S. Manzato

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.

Damage detection in wind turbine blades by using operational modal analysis

In this paper a vibration-based approach to identify a crack in a wind turbine blade is described and demonstrated numerically and experimentally. Operational modal analysis has been performed before and after a buckling test, and vibration data, gathered from some accelerometers placed along the blade, was used to monitor the integrity of the structure, since the modal parameters are directly influenced by the physical properties of the structures. Additionally a numerical prediction has been done both with a full-scale model and with a one-dimensional model. The results show that this approach is able to estimate successfully the presence of damage and a good numerical and experimental correlation has been found. Finally, some considerations regarding the rotation of the blade in the undamaged and damaged cases have been done.

A Multiphysical Modelling Approach for Virtual Shaker Testing Correlated with Experimental Test Results

A virtual shaker testing simulation environment aims to predict the outcome of a spacecraft vibration test numerically, prior to its physical execution at the environmental test facility. Therefore, it needs to comprise the complex dynamical characteristic of the test setup and facility in order to calculate and predict the interaction between the electrodynamic shaker system, test specimen, and vibration controller as it occurs during tests of large spacecraft. A currently derived one-dimensional multiphysical shaker model with three degrees-of-freedom, e.g. the 160 kN electrodynamic shaker of the European Space Agency (ESA), is based on a tailored experimental system identification methodology to estimate the system’s parameter. The model is validated by subsequent simulations to recalculate and predict the test results. The main focus of the paper is the enhancement of the shaker model to encompass additionally, lateral and rotational dynamics of the shaker table as well as the coupling with test specimen dynamics and control system performance. The improvements are based on analytical modelling steps in conjunction with the exploitation of experimental test results of hammer impact excitations, and closed-loop random and sine control testing performed on the shaker, loaded with a dummy specimen and excited with different excitation levels.

Order Based Modal Analysis Versus Standard Techniques to Extract Modal Parameters of Operational Wind Turbine Gearboxes

Rotating machineries generally operate under very dynamic and complex conditions, during which structural nonlinearities, tonalities or other dynamic related phenomena may arise, affecting the assumptions made in the design phase. Techniques that allow accurately and confidently identifying the dynamic response are then of paramount importance in such complex scenarios. Wind turbine gearboxes are a typical example of such machines, as they operate under strong transient conditions caused by the turbulent and non-stationary wind speed as well as fluctuations in the electrical grid. If models can help predict these interactions, dedicated experiments need to be foreseen to characterize operational response and validate/improve the developed numerical models.

A Mission Synthesis Procedure for Sine-on-Random Excitations in a Helicopter Application

This paper considers a so-called “Mission Synthesis” procedure where a qualification test specification is derived for an electronic component (i.e., a VHF radio) mounted on a console inside a helicopter cockpit. The environmental vibrations, which are measured during various flight maneuvers, consist of deterministic sine tones superimposed on Gaussian random excitations, i.e. so-called “Sine-on-Random” (SoR) vibration. The Fatigue Damage Spectrum (FDS) model is utilized for analyzing the fatigue damage potential of these SoR excitations. A particular objective in this application example is to assess the induced fatigue damage for two design alternatives of the radio console, which is achieved by means of an FDS comparison. Furthermore, it is demonstrated that experimental modal analysis can provide valuable insights on the physical root causes for large induced fatigue damage, such that appropriate design modifications can be prescribed. Finally, the Mission Synthesis procedure enables the derivation of an accelerated but damage-equivalent shaker qualification test. For the component considered in this example it is demonstrated that the derived test specification is more severe than a MIL-STD standardized SoR test specification.

Best Practices for Using Order-Based Modal Analysis for Industrial Applications

The Order-Based Modal Analysis (OBMA) technique shows to be very powerful for identifying the modal parameters in operational conditions in case of rotating machineries during transient operations. The main idea behind the method is that instead of estimating the spectra and apply Operational Modal Analysis (OMA) by using them, the so-called orders can be extracted and used as input for the OMA technique. It can be assumed that the measured responses are mainly caused by the rotational excitation. In this case, run-up and coast-down events can be assimilated to multi-sine sweep excitation in the frequency band of interest. Several studies have been performed to identify the best practice for OBMA both in terms of Order Tracking (OT) techniques and OMA techniques. Based on the boundary conditions, on the structure under-test and on the effective operational conditions a technique can be more powerful than another one. Basically there are two fundamental steps: a very good measurement of the tachometer signal and the correct extraction of the orders, both in amplitude and phase. For the first step several alternatives are possible. The sensors to be used for measuring the rotational speed are depending from both the application and the objective of the study. For this reason, several sensors can be found in the market with a huge variety of costs and performances. The best sensor can then be selected for each individual application based on the type of analysis, the accessibility of the shaft, the ease of instrumentation and the required accuracy or level of detail. For the second step, several techniques are available in commercial software and some others have been implemented in a research environment. Each of them has its own advantages and drawbacks. The final aim of the work is to provide guidelines for the correct use of the OBMA technique in an industrial context. Several cases will be shown: a locomotive cabin and a car during engine run-ups and a 3.2 MW wind turbine gearbox during controlled run-ups on a test rig.

Analyses of Target Definition Processes for MIMO Random Vibration Control Tests

In Random Vibration environmental testing, it is a common practice to specify the requirements as acceleration power spectral densities (PSDs) that need to be reproduced at user-defined control channels. Such a test is typically performed in a single-axis setting, where the test article is subjected to vibrations in one direction only. If more than one direction is of interest sequential single-axis tests are performed after rotating the test article or using a slip table configuration. This way to perform multi-axial Random Vibration tests is out of date: there is definitely some lack of realism in sequential single-axis testing, as the stress loading and boundary conditions will significantly differ from the true three-dimensional environment. For very heavy structures, often the excitation level safely reachable by a single shaker is not even sufficient, the limitation being the risk of damaging the sometimes very expensive and fragile test articles due to high concentrated stresses. All these limitations are overcome if a Multiple-Input-Multiple-Output (MIMO) Random Vibration Control test is performed. Even though the benefits of MIMO tests are clear and accepted by the environmental engineering community, their practice still needs to grow. This is mainly due to the high degree of expertise needed to perform these tests. The challenges of MIMO Random Control start even before the actual test, in the test definition phase. The target that needs to be reached during the test is a full Spectral Density Matrix where the cross terms are as important as the diagonal ones. Defining this matrix with no a-priori knowledge of the cross-correlation between control channels is very challenging: filling in the off-diagonal terms, in fact, must guarantee that the target has a physical meaning. This is translated in the algebraic constraint that the target matrix needs to be positive (semi)-definite. On the other end the pushing driver of any Random Vibration Control test is to be able to replicate specific PSDs, given, for instance, by qualification specifications or optimal profiles (in terms of fatigue damage or comfort requirements). In defining the target matrix the main challenge is to guarantee a physically realizable full target spectral density matrix that has fixed PSD terms. Several authors tackled the problem of defining the best target possible (in terms of minimum drives energy, in terms of control performances,) even though few works can be addressed that tackle the problem of defining a realizable target first. This leads to a gap in the standards about a generally accepted and robust procedure to define the MIMO Random target matrix. The purpose of this work is to investigate different target generation procedures pointing out the advantages and the challenges in terms of physical meaning and their impact on the random control strategy. Alternative solutions based on on-going research topics will also be considered to propose alternative robust target definition routines in order to aim to a well-defined automatic procedure to include in the standard practice.

CR-Calculus and adaptive array theory applied to MIMO random vibration control tests

Performing Multiple-Input Multiple-Output (MIMO) tests to reproduce the vibration environment in a user-defined number of control points of a unit under test is necessary in applications where a realistic environment replication has to be achieved. MIMO tests require vibration control strategies to calculate the required drive signal vector that gives an acceptable replication of the target. This target is a (complex) vector with magnitude and phase information at the control points for MIMO Sine Control tests while in MIMO Random Control tests, in the most general case, the target is a complete spectral density matrix. The idea behind this work is to tailor a MIMO random vibration control approach that can be generalized to other MIMO tests, e.g. MIMO Sine and MIMO Time Waveform Replication. In this work the approach is to use gradient-based procedures over the complex space, applying the so called CR-Calculus and the adaptive array theory. With this approach it is possible to better control the process performances allowing the step-by-step Jacobian Matrix update. The theoretical bases behind the work are followed by an application of the developed method to a two-exciter two-axis system and by performance comparisons with standard methods.

Structural Health Monitoring challenges on the 10-MW offshore wind turbine model

The real-time structural damage detection on large slender structures has one of its main application on offshore Horizontal Axis Wind Turbines (HAWT). The renewable energy market is continuously pushing the wind turbine sizes and performances. This is the reason why nowadays offshore wind turbines concepts are going toward a 10 MW reference wind turbine model. The aim of the work is to perform operational analyses on the 10-MW reference wind turbine finite element model using an aeroelastic code in order to obtain long-time-low- cost simulations. The aeroelastic code allows simulating the damages in several ways: by reducing the edgewise/flapwise blades stiffness, by adding lumped masses or considering a progressive mass addiction (i.e. ice on the blades). The damage detection is then performed by means of Operational Modal Analysis (OMA) techniques. Virtual accelerometers are placed in order to simulate real measurements and to estimate the modal parameters. The feasibility of a robust damage detection on the model has been performed on the HAWT model in parked conditions. The situation is much more complicated in case of operating wind turbines because the time periodicity of the structure need to be taken into account. Several algorithms have been implemented and tested in the simulation environment. They are needed in order to carry on a damage detection simulation campaign and develop a feasible real-time damage detection method. In addition to these algorithms, harmonic removal tools are needed in order to dispose of the harmonics due to the rotation.

Wind Turbine Gearbox Dynamic Characterization Using Operational Modal Analysis

The aim of this paper is to characterize the dynamic behavior of a wind turbine gearbox installed on a dynamic test rig to replicate operational conditions. Wind turbines and gearboxes operate under very dynamic and complex conditions, caused by turbulent wind, fluctuations in the electricity grid etc. In those conditions, structural nonlinearities in bearings and gears cause natural frequencies to be significantly influenced by the operational conditions. To verify the dynamic response of a multi-megawatt gearbox, a comprehensive test campaign has been performed in the context of the European project ALARM at the ZF Wind Power test rig. Accelerations have been measured at more than 250 locations on the test rig and for different load levels and operating conditions. This paper focuses on the influence of the torque levels on the identified modal parameters. The acquired time histories during run-ups have been processed using different Operational Modal Analysis techniques. The aim is to provide a modal model that can be used for correlation and updating of a flexible nonlinear multibody model of the whole test rig as well as vibration levels to estimate structure-borne noise in the different operating conditions.