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See detailLOCALIZATION OF ENERGY IN A PERFECTLY SYMMETRIC BLADED DISK ASSEMBLY DUE TO NONLINEARITIES
Georgiades, Fotios; Peeters, Maxime ULg; Kerschen, Gaëtan ULg et al

in ASME Internaional Mechanical Engineering Congress and Exposition, Seattle, 2007 (2007, November)

Although a bladed disk is typically designed to have identical blades, manufacturing tolerances, wear, and other causes may cause random deviations among the blades. The blade-to-blade discrepancies ... [more ▼]

Although a bladed disk is typically designed to have identical blades, manufacturing tolerances, wear, and other causes may cause random deviations among the blades. The blade-to-blade discrepancies, denoted as mistuning, lead to vibratory responses mostly concentrated in small regions of the bladed-disk assembly, according to a phenomenon called localization. The resulting spatial confinement of the vibration energy causes the responses of some blades to become dangerously high and increases the amplitude of the bladed-disk assembly's overall response. The attendant increase in stresses can lead to premature high cycle fatigue (HCF) of the blades. In this study we investigate whether vibration localization in a perfectly symmetric bladed disk assembly may occur in the presence of nonlinearity. To this end, the nonlinear normal modes (NNMs) of a simplified model of a bladed disk assembly are computed. The NNMs are then carefully examined to highlight possible vibration localization phenomena. [less ▲]

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See detailSuppressing aeroelastic instability by means of broadband targeted energy transfers, part 2: Experiments
Lee, Y. S.; Kerschen, Gaëtan ULg; McFarland, D. M. et al

in AIAA Journal (2007), 45(10), 2391-2400

This paper presents experimental results corroborating the analysis developed in the companion paper, Part I (Lee, Y., Vakakis, A., Bergman, L., McFarland, M., and Kerschen G., "Suppression Aeroelastic ... [more ▼]

This paper presents experimental results corroborating the analysis developed in the companion paper, Part I (Lee, Y., Vakakis, A., Bergman, L., McFarland, M., and Kerschen G., "Suppression Aeroelastic Instability Using Broadband Passive Targeted Energy Transfers, Part 1: Theory," AIAA Journal, Vol. 45, No. 3, 2007, pp. 693-711), and demonstrates that a nonlinear energy sink can improve the stability of an aeroelastic system. The nonlinear energy sink was, in this case, attached to the heave (plunge) degree of freedom of a rigid airfoil which was supported in a low-speed wind tunnel by nonlinear springs separately adjustable in heave and pitch. This airfoil was found to exhibit a at flow speeds above the critical ('flutter") speed of 9.5 m/s, easily triggered by an initial heave displacement. After attachment of a single degree of freedom, essentially nonlinear energy sink to the wing, the combined system exhibited improved dynamic response as measured by the reduction or elimination of limit cycle oscillation at flow speeds significantly greater than the wing's critical speed. The design, application, and performance of the nonlinear energy sink are described herein, and the results obtained are compared to analytical predictions. The physics of the interaction of the sink with the wing is examined in detail. [less ▲]

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See detailBroadband passive targeted energy pumping from a linear dispersive rod to a lightweight essentially non-linear end attachment
Georgiades, Fotios; Vakakis, Alexander F.; Kerschen, Gaëtan ULg

in International Journal of Non-Linear Mechanics (2007), 42(5), 773-788

We examine non-linear resonant interactions between a damped and forced dispersive linear finite rod and a lightweight essentially non-linear end attachment. We show that these interactions may lead to ... [more ▼]

We examine non-linear resonant interactions between a damped and forced dispersive linear finite rod and a lightweight essentially non-linear end attachment. We show that these interactions may lead to passive, broadband and on-way targeted energy flow from the rod to the attachment, which acts, in essence, as non-linear energy sink (NES). The transient dynamics of this s stem subject to shock excitation is examined numerically using a finite element (FE) formulation. Parametric studies are performed to examine the regions in parameter space where optimal (maximal) efficiency of targeted energy pumping from the rod to the NES occurs. Signal processing of the transient time series is then performed. employing energy transfer and/or exchange measures, wavelet transforms, empirical mode decomposition and Hilbert transforms. By computing intrinsic mode functions (IMFs) of the transient responses of the NES and the edge of the rod, and examining resonance captures that occur between them, we are able to identify the non-linear resonance mechanisms that govern the (strong or weak) one-way energy transfers from the rod to the NES. The present study demonstrates the efficacy of using local lightweight non-linear attachments (NESs) as passive broadband energy absorbers of unwanted disturbances in continuous elastic structures, and investigates the dynamical mechanisms that govern the resonance interactions influencing this passive non-linear energy absorption. (c) 2007 Elsevier Ltd. All rights reserved. [less ▲]

