text files and stores them in a desired directory. The numerical simulation is being run in parallel to installed sensors measurements and is as well controlled by the same Visual Basic applica tion. By having actual measurements and numerical simulation results one will be able to co mpare those two outcomes. In particular, if the numerical model is tailored to the physical one, da ta comparison will allow identifying malfunctioning due to component damage or extreme w orking condition.

Contact versus noncontact measurement of a large composite fuselage panel

This paper presents a comprehensive study between accelerometer, laser vibrometer and microflown probe measurements aimed at comparison of modal model quality assessment. Object of an investigation was a large composite fuselage panel. An extensive test campaign was performed with application of SIMO, MIMO, random and harmonic excitation, velocity and acceleration sensors, contact and non-contact measurement techniques. Advantages and disadvantages of applied instrumentation are discussed taking into account test data variability and the trade-off between workload and test data quality. Presented are real-life measurement problems related to the specific set up conditions. Finally a statistical analysis of estimated models is evaluated to bring to light general assessment of test campaign. Such assessment has a vital importance of successful fault detection based on modal parameters observation as well as in uncertain non-deterministic numerical model updating.

Uncertainty Quantification of the Main Rotor Blades Measurements

This chapter provides a novel approach in the field of uncertainty assessment of the experimental investigation of mechanical properties of load carrying structures made of composite material. A combination of experimental methods are used hereto. In the experimental part, different test configurations were taken into account. The excitation was performed by means of random and harmonic, single and multi-point stimuli while the response measurement was done through contact and non-contact methods. The test results are applied for the structural identification of the object according to a range of experimental models obtained from test. The sources of the test data variabilities were related to the excitation and measurement technique applied for the investigated object. It takes into account test data variability resulting from structural dynamic properties measurement. As a result of presented research set of experimental models was developed with an introduction of quality function assessing the influence of particular factors. A set of variable test data was assessed with a criterion of its application in industrial cases.

Numerical Model Quality Assessment of Offshore Wind Turbine Supporting Structure Based on Experimental Data

As a structure degrades some changes in its dynamical behavior can be observed, and inversely, observation and evaluation of these dynamical changes of the structure can provide information of structural state of the object. Testing of the real structure, besides of being costly, can cover only limited working states. It is particularly considerable in case of hardly accessible, and randomly/severely dynamically loaded offshore structures. As a testing instrument, numerical simulations are not limited as much, however a quality of answers depends significantly on an excellence of numerical model, and how in reality it reassembles the actual structure and the test results. One of the strategies is to correlate and update numerical models basing on the experimental data. Presented outcomes were obtained in frame of “AQUILO” project that aims to create a knowledge base, from which the investor will be able to decide on the best type of support structure for offshore wind farm specific location in Polish maritime areas. The examined object is laboratory tripod type support model (scaled) of the offshore wind turbine supporting structure, with appended flange on the one of the branches, allowing simulation of a fatigue cracking process. For the assessment and comparison with numerical model calculation results of the dynamical state of the structure the Experimental Modal Analysis (EMA) approach was selected. After several measuring campaigns a database of results including varying type of supporting condition, and crack opening stages, was obtained. The numerical model was constructed with use of Finite Element Method (FEM) approach. The quality of FE model was assessed using Modal Assurance Criterion (MAC) that compares both models modal vectors, that is modal deformation shapes. Also the differences in frequencies of modes was assessed and taken as quantification of an compatibility. The results (EMA) show importance of applying and modelling (FEM) of supporting condition. An additional senility analysis directs indicated best fit parameters for a start of FE optimization process. doi: 10.12783/SHM2015/349

Uncertainty Quantification in Experimental Structural Dynamics Identification of Composite Material Structures

Aerospace and wind energy structures are extensively using components made of composite materials. Since these structures are subjected to dynamic environments with time-varying loading conditions, it is important to model their dynamic behavior and validate these models by means of vibration experiments. Modal testing results depend on numerous factors introducing uncertainty to the measurement results. Different experimental techniques applied to the same test item or testing numerous nominally identical specimens yields different test results. This paper presents a systematic approach for uncertainty evaluation in experimentally estimated models. Investigated structures are plates, fuselage panels and helicopter main rotor blades as they represent different complexity levels ranging from coupon, through sub-component up to fully assembled structures made of composite materials. To evaluate the variability of the identified parameters, a statistical method is implemented for processing an extensive collection of experimental data.

Bush Skewness Defect in Journal Bearing as an Example of Knowledge Base Development

In the paper presented are course and results of investigations of the influence of slide bearing bush and shaft mutual skewness on the dynamic properties of the 200 MW 13K215 turboset. The defect was simulated by means of numerical model of the turboset. Simulation was performed for each bearing individually in both, horizontal and vertical planes. Maximum values of the skewness angle were calculated for each bearing at both planes. The influence of the defect on dynamic properties of turboset was presented. Basing on the results of the simulation multiple defect-symptom diagnostic relations were built. Diagnostic relations were used as learning and testing data sets for a knowledge base of an expert system based on the inverse diagnostic model. This expert system is implemented at the unit 7 at the Kozienice Power Station as a first domestic diagnostic system.