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D. J. Ewins - One of the best experts on this subject based on the ideXlab platform.
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extending Modal Testing technology for model validation of engineering structures with sparse nonlinearities a first case study
Mechanical Systems and Signal Processing, 2017Co-Authors: Arnaldo Delli Carri, Dario Di Maio, B Weekes, D. J. EwinsAbstract:Abstract Modal Testing is widely used today as a means of validating theoretical (Finite Element) models for the dynamic analysis of engineering structures, prior to these models being used for optimisation of product design. Current model validation methodology is confined to linear models and is primarily concerned with (i) correcting inaccurate model parameters and (ii) ensuring that sufficient elements are included for these cases, using measured data. Basic experience is that this works quite well, largely because the weaknesses in the models are relatively sparse and, as a result, are usually identifiable and correctable. The current state-of-the-art in linear model validation has contributed to an awareness that residual errors in FE models are increasingly the consequence of some unrepresented nonlinearity in the structure. In these cases, additional, higher order parameters are required to improve the model so that it can represent the nonlinear behaviour. This is opposed to the current practice of simply refining the mesh. Again, these nonlinear features are generally localised, and are often associated with joints. We seek to provide a procedure for extending existing Modal Testing to enable these nonlinear elements to be addressed using current nonlinear identification methods directed at detection, characterisation, location and then quantification – in order to enhance the elements in an FE model as necessary to describe nonlinear dynamic behaviour. Emphasis is placed on the outcome of these extended methods to relate specifically to the physical behaviour of the relevant components of the structure, rather than to the nonlinear response characteristics that are the result of their presence.
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a systematic approach to Modal Testing of nonlinear structures
31st International Modal Analysis Conference on Structural Dynamics IMAC 2013, 2014Co-Authors: D. J. Ewins, Delli A CarriAbstract:The application of experimental Modal analysis methods to nonlinear structures (sometimes referred to as “nonlinear Modal Testing” – NLMT) is not a new field, but only in the past few years has it become mature enough to be approached in a systematic way. Many methods have been developed over the years for dealing with nonlinearities in structural dynamics, but nonlinearity is an extremely complex phenomenon with so many aspects and consequences that is not possible to have a single method capable to deal with all of them. Rather than taking a holistic approach, it is perhaps useful for the engineer to have a set of mathematical tools to analyse separate subsets of the whole problem, i.e. one being within the scope of each individual investigation. The main objective of this paper is to provide a modular framework from which the engineer can choose the most appropriate method to retrieve information about an examined nonlinearity, based on the type of information needed and the available data set. This is achieved by performing a breakdown of the nonlinear Modal analysis process into four main stages: detection, localisation, characterisation and quantification – each of these providing a different level of insight into the problem. A review of currently-available algorithms applicable for these four categories is presented, as well as their application to two simple case studies.
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experimental non linear Modal Testing of an aircraft engine casing assembly
31st IMAC A Conference on Structural Dynamics 2013, 2013Co-Authors: Dario Di Maio, Paul N Bennett, C W Schwingshackl, D. J. EwinsAbstract:This paper aims to present experimental work on an aircraft engine casing assembly. Nowadays single components of casings can be modeled with such high accuracy that they can be validated by carrying out the model validation process using measured data from a sector of the entire assembly. This smart validation process can be achieved by carrying out the Modal analysis with a Scanning LDV (Laser Doppler Vibrometer) system which allows good spatial resolution of the measured mode shapes. The validation process can be assumed valid under linear response conditions obtainable for low vibration amplitudes. Casings are typically connected together by joints which may or may not respond non-linearly under high levels of vibration. Therefore, prior to conducting any non-linear validation, the mode(s) responding non-linearly must be identified beforehand in order to correctly specify the non-linear Modal Testing required. The work presented here will use a large civil engine casing assembly comprising a Combustion Chamber Outer Casing (CCOC), High Intermediate Pressure Turbine Casing (HIPTC) and Low Pressure Turbine Casing (LPTC.) The Fine Mesh Finite Element Model (FMFEM) was successfully validated using linear Modal analysis test data. One of the objectives of this work is to define the key points for conducting non-linear Modal Testing of such large casing assemblies and sub-assemblies. One outcome of the experimental work was a set of recommendations for performing measurements, which should be carried out within the frequency bandwidth selected during the model validation process. Experimentally derived non-linear response curves are presented in this paper.