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See detailComplex dynamics and targeted energy transfer in linear oscillators coupled to multi-degree-of-freedom essentially nonlinear attachments
Tsakirtzis, Stylianos; Panagopoulos, Panagiotis; Kerschen, Gaëtan ULg et al

in Nonlinear Dynamics (2007), 48(3), 285-318

We study the dynamics of a system of coupled linear oscillators with a multi-DOF end attachment with essential (nonlinearizable) stiffness nonlinearities. We show numerically that the multi-DOF attachment ... [more ▼]

We study the dynamics of a system of coupled linear oscillators with a multi-DOF end attachment with essential (nonlinearizable) stiffness nonlinearities. We show numerically that the multi-DOF attachment can passively absorb broadband energy from the linear system in a one-way, irreversible fashion, acting in essence as nonlinear energy sink (NES). Strong passive targeted energy transfer from the linear to the nonlinear subsystem is possible over wide frequency and energy ranges. In an effort to study the dynamics of the coupled system of oscillators, we study numerically and analytically the periodic orbits of the corresponding undamped and unforced hamiltonian system with asymptotics and reduction. We prove the existence of a family of countable infinity of periodic orbits that result from combined parametric and external resonance interactions of the masses of the NES. We numerically demonstrate that the topological structure of the periodic orbits in the frequency-energy plane of the hamiltonian system greatly influences the strength of targeted energy transfer in the damped system and, to a great extent, governs the overall transient damped dynamics. This work may be regarded as a contribution towards proving the efficacy the utilizing essentially nonlinear attachments as passive broadband boundary controllers. [less ▲]

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See detailEnhancing Robustness of Instability Suppression by Means of Multi-Degree-of-Freedom Nonlinear Energy Sinks
Lee, Young S.; Vakakis, Alexander F.; Bergman, Lawrence A. et al

in 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Honolulu, 2007 (2007, April)

Multi-degree-of-freedom nonlinear energy sinks (MDOF NESs) are utilized to improve robustness of suppression of limit cycle oscillations (LCOs) due to aeroelastic instability. Bifurcation analysis by a ... [more ▼]

Multi-degree-of-freedom nonlinear energy sinks (MDOF NESs) are utilized to improve robustness of suppression of limit cycle oscillations (LCOs) due to aeroelastic instability. Bifurcation analysis by a numerical continuation technique shows that controlling occurrence of a limit point cycle (LPC or saddle-node) bifurcation point above a Hopf bifurcation point is crucial to enhancing robustness. Not only greatly can MDOF NESs enhance the robustness of suppression against even strong external disturbances, but they can also yield a similar e±ciency even with a smaller mass, compared to the SDOF NESs with the same parameter conditions. Nonlinear modal interactions between the aeroelastic modes and the MDOF NES are examined to demonstrate e±ciency of the MDOF NES. [less ▲]

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See detailTheoretical and experimental study of multimodal targeted energy transfer in a system of coupled oscillators
Kerschen, Gaëtan ULg; Kowtko, Jeffrey J.; McFarland, D. Michael et al

in Nonlinear Dynamics (2007), 47

The purpose of this study is the theoretical and experimental investigation of targeted energy transfers from a two-degree-of-freedom primary structure to a nonlinear energy sink (NES). It is demonstrated ... [more ▼]

The purpose of this study is the theoretical and experimental investigation of targeted energy transfers from a two-degree-of-freedom primary structure to a nonlinear energy sink (NES). It is demonstrated that an NES can resonate with and extract energy from both modes of the primary structure. By facilitating these energy transfers, notably through excitation of appropriate periodic and quasi-periodic orbits, one can promote dissipation of a major portion of externally induced energy in the nonlinear attachment. [less ▲]

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See detailIdentification of nonlinear modal inter. between elastic continua nonlinear attachments
Geogiadis, F.; Tsakirtzis, S.; Panagopoulos, P. et al

in 1st int. Conf. on Computational Dynaics and Earthquake Eng., Rethymno, 2007 (2007)

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See detailSuppressing aeroelastic instability by means of broadband passive TET, part 1, theory
Lee, Y.; Vakakis, Alexander F.; Bergman, L. A. et al

in AIAA Journal (2007), 45

We study passive and nonlinear targeted energy transfers induced by resonant interactions between a singledegree- of-freedom nonlinear energy sink (NES) and a 2-DOF in-flow rigid wing model. We show that ... [more ▼]

