Marcin Luczak’s research while affiliated with Gdansk University of Technology and other places

The establishment of a Digital Twin of an operating engineered system can increase the potency of Structural Health Monitoring (SHM) tools, which are then bestowed with enhanced predictive capabilities. This is particularly relevant for wind energy infrastructures, where the definition of remaining useful life is a main driver for assessing the efficacy of these systems. In order to ensure a proper representation of the physical structure, the monitored response of the Digital Twin should match the one experienced by the actual system throughout the complete spectrum of its operational conditions. In most typical SHM configurations, it is only possible to rely on output-only measurements, acquired from finite positions within a structure, which naturally raises the challenge of recovering the full-field operational response, including unmeasured locations. This problem, also known as Virtual Sensing (VS), has been treated using different schemes, including Bayesian filtering and Modal Expansion (ME). In this paper, the Augmented Kalman Filter (AKF) is exploited to this end; a tool which allows for simultaneous full-field response and unmeasured input prediction. The common issue of Bayesian filtering relies on calibration of the filters defining parameters, namely the assumed measurement and process noise covariance levels. While the first is directly related to the accuracy of the employed physical sensors, the latter often acts as a tuning parameter for improving the reliability of the prediction. The process noise covariance adjustment is often performed in an offline fashion, either by making use of regularization methods, e.g., the L-curve method, or via trial and error. In this work, we propose a methodology for automated process noise covariance adaptation, relying on response estimates recovered by means of an improved ME approach. The method is validated on experimental data from a large scale research Wind Turbine (WT) blade made of glass fiber reinforced plastics.

The uncertainty afflicting modal parameter estimates stems from e.g., the finite data length, unknown, or partly measured inputs and the choice of the identification algorithm. Quantification of the related errors with the statistical Delta method is a recent tool, useful in many modern modal analysis applications e.g., damage diagnosis, reliability analysis, model calibration. In this paper, the Delta method-based uncertainty quantification methodology is validated for obtaining the uncertainty of the modal parameter and the modal indicator estimates in the context of several well-known subspace identification algorithms. The focus of this study is to validate the quality of each Delta method-based approximation with respect to the experimental Monte Carlo distributions of parameter estimates using a statistical distance measure. On top of that, the accuracy in obtaining the related confidence intervals is empirically assessed. The case study is based on data obtained from an extensive experimental campaign of a large scale wind turbine blade tested in a laboratory environment. The results confirm that the Delta method is, on average, adequate to characterize the distribution of the considered estimates solely based on the quantities obtained from one data set, validating the use of this statistical framework for uncertainty quantification in practice.

Modal testing of different test artefacts requires a multitude of decisions to be taken before the test, active experiment and analyses stages of the entire testing campaign. Modal test is often performed only once, and one of its major application is to validate the numerical model. The test-related decisions may impair the quality of the test results and in turn the following correlation and model updating procedure. The current paper provides a systematic approach for the parametric study of the test setup influence on the blade modal properties. The same wind turbine blade has been configured in four different test setups, including heavy instrumentation of the blade for the fatigue certification tests. The main objective of the research was to provide a comprehensive assessment of the impact of the test setup on the modes. Experimentally supported evidence has been found that mounting blade on the test block modifies the system structural dynamics. Not only existing global blade modes receive a shift in frequencies, but also new coupled modes may emerge. A new finding of the presented research is an experimental based characterisation of the impact of heavy excitation equipment for fatigue test. Adding exciters of a mass nearly equal of the blade mass has little influence on the identified natural frequencies of the first two modes, while lower all frequencies from the third mode onwards. The structural damping ratios are also affected, and generally increased. A vibration engineering standard free-free boundary conditions with the impact hammer excitation serves as a baseline model, to which the other test setup are compared. The estimated modal models of the intact blade will serve as a reference for the modal based damage detection methods application.

In this work, wind turbine blade numerical models have been developed with two different finite element software-DTU Wind Energy HAWCStab2 with Timoshenko beam elements and MSC NASTRAN with solid elements. A correlation analysis has been performed comparing both of these models with experimentally measured data in order to validate the models. For the experimental part, we have performed the experimental and operational modal analysis of a 14.3 m long composite blade clamped at the root. The natural frequencies have been extracted from the measured acceleration responses with three different methods-PolyMAX, MOESP and CWT and compared with the ones computed by the two software, while the mode shapes were correlated with MAC and COMAC metrics. Although the frequency values are in good agreement, the ordering of the 5 th flapwise mode shape has been swapped with the 1 st simulated torsional mode in the solid model, while this was not the case with mode shape ordering in the beam model.

The aim of this paper is to explore alternative ways to determine the vibration characteristics of wind turbine blades to reduce the amount and duration of needed tests without compromising the quality of the retrieved information. Applied test setup configuration, test specimen mounting and measurement equipment are known to affect the test results. The investigation concerns the use of Operational Modal Analysis (OMA) method together with the existing instrumentation for fatigue and static testing. The wind turbine blade is clamped to the support structure and instrumented by means of many strain gauges. A so-called pull and release test can be performed,and the modal parameters can be obtained by analyzing the measured strains during this type of test. Numerical correlation can then be performed with respect to the validated FE model of the clamped blade allowing to enrich the acquired information.

The paper presents a comparison of two vibration-based methods for the damage detection of a laboratory scale model of a tripod. Tripods are a part of the supporting structures for offshore wind turbines. The tripod model structure allows the investigation of the propagation of a circumferential representative crack in one of the cylindrical upper braces of the tripod itself. The first damage detection method addresses the use of acceleration signals in a genuine experimental modal analysis (i.e. input-output modal analysis) while the second one is based on operational modal analysis (i.e. output only modal analysis). The progressive damage is monitored by the calculation of the modal parameters and following their deviations. Both methods were performed on the undamaged and damaged structure for different support conditions and excitations (shaker, hammer, in water basin under wave excitation). The results suggest that both the methods can be considered useful tools for damage detection in dry and in-water conditions for offshore support structures. The presented technique proves to be effective for detecting and assessing the presence of representative cracks.

Modern wind turbine blades are being tested for certification purposes in accordance to the IEC-64100 standard. Part 23 of the norm details the requirements for the full scale structural testing of rotor blades. As a minimum, it requires measurement of the first and second flap wise and first edge wise natural frequencies. It lists damping and mode shapes as other blade properties which may be of interest and optionally measured. The paper presents the modal model parameters estimation based on the experimental modal analysis. In two tests performed, the input force has been introduced through impact hammer and two electrodynamic shakers excitation. Several first modes had been identified for both excitation methods, including first torsional mode of the investigated blade. Results of the modal tests can be used to (a) provide more detailed information about the structural dynamics characteristics of the blade and (b) improve the design by adjusting the dynamic properties of the blade to some desired condition.

DTU Wind Energy continues the experimental investigation of the wind turbine blades to assess innovative designs of long and slender blades. This paper presents an experimental structural dynamics identification and structural model validation of the 14.3m long research blade. Unique feature of the blades is that its internal layup design has been highly optimized w.r.t. stretching the rotor and substantial mass reduction at the same time. As the result, the blade is more flexible than the traditional one. The results of the modal tests following analyses were performed: (i) Uncertainty Quantification of the experimental modal parameters for the blades, (ii) non-linearity assessment, (iii) numerical model correlation – frequencies and mode shapes of the experimental model comparison with those from Finite Element (FE). Finally, the outlook for the future experimental blade research activity is outlined.