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continuous scan a method for performing Modal Testing using meaningful measurement parameters part i
Mechanical Systems and Signal Processing, 2011Co-Authors: Dario Di Maio, D. J. EwinsAbstract:Abstract This paper presents the first part of a work about Modal Testing using meaningful measurement parameters. Scanning Laser Doppler Vibrometer (SLDV) systems are becoming largely used both in industry and university for performing vibration measurements. A reason for the success of SLDV systems can be found in their capability of measuring vibration remotely and under different environmental conditions which, when hostile, can inhibit other transducers to work correctly. Hence, SLDV system can be very practical and useful in many engineering applications. SLDV systems are being used as a contactless transducer measuring vibrations from a discrete number of measurement positions marked on the specimen whenever an optical access to it is available. Hence, the advantage of a Modal test carried out using accelerometers and one carried out using a SLDV system can be: (i) the automation of the measurements and (ii) the increase of the spatial resolution of the measured modes. This suggests that SLDV systems can be used as a practical replacement of accelerometers operating the same measurement method. Continuous Scanning method is a novel approach of using contactless transducers for measuring vibrations. The most important difference between a discrete and a continuous approach is the method of measuring a vibration pattern. A discrete method measures the level of vibrations at discrete positions on a structure whereas a continuous method captures the modulation of the vibrations produced by the excited modes. This is possible when a transducer can travel across a vibrating surface. This first part of the work presents a new approach of continuous scanning measurement method using a multi-tonal excitation waveform. The paper starts from a comparison between a step and continuous scan mode to introduce a novel approach of continuous scan and multi-tonal excitation waveform. The objective of this first part of work is to present and understand that measurement parameters, such as measurement positions, and can be carefully chosen to improve the measurement technique. A laboratory test piece and a helicopter tail cone are used as examples for the application of this new measurement method approach.
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Modal analysis and Testing of rotating structures
Philosophical Transactions of the Royal Society A, 2001Co-Authors: Izhak Bucher, D. J. EwinsAbstract:This paper surveys the state of the art of Modal Testing or experimental Modal analysis of rotating structures. When applied to ordinary, nonrotating structures, Modal Testing is considered to be w...
Thomas G. Carne - One of the best experts on this subject based on the ideXlab platform.
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Modal Testing for Validation of Blade Models
Wind Engineering, 2020Co-Authors: D. Todd Griffith, Thomas G. Carne, Joshua A. PaquetteAbstract:The focus of this paper is a test program designed for wind turbine blades. Model validation is a comprehensive undertaking which requires carefully designing and executing experiments, proposing appropriate physics-based models, and applying correlation techniques to improve these models based on the test data. Structural models are useful for making decisions when designing a new blade or assessing blade performance, and the process of model validation is needed to ensure the quality of these models. Blade Modal Testing is essential for validation of blade structural models, and this report discusses Modal test techniques required to achieve validation. Choices made in the design of a Modal test can significantly affect the final test result. This study aims to demonstrate the importance of the proper pre-test design and test technique for validating blade structural models.