We study passive and nonlinear targeted energy transfers induced by resonant interactions between a singledegree- of-freedom nonlinear energy sink (NES) and a 2-DOF in-flow rigid wing model. We show that it is feasible to partially or even completely suppress aeroelastic instability by passively transferring vibration energy from the wing to the NES in a one-way irreversible fashion. Moreover, this instability suppression is performed by partially or completely eliminating its triggering mechanism. Numerical parametric studies identify three main mechanisms for suppressing aeroelastic instability: recurring burstout and suppression, intermediate suppression, and complete elimination.Weinvestigate these mechanisms both numerically by the Hilbert–Huang transform and analytically by a complexification-averaging technique. Each suppression mechanism involves strong 1:1 resonance capture during which the NES absorbs and dissipates a significant portion of energy fed from the flow to the wing. Failure of suppression is associated with restoring the underlying triggering mechanism of instability, which is a series of superharmonic resonance captures followed by escapes from resonance. Finally, using a numerical continuation technique, we perform a bifurcation analysis to examine sensitive dependence on initial conditions and thus robustness of instability suppression. [less ▲]

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See detailIdentification of Linear and Nonlinear Systems using Signal Processing Techniques
Kerschen, Gaëtan ULg; Poncelet, Fabien ULg; Golinval, Jean-Claude ULg et al

in 26th Benelux Meeting on Systems and Control, Lommel, 2007 (2007)

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See detailWING-FLUTTER MITIGATION BY TARGETED ENERGY TRANSFERS INDUCED BY AN ESSENTIALLY NONLINEAR ATTACHMENT
Lee, Young S.; McFarland, D. Michael; Kerschen, Gaëtan ULg et al

in International Symposium on Recent Advances in Mechanics, Dynamical Systems and Probability Theory, Palermo, 2007 (2007)

The problem to be addressed here is the mitigation of limit cycle oscillations (LCO) in a rigid in-°ow wing with nonlinear heave and pitch sti®nesses in quasi-steady °ow, using targeted energy transfer ... [more ▼]

The problem to be addressed here is the mitigation of limit cycle oscillations (LCO) in a rigid in-°ow wing with nonlinear heave and pitch sti®nesses in quasi-steady °ow, using targeted energy transfer (TET). We show through simulation and Wavelet transforms that LCO formation is a process of resonance capture, with heave response triggering the pitch LCO. With the addition of a lightweight, fully passive attachment { a nonlinear energy sink (NES) { to the wing, we analytically predict three distinct mechanisms for LCO mitigation: repeated suppressed burst-outs, partial and complete suppressions of aeroelastic instability. Subsequent wind-tunnel experiments conducted in the Nonlinear Aeroelastic Test Apparatus (NATA) at Texas A&M University fully support these results. [less ▲]

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See detailComputer-aided analysis of rigid and flexible multibody systems
Verlinden, Olivier; Bruls, Olivier ULg; Conti, Calogero et al

Learning material (2007)

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See detailPhysical interpretation of independent component analysis in structural dynamics
Kerschen, Gaëtan ULg; Poncelet, Fabien ULg; Golinval, Jean-Claude ULg

in Mechanical Systems & Signal Processing (2007), 21

This paper focuses on the relation between the vibration modes of mechanical systems and the modes computed through a blind source separation technique called independent component analysis (ICA). For ... [more ▼]

This paper focuses on the relation between the vibration modes of mechanical systems and the modes computed through a blind source separation technique called independent component analysis (ICA). For free and random vibrations of weakly damped systems, a one-to-one relationship between the vibration modes and the ICA modes is demonstrated using the concept of virtual source. Based on this theoretical link, a time-domain structural system identification technique is proposed and is illustrated using numerical examples. [less ▲]

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See detailOutput-only modal analysis using blind source separation techniques
Poncelet, Fabien ULg; Kerschen, Gaëtan ULg; Golinval, Jean-Claude ULg et al

in Mechanical Systems & Signal Processing (2007), 21(6), 2335-2358

The present study carries out output-only modal analysis using two blind source separation (BSS) techniques, namely independent component analysis and second-order blind identification. The concept of ... [more ▼]

The present study carries out output-only modal analysis using two blind source separation (BSS) techniques, namely independent component analysis and second-order blind identification. The concept of virtual source is exploited and renders the application of these BSS techniques possible. The proposed modal analysis method is illustrated using numerical and experimental examples. (c) 2007 Elsevier Ltd. All rights reserved. [less ▲]

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See detailUsing Targeted Energy Transfer to Stabilize Drill-string Systems
Viguié, Régis ULg; Kerschen, Gaëtan ULg; Golinval, Jean-Claude ULg et al

in 25th International Modal Analysis Conference, Orlando, 2007 (2007)

Torsional vibration of the drill strings used in drilling oil and gas wells arises from a complex interaction of the dynamics of the drilling structure with speed-dependent effective rock-cutting forces ... [more ▼]

Torsional vibration of the drill strings used in drilling oil and gas wells arises from a complex interaction of the dynamics of the drilling structure with speed-dependent effective rock-cutting forces. These forces are often difficult to model, and contribute substantially to the problems of controlling the drilling operation so as to produce steady cutting. We consider here the use of a nonlinear energy sink (NES) , an attachment which has been shown to be effective in reducing or even eliminating self-excited motions in van der Pol and aeroelastic systems. The NES is a completely passive, inherently broadband vibration absorber capable of attracting and dissipating vibrational energy from primary structures, in this case nonlinear discontinuous models of drill-string systems. In this paper we describe a prototypical drill string-NES system, briefly discuss some of the analytical and computational tools suitable for its analysis, and then concentrate on mathematical results on the efficacy of the NES in this application and their physical Interpretation. [less ▲]

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See detailBLIND SOURCE SEPARATION TECHNIQUES FOR MODAL ANALYSIS: EXPERIMENTAL DEMONSTRATION
Poncelet, Fabien ULg; Kerschen, Gaëtan ULg; Golinval, Jean-Claude ULg et al

in International Conference on Engineering Dynamics, Carvoeiro, 2007 (2007)

The present study carries out output-only modal analysis using two blind source separation (BSS) techniques, namely independent component analysis and second-order based identification. It is shown under ... [more ▼]

The present study carries out output-only modal analysis using two blind source separation (BSS) techniques, namely independent component analysis and second-order based identification. It is shown under which circumstances the normal coordinates of the vibration modes may be interpreted as virtual sources, which renders the application of these BSS techniques possible. To support the theoretical findings, numerical experiments are first carried out using a discrete system of three masses. Secondly the method is applied to a real-life structure which is a blade extracted from a turbojet engine stator. The results are compared with the stochastic subspace identification method. [less ▲]

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See detailBlind source separation techniques - Another way of doing operational modal analysis
Poncelet, Fabien ULg; Kerschen, Gaëtan ULg; Golinval, Jean-Claude ULg et al

in International Operational Modal Analysis Conference, Copenhague, 2007 (2007)

In many practical applications, it is difficult or even impossible to measure the force applied to a structure. This is especially the case in civil engineering applications. Therefore, this paper ... [more ▼]

In many practical applications, it is difficult or even impossible to measure the force applied to a structure. This is especially the case in civil engineering applications. Therefore, this paper proposes to explore the utility of blind source separation (BSS) techniques for operational modal analysis. The basic idea of BSS is to recover unobserved source signals from their observed mixtures. The feasibility and practicality of the proposed method are demonstrated using an experimental application. [less ▲]

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See detailLEODIUM project and student space activities at the University of Liege
Vandenrijt, Jean-François ULg; Rochus, Pierre ULg; Kerschen, Gaëtan ULg et al

Conference (2007)

the origin of the LEODIUM project which stands for Low Earth Orbit Demonstration of Innovations in University Mode. This project aims to create student pico- or nano-satellites. There is a strong ... [more ▼]

the origin of the LEODIUM project which stands for Low Earth Orbit Demonstration of Innovations in University Mode. This project aims to create student pico- or nano-satellites. There is a strong motivation coming from the students and from the academic members to promote space. Being unexperienced in complete satellite design, as a first step in this direction, we started working with the ESA funded SSETI association. At the moment, two teams of students are working on SSETI satellites. One is working as the MECH team which is in charge of the design of the sun-pointing solar panels of the European Student Earth Orbiter (ESEO) satellite mechanisms, the other team is working on the phase-A studies of the Narrow Angle Camera (NAC) which will be the core payload of the European Student Moon Orbiter (ESMO) satellite, both in collaboration with local industrials. The students are mainly working on ESEO and ESMO as part of their master thesis. In addition to that, two PhD students are in charge of the coordination of the teams (one for the ESEO team and another for the ESMO team). They will also be the link between previous and new students every times an overlap will occur and ensure that information is not lost during the transition. In this paper, we will describe the current design and the achievements on the ESEO solar panels and the ESMO NAC camera. Explanation will also be given on how the collaboration between the local student and the SSETI association is managed. Finally, we will briefly introduce the prospective for the LEODIUM project. [less ▲]

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See detailPractical Computation of Nonlinear Normal Modes Using Numerical Continuation Techniques
Peeters, Maxime ULg; Viguié, Régis ULg; Kerschen, Gaëtan ULg et al

in International Workshop: Advancements in Design Optimization of Materials, Structures and Mechanical Systems, Xi'An, 2007 (2007)

The concept of nonlinear normal modes (NNMs) is considered in the present paper. One reason of the still limited use of NNMs in structural dynamics is that their computation is often regarded as ... [more ▼]

The concept of nonlinear normal modes (NNMs) is considered in the present paper. One reason of the still limited use of NNMs in structural dynamics is that their computation is often regarded as impractical. However, when resorting to numerical algorithms, we show that the NNM computation is possible with limited implementation effort, which paves the way to a practical method for determining the NNMs of nonlinear mechanical systems. The proposed algorithm relies on two main techniques, namely a shooting procedure and a method for the continuation of NNM motions. The algorithm is demonstrated using two different mechanical systems, a nonlinear two-degree-of-freedom system and a nonlinear cantilever beam discretized by the finite element method. [less ▲]

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See detailPARTICIPATION OF ULG STUDENTS TO THE SSETI MICROSATELLITE PROGRAMME USING SAMCEF
Bolland, M.; COLLIGNON, G.; VANDENPLAS, X. et al

Conference (2007)

The objective of this presentation is to discuss the participation of students at the University of Liège to the educational spacecraft project SSETI-ESEO proposed by the European Space Agency. SSETI ... [more ▼]

The objective of this presentation is to discuss the participation of students at the University of Liège to the educational spacecraft project SSETI-ESEO proposed by the European Space Agency. SSETI students are now mainly involved in the ESEO satellite for which the University of Liège is in charge of the MECH work package. The students have to propose a design for the structure of the solar panels (including the geometry and materials) and their deployment and pointing mechanisms (including electrical circuits, motors, hinges and actuators). Through detailed finite element computations in the SAMCEF software, they have to verify the structural integrity of the panels during the launch phase and to guarantee a safe deployment once on-orbit. Another task is to ensure an appropriate orientation of the solar panels to collect the maximum power from the sun. An industrial partner specialized in the development of computer-aided engineering solutions, SAMTECH, and a research center, Liège Space Center (CSL), also bring their expertise to the project. [less ▲]

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See detailPASSIVE SUPPRESSION OF AEROELASTIC INSTABILITIES OF IN-FLOW WINGS BY TARGETED ENERGY TRANSFERS TO LIGHTWEIGHT ESSENTIALLY NONLINEAR ATTACHMENTS
lee, Young S.; Vakakis, Alexander F.; Bergman, Lawrence A. et al

in International Forum on Aerolasticity and Strucural Dynamics, Stockholm, 2007 (2007)

Theoretical and experimental suppression of aeroelastic instabilities by means of broadband passive targeted energy transfers has been recently studied. A single-degree-offreedom (SDOF) nonlinear energy ... [more ▼]

Theoretical and experimental suppression of aeroelastic instabilities by means of broadband passive targeted energy transfers has been recently studied. A single-degree-offreedom (SDOF) nonlinear energy sink (NES) was coupled to a 2-DOF rigid wing modeled in the low-speed, subsonic regime with quasi-steady aerodynamic theory. The nonlinear attachment was designed and optimized to suppress the critical nonlinear modal energy exchanges between the flow and the (pitch and heave) wing modes, thus suppressing the (transient) triggering mechanism of aeroelastic instability. We performed bifurcation analysis to find regions of robust passive aeroelastic suppression in parameter space. Then, we employed multi-degreeof-freedom nonlinear energy sinks (MDOF NESs) to improve robustness of the aeroelastic instability suppression. Bifurcation analysis by a numerical continuation technique demonstrated that controlling the occurrence of a limit point cycle (LPC or saddle-node) bifurcation point above a Hopf bifurcation point is crucial to enhancing suppression robustness. MDOF NESs not only can enhance robustness of suppression against even strong gust-like disturbances, but they require lower NES mass compared to SDOF NES designs. The validity of the theoretical findings was proven by a series of wind tunnel experiments. [less ▲]

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