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Rotating Machinery, Vibro-Acoustics & Laser Vibrometry, Volume 7: Proceedings of the 36th IMAC, A Conference and Exposition on Structural Dynamics, 2018, the seveth volume of nine from the Conference brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Rotating Machinery, Hybrid Testing, Vibro-Acoustics & Laser Vibrometry, including papers on:

Rotating Machinery


Experimental Techniques

Scanning Laser Doppler Vibrometry Methods



Chapter 1. Summarizing Results for Scaling OMA Mode Shapes by the OMAH Technique

Methods for scaling mode shapes determined by operational modal analysis (OMA) have been extensively investigated in the last years. A recent addition to the range of methods for scaling OMA mode shapes is the so-called OMAH technique, which is based on exciting the structure by harmonic forces applied by an actuator. By applying harmonic forces in at least one degree-of-freedom (DOF), and measuring the response in at least one response DOF, while using at least as many frequencies as the number of mode shapes to be scaled, the mode shape scaling (modal mass) of all modes of interest may be determined. In previous publications on the method the authors have proven that the technique is easy and robust to apply to both small scale and large scale structures. Also, it has been shown that the technique is capable of scaling highly coupled modes by using an extended multiple reference formulation. The present paper summarizes the theory of the OMAH method and gives recommendations of how to implement the method for best results. It is pointed out, as has been shown in previous papers, that the accuracy of the mode scaling is increased by using more than one response DOF, and by selecting DOFs with high mode shape coefficients. To determine the harmonic force and responses, it is recommended to use the three-parameter sine fit method. It is shown that by using this method, the measurement time can be kept short by using high sampling frequency and bandpass filtering whereas spectrum based methods require long measurement times. This means that even for structures with low natural frequencies, the extra measurement time for scaling the mode shapes can be kept relatively short.

Anders Brandt, Marta Berardengo, Stefano Manzoni, Marcello Vanali, Alfredo Cigada

Chapter 2. Delamination Identification of Laminated Composite Plates Using a Continuously Scanning Laser Doppler Vibrometer System

Delamination frequently occurs in a laminated composite structure and can cause prominent local anomalies in curvature vibration shapes associated with vibration shapes of the composite structure. Spatially dense vibration shapes of a structure can be rapidly obtained by use of a continuously scanning laser Doppler vibrometer (CSLDV) system, which sweeps its laser spot over a vibrating surface of the structure. This paper extends two damage identification methods for beams to identify delamination in laminated composite plates using a CSLDV system. One method is based on the technique that a curvature vibration shape from a polynomial that fits a vibration shape of a damaged beam can well approximate an associated curvature vibration shape of an undamaged beam and local anomalies caused by structural damage can be identified by comparing the two curvature vibration shapes, and the other is based on the technique that a continuous wavelet transform can directly identify local anomalies in a curvature vibration shape caused by structural damage. In an experimental investigation, delamination identification results from the two methods were compared with that from a C-scan image of a composite plate with delamination.

Da-Ming Chen, Y. F. Xu, W. D. Zhu

Chapter 3. Rapid and Dense 3D Vibration Measurement by Three Continuously Scanning Laser Doppler Vibrometers

This paper presents an investigation of rapid and dense three-dimensional (3D) vibration measurement by three continuously scanning laser Doppler vibrometers (CSLDVs) in a synchronously scanning mode, which means that three laser spots from three CSLDVs continuously move along the same scan trajectory and synchronously measure vibration of the same point on a surface. Three CSLDVs, i.e., Top, Left, and Right CSLDVs, are from a Polytec PSV-500-3D system and input signals to scan mirrors of each CSLDV are controlled by a dSPACE MicroLabBox control unit. The whole system is called a 3D-CSLDV system. Rotation angles of scan mirrors of Left and Right CSLDVs can be obtained from those of the Top CSLDV based on positional relations among three CSLDVs. With the proposed methodology, three laser spots from three CSLDVs can continuously move in a synchronously scanning mode. An experiment is conducted to obtain 3D operational deflecting shapes (ODSs) of a composite plate in a measurement coordinate system (MCS) by a 3D-CSLDV system. Results of 3D ODSs in the MCS are in good agreement with those from a PSV-500-3D system in a step scanning mode.

Da-Ming Chen, W. D. Zhu

Chapter 4. Modal Control of Magnetic Suspended Rotors

The present work is devoted to the conception of a Systematic Approach for the Robust Design of an Active Modal Controller applied to Supercritical Rotors supported by Magnetic Bearings. For this purpose, all the steps involved in the design of a robust controller based on the multimodel eigenstructure assignment technique is discussed. The first step is the development of a numeric/computer model in MATLAB/SIMULINK environment, based on the specifications of the test rig used in the experimental part of this work (which are provided by the test rig manufacturer). The first stage of the controller synthesis has to do with the specification of all design requirements. Then, all the phases involved in the process of the multimodel eigenstructure assignment are addressed in detail, starting from the determination of the plant dominant poles, so that, finally, the model based analysis of the system stability and performance are studied. The performance of the controller was evaluated through the analysis of its closed loop Transfer Functions and by investigating the unbalance response of the rotating system. It is worth mentioning that for all presented investigations, comparisons between two architectures of the controller, namely, the modal controller and the PI controller were performed, accordingly. The following natural step is the analysis of the correlation of theoretical and experimental results for validation purposes, which will be presented in a further contribution.

Marcus Vinicius Fernandes de Oliveira, Felipe Carmo Carvalho, Adriano Borges Silva, Aldemir Ap Cavalini, Valder Steffen

Chapter 5. On the Implementation of Metastructures in Rotordynamics

This paper presents the possibility to apply distributed vibration absorbers to reduce the vibration amplitude in axially symmetric components. The kind of metastructures here presented does not require complicated and expensive production processes such as additive manufacturing, but can be manufactured with more conventional means, for example with wire electrical discharge machining (EDM). Flexible rotating structures have a lot of natural frequencies in the operative range and the geometry changing approach is often not sufficient to improve the dynamic behavior. Using structural damping becomes fundamental to increase the transmitted power and the system life. In the present paper, the authors investigate the possibility to apply different geometrical configurations, each designed and tuned to suppress a particular mode shape, by means of the metastructure concept applied to flexible rotating devices. Their performances are analyzed with and without a balanced mistuning added to the metastructure. A number of distributed absorbers is set in order to act on each mode to be damped. Their vibration amplitude is firstly compared by keeping the total mass constant with respect to the original component, but also cases with higher and lower mass are analyzed. The frequency response of the proposed configurations is obtained from three-dimensional (3D) finite element models and some interesting results are evinced.

Carlo Rosso, Elvio Bonisoli, Fabio Bruzzone

Chapter 6. Analysis of the Dynamic Response of Coupled Coaxial Rotors

The fundamental dynamics of a single rotor are very well understood, and extensively covered in the literature. However, many rotating machines such as aircraft engines consist of multiple shafts, which are often directly coupled by inter-shaft bearings. This paper aims to provide better insight into the underlying dynamics of such systems by analysing a simple but representative model of a rigid dual-rotor system. The modes and natural frequencies were computed numerically and it was found that the different modes could be classified by the following criteria: (i) relative phase of the motion of each rotor, (ii) whirl direction of the rotors, and (iii) presence of rotational or translational motion. The high-speed mode shapes could also be classified into (i) “static” modes with very low frequencies, (ii) “flat” modes which tend towards constant frequencies, and (iii) “precessional” modes which have a frequency which linearly increases with speed. A parameter study was performed in order to obtain a better understanding of the sensitivity of the modal properties. It was found that increasing the inter-shaft bearing stiffness can raise the natural frequencies of the modes at low speeds as well as the critical speeds, but has less influence at high speeds. The speed ratio influences the whirl direction of the modes and hence plays a crucial role in determining how each mode varies with speed. Since the speed ratio also controls the frequency of excitation from unbalances, it has a particularly profound effect on the critical speeds, and extra ones can arise. The importance of considering the dynamics of the complete system in the design of turbomachinery with multiple-shafts was highlighted.

Alexander H. Haslam, Christoph W. Schwingshackl, Andrew I. J. Rix

Chapter 7. Operational Modal Analysis of Rotating Machinery

Harmonic excitation of structures caused by rotating equipment is a problem faced by many engineers in the field of Operational Modal Analysis (OMA). Several methods to discard the influence of harmonic inputs over systems natural responses has been proposed in the literature and implemented in various software solutions. This paper recalls some of the most used techniques and uses a new time domain method for removing harmonics from measurements. Deployed method does not rely on filtering, statistical detection nor on non-linear fitting. Instead, it predicts the harmonic part of the time series and deploys an orthogonal projection of the latter onto the raw measurements to remove the harmonic part of the signal. The new technique is a part of an semi-automated framework for OMA of structures contaminated with harmonics, whose flow is presented in this paper. The merit of the framework is discussed in the context of OMA of a full scale operating ship with rotating machinery on-board.

S. Gres, P. Andersen, L. Damkilde

Chapter 8. Characterization of Torsional Vibrations: Torsional-Order Based Modal Analysis

Torsional vibrations are angular vibrations of a rotating machine, typically a shaft along its axis of rotation. When these vibrations are amplified, they could lead to comfort, efficiency, noise or coupling wear problems. Torsional vibrations are important in cases in which the power needs to be transmitted using a rotating shaft or couplings, such as in the case of automotive, truck and bus drivelines, marine drivelines or power-generation turbines. Their characterization both in terms of frequency and damping ratios is quite difficult since they are influenced by several parameters such as material properties and operating conditions (temperature, load, rpm, etc). New powertrain designs, such as start-stop systems, downsized engines and lighter powertrains, increase the importance of developing an in-depth understanding of torsional vibrations. These are the main reasons behind the development of the so-called Torsional-Order Based Modal Analysis (T-OBMA) technique. As its name suggests, this technique is base upon the Order-Based Modal Analysis (OBMA) technique that showed to be very powerful for identifying the modal parameters in operational conditions in case of rotating machines during transient operations. The classical OBMA is performed by measuring both the rotational speed (by means of zebra tapes or digital encoders) and some other quantities in several points on the structure itself (accelerations, deformations, etc.). On the other hand, T-OBMA is focused on the identification of the torsional modal parameters by measuring only the rotational speed in two or more points along a driveline. The technique has been validated both in a simulation environment and in a real test scenario. In this paper the main outcomes of the simulations will be explained and the guidelines to apply it in a rotating machinery context for torsional vibrations identification will be listed.

Emilio Di Lorenzo, C. Colantoni, F. Bianciardi, S. Manzato, K. Janssens, B. Peeters

Chapter 9. Long-Term Automatic Tracking of the Modal Parameters of an Offshore Wind Turbine Drivetrain System in Standstill Condition

Modal behavior of a wind turbine is an important design aspect for tackling noise, vibration, and harshness (NVH) issues and validating complex simulation models. This paper focusses long-term modal analysis on an offshore wind turbine (OWT) in stand still conditions. It presents the results of an automated procedure to track the variation of the modal parameters of the drivetrain system of the OWT. The tracking focuses on the continuous monitoring of the resonant frequencies and damping values of the most dominant modes of the drivetrain unit during more than half a day of stand still. The long-term tracking of the natural frequencies and modal damping of the drivetrain vibration modes under distinct ambient conditions allows to better understand the dynamics of the drivetrain by gaining confidence in modal parameters estimated over multiple measurement blocks and helps in gaining understanding in the dynamics of the OWT. The used automatic tracking procedure is based on the well-known parametric operational modal analysis algorithm, pLSCF estimator. The experimental data used in this paper has been obtained during a long-term measurement campaign lasting 6 months on an offshore wind turbine with instrumentation directly mounted on the drivetrain. Both eigenfrequencies and damping values are of particular interest.

Mahmoud El-Kafafy, Nicoletta Gioia, Patrick Guillaume, Jan Helsen

Chapter 10. Dynamic Modelling and Vibration Control of a Turbomolecular Pump with Magnetic Bearings in the Presence of Blade Flexibility

This article presents the development of a tridimensional phenomenological model of a turbomolecular pump taking into account the blades dynamics as well as the gyroscopic effects, centrifugal stiffening and spin softening. Rotor and blades behavior is approximated by Euler-Bernoulli beams. Rayleigh-Ritz method is used to approximate continuous displacements and an energetic approach is used to obtain the equations of motion. The global dynamics of the system is analyzed by assuming linear bearings, whose stiffness is experimentally defined. Mistuning of the blades is experimentally identified and simulated so that its influence on blades modes organization could be studied. The obtained model has shown good correlation with experimental results.

Alysson B. Barbosa Moreira, Fabrice Thouverez

Chapter 11. Pushing 3D Scanning Laser Doppler Vibrometry to Capture Time Varying Dynamic Characteristics

3D scanning laser Doppler vibrometry (LDV) systems are well known for modal testing of articles whose excited dynamic properties are time-invariant over the duration of all scans. However, several potential test situations can arise in which the modal parameters of a given article will change over the course of a typical LDV scan. One such instance is considered in this work, in which the internal state of a thermal battery changes at different rates over its activation lifetime. These changes substantially alter its dynamic properties as a function of time. Due to the extreme external temperatures of the battery, non-contact LDV was the preferred method of response measurement. However, scanning such an object is not optimal due to the non-simultaneous nature of the scanning LDV when capturing a full set of data. Nonetheless, by carefully considering the test configuration, hardware and software setup, as well as data acquisition and processing methods it was possible to utilize a scanning LDV system to collect sufficient information to provide a measure of the time varying dynamic characteristics of the test article. This work will demonstrate the techniques used, the acquired results and discuss the technical issues encountered.

Bryan Witt, Brandon Zwink

Chapter 12. Dynamic Measurements on Miniature Springs for Flaw and Damage Detection

Small components are becoming increasingly prevalent in today’s society. Springs are a commonly found piece-part in many mechanisms, and as these components become smaller, so do the springs inside of them. Because of their size, small manufacturing defects or other damage to the spring may become significant: a tiny gouge might end up being a significant portion of the cross-sectional area of the wire. However, their small size also makes it difficult to detect such flaws and defects in an efficient manner. This work aims to investigate the effectiveness of using dynamic measurements to detect damage to a miniature spring. Due to their small size, traditional instrumentation cannot be used to take measurements on the spring. Instead, the non-contact Laser Doppler Vibrometry technique is investigated. Natural frequencies and operating shapes are measured for a number of springs. These results are compared against springs that have been intentionally flawed to determine if the change in dynamic properties is a reasonable metric for damage detection.

Daniel P. Rohe

Chapter 13. Using High-Resolution Measurements to Update Finite Element Substructure Models

Many methods have been proposed for updating finite element matrices using experimentally derived modal parameters. By using these methods, a finite element model can be made to exactly match the experiment. These techniques have not achieved widespread use in finite element modeling because they introduce non-physical matrices. Recently, Scanning Laser Doppler Vibrometery (SLDV) has enabled finer measurement point resolution and more accurate measurement point placement with no mass loading compared to traditional accelerometer or roving hammer tests. Therefore, it is worth reinvestigating these updating procedures with high-resolution data inputs to determine if they are able to produce finite element models that are suitable for substructuring. A rough finite element model of an Ampair Wind Turbine Blade was created, and a SLDV measurement was performed that measured three-dimensional data at every node on one surface of the blade. This data was used to update the finite element model so that it exactly matched test data. A simple substructuring example of fixing the base of the blade was performed and compared to previously measured fixed-base data.

Daniel P. Rohe

Chapter 14. Determination of Representative Offshore Wind Turbine Locations for Fatigue Load Monitoring by Means of Hierarchical Clustering

A fundamental issue during the planning of offshore wind farms is to determine representative locations for fatigue load monitoring, which can be used to reduce maintenance costs. The contribution of this work is an integrated concept based on geometry variations of the jacket substructure. A hierarchical clustering algorithm, using distance measures between these variations, aims to group turbines according to similar fatigue behavior under consideration of local environmental conditions such as wind speed, water depth, and foundation stiffness. Based on this procedure, common jacket designs for each cluster are determined. Next, one location for each cluster is identified to be most suitable for monitoring. At last, uncertainties in fatigue lifetime for other locations in the cluster are given.

Andreas Ehrmann, Cristian Guillermo Gebhardt, Raimund Rolfes

Chapter 15. Effect of Friction-Induced Nonlinearity on OMA-Identified Dynamic Characteristics of Offshore Platform Models

The identification of the modal characteristics of engineering systems under operational conditions is commonly conducted with the use of the Operational Modal Analysis (OMA), being a class of useful tools employed within various fields of structural, mechanical as well as marine and naval engineering. The current OMA methods have been advanced on the basis of two fundamental, though, restrictive assumptions: (i) linearity and (ii) stationarity. Nevertheless, there are several applications that are inherently related to various nonlinear mechanisms, which, in turn, violate the two cornerstones of OMA and hence, question its robustness and efficiency. Along these lines, the current study addresses the effect of friction-induced nonlinearity on OMA-identified dynamic characteristics of an experimental set up consisting of a pair of reduced scale offshore platform models that are connected through a friction-based mechanism. Both time-domain and frequency-domain methods were employed to assess the effect of the varying friction-induced nonlinearity on the OMA-identified modal characteristics. The findings of this study reveal that OMA-based methods provide reasonable identification results implying that nonlinear and nonstationary systems can be described by underlying linear systems, even though, in principles, the basic assumptions of linearity and stationarity are violated.

Tobias Friis, Antonios Orfanos, Evangelos Katsanos, Sandro Amador, Rune Brincker

Chapter 16. Remote Damage Detection of Rotating Machinery

Condition-based monitoring (CBM) is a method of damage detection that actively monitors continuously operating machines to identify the earliest signs of deteriorating performance. There are many accepted methods of CBM, however this paper focuses on remote sensing of the vibration signatures of rotating machinery, e.g. an AC motor. Traditionally, vibration-based CBM requires accelerometers to be mounted directly on the machinery of interest; however, certain operating conditions may prevent such direct accelerometer access. This research aims to understand how to improve upon existing CBM methods by developing a transfer function which accounts for the propagation environment between the machinery of interest and an accelerometer placed some distance away, and then use this transfer function to perform damage detection. By taking baseline measurements at the start of the life on an AC motor as well as a spatially separated location, a transfer function between the two measurements can be developed in a similar fashion to a frequency response function. Using the developed transfer function and spectral analysis, remote measurements can then be used to reconstruct the vibration signatures at the motor. Furthermore, it will be shown that using this transfer function improves performance of a damage detector using a matched filter. These methods were tested using a motor mounted to a plate that is mounted on a wooden table as well as with a characterization motor loosely mounted on compliant padding. Also discussed are the damage detection results, as evidenced by the improvement in the receiver operating characteristic (ROC) curves of four damage modes, showing that capturing the propagation environment through the transfer function improves remote damage detection performance.

Peter H. Fickenwirth, Charles H. Liang, Tyrel C. Rupp, Eric B. Flynn, Adam J. Wachtor

Chapter 17. Experimental Demonstration of a Tunable Acoustoelastic System

Acoustoelastic coupling occurs when a hollow structure’s in-vacuo mode aligns with an acoustic mode of the internal cavity. The impact of this coupling on the total dynamic response of the structure can be quite severe depending on the similarity of the modal frequencies and shapes. Typically, acoustoelastic coupling is not a design feature, but rather an unfortunate result that must be remedied as modal tests are often used to correlate or validate finite element models of the uncoupled structure. Here, however, a test structure is intentionally designed such that multiple structural and acoustic modes are well-aligned, resulting in a coupled system that allows for an experimental investigation. Coupling in the system is first identified using a measure termed the magnification factor and the structural-acoustic interaction for a target mode is then measured. Modifications to the system demonstrate the dependency of the coupling with respect to changes in the mode shape and frequency proximity. This includes an investigation of several practical techniques used to decouple the system by altering the internal acoustic cavity, as well as the structure itself. Furthermore, acoustic absorption material effectively decoupled the structure while structural modifications, in their current form, proved unsuccessful. The most effective acoustic absorption method consisted of randomly distributing typical household paper towels in the acoustic cavity; a method that introduces negligible mass to the structural system with the additional advantages of being inexpensive and readily available.

Deborah Fowler, Garrett Lopp, Dhiraj Bansal, Ryan Schultz, Matthew Brake, Micah Shepherd

Chapter 18. Numerical Modeling of an Enclosed Cylinder

Finite element models are regularly used in many disciplines to predict dynamic behavior of a structure under certain loads and subject to various boundary conditions, in particular when analytical models cannot be used due to geometric complexity. One such example is a structure with an entrained fluid cavity. To assist an experimental study of the acoustoelastic effect, numerical studies of an enclosed cylinder were performed to design the test hardware. With a system that demonstrates acoustoelastic coupling, it was then desired to make changes to decouple the structure from the fluid by making changes to either the fluid or the structure. In this paper, simulation is used to apply various changes and observe the effects on the structural response to choose an effective decoupling approach for the experimental study.

Ryan Schultz, Micah Shepherd

Chapter 19. Exploiting Laser Doppler Vibrometry in Large Displacement Tests

Large-displacement vibration testing has assumed increasing importance to answer the need of predicting damages for excessive real life vibration. Moreover, the need for reducing weight is pushing towards the use of non-contact methods in such tests. Discrete Scanning Laser Doppler Vibrometry (SLDV) has resulted to be an important tool for contactless measuring the dynamic behavior of structures. However, the method suffers when high displacements are reached because the laser spot continuously moves on the target surface with an associated uncertainty on the actual measurement point location that increases proportionally to the increase of the deformation of the structure. Such a condition causes the vibration signal to look alike the one recorded by a Continuous Scanning Laser Doppler Vibrometry (CSLDV) approach, with sidebands that are more pronounced the higher the structure deformation is.This paper describes how SLDV can still be exploited for measuring on structures undergoing large displacements, with the further benefit of extracting information related to additional degrees of freedom deriving from the continuous scanning look-alike behavior of the measurement. This results in a better capability of the technique to extract incipient/potential damages on the structure under test. The concept is explored on simulated data representing a cantilever beam pushed to move at high displacements.

E. Copertaro, P. Chiariotti, M. Martarelli, P. Castellini

Chapter 20. A Rational Basis for Determining Vibration Signature of Shaft/Coupling Misalignment in Rotating Machinery

Shaft misalignment is the most common fault in rotating machinery besides unbalance. A poorly aligned machine can cost a factory upward of 30–40% in machine down time, replacement parts, inventory, and energy consumption. Vibration analysis is prompted as a most common methodology for determining misalignment while a machine is in operation. Considering the importance of alignment, the vibration spectrum of misalignment lacks a consensus and is elusive. This work is an evolution from the research performed on a large body of the vibration data to determine a unique vibration signature for shaft/coupling misalignment while operating under varying conditions such as speed, type and level of misalignment, coupling types and machinery dynamic stiffness. The data is analyzed from tests conducted on different machinery fault simulators operated at several shaft speeds, types of couplings, shaft diameters, structural stiffnesses, and multiple misalignment configurations. The results indicate a confusing picture of misalignment vibration signature. In this paper we present the results of vibration data analysis and outline an approach for vibration analysis of the shaft/coupling misalignment of rotating machines. This includes uses of rotor frequency response function and physics based predictive model.

Changrui Bai, Surendra (Suri) Ganeriwala, Nader Sawalhi

Chapter 21. Parametric Experimental Modal Analysis of a Modern Violin Based on a Guarneri del Gesù Model

Mechanical effects and dynamic behaviour of a modern violin, built on a Guarneri del Gesù model, were examined via parametric experimental modal analysis. The soundpost, a mobile component of a violin, has a particular relevance for the final acoustic performance, even if its dimensions are extremely small. Therefore, the aim of this first research is to understand the violin sensitivity to the soundpost position considering its influence on the overall structural-vibrational behaviour.Experimental modal analysis was performed in six different configurations, related to different positions of the soundpost inside the violin, including both the outer possible locations and optimal position, generally defined by the violinist according only to the perceived best acoustic performance. Six “Signature” modes were identified and tracked in all configurations, comparing mode shapes, damping and natural frequencies of involved modes, in order to find a correlation between mechanical vibrations and acoustic performance of the instrument.The effects of the soundpost position on the modal properties of the “Signature” mode shapes are highlighted and discussed. Finally, the potential role of soundpost as a practical engineering tool to improve the signature and sound quality is discussed.

Elvio Bonisoli, Marco Casazza, Domenico Lisitano, Luca Dimauro

Chapter 22. Influence of the Harmonics on the Modal Behavior of Wind Turbine Drivetrains

In the last decades, noise, vibration and harshness (NVH) problems became critical issues to be tackled by the wind industry. They have been caused by the upscaling trend that has imposed bigger (not quasi-static) loads on turbine subcomponents: the dynamic loads are significantly influencing the fatigue life of the wind turbine components and the tonalities generated. To validate complex simulation models, it is of high interest to continuously track the modal parameters of the fundamental modes of a wind turbine during operating conditions. At this purpose, operational modal analysis (OMA) represents a powerful tool.The work investigates and implements a completely automated OMA technique for continuously tracking the modes of a wind turbine drive train under normal operating conditions. The methodologies implemented are illustrated using data acquired during a long-term monitoring campaign of an offshore wind turbine. Modal estimation is based on the state-of-the-art pLSCF algorithm. To make it suitable for continuous analysis, the algorithm is improved by eliminating all the human interactions required. The procedure is then coupled with a method that automatically tracks the modal parameters along different data sets. Since this work focuses on the application of OMA on rotating machines, harmonics need to be dealt with. At this purpose, the use of a cepstrum lifter is analyzed and implemented. Modal estimates obtained from an automated analysis on stand still data and rotating turbine data are compared. The data coming from the rotating machine are pre-processed by means of a cepstrum-based procedure. It is shown that the automatic procedure is able to detect modes close to narrow harmonic components, while it still fails in case of broader harmonics. The cepstrum lifter is able to properly filter out also the harmonics influencing broader frequency bands, making OMA possible on the complete frequency band of interest.

N. Gioia, P. J. Daems, C. Peeters, M. El-Kafafy, P. Guillaume, J. Helsen

Chapter 23. The Influence of Geometrical Correlation in Modal Validation Using Automated 3D Metrology

Structural analysis is a major part of all manufacturing and testing industries. The need for high level accuracy of the results in the testing field has increased progressively, resulting in development of advanced state of art techniques. In order to acquire the vibrational characteristics of a structure, a detailed Finite Element Analysis (FEA) modelling is performed. Also, Experimental Modal Analysis (EMA) is conducted to extract the dynamic characteristics of a structure. The results obtained from both the processes are correlated for validation purposes. Based on the correlation (good or bad) the structural analysis is validated. In most cases the correlation is not satisfactory; it is mainly because of the boundary conditions that differ in FE and EMA.This research study explains in detail how important the boundary conditions are for modal validation. But the most imperative part, as the first step of correlation, is the geometry analysis. If the geometrical correlation is not accurate, the later part of correlation will turn out to be an assumption based on inaccuracies. Assumption of a geometrical correlation, without being sure of the differences, will lead to inaccurate results for validation.A reference plate is tested and simulated by using EMA and FEA techniques respectively. EMA is conducted by using a 3D SLDV for measuring the output response and the input force of excitation is induced by a Scalable Automatic Modal hammer (SAM). This plate is then scanned using ATOS Triple Scan II GOM 3D geometry scanner. The scanned results are compared with the FE model of the reference plate.The results presented show the importance of geometrical correlation for modal validation and provide results of deviations that were observed on a reference plate. With these conclusions, working on modal validation can be developed by reducing the inaccuracies for the presentation of correlation.

Tarun Teja Mallareddy, Daniel J. Alarcón, Sarah Schneider, Peter G. Blaschke
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