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the inception of oma in the development of Modal Testing technology for wind turbines
Mechanical Systems and Signal Processing, 2010Co-Authors: Thomas G. Carne, George H. JamesAbstract:Wind turbines are immense, flexible structures with aerodynamic forces acting on the rotating blades at harmonics of the turbine rotational frequency. These harmonics are comparable to the Modal frequencies of the structure. Predicting and experimentally measuring the Modal frequencies of wind turbines have been important to their successful design and operation. Performing Modal tests on wind turbine structures over 100 m tall is a substantial challenge, which has inspired innovative developments in Modal test technology. For wind turbines, a further complication is that the Modal frequencies are dependent on the turbine rotation speed. The history and development of a new technique for acquiring the Modal parameters using output-only response data, called the Natural Excitation Technique (NExT), will be reviewed, showing historical tests and techniques. The initial attempts at output-only Modal Testing began in the late 1980s with the development of NExT in the 1990s. NExT was a predecessor to Operational Modal Analysis (OMA), developed to overcome these challenges of Testing immense structures excited with natural environmental inputs. We will trace the difficulties and successes of wind turbine Modal Testing from 1982 to the present.
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The Development of Modal Testing Technology for Wind Turbines: A Historical Perspective
2007Co-Authors: George H. James, Thomas G. CarneAbstract:Wind turbines are very large, flexible structures, with aerodynamic forces on the rotating blades producing periodic forces with frequencies at the harmonics of the rotation frequency. Due to design consideration, these rotational frequencies are comparable to the Modal frequencies; thus avoiding resonant conditions is a critical consideration. Consequently, predicting and experimentally validating the Modal frequencies of wind turbines has been important to their successful design and operation. Performing Modal tests on flexible structures over 120 meters tall is a substantial challenge, which has inspired innovative developments in Modal test technology. A further trial to the analyst and experimentalist is that the Modal frequencies are dependent on the turbine rotation speed, so Testing a parked turbine does not fully validate the analytical predictions. The history and development of this Modal Testing technology will be reviewed, showing historical tests and techniques, ranging from two-meter to 100-meter turbines for both parked and rotating tests. The NExT (Natural Excitation Technique) was developed in the 1990's, as a predecessor to OMA to overcome these challenges. We will trace the difficulties and successes of wind turbine Modal Testing over the past twenty-five years from 1982 to the present.
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support conditions for free boundary condition Modal Testing
Sound and Vibration, 2006Co-Authors: Thomas G. Carne, Daniel Todd Griffith, Miguel E CasiasAbstract:When Modal Testing a structure for model validation, free boundary-conditions are frequently approximated in the lab to compare with free boundary-condition analyses. Free conditions are used because they are normally easy to simulate analytically and easier to approximate experimentally than boundary conditions with fixed conditions. However, the free conditions can only be approximated in the lab because the structure must be supported in some manner. This paper investigates and quantifies the effects of the support conditions on both the measured Modal frequencies and damping factors. The investigation has determined that the measured Modal damping is significantly more sensitive to the support system (stiffness and damping) than the measured Modal frequency. Included in the paper are simple formulas which can be used to predict the effect on the measured Modal parameters given the support stiffness and damping.
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Modal Testing of wind turbines, then (1982) and now (2002)
2002Co-Authors: Thomas G. CarneAbstract:Wind turbines are very large, flexible structures which rotate in the wind with constant rotation speed. The aerodynamic inputs on the rotating blades produce periodic forces with frequencies equal to the rotation frequency and its harmonics. Typically, the turbine rotational frequencies are comparable to the structural Modal frequencies, thus resonance is a dangerous possibility. Consequently, predicting and experimentally verifying the Modal frequencies of wind turbines has always been critical to their successful design. Performing tests on structures that are as large as 100 meters tall and that require massive forces to excite is a challenge for Modal Testing. A further challenge to the analyst and experimentalist is that the Modal frequencies are dependent on the turbine rotation speed, so Testing a parked turbine does not fully validate the analytical predictions. The history and development of wind turbine Modal Testing will be reviewed, showing results from two-meter to 100-meter sized turbines for both parked and rotating tests. In 1982, Modal Testing hardware and software technology was in its infancy as compared to the present day. We will trace the difficulties and successes of Modal Testing over the past twenty years.
George H. James - One of the best experts on this subject based on the ideXlab platform.
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the inception of oma in the development of Modal Testing technology for wind turbines
Mechanical Systems and Signal Processing, 2010Co-Authors: Thomas G. Carne, George H. JamesAbstract:Wind turbines are immense, flexible structures with aerodynamic forces acting on the rotating blades at harmonics of the turbine rotational frequency. These harmonics are comparable to the Modal frequencies of the structure. Predicting and experimentally measuring the Modal frequencies of wind turbines have been important to their successful design and operation. Performing Modal tests on wind turbine structures over 100 m tall is a substantial challenge, which has inspired innovative developments in Modal test technology. For wind turbines, a further complication is that the Modal frequencies are dependent on the turbine rotation speed. The history and development of a new technique for acquiring the Modal parameters using output-only response data, called the Natural Excitation Technique (NExT), will be reviewed, showing historical tests and techniques. The initial attempts at output-only Modal Testing began in the late 1980s with the development of NExT in the 1990s. NExT was a predecessor to Operational Modal Analysis (OMA), developed to overcome these challenges of Testing immense structures excited with natural environmental inputs. We will trace the difficulties and successes of wind turbine Modal Testing from 1982 to the present.
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The Development of Modal Testing Technology for Wind Turbines: A Historical Perspective
2007Co-Authors: George H. James, Thomas G. CarneAbstract:Wind turbines are very large, flexible structures, with aerodynamic forces on the rotating blades producing periodic forces with frequencies at the harmonics of the rotation frequency. Due to design consideration, these rotational frequencies are comparable to the Modal frequencies; thus avoiding resonant conditions is a critical consideration. Consequently, predicting and experimentally validating the Modal frequencies of wind turbines has been important to their successful design and operation. Performing Modal tests on flexible structures over 120 meters tall is a substantial challenge, which has inspired innovative developments in Modal test technology. A further trial to the analyst and experimentalist is that the Modal frequencies are dependent on the turbine rotation speed, so Testing a parked turbine does not fully validate the analytical predictions. The history and development of this Modal Testing technology will be reviewed, showing historical tests and techniques, ranging from two-meter to 100-meter turbines for both parked and rotating tests. The NExT (Natural Excitation Technique) was developed in the 1990's, as a predecessor to OMA to overcome these challenges. We will trace the difficulties and successes of wind turbine Modal Testing over the past twenty-five years from 1982 to the present.
David J. Ewins - One of the best experts on this subject based on the ideXlab platform.
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Modal Testing: Theory, Practice and Application
Book, 2000Co-Authors: David J. EwinsAbstract:During the 1980s the technology of Modal Testing became very widely practised in all those engineering disciplines where vibration and other dynamic phenomena affect the behaviour and performance of structures and machines. The techniques involved in carrying out a Modal test were developed to a high degree of sophistication while the applications to which the results of these tests could be put became more numerous and more powerful. At the same time as the advantages of Modal Testing were being enjoyed by an increasing audience, some of the drawbacks of inexpert use of the technology were being learned and recorded. These experiences reinforced the need for a thorough understanding of fundamentals upon which Modal Testing is based, and of the detailed workings of the various phases and processes which make up a successful test. In this book, all the steps involved in planning, executing, interpreting and applying the results from a Modal test are described in straightforward terms. Efforts are made throughout to ensure that the reader understands the physics of the various stages as well as (if not before) the mathematics. This edition has brought the previous book up to date by including all the new and improved techniques which have emerged during the 15 years since the first edition was written. The more powerful applications are developed in more detail than previously and some new topics have been introduced, notable amongst which are the application of Modal Testing to rotating machinery and the use of the scanning laser vibrometer.
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THE SCANNING LASER DOPPLER VIBROMETER APPLIED TO IMPACT Modal Testing
IMAC XVII - 17th International Modal Analysis Conference, 1999Co-Authors: A. B. Stanbridge, Milena Martarelli, David J. EwinsAbstract:Traditionally, continuous scanning laser Doppler vibrometer (LDV) test techniques involve sinusoidal Testing of structures. These techniques are extended in this article to impact Testing. In an impact test, the response of a scanning LDV shows peaks in the frequency domain, which are spaced at multiples of the frequency of the scan, on both sides of each natural frequency. Modal analysis can be applied, and the resulting Modal constants can be processed to give mode shapes, angular vibration, or translational components of vibration at a point. The method appears to be comparable to normal impact Testing in speed and effectiveness, but with considerable advantages for Modal Testing.
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A closed-loop model for single/multi-shaker Modal Testing
Mechanical Systems and Signal Processing, 1991Co-Authors: Wai Ming To, David J. EwinsAbstract:Abstract The traditional form of a frequency response function (FRF) measurement for a single-shaker Modal Testing (also for multi-shaker Modal Testing) using random excitation is an open-loop system. However, inevitable physical constraints cause such FRF measurements to be inherently of a closed-loop form. This paper presents the effect of hidden feedback paths on some current FRF estimators. Attention has been drawn to the effects of noise and leakage on these estimators. The closed-loop model is generalised to explore the effect of shaker-structure interaction in two-shaker Modal Testing. The complexity of multi-shaker Modal Testing is illustrated by using a multi-input multi-output model from which a frequency domain technique is developed to give noise-free estimates of the FRFs. The applicability of the closed-loop model in a two-shaker sine dwell test is validated using results from experiments carried out on a circular disk.
Alemdar Bayraktar - One of the best experts on this subject based on the ideXlab platform.
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Vibration based Modal Testing of a scaled reinforced concrete building for construction stages
Bulletin of Earthquake Engineering, 2017Co-Authors: Temel Türker, Alemdar BayraktarAbstract:The dynamic behavior of buildings can change during construction stages. The brick walls cause a considerable effect on the Modal behavior depending on the wall configuration. In this paper, it is aimed to present the effects of construction stages (bare frame, brick-walled and coated cases) on the Modal parameters of reinforced concrete (RC) buildings. For this purpose, a three-storey RC building model with a 1/2 scale was constructed in the laboratory of Civil Engineering Department at Karadeniz Technical University. The Modal Testing measurements were performed by using operational Modal analysis method for the bare frame, brick walled and coated cases of the building model. Randomly generated loads by impact hammer were used to vibrate the building model; the responses were measured by uni-axial seismic accelerometers as acceleration. The building’s Modal parameters at these construction stages were extracted from the processed signals using the enhanced frequency domain decomposition technique. The natural frequencies of the building model varied depending on the construction stages considerably. The frequencies decreased by increasing number of stories. But the brick walls caused a significant increase in the natural frequencies and affected the Modal behavior that the longitudinal response occurred at first mode.
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finite element model calibration of berke arch dam using operational Modal Testing
Journal of Vibration and Control, 2011Co-Authors: Baris Sevim, Alemdar Bayraktar, Ahmet Can AltunisikAbstract:This paper presents the finite element calibration of Berke Arch Dam by using Operational Modal Testing. Achievement of this purpose involves structural vibration characteristics of Berke Arch Dam using analytical and Operational Modal Analyses. Therefore, the study has two parts — analytical and experimental. In the analytical part of the study, the authors developed a 3D finite element model of Berke Arch Dam-reservoir-foundation system using ANSYS software, and analytically determined vibration characteristics such as natural frequencies and mode shapes. In the experimental part of the study, sensitive accelerometers were placed to several points of Berke Arch Dam, and ambient vibration tests were conducted over four days to obtain dynamic characteristics. The Enhanced Frequency Domain Decomposition technique is used to estimate natural frequencies, mode shapes and damping ratios of the Berke Arch Dam experimentally. Results showed that there were some differences between analytical and experimental